How does the thickness of a wing affect its aspect ratio?

Indeed, the aspect ratio is defined as you describe. It is a way to define how slender the wing is for a given wing area, which is the main parameter in determining how to generate enough lift to keep the aircraft aloft. An area is two-dimensional, and a broadly rectangular shape like a wing area is defined by the length and the breadth of it: span and chord.

Aspect ratio describes the proportions of span and chord: a short stubby wing, or a long and slender one? This is one of the main parameters in how much drag the wing creates: a long slender wing minimises one particular form of aerodynamic drag, called induced drag.

 Image source

But a wing is a cantilever beam, subjected to aerodynamic lift. The lift forces bend the wing upwards, and the wing structure must produce internal forces that cancel out the aerodynamic lift. In a hollow cantilever beam like a wing, the skin mainly does that: the upper skin resists compression, and the lower skin resists tension. The further away the upper and lower skin are, the lower the stresses. A structural engineer wants to give as much distance between the plates as possible: he wants the wing to be very thick, in order for it to be light in weight.

But again aerodynamics puts a limit to this: a very thick wing would create too much form drag, so the structures people are given the maximum distance that the aerodynamics can support. Wing thickness affects aspect ratio in defining the maximum AR that can be achieved.

The wing thickness is not part of the induced drag, and does not appear in the definition of the aspect ratio. It is an important defining parameter in wing weight and other forms of drag.


How to do Weight & Balance for Cessna 172?

The Calculation section of “Center of gravity of an aircraft” on Wikipedia gives an overview of the steps.

  Determine the weights and arms of all mass within the aircraft.
  Multiply weights by arms for all mass to calculate moments.
  Add the moments of all mass together.
  Divide the total moment by the total weight of the aircraft to give an overall arm.

The Basic Empty Weight (BEW) of the aircraft will come from the Weight & Balance information that is unique to each aircraft. The computed arm for the BEW may have some rounding error, so treat it as informational. Use the precise figures for Weight and Moment from your aircraft’s W&B.

The arms, distances from some reference datum, will also come from your aircraft’s W&B info: fuel, front seats, rear seats, and baggage areas on a Cessna 172. Copy those into your table as well.

Avgas (100LL) weighs 6 pounds per gallon, so the weight is 6 times the number of gallons.

I like to work top-to-bottom and left to right. For the grand finale, divide the sum of the Moment column by the sum of the Weight column to get the overall center of gravity.

The example below is from my PP-ASEL checkride taken in a Cessna 172. The items in boldface are computed values. The digits after the decimal are there for typographical purposes and do not necessarily indicate significant figures.

$$\begin{array}{c|r|r|r|}
                     & \textit{Weight} & \textit{Arm} & \textit{Moment} \\ \hline
\text{Airplane}      & 1{,}665.6     & 39.2       & 65{,}928.93      \\
                     &               &            &                  \\
\text{Fuel (50 gal)} & \bf{300.0}    & 48.0       & \bf{14{,}400.00} \\
                     &               &            &                  \\
\text{Bacon}         & 180.0         & 37.0       & \bf{6{,}660.00}  \\
\text{Shelton}       & 172.0         & 37.0       & \bf{6{,}364.00}  \\
                     &               &            &                  \\ \text{Duffel}        & 10.0          & 73.0       & \bf{730.00}      \\
\text{Shelton bag}   & 10.0          & 73.0       & \bf{730.00}      \\
                     &               &            &                  \\
\text{Backpack}      & 15.0          & 93.3       & \bf{1{,}399.50}  \\ \hline
                     & \bf{2{,}352.6}& \bf{40.9}  & \bf{96{,}212.43}
\end{array}$$

In this particular case—given for information only—the center of gravity (40.9 ≈ 96,212.43 / 2,352.6) at the total weight is within limits for the Normal category as given in the aircraft’s Pilot Operating Handbook (POH). Your case may vary. The Pilot-in-Command is responsible for computing Weight & Balance for each flight.

As a point of interest, my DPE was Clyde Shelton who recently flew his ten thousandth checkride.


How to communicate a circling approach over CTAF?

"Overflying the field at [whatever] MSL to enter the left downwind for runway 18"
is what I would call at a nontowered field.  It's also the phraseology that tower controllers have used when I've done that particular approach at a towered field.


What are ATC's considerations when they instruct to intercept final approach course instead of simply clearing for approach?

The main reasons I will instruct a pilot to intercept and follow the localiser instead of clearing him for the ILS are separation and minimum safe altitudes.

In a busy approach environment, it is often beneficial to let two aircraft intercept the approach with 1000 ft vertical separation. Maybe the aircraft are slightly too close horizontally but the first one is faster than the second one. I can then have then both follow the approach course, and descent number two on the approach once horizontal separation exists.

As for minimum safe altitudes, if you are approaching an airport on a heading straight in toward the runway, maybe there is room for you to intercept the approach course a large distance from touchdown. However, at this distance it might not be possible to descend to the approach altitudes, due to terrain or airspace restrictions. In this case I will also clear you to follow the localiser and then give the approach clearance later.


How are airlines able to work out gate allocations so far in advance?

Route times are pretty well known. If you know when the plane should depart and how long the flight should last, it's pretty easy to calculate when it will be arriving. Give a few minutes leeway on either side, and you should be good. If the plane is not there on time, you send it somewhere else.

Most large aircraft have some sort of ACARS that allows them to talk to company dispatch and let them know if they're going to be early or late, among other things. Barring that, they almost always have a company radio frequency that they can relay messages on one way or another.


You should wash your aircraft in accordance with the manufacturer's instructions.

We could all offer opinions and product recommendations in response to this question. However, the best answer is: in accordance with the manufacturer's instructions. Since you ask the question generically, without any information on what aircraft you might have, I will answer generically.

Your manufacturer's instructions can generally be found in at least two places: the owner's manual/AFM and the maintenance manual. For example, in the owners manual for a 1975 Cessna 172M, you will find Section V: Care of The Airplane containing instructions on how to wash painted surfaces, bare aluminum surfaces, interior surfaces, and the windscreen and windows. The maintenance manual for the same aircraft contains nearly—if not exactly—identical information in chapter 2.

Now, for that aircraft and others, Cessna recommends use of mild detergent in certain applications; you might not find that overly helpful in choosing what to use. Based on my research and experience, and time honored practice that I've observed, Dawn dish soap is widely used for cleaning painted Cessna aircraft with no apparent long term harmful effects. The airframe that I fly for work has tens of thousands of hours on it, and while I cannot vouch for how often it was washed in the past, I try to wash parts of it a few times a week. There is no apparent washing damage to the paint.

I will also highlight Cessna's instruction to use Stoddard solvent on areas of stubborn oil or grease; this can be especially helpful in cleaning the belly with it's typical collection of oil and dirt. My practice is typically to wipe down the belly with solvent first, and then follow with a water and detergent wash.

If the manufacturer's instructions are not clear, or are otherwise insufficient to answer your question regarding your specific aircraft, I would recommend asking your maintenance professional. He or she should have both the knowledge and experience to steer you in the right direction.

For more insight into the world of aircraft cleaning, and especially for the specific product recommendations you want, you might take a look at this AvWeb article by Kim Santerre: Aircraft Cleaning.


If both wings have the same incidence and the wings are not staggered (= they sit at the same lengthwise location along the fuselage), then yes, their lift is split equally between both.

Most biplanes were (and sometimes still are) single- or two-seaters and in order to give the pilot good forward and upward visibility, the upper wing is shifted forward a bit (positive stagger). This will place it closer to the upwash (flow field ahead of the wing with increased angle of attack) of the bottom wing and in turn place the bottom wing closer to the downwash (flow field past the wing with reduced angle of attack) of the top wing. This effect becomes stronger with increasing angle of attack of the whole biplane, in line with the lift coefficient. With positive stagger, the upper, forward wing carries the majority of the load. How much that is depends obviously on the amount of stagger.

The formula for the interference drag is the Biot-Savart law which describes the effect of one lift-creating object on its surroundings. Since both wings interfere mutually, the solution is not straightforward but needs several steps:

First, the location of a lift-creating vortex and its two trailing vortices is defined. A wing should be split into several sections which all have their own vortex strength. The more sections, the more precise the result will be. For a biplane this needs to be done twice, of course.
Then a number of points where the local flow direction is prescribed must be defined. Whereas the vortex usually is located at the quarter of the wing chord, those points are best placed at three quarters of the wing chord; again spread over the span of the wings. They are called control points.
Now the influence of a particular vortex on a particular control point is described using the Biot-Savart law. After this has been done for all vortices and all control points, those influences define a matrix.
By solving the matrix equation for tangential flow at the control points, the strength of the single vortices in the influence of all other vortices can be calculated.

A more elaborate description of the process can be found in NASA Technical Note D-5335.


Why should the centre of mass be ahead of the centre of pressure?

On most airplanes, the horizontal stabilizer provides a downward lift. The center of mass is between the nose gear and the main gears.

If the center of mass is too aft:

Stall (and spin) recovery will be difficult or even impossible
Increases the chance of tail strike
The plane will tip on its tail if the main gears are not moved aft

If center of mass is aft of center of pressure, then the horizontal stabilizer must provide positive (upward) lift; the tail structure must be made stronger to support the higher lift. The wings will be moved forward of the main gears to keep the aircraft balanced on the ground. Such designs are called Tandem wing. See also this question.


How do engineers log airplane maintenance?

Natural language is concrete enough for this purpose, the logs aren't meant for machine parsing but instead for pilots, inspectors and other mechanics.

Having such a code system requires mechanics to memorize all of the codes or have a massive list handy when filling out the logs.

Also making such an international code system requires agencies of all countries/manufacturers to sit together and set those codes for all possible maintenance actions. There is a very high chance an action will be missed so a natural language box for "additional comments" is still required. It also needs to be rolled out to the actual mechanics filling out the logs.


What mechanism did the turbine reversers of early high-bypass turbofans use?

You're asking about turbine (hot) reversers of early high-bypass turbofans.
747

flickr.com and twitter.com

It had the same mechanism as the cold bypass (blocking doors), similar to what the C-17 (right image) uses nowadays.
L-1011

Lockheed L-1011 TriStar - Airliner Tech Vol. 8

It had target-type, but only during development.
DC-10

Design Features of the CF6 Engine Thrust Reverser and Spoiler (sae.org) and airliners.net

It had– "something else."


Why is a strake placed on the fuselage in front of the tailplane on the Hawk aircraft?

You may have noted that the tailplane of the Hawk is a full-flying surface, also called an all-moving surface. When the stick is pulled aft, the leading edge will align with the strake, forming in effect a leading edge extension. This will produce a vortex over the lower surface of the inner tailplane and prevent early flow separation at the tailplane root at low speed.

The obvious next question is: Why is it not part of the tailplane itself? Adding it to the tailplane would increase drag and produce nonlinear control forces. Since it was added later in the life of the design, fixing it to the fuselage allowed to keep the tail surfaces and their actuation unchanged.

Note that early Hawk versions (Mk.1, Hawk 50) lacked this detail. In flight testing, it was found that the Hawk would suddenly nose-dive with full flaps and gear up. This was dubbed "phantom dive". Cutting back the flaps solved that behavior, but later, heavier versions of the Hawk needed more lift, so a different fix had to be found. From the Hawk story by Harry Fraser-Mitchell:

  It was shown with the half model of the Hawk at Hatfield that high
  local downwash at the tail, coupled with the very large nose down
  pitching moment induced by the flap, was causing the tailplane to
  stall on its lower surface, so that it could no longer provide
  adequate balancing power. It needed more lift, extended to higher
  angles of attack.

  In the case of the F-4K Phantom aircraft this was achieved by
  installing a fixed leading edge slot to the tailplane, harking back to
  demonstrations of such devices by Handley Page on wings in the early
  Twenties! A fixed slot with its associated drag was not an option on
  the Hawk although a cambered tailplane was tried on the model with
  some success. Removal of the outboard vane of the flap reduced the
  flap pitching moment to such a value that the standard tailplane could
  cope, so this was the quick solution for the RAF. However, for the US
  Navy VTX project, (and for later combat versions of the Hawk) the
  maximum possible lift was required, so that at least the outer flap
  vane had to be replaced. The dive phenomenon had to be fixed.

  An example of the cross-fertilization of knowledge due to the matrix
  working of the design department now occurred. Barry Pegram, then
  Section leader of the Fluid Dynamics section of the Aerodynamics
  Department, had been working on the adoption of leading edge root
  extensions (LERX) for the “Harrier” wing, and this work had shown that
  these devices extended the lift of the wing to higher angles of attack
  by virtue of the non-linear lift developed by the vortex flow they
  created. He proposed that these should be added to the tailplane of
  the Hawk model, but this would have had a serious effect on the
  tailplane hinge moments. The author, who was working with Barry in the
  V/STOL tunnel at Hatfield, suggested that the ‘tailplane canard vane’
  (TCV), as it was called, could be fixed to the fuselage at such a
  position that it was lined up with the flow at normal conditions, but
  with its trailing edge adjacent to, with a small clearance, the
  leading edge of the tailplane at its maximum nose down position.

  Experimenting showed that these vanes could be made quite small and
  they gave a complete cure to the problem in the wind tunnel, with very
  little drag in the normal flight regime. To prove the concept in
  flight, some temporary vanes were manufactured which could rapidly be
  fitted to one of the test aircraft which had flaps with the full vane.
  First, the aircraft was flown without the TCV to establish the
  conditions under which the ‘Phantom Dive’ occurred on that particular
  aircraft. On a later flight, the TCVs were fitted and the aircraft
  flown again to the identical conditions as before. Despite every
  attempt by the pilot to instigate the phenomenon, it did not occur –
  the vanes were a complete success, even though they looked
  inconspicuously small for such a large effect.


Are STOL kits that use leading edge slats available for 'typical' GA aircraft?

Lowering stall speed is not a benefit in itself, it's only a benefit if it you need it. There are STOL kits for many airplanes, you don't see them much on most airplanes because there's no real benefits in normal flying and they cost a fair chunk of money to install. STOL kits only get you so far, adding a STOL kit onto an airplane with a 50 kt stall speed will not give you a 25kt stall speed, it's a lot less than that. If you have a bush airplane, fly into very short fields or in high density altitudes then a STOL kit can give you extra capabilities. If you don't fly into very short fields then a STOL kit is extra weight and costs for capabilities you won't use.

It's also not as simple as simply sticking some extra bits on the front of the wing, there's many considerations. For one, if you drop an airplane's stall speed substantially then you will run into control authority issues at slow speed. You will have to beef up the ailerons, rudder and elevator to compensate, which is a substantial re-design, adding costs. The most popular STOL kits are for airplanes which already are good STOL aircraft to begin with and have adequate control authority.


Is it common for carriers to have separate certification for analog versus glass cockpits?

Pilot type ratings are usually same for both versions (analog and digital cockpits) of the same aircraft as far as regulations are concerned. For transition to glass cockpit, you may have to take a Electronic Flight Instrumentation Course (or differences training course).

In some cases of transition (for example, EASA  ATR 42/72 (glass cockpit, non
ECL) to any other ATR 42/72 'classic'), a full type rating training course is required; however, the type rating is still the same.

For example, see the FAA and EASA type rating lists.


Referring to the B737NG, at what point (height) does a TOGA revert to LVL CHG after the takeoff?

LNAV & VNAV can be armed before takeoff; HDG SEL cannot be similarly armed.
If VNAV is armed & LNAV is not, then when VNAV engages at 400', the lateral mode switches to HDG SEL. But that's because the pilots had armed VNAV.
Taking off in TO/GA, the system won't revert on its own to LVL CHG. You can climb into the flight levels still in TOGA, although it's hard to think of a good reason to do do. If TOGA is active when the autopilot is engaged, the vertical mode will change at that point to LVL CHG, with V2 + 20 as the commanded airspeed.
As to why there is no option to arm LVL CHG on the ground, there is no need to. TOGA gives you speed guidance to bugged speeds (starting with V2 + 20) anyway, and VNAV gives you guidance to the profile speeds. Why offer another mode change that adds nothing to what you already have?


Where is the toilet smoke detection indication displayed for a B777-200/300?

From the 777 manual:

  Each lavatory has a single smoke detector. If smoke is detected, an aural alert
  sounds in the lavatory and in the cabin. In addition, the lavatory call light
  flashes and the master call light at the associated attendant station illuminates.
  The EICAS advisory message SMOKE LAVATORY indicates smoke is detected in
  one of the lavatories.

The EICAS is a display in the cockpit.

It is treated as a fire, and the cabin and flight crews follow the set procedures to handle it.


What is the brace position for cockpit crew?

You don't brace -- you're still operating the aircraft!  The pilot seats have shoulder harnesses; in many cases you can lock them, plus they also have inertial reels that keep you from slamming forward at an impact.  Additionally, the pilots can presumably see the moment of impact coming, and tense for exactly that moment in a way that passengers cannot.


In the U.S., what are the requirements for being an Executive airport?

This is a marketing distinction -- generally applied to a major GA reliever airport designed to serve light/business jets in design categories B-II and C-II or perhaps C-III.  This is distinguished from general purpose GA relievers as a light GA reliever may be designed to service a maximum design category of A-II, B-I, or B-II; it is also distinguished from County, Municipal, or Regional airports as while they are similarly sized in terms of design category, a County, Municipal, or Regional airport is the primary airport for that area as opposed to being a reliever for a larger scheduled service airport.


How is drone (UAS) traffic regulated in the US?

Regulations regarding UAVs are very restrictive in controlled airspace. Whenever a company wants to test out a UAS, they have to request a block of airspace from the FAA (or local ATC body). This block is restricted to UAS traffic. No manned aircraft are allowed in that area for the duration of the test.

Currently companies are working with the FAA to integrate UAVs into the regular airspace system. Recently, the FAA designated 6 states as test sites for the FAA to test UAVs.


Would a headwind result in a faster propeller rpm?

The problem is you're holding the aircraft with your hand.

Imagine a windmill. It has no power at all, it spins freely. What happens when wind is blowing at it? It turns.

Now imagine a propeller with no power connected to it, so it acts like a windmill. If wind is blowing at it, it will turn just the same. Why? Because a propeller pushes wind backward when it turns. When it is facing a headwind, the propeller wants to turn such that it is pushing air at the same speed as the arriving air.

In your case, I suppose the fan is blowing air at a faster speed, compared to the propeller is pushing air on its motor. Say the motor pushes air backward at 1 m/s. The fan blows air at 2 m/s. That's why it spins faster with the fan on.

But that's only true as long as you're holding the aircraft with your hand.

In flight, nothing is holding the aircraft at a constant ground speed. For example, assume in a no wind condition, an aircraft is flying at 80 knots. Suddenly it encounters a 10 knots headwind, and continues to fly into this headwind for the remainder of the flight. What happens?

At the moment of encountering the headwind, the airspeed indicator in the cockpit jumps from 80 to 90. The propeller rpm will also suddenly jump to a higher value. But that is not going to last. The aircraft is getting pushed backward by the wind, the airspeed is going to slowly drop back to 80. There is not enough power to keep the propeller at the higher rpm which pushes air at 90, it can only to 80. After a while, it is flying at 80 knots relative to the air, and 70 knots relative to the ground.


What is the highest number of airliners shown flying together in a single photo?

The image found here shows 13 DC-3's in formation which were used as airliners at one point but these are in military livery in the photo.
Here is the kind of photo you describe but only 3 aircraft are in view. Keep in mind aircraft are generally turned onto the final approach path and only a few may be lined up straight off the runway at any given time.
The airbus operation you link, may be the biggest instance of passenger jets flying in tight formation.


What is the minimum separation between two landing A380s?

In general, the heavier an aircraft is, the greater the wake it produces but, it is also less affected by the wake of other aircraft.  In particular, an aircraft can always resist the wake turbulence of a lighter category aircraft.

It was found that the A380 was not affected by wake turbulence from any aircraft in front, including another A380, down to the minimum RADAR separation used on approaches which is 3NM.

The RADAR separation used on any particular approach might be greater, but not less than this minimum therefore, there is no need to apply wake turbulence separation to a following A380 and the required RADAR separation for the approach is applied.


Is it common for panels on a plane's body to not be perfectly aligned?

It looks like you were seeing simple skin lap joints.  Flush skin butt joints are generally used on high speed aircraft, but these joints are more complex to make (you have to use a joining splice strip under the butt joint).

A lot of the time the manufacturer determines that the local airflow isn't smooth right at the surface (there is a turbulent boundary layer that is maybe 1/8" deep) and a small step in the surface has a negligible effect on drag.  If this is the case, a simple lap joint, that doesn't require the joining splice strip underneath, is used.

There usually isn't a hard 90 degree step on lap joints because it's normal practice to put a fillet of sealant along the edge to make the higher skin taper toward the lower one and this mitigates any drag effects further.


What special handling characteristics or techniques do biplanes have?

Short answer:

For clarification: Equivalence between biplane and monoplane means than both have the same wing area and the same engine. Then the main differences in maneuvering are:

The biplane has better roll acceleration than an equivalent monoplane.
The biplane has a higher roll rate than an equivalent monoplane at the same speeds.
All biplane flying takes place at lower speeds, resulting in a lower space requirement for all maneuvers. This also means that inertial effects are less pronounced: When pulling up, there is less kinetic energy available for climbing, so (for example) hammerhead turns will end with less altitude gain.

Differences in handling: The biplane has

lower aileron forces for the same roll rate at the same speed
lighter control forces overall due to lower flight speed

Differences in performance:

shorter take-off and landing distances
a lower stall speed
a much lower maximum speed
a lower optimum cruise speed and range
a lower power requirement due to the lower flying speeds, or if both use the same engine, a better power-to-weight ratio

when compared to an equivalent monoplane. These differences are most pronounced if the airplane carries just the pilot and not much payload.

Flying techniques are the same as for monoplanes. Indirectly, differences are likely due to differences in design. Example: Few biplanes profit from having a retractable landing gear while gear retraction makes sense for monoplanes with higher power loading (installed power relative to wing area).

Explanation

Biplanes have two major differences:

Smaller wing span at the same wing area, and
Wire bracing results in very lightweight biplane wings.

The smaller span reduces roll damping and roll inertia, so a biplane will accelerate into a roll more quickly than an equivalent monoplane and will reach a higher roll rate. This is the main difference in maneuvering.

The smaller wing span results in more induced drag if both have the same mass and the same speed. With wire bracing, this condition is unrealistic, and an equivalent biplane will be much lighter. If the structure is a substantial part of the aircraft's mass (this is typical for aerobatic airplanes), the result can easily be less induced drag, despite the lower span, and also lower wing loading. This in turn means that both fly at different speeds: The biplane will be able to fly much slower, but the aerodynamic drag of the bracing will restrict it to low speeds. This also means inertial effects are less pronounced: The lower mass and lower speed of the biplane combine for a marked difference to the equivalent monoplane.

For aerobatic displays this is ideal: All action takes place close to the audience, and the biplane will need a much smaller area for all maneuvers than an equivalent, but heavier monoplane. The downside are low maximum speed and low range.

Another difference in performance are much shorter take-off and landing distances due to the lower wing loading, which results in a lower stall speed. The optimum endurance and optimum range speeds are lower than those of an equivalent monoplane as well, so all biplane flying happens at lower speeds, which is beneficial for training aircraft.

Since control forces are proportional to dynamic pressure, a biplane will have lower control forces than an equivalent monoplane. Here equivalence also means that the relative chord of all control surfaces is the same. In reality, a good designer will select a higher relative chord for the biplane's control surfaces to ensure that control forces are above their required minimum.

The heavy, unreliable engines of the early years made biplanes the ideal way of taking to the air. Once engines became more powerful and allowed higher payloads, the monoplane became better suited to carrying passengers and freight at higher speeds and over longer distances.


How are winter weather operations different if they are based on an aircraft carrier instead of a runway on land?

It's true, carriers typically don't have snow removal and deicing equipment (like trucks, but they do have fluid and spray equipment). They don't have this because they regularly avoid weather that could cause significant icing or snow (carriers move pretty quick and weather is somewhat predictable).
But it certainly isn't unheard of that they encounter snow or ice.
The carrier deck
The deck is somewhat cleared using tools like shovels:

Source: TheDrive.com
They also have a "sweeper" truck that can be used to sweep smaller amounts of snow off the deck:

Source: TheDrive.com
This is an older picture, but the equipment is still on modern carriers. They use it primarily for cleaning the deck and after incidents requiring broad FOD removal.
So one thing is in abundant supply on carriers, manual labor, and a lot of snow is removed doing just that. Teams working with shovels and dumping it overboard.
The Russians have a tool primarily used for FOD clearing, but is pretty effective at deicing the carrier deck:

This is basically a jet engine (same one used in MIG-15's) that is mounted to the front of a tractor with a diffuser on the front.
Aircraft
The aircraft have an advantage that most of them can be stored inside when they have icing conditions. Aircraft that are caught outside are manually cleaned using things like brooms and even baseball bats.

Source: TheDrive.com
They also have manual de-icing spray systems:

Source: US National Archives


What is the exact meaning of CAVOK and CAVU? How are they used by pilots and ATC?

CAVOK is formally defined in ICAO Annex 3 ( APP3-2 2.2 Use of CAVOK ) . You can hear it at the airports where there is no ATIS , when ATC communicates the weather info.
CAVU is not formally defined in Annex 3 .

When the following conditions occur simultaneously at the time of observation:
 a) visibility, 10 km or more, and the lowest visibility is not reported;
 b) no cloud of operational significance;
 c) no weather of significance to aviation as given in 4.4.2.3 and 4.4.2.6;

information on visibility, runway visual range, present weather and cloud amount, cloud type and height of cloud base shall be replaced in all meteorological reports by the term “CAVOK”.


Can a helicopter be "stalled" by pulling too hard?

About High Energy Maneuvers in a Helicopter: not quite like fixed wing flying

  ... if the player banks the helicopter then make a sudden and
  hard pull on the cyclic to make a turn while max collective is
  applied, the low rpm warning will sound, followed by a loss of
  power and a few seconds later loss of attitude control.

I don't understand what you mean by "loss of power" here (engines are red lined and don't produce any more power?) ... but to proceed

  The aircraft can be recovered by:(recovery steps)

While the recovery steps are rational, the point is to not wait that few seconds if you droop Nr.  I am not sure if the simulation is trying to model retreating blade stall or not, but the response to droop in a max performance maneuver cannot be delayed.

With max collective and a maneuver that induces low NR you don't wait: you reduce the load on the rotor head.  You do that because the engines aren't able to keep the rotor turning based on the load you've put on the system.  If you keep the load on you just make the problem worse.  (In that regard, the simulation is more or less faithful to real life ...)

  My guess explanation is as follows:

1. When max collective is applied, the blades are working hard by cutting air at a large angle of attack
Yes.  Higher angle of attack.
2. When the aircraft is banked and aft cyclic is applied, the airflow relative to the aircraft is like when it is in a very high rate descend
No. Not the same.
3. The critical angle of attack is exceed on the rotor blades, they stall and cease to produce lift
While technically true, the reduction of Rotor RPM (Nr) is your cue to Stop Doing That! (whatever "that" is) so that Nr does not decay further.  Note that the retreating blade will tend to stall first, which is a different stall mode than a fixed wing aircraft stalling due to high AoA.
4. Due to the greatly increased drag, the main rotor is unable to keep rpm up with its power, therefore rpm decreases
It's the Engines that can't keep the rotors turning that is causing the Nr to droop ...  and that's due to the loads you are putting on the drive system.
5. As rpm decreases, less lift is produced and the stall worsens.
Less lift is produced and you may approach the stall rpm (somewhere around 70-80% of rated Nr depending on the helicopter) if you don't do something to restore Nr.  That means lessen the severity of the maneuver and stop overloading your engines' ability to keep the rotors turning as soon as the Nr droops.
6. By reducing collective, the drag of the blades is reduced, and rotor power is enough to keep rpm at normal range again.
No, the Engine Power is enough (or becomes enough) to keep the Nr in its proper range, but reducing collective is a key first step (as is leveling the wings) to retain control of your rotor system.
Is my explanation correct?
No
Can this happen in real life?
Flying a helicopter like it's an F-18 in a Blue Angels flight demonstration is a good way to kill yourself.


What does a "hard stand departure" mean?

A hardstand is a type of parking spot separate from the terminal.  Wide/narrow body gates are associated with the connection to the terminal, where a hardstand is just an area of pavement reinforced for heavy vehicles (or airplanes).


A conventional quadcopter yaws by speeding up two rotors and slowing down the other two.  Once those two have slowed down to zero RPM, it can't yaw any faster.
A formula for this would involve the rotational moments of inertia of each motor+rotor and of the quadcopter's chassis, and the distance from the quadcopter's center of mass to each rotor's axis.  (If the rotors aren't coplanar, it gets messy.)
If those two rotors can be reversed, then they are (inefficiently) thrusting up instead of down.  The limit then may be when they overpower the thrust of the first two, forcing the quadcopter to descend.  Or, if the rotors are spectacularly inefficient when reversed, the limit is when the rotors reach maximum RPM.


What happens during Engine Performance Restoration

An engine performance restoration is a scope of work ("workscope") for a turbofan engine shop visit, where the engine is removed from the aircraft and sent to a engine maintenance facility and the engine case is opened.

An engine performance restoration involves the inspection, rebalancing or replacement of specified parts in the core gas flow path, that is, HPC, combustor, and HPT. These parts are most critical for the fuel burn and EGT margin of the engine. If any of these parts are life-limited, they will be changed at this point.

It also should include any mandatory inspections or replacements, covering the whole engine, dictated by the manufacturer or regulators.

It excludes any optional refurbishment or inspection of the fan, LPC, LPT, and gearbox. It may also exclude inspection of parts deemed non-performance critical in the core.

Because of this, the cost of the visit is reduced, but the engine will not be as efficient and potentially require another visit sooner than a "full overhaul". Thus, an engine performance restoration is the most basic scheduled engine shop visit. It is common that a performance restoration be alternated with a full overhaul.

See Engine Maintenance Concepts or Basics of Aircraft Maintenance Reserve Development and Management for a more detailed overview.


Are flight schools required to verify citizenship of all students?

You are required to verify the identity of people undergoing initial Sport Pilot, Recreational Pilot, Private Pilot, Instrument, and Multi Engine training.  Any other kind of dual instruction does not require verification of US citizenship or TSA approval of resident aliens.  See 49 CFR 1552.3(h).

The CFI must inspect the original documents and verify they are authentic.  The CFI must endorse the CFI’s own logbook with an endorsement per AC 61-65 indicating they have done such and verify the student is a US Citizen.  These records must be kept for a minimum of 5 years.  If the CFI cannot validate the student as a citizen, be done, they must submit the student’s information and immigration documents to the TSA and with for their approval prior to beginning flight training.

https://www.aopa.org/advocacy/pilots/alien-flight-training-program/us-citizens-seeking-flight-training


Would it be viable to install a screen in front of jet intakes to prevent them from sucking in birds?

Modern high-bypass turbofan engines work by pulling in immense volumes of air and accelerating it.  A screen as proposed would make that task far more difficult, which would ruin the efficiency of the engine.  It's also pretty well unnecessary.

Jets worldwide take plenty of birdstrikes every day.  Unless the pilots see it (hitting the windscreen will do that) or hear it (hitting something near the cockpit) or the bird goes into the engine core (which causes the smell of burned bird in the air conditioning), the chance that the crew knows about it before the next walk-around inspection is pretty slight.  The vast majority of birdstrikes are utterly inconsequential.  (Except for the bird -- ruins his day pretty thoroughly!)

Yes, there are a small number of very highly visible events where birds do disable an aircraft -- the "miracle on the Hudson" and the loss of an AWACS in Alaska come to mind, and there are others.  And it's possible for a birdstrike on an engine to cause significant damage IF the bird is large enough AND it goes into the core, but in the big picture, that is actually pretty uncommon.

Thus, the aircraft manufacturers and operators end up accepting the risk of a catastrophic birdstrike, because the alternative of protecting the engine as suggested is too expensive when the effects on the efficiency of the engine are considered.

One other point, bird flesh is pretty easy for an engine to chop up and digest.  If a big bird hits a metal shield at high speed, there would be the risk that the shield could be damaged enough that some of the metal would break apart and go into the engine as well, and THAT would cause far more damage than just the bird.

Fun video of stuff being shot into an engine during testing


When is a high pressure system associated with IFR/MVFR conditions?

In the temperate zone that covers the US and Canada, weather systems are generally related to frontal boundaries between warm/wet and cold/dry air masses, where the cold air pushes under the warm air and forces it up.

But in an area with a large weak high pressure system, you can get a lot of weather caused just by solar convection, depending on the humidity and lapse rate of the air in the mass.  The weather systems will be kind of randomly distributed around, instead of being arrayed along a frontal boundary.

You can see it in the chart you posted.  The weather along the Gulf coast is probably related to the stationary frontal boundary that is there, whereas the weather in the central states is from local convective conditions.

The absolute best practical book on aviation weather theory I've ever read is called the RCAF Weather Manual, written by the Royal Canadian Air Force as its primary weather training guide for pilots.  It is so clearly and logically presented I cannot recommend it enough.


What are the printouts/paperwork a captain does at the check-in desk?

The paperwork he was looking at is the dispatch release.  This paperwork will have

Crew names
Fuel information
Filed route of flight
Alternates as needed
Current weather
Forecast weather at destination and alternates
NOTAMS for departure, destination, alternates, navaids and anything along the route of flight.
Deferred MEL items

There are also probably a few things I'm forgetting at the moment.

When the captain gets the release the first thing to do is make sure it is  for you -- correct crew names and airplane.  You'll check the route of flight and make sure it will work (weather) and evaluate the ordered fuel.  You'll review the destination weather, see if an alternate is needed and make sure a suitable one was filed.  You'll also check departure weather and make sure a takeoff alternate is specified if needed.  If the paperwork isn't right, you call your dispatcher, otherwise you sign it and take it down to the airplane.

The one-page item the captain got may have been a pre-departure clearance.  These are generally available via ACARS but can be printed if needed at a terminal.  They could also be other information such as ground delay programs or ground stop information that affects the flight.


What is the difference between a private and a commercial flight according to the US Customs and Border Protection?

According to the CBP eAPIS FAQ, this definition is in 19 CFR 122.1:

  (d) Commercial aircraft. A “commercial aircraft” is any aircraft
  transporting passengers and/or cargo for some payment or other
  consideration, including money or services rendered.

  [...]

  (h) Private aircraft. A “private aircraft” is any aircraft engaged in
  a personal or business flight to or from the U.S. which is not:

  (1) Carrying passengers and/or cargo for commercial purposes;

  (2) Leaving the U.S. carrying neither passengers nor cargo in order to
  lade passengers and/or cargo in a foreign area for commercial
  purposes; or

  (3) Returning to the U.S. carrying neither passengers nor cargo in
  ballast after leaving with passengers and/or cargo for commercial
  purposes;


What's the cabin pressure of a Bombardier CRJ-900?

At max differential it's 8000 ft.  The controller operates on an automatic program schedule that lets the cabin pre-pressurize to about 300 ft below field elevation at the start of the takeoff to minimize the initial pressure drop sensation following liftoff.  It then lets the cabin altitude rise 500-800 fpm, about 1/4 of the airplane's actual climb rate, until it gets to the max differential of just over 8 psi, which gives 8000 ft in the cabin at 41000 ft.


Was the miracle on the Hudson saved exclusively by the APU?

Yes, the solution to start APU was important.

The ditching procedure directs the use of maximum available slats and flaps for the final approach and touchdown (source, chapter 10.3). This is not possible without APU, as Airbus A320-214 cannot move flaps if only powered by RAT (only blue hydraulic line, same source, chapter 9.3). The running APU adds the green hydraulic line that can also move flaps.

Hence, without running APU, the optimal ditching procedure is not possible; that gives less chances for success.


Is there a particular reason that EasyJet uses U2 as their code?

Easyjet got their IATA code fairly late on, and as such could not use EZ (which was in use by Sun Air), so they were randomly issued their IATA code of U2.

They do have the three letter code of EZY, just as British Airways has BAW as well as BA.


ETOPS 138 and 207 are 15% extensions of ETOPS 120 and 180.

The reason for that is that with the ETOPS 120 rule, most of the Atlantic could be covered when using Kangerlussuaq (Greenland), Gander (Newfoundland), Lajes Field (Azores) and Keflavik (Iceland) as alternative airports. Only some small triangles of the Atlantic were outside the 120 minutes flight time limit from these airports. To cover the whole Atlantic, the next step up in ETOPS (180) would be necessary. But since that is such a big difference, a 15% extension of 120 minutes, ETOPS 138, was approved which allows for covering all of the Atlantic with few extra requirements.

For ETOPS 180 it is similar, but then for the Pacific. Normally ETOPS 180 is sufficient to cross the Pacific, but ETOPS 207 allows the 777-200ER/-200LR/-300ER to cross the Pacific ocean even when Eareckson Air Station on the western end of the Alaskan Aleutian Islands is closed.


Are electronic copies of aircraft logbooks acceptable for reconstruction?

According to FAA's advisory AC 120-78A, it is perfectly acceptable to maintain electronic records for airframe/engine.


The Benedek 10355 clearly stalls with separation starting from the trailing edge.
There is some discussion whether a laminar-to-turbulent transition due to rising pressure will always involve a laminar separation bubble, however short. But that bubble does not constitute a leading edge stall.
XFOIL is not meant for flow simulation deep into the stall region. It does amazingly well to capture the initial stall but cannot possibly model the extended separated flow well into the stall accurately. I am actually surprised that it still produces converging results at such high angles of attack.
In short, I would not expect the XFOIL results above 12° AoA to model accurately what is happening in reality. Especially that sharp drop in lift at around 21° looks very much like the code settling on another solution but not something that can be expected in real life.


How many pilots are required in an Airbus A380?

In general, it is perfectly possible for a single person to fly modern aircraft like A380. However, regulations require the presence of 2 pilots (PF and PNF). In other words, technically yes, legally no.

In long distance flights, the aircraft usually carries two sets of cockpit crew (4, each capable of flying the aircraft) as the flight hours may be more than the permitted flying hours.

FAA Advisory Circular 25.1523-1 deals with the minimum flight crew in transport category aircraft. According to Section 5 Certification Procedures, Subsection c, Testing (vi) Incapacitated Crew member,

  Whenever the applicable operating rule requires a minimum flightcrew of at least two pilots, the certification program should include a demonstration
  of operations during the total incapacitation of a crewmember at any point in a given flight. It must be shown that the airplane can be operated safely and landed safely with the remaining crew at a planned or unplanned destination.


How does a Mach Meter determine the speed of sound at a given altitude?

An (analog) machmeter looks something like this:

So it's more like an more complex version of the airspeed indicator, in this case correcting for the altitude in the process. That being said, I found this extract apparently from an FAA publication:

  Some older mechanical Machmeters not driven from an air data computer use an altitude aneroid inside the instrument that converts pitot-static pressure into Mach number. These systems assume that the temperature at any altitude is standard; therefore, the indicated Mach number is inaccurate whenever the temperature deviates from standard. These systems are called indicated Machmeters. Modern electronic Machmeters use information from an air data computer system to correct for temperature errors. These systems display true Mach number.

Most systems today use more detailed data from sensors to give a correct value through a variety of (complex) calculations.

A little more discussion is available on PPruNe.

Side note: Speed of sound ($a$) itself is solely determined by temperature (that being said, you are able to determine it from pressure, as pressure is a function of temperature) hence the problem with the analog system above.

For air:

$$a=\sqrt{R{\gamma}T}~m/s$$

Where:

$R=287$  Specific Gas constant [dimensionless]

$\gamma=1.4$  Specific heat ratio [dimensionless]

$T=$ Absolute temperature [K]

Remember that you're reading off indicated airspeed [IAS] in knots in the cockpit, which is not the same as True airspeed [TAS] converted to m/s, in case you're trying to work out your mach speed manually ($M=\frac{TAS}{a}$)

For use without knowledge of airspeed & temperature, Wikipedia gives the following formula for subsonic flows:

$$M=\sqrt{5((\frac{P_T}{P}+1)^\frac27-1)}$$

Where:

$P_T=$ Total Pressure

$P=$ Static Pressure


Comparing the Lilienthal and Wright gliders, which one had a better glide ratio?

Otto Lilienthal saw flying as something similar to bicycling: A sport and an opportunity to spend time out in Mother nature. Therefore, he designed all his airplanes such that they would fit through the door of a standard Reichsbahn freight wagon. He used a central truss (Gestellkreuz) to which willow stakes could be bolted such that they would fold backwards. He covered this structure with waxed cotton cloth and used wooden ribs to shape the airfoil of these wings. Consequently, the wingspan of his designs was limited.

The next limitation was his way of controlling his aircraft only by weight shifting. Since his hands were already busy with holding on to the aircraft, he could not use them to move control surfaces. His drawings and his correspondence show that he considered control surfaces and even ailerons, but he did not incorporate them into his designs.

Only his type 18 glider had a rectanguar inner wing which could be pulled close using a scissor link spar. However, with the bigger wing his weight shifting control showed its limits, as this did not lend itself to controlling gliders with bigger wingspans.

Another limit was his insistence that his aircraft would not need a runway or a launching device. Instead, he would grab handles in the Gestellkreuz, stand upright and use his legs as a landing gear. This limited not only the size, but also the wing loading of his designs.

The Wrigths, on the other hand, strived for the mechanically simplest and most lightweight designs and arrived at a biplane of constant chord and much higher aspect ratio. The 1902 glider could already be flown in a prone position and the pilot could control it with control surfaces.

Drag area of the human body in different poses, from Sieghart Hoerner's Fluid Dynamic Drag. The pilot of the Wright flyer caused only one fifth of the drag of the pilot in Lilienthal's designs.

Because the wingspan of the Wright glider was much higher (9.6 m versus 6.7 m for Lilienthal's Normalsegelapparat) and the much higher drag of the pilot's position in case of the Lilienthal design, the Wright glider certainly had a much better glide ratio.


If this is not a Lippisch L-13, what is it?

Alexander Lippisch did develop a similar-looking wind tunnel model, see photo. According to his Errinerungen (memoirs), Page 218, this was in 1943 and it was tested in 1944. Dan Sharp; Secret Projects of the Luftwaffe: Vol. 1 Jet Fighters 1939-1945 (Mortons 2020) shows a couple of photos on Page 219 and says that it marked the start of his delta-winged P.12/P.13 project, including the DM-1 glider which was actually built and flown. But this early wind tunnel model had only a thin central spine, with no cockpit and, like all his delta aeroplanes, was designed to fly apex foremost.

The illustration on the sticker or card is, as you note, back to front. Consequently its supersonic performance would be appalling. It is probably based on a photo of the model, with its identification based on remarks about the P.13 (or Projekt 13), but is otherwise total fantasy. Presumably there is no suggestion that the author of the article actually flew it.


What are the visual differences between the F/A-18 Hornet/Super Hornet and the EA-18G Growler?

You asked about visual differences. The main visual difference is that there are no guns on the EA-18G, just AIM-120 AMRAAM or AGM-88 HARM missiles. Other than that the visual differences are minimal. The airframe is still a F/A-18F Super Hornet, where -F stands for twin-seat. Engines are also the same General Electric F414-GE-400, however I suspect there could be some differences in regards to the engine auxiliary equipment, such as the electrical generator. I also suspect there is additional equipment added for the sole purpose of conditioning the electrical power and insulating everything to keep interference minimal. It is even possible that there are optical fibers in areas where on the F/A-18F copper cables are still used.

Main differences:

Dedicated electronic attack equipment, e.g. what makes the aircraft an EA-18G, has been put in the place of the cannon.
The EA-18G has one extra protruding antenna on the back, along with the satcom antenna behind it.
The missile launch rails on the ends of the wings seem to be permanently modified to support the ALQ-218 Tactical Jamming Receiver, which is the same pod as on the EA-6B Prowler.

Here is an image of the EA-Growler showing some of the external modifications and what they’re for:

Source: airforce.gov.au/Archive.org


How is airspace in the Persian Gulf determined?

The map you see is for Flight Information Regions (FIR's), that's different from sovereign airspaces.

Sovereign airspace by international law "corresponds with the maritime definition of territorial waters as being 12 nautical miles out from a nation's coastline. Airspace not within any country's territorial limit is considered international, analogous to the "high seas" in maritime law."

Can military planes fly across this "international airspace"? Yes. An example is Russian war planes flying very close to the UK airspace, passing through many FIR's.

An example for a disagreement is between Greece and Turkey, because they have so many islands between them. According to some sources just last month Turkey violated Greek airspace 141 times in one day. The point is, there are disputes over sovereign airspace just like with land borders.

  Why, for instance, does Bahrain have such a disproportionately large amount of airspace [FIR] for its size?

It's a matter of "administrative convenience". Both Bahrain and Qatar gained their independence from the UK in 1971. It's quite likely the current FIR shapes were carried over from before and based on where the radars were initially installed.

  [...] Qatari flights fly over Das island, despite it being part of UAE territory.

Checking the official Qatari aeronautical chart above, the Emirati islands are not part of Qatari airspace.

See below the official FIR chart for Bahrain, despite covering the area north of Qatar, airplanes in and out of Qatar still fly north.

To illustrate the operational and administrative convenience I've split the commonly traveled east-west airways between Saudi Arabia, Bahrain, Qatar, and UAE. Now you can imagine the impact it will have on ATC, flight planning, flying, etc.

To further illustrate the region, abeam the four countries the distance is a mere 150 NM, or 20 minutes in cruise. If the FIR is split, a single flight would need to contact 4 different frequencies in such a short time. And if that flight is descending/climbing to/from one of the four countries, multiplied by the amount of flights, then you can imagine the substantially higher workload on both ATC and pilots with regard to coordination and hand-offs.

A similar oversized FIR example is Singapore FIR (shown below).

(Source)


What is the difference between SoCal Approach, SoCal Departure and SoCal TRACON?

SoCal TRACON (Terminal Radar Approach CONtrol) is the name of the facility.
SoCal Approach and SoCal Departure are the call signs used over the radio by that facility, depending on whether the particular aircraft they’re talking to is arriving or departing. They’re the same controllers on the same frequencies.


About PCM

  I do not understand which are the advantages of using PCM [in a transponder]

Pulse Code Modulation (PCM) is a method used in codecs, while modulation is a method used in modems. PCM with modulation in the name is a misnomer, PCM doesn't relate to modulation in any way.

PCM is used in audio processing to take an analog signal and encode it into a digital stream by sampling, quantization and companding. It is for instance used for audio storage on a Compact Disc and in a Waveform file (.wav). It can be used for transmission, but CPM is only the portion of the chain which prepares the digital data to be sent.

Some forms of PCM are oriented for audio compression (e.g. DPCM or ADPCM), this aspect might have some interest regarding the larger mode S data blocks, but this compression is lossy, meaning some data would not be transmitted, which is indeed not an option for SSR.

To clear any doubt about PCM (and associated encoding delta modulation, delta-sigma, and differential PCM) not being a modulation technique, from Introduction to Analog and Digital Communications:

  A point that needs to be stressed one last time: In reality, PCM, DM,
  and DPCM are source-encoding strategies, whose purpose is to convert
  analog signals into digital form. For actual transmission of the
  encoded data over a communication channel, the discrete form of
  pulse-amplitude modulation (PAM) is typically used.

Afaik, there are no PCM application in a transponder or in the secondary surveillance radar to which the transponder replies. It would be interesting to see the document where PCM is mentioned in the context of SSR or modulation.

About transponder modulation

  How does the modulation of the transponder work?

Usually a SSR (secondary surveillance radar) ground station or an anti-collision system interrogates aircraft transponders by sending interrogation pulses on 1030 MHz. The transponders answer on 1090 MHz. There are several modes which are used to obtain different information from a compatible transponder. Civil aviation modes are:

Mode A to get the 12-bit identification/squawk code.
Mode C to get the 12-bit identification and the altitude.
Mode S to get 24-bit ICAO identification and various data. In this mode, the transponder also regularly broadcasts data spontaneously for ACAS and/or ADS-B uses.

Modes A/C can be identified by the time elapsed between pulses P1 and P3 (more here) and mode S by the presence of pulse P6 datablock. Transponders answer if they are compatible with the interrogation mode only.

Type of modulation

The basic type of modulation used for everything except the datablock of mode S interrogation (pulse P6) is the On-Off Keying (OOK), a type of Amplitude Shift Keying (ASK) for which amplitude can have two values. Bits of information are transmitted one by one, the carrier is sent or interrupted, according to the bit value:

OOK is a simple class of modulations, used to transmit information at low speed.

Mode S

Mode S (Select) allows the interrogator to specify the ID of the interrogated transponder, to prevent all other transponders to answer. This protocol requires a significantly more information both for the interrogation and the reply. This information is not transmitted as simple pulses using OOK, some form of synchronization must be added, specially when long chain of 0s or 1s are transmitted.

Instead of representing 0 by the absence of carrier and 1 by the presence of carrier (OOK), bits in the P6 datablock are represented by a presence of carrier which some characteristic is changed:

For interrogation, pulse P6 contains information for the Synchronization Phase Reversal (SPR) signal, the interrogation command, the target transponder ID, and 24 bits of the cyclic redundancy check (CRC).

P6 bits are sent using a variant of Differential Phase Shift Keying, the Differential Binary Phase Shift Keying (DBPSK) with phase shift being 0° for 0 and 180° for 1, i.e. consisting in inverting the carrier when bit value is 1:

DBPSK used for in interrogation data

The bit rate is 4 Mbps. In addition, the difference of amplitude of pulse P5 (sent in all directions) and pulse SPR (sent only forwards) is sensed to mute transponders not in front of the interrogator antenna.
Reply data blocks are encoded using Pulse Position Modulation (PPM) which consists in sending a pulse of carrier with a time shift dependent on the value. This ensures a return to zero which is a form of synchronization. For binary values, the pulse is sent either in the first half of the time slot for the bit (for bit 1), or the second half (for bit 0).

PPM with only two values is therefore equivalent to first coding data using Manchester code, then modulating the carrier using OOK already used for mode A/C. This choice was done to simplify airborne electronics when mode S was adopted by FAA.

PPM used for reply data

The bit rate is 1 Mbps. The content of the data block depends on the type of interrogation, or the type of automatic broadcast used. Example for ADS-B squitter sent automatically:

Source

More on ATC interrogation and reply formats here.


Are plane's black boxes essentially useless tool in the modern age and how to improve upon it?

We have this topic well covered here, here and here (which is the granddaddy of them all).

Basically, it comes down to two issues:

Regulation: It takes a long time for innovations to be approved, especially in safety-related applications.
On long-range flights there are long stretches of no radio contact (except maybe for a satellite connection), so streaming becomes impractical due to the data volume.

Black boxes have been improved a lot since their introduction and they continue to get better. Please let the investigators find and analyze the recordings first before jumping to conclusions.


What is typical cruise altitude in the Himalayas?

I am sure I have no knowledge for reading charts but en-route charts for Nepal indicate something like F170 which I translate to FL170 or 17,000 ft. Pls correct me if I misunderstand it.

Here is the source


Can an airplane lift off the ground when lift is equal to weight?

Yes they can, in fact aircraft regularly lift off the ground when lift is less than weight. They achieve this by rotating the nose, which provides a vertical component of the thrust vector. See the excellent answers to  this question for a detailed explanation.


Can fuel sloshing in the tanks cause a stall?

There are dividers (baffles) in the fuel tank precisely to stop the sloshing. They are just large plates that will stop the fuel from flowing across.

High maneuver jets will use bladder tanks (so no air is in the tank with the fuel) as a way to contain the fuel.


What is the difference between a waypoint and a significant point in an airspace?

According to ICAO PANS-ATM (Doc 4444) 16th Edition,
Waypoint is a type of significant point.
Mentioned in Chapter 1 - Definition as

Significant point :- A specified geographical location used in defining an ATS route or the flight path of an aircraft and for other
navigation and ATS purposes.  Note.— There are three categories of
significant points: ground-based navigation aid, intersection and
waypoint. In the context of this definition, intersection is a
significant point expressed as radials, bearings and/or distances from
ground-based navigation aids.
Waypoint is defined as a specified geographical location used to define an area navigation route or the flight path of an aircraft
employing area navigation.

So in short waypoint is a significant point used to define an area navigation route or the flight path of an aircraft employing area navigation


METAR

The METAR always refers to a specific airport and is obtained right then and there. That is at the airport.

It is a report of current conditions, usually hourly.

In the US, the FAA polls all systems remotely and disseminates the reports in METAR format. No word on wether this is via phone, mobile or radio, if that is what you are asking.

In Germany, the DWD, the german weather service, provides the reports. It can be assumed, that the situation is similar and that the stations are polled.

Weather stations have wide-ranging capabilities and operators, so the reported parameters may be different.

METAR reports may be reviewed by an individual before being transmitted, but the vast majority are reported automatically.

METAR reports are used for aviation weather, by definition, as a "Meteorological Terminal Area Report".

TAF

The TAF is a forecast. It is updated only 4 times a day and is provided by a human forecaster on the ground. Most larger airports have a meteorologist.

FORMAT

Both METAR and TAF use similar format, but each contains elements distinct from the other.  For example, a TAF may contain a FM (from) or BECMG (becoming), but a METAR never will.

DATA SOURCES

You can use https://www.checkwx.com/ to check both.
Or http://tgftp.nws.noaa.gov/weather/current/ if you want the raw data.


Is drag coefficient lowest at zero angle of attack?

Obviously, drag should be smallest for symmetrical airfoils at zero angle of attack.

However, most airfoils have camber, and then the lowest drag is at positive lift coefficients in case of positive camber. Where that point is exactly depends on many parameters; in case of laminar airfoils even local imperfections can have a noticeable effect. Generally, the lowest drag can be found at the angle of attack where the stagnation point is exactly at the center of the leading edge, where the local curvature is highest. A deviation from this point will force the flow on one side to negotiate this point of highest curvature all by itself, resulting in a suction peak which will increase the losses in the boundary layer.

This is a theoretical drag polar (calculated with XFOIL) of an airfoil with a 20% camber flap at different settings and a Reynolds number of 1.5 million. The laminar bucket is clearly visible and produces a range of lift coefficients with nearly identical drag. The small waviness at the lower end of the laminar bucket is an artificial result of smoothing the plot.

What is obvious is how camber shifts the area of minimum drag up and down. If you use the right side of the plot to find the angle of attack of minimum drag, you will find that this is not constant but goes down as flap angles go up. For the 0° flap polar it is at about -2° AoA. This is caused by the induced angle of attack which increases with the lift coefficient.

The 6-series NACA airfoils were the first to be systematically designed with the pressure distribution in mind, and the design lift coefficient is where the condition of the ideal stagnation point location is met. This is indicated by the digit right after the hyphen in the airfoil name: Divide this digit by 10 and you have the lift coefficient of minimum drag.

Example: The $63_1-412$ airfoil has its lowest drag at a lift coefficient of 0.4.

If you want to know the angle of attack with the lowest drag of a whole airplane, this is a very different matter and needs to include the drag due to lift, which is of course smallest at the zero lift polar point.


What control issues did the X-1 have to overcome to achieve supersonic flight?

The control problems are caused by a shift of the aerodynamic forces. In subsonic flight, they are mostly created in the forward part of the wing (the lift increase with angle of attack works at the quarter chord point of a wing, regardless of sweep), while in supersonic flow they work equally over the full chord. The center of lift in pure supersonic flow is at mid chord, and in reality the aerodynamic center shifts slowly back when the aircraft accelerates through Mach 1 and more of the surface is exposed to supersonic flow.

Since the c.g. position is mostly fixed (Concorde pumped fuel between tanks to shift the c.g.), this shift must be compensated with elevator deflection. The elevator must move trailing edge up, which creates a break in the airfoil contour, which in turn creates a heavy shockwave at high subsonic and supersonic flight speed. This shock causes flow separation and can lead to elevator reversal. Only when the fixed part of the horizontal tail can be moved, too, or you have a full-flying tail (the whole stabilizer is moved, not only the rear part of it) to create the needed lift change without a contour break, the aircraft can be trimmed for both subsonic and supersonic speed.

The benefit of the X-1 was a stabilizer with variable incidence, which the P-51 lacked. Sweep would have helped, too, but was not really necessary.


How do you visually identify aircraft from the ground?

The canonical reference for aircraft recognition would be Jane's All the World's Aircraft.

http://www.airliners.net is also a useful resource, but generally means working from photos - the line drawings in Jane's are often easier to use because they abstract away things like airline liveries and distill the aircraft down to its basic shapes.


Why don't most fighters have thrust reversers?

Thrust reversal is a complex system (equals money) and it uses significant amount of fuel (also equals money). It is not a very efficient means to reduce the speed of a landing aircraft either, and other methods (e.g. brakes or drogue parachutes) are much better suited.

The biggest advantage of thrust reversal is that it cancels out the idle forward thrust working against the aircraft. Canceling out idle thrust isn't an issue on most fighters, as they have an engine nozzle that is open at ground idle to prevent any thrust from being produced. This nozzle probably also makes it a bit difficult to integrate Thrust reversals, but the Panavia Tornado proves it can be done.


I analyzed 2 nearly identical flying wing in XFLR5, how come the one with the thinner tip chord has a higher cD?

It may be inaccuracies in your modeling method; it's hard to say without more info.

First, this isn't quite an apples-to-apples comparison. The aspect ratio of the first wing is higher than the second wing, so you're varying more parameters than just the tip chord.

The way to think about this is that the CD of a wing is going to be strongly dependent on the aspect ratio $AR$, the taper ratio $\lambda_T$, the chord and twist distribution along the wing, as well as the airfoil distribution. Is the airfoil t/c constant across both wings? That will also have an effect.

It would be clearer to look at a polar of CL plotted against CD; the best point of comparison is to look at the drag generated at the same CL, not the same angle of attack (because the zero-lift angle of attack will change with wing design as well). The second wing is generating less drag, but also less lift at a given angle of attack; the comparison isn't really fair. If you're using a VLM what you're measuring is the CDi of a wing geometry, which is going to be most strongly dependent on CL.


What happens if fuel tanks are hit by gunfire?

OK, let me expand my comments about inert gas to Peter Kämpf's answer to an own answer.

Argon or Nitrogen ?

It was asked if Argon is a suitable inert gas. While in principle it is, it is also a rare element and so expensive. A better choice is nitrogen. It makes up 79% of the air and is much cheaper.

For example, I found ~50€ for a 50l bottle of nitrogen and ~100€ for argon.
But one also has to keep in mind that the production capacity for argon is not very high, so if aviation starts to buy large amounts to use it as inert gas, the price would increase soon.

Argon and nitrogen are gases which can not be liquefied at normal temperatures, so they are stored under pressure in bottles like this:

They are made of steel to  withstand the pressure of up to 200-300 bar (atmospheres). So, a big 50l bottle has a weight of 77kg without filling (sorry, german).

Nowadays, also civilian aircraft are often equipped with an inert gas system, since the central wing tank of TWA800 exploded most likely due to electrical short circuits

But when it comes to larger aircrafts with larger tanks, gas bottles become a problem: The A380 has a fuel capacity of 325,000 liters. Imagine you want to fill its tanks from the 50l bottles mentioned above. The content of one bottle expands to 10,000-15,000 liters at sea level pressure, so you need 20-32 bottles for the A380.

This bottles have a weight of almost 2.5 tons and need some space. The high pressure is also an issue, and of course, you have to refill the bottles after each flight.

So, it's not a good idea to use this system on larger aircrafts.

I was quite sure that I once read that at least small aircrafts like fighters use bottles, but I don't find any evidence. On the other side, a F35 can carry up to 10,000l in its internal tank, so it would also need a 50l bottle. So, I think I misunderstood something.

CO2 ?

The benefit of CO2 is that it becomes liquid under pressure, You won't need so many bottles and the bottles themselves would be lighter. But CO2 may act corrosive if it finds some drops of water, as it turns to carbonic acid.

Exhaust gas?

A gas that is the product of a fire doesn't burn any more.
This is indeed used to fill the tank of large ships, but you can not take it from the main engine: Those are diesel engines always working with an excess of air, so the exhaust gas contains oxygen. The ships burn some fuel with a balanced amount of air in a inert gas generator.

As fans are similar to diesel engines in this context, exhaust gas is no solution. The gas also contains CO2 and water, so may be no good idea in general for aviation.

Halon

Many older combat aircrafts like the F16 use halon to extinguish fire, but also to fill the tanks.
As halon is one of the worst greenhouse gases, its production has been abandoned 1994 and this document describes the search for alternatives working largely the same way:

To explain it in short: Inside a fire, molecules of the burning material break up and connect to oxygen atoms. This connection is what gives the energy we want in our engines and we want not in the tank.
Halon jumps into the reaction and connects itself to the open ends. The reaction between oxygen and the material, which gives the energy, can not take place any more.

It is said that about 6% of halon in air is enough to extinguish fire, 10% and more can be survived, if exposure time is not too long.

Halon was not used as inert gas, but it was injected into the tanks shortly before a fight was expected

On-Board Inert Gas Generation System(OBIGGS)

Today's solution is to use a On-Board Inert Gas Generation System(OBIGGS) developed by NASA/Boeing

It uses some kind of membrane which can be passed by nitrogen more easily than by oxygen. Bleed air is lead to this membrane with some pressure. The oxygen is kept back and you get air with a significantly smaller fraction of oxygen behind the membrane. There are devices on the marked which produce 2lb/min with up to 1% remaining oxygen

The goal is to have not more than 9-12% in the tank, see the link to NASA above.

By the way: In WW I, Germany sent hydrogen filled airships to England. One could guess just shooting at them would take them down, but the holes were too small to cause a reasonable loss of hydrogen. And unless the bullets made sparks when hitting some steel in the ships, they also did not set the ships on fire. It turned out that firing standard munition at airships was quite useless.

The solution was to use phosphor based tracker munition which set the hydrogen streaming out of the leaks from the other bullets on fire.


What airport fees are charged to airlines in the case of operations following an unscheduled landing?

A while back I was on my way to Frankfurt on Delta for a business meeting when a frail passenger (who quite honestly should have never been allowed to board in the first place) passed away before getting to the midpoint of the trip. The captain turned the aircraft around and landed at Gander International in Canada, where we spent the night before continuing on our way.

Because I've seen this in the news once in a while, I decided to check the airport's web site to see what is their fee structure. Lo and behold, they publish a comprehensive list of fees that covers everything related to an occurrence of this type.

The fee structure even covers "air rage" incidents under the category of "Emergency" and is charged at so-many-dollars per 1000 kg!

  Emergency
  Including medical, mechanical, air rage, etc.
  $12.57 per 1000kg


Does the centre of pressure of a cambered airfoil go ahead of the quarter chord point when angle of attack is increased?

The lower one is better. Details depend on the camber.
A flat plate or a symmetric airfoil have their center of pressure at the quarter chord point. Angle of attack changes change the lift produced, but the center of pressure stays at the quarter point (as long as the flow does not separate). This is exactly true for two-dimensional flow; as soon as we look at real wings with limited span, the exact location of the center of pressure moves forward at the same angle of attack with lower aspect ratio. The center of pressure of a slender body is approaching the leading edge. But your question is about airfoils, so the rest of this answer looks at 2D flow again.
Positive camber contributes lift independent of angle of attack, and its center of pressure location depends on the shape of the camber. A camber line shaped like a segment of a circle has its center of pressure at 50% of chord, and the camber line of rear-loaded airfoils is even farther back.
The resulting location of the center of pressure is the sum of both, camber and angle of attack effects. At low angle of attack the camber line dominates this location, so it is somewhere at mid chord. At high angle of attack the center of pressure shifts forward towards the 25% point. Note that this behavior will make the isolated airfoil unstable in pitch.
A negative camber will reduce lift and result in a center of pressure location ahead of the 25% point. Increasing the angle of attack will now shift the center of pressure back, again towards the 25% point. Now you have a stable pitch behavior (when the airfoil pitches up, the center of pressure moves back and creates a correcting moment change), such that a conventional airplane which loses its horizontal tail can still fly on when inverted. The center of gravity will determine the trim speed, and control may still be possible either by moving the wing flaps or by shifting the center of gravity. Theoretically, at least. I have witnessed an experienced model aircraft pilot doing this after the horizontal tail of his plane was lost in flutter, but the resulting glide speed did not lend itself to a smooth landing.


What are the standards on displaying Crew Awareness Messages?

Yes, certainly there are standards for crew awareness messages. The number of messages displayed is normally limited by screen space allotted, which is may vary from around 5 to 15 messages  and when there is a 'spillover' the messages continue on the next page. The pages are selectable via a simple cancel/recall push button and may also automatically change as corrective actions are taken by the crew or the cause/fault ceases for whatever reason. There is Colour Coding, eg red for Warnings and amber for Cautions. Green and white are also typically used for normal but temporarily used systems eg 'Anti-Ice' or Passenger signs.
All the above describe some of the features of modern crew alerting systems, for details refer to the following document:
(You might find it useful to go to Appendix 5 (Definitions), right at the end of this Advisory Circular, before reading the main body of the document. Appendix 5 makes mention using diagrams as to where in the pilots field vision the visual alert should ideally be.)
Click here for the FAA AC 25.1322-1 'CREW ALERTING'
The above linked AC shows us acceptable means of complying with FAA Rule which is - 14 CFR Part 25.1322 reproduced below.

§ 25.1322 Flightcrew alerting. (a) Flightcrew alerts must:
(1) Provide the flightcrew with the information needed to:
(i) Identify non-normal operation or airplane system conditions, and
(ii) Determine the appropriate actions, if any.
(2) Be readily and easily detectable and intelligible by the
flightcrew under all foreseeable operating conditions, including
conditions where multiple alerts are provided.
(3) Be removed when the alerting condition no longer exists.
(b) Alerts must conform to the following prioritization hierarchy
based on the urgency of flightcrew awareness and response.
(1) Warning: For conditions that require immediate flightcrew
awareness and immediate flightcrew response.
(2) Caution: For conditions that require immediate flightcrew
awareness and subsequent flightcrew response.
(3) Advisory: For conditions that require flightcrew awareness and may
require subsequent flightcrew response.
(c) Warning and caution alerts must:
(1) Be prioritized within each category, when necessary.
(2) Provide timely attention-getting cues through at least two
different senses by a combination of aural, visual, or tactile
indications.
(3) Permit each occurrence of the attention-getting cues required by
paragraph (c)(2) of this section to be acknowledged and suppressed,
unless they are required to be continuous.
(d) The alert function must be designed to minimize the effects of
false and nuisance alerts. In particular, it must be designed to:
(1) Prevent the presentation of an alert that is inappropriate or
unnecessary.
(2) Provide a means to suppress an attention-getting component of an
alert caused by a failure of the alerting function that interferes
with the flightcrew's ability to safely operate the airplane. This
means must not be readily available to the flightcrew so that it could
be operated inadvertently or by habitual reflexive action. When an
alert is suppressed, there must be a clear and unmistakable
annunciation to the flightcrew that the alert has been suppressed.
(e) Visual alert indications must:
(1) Conform to the following color convention:
(i) Red for warning alert indications.
(ii) Amber or yellow for caution alert indications.
(iii) Any color except red or green for advisory alert indications.
(2) Use visual coding techniques, together with other alerting
function elements on the flight deck, to distinguish between warning,
caution, and advisory alert indications, if they are presented on
monochromatic displays that are not capable of conforming to the color
convention in paragraph (e)(1) of this section.
(f) Use of the colors red, amber, and yellow on the flight deck for
functions other than flightcrew alerting must be limited and must not
adversely affect flightcrew alerting.


Why was the FDR of flight ET409 extracted from ocean transported dry instead of in water?

As posted on Avherald, Lebanon's CAA released the final report in January 2012, detailing the investigation and its findings. The link on Avherald is broken, the current site for the CAA can be found here. The front page still links to a page on the report, where it can be downloaded.

Section 1.11 describes the recovery and contents of the flight recorders, and describes both recorders as being "immediately packed in water.".

See this fragment:

  The DFDR was recovered from the Mediterranean Sea by the Lebanese Navy divers and turned over to the IIC in presence of members from the IC on 7 February, 2010. The DFDR was immediately packed in water to prevent/delay the onset of corrosion and transported to the BEA laboratory in Paris France under the custody of the IIC accompanied by a Lebanese and an Ethiopian IC members.

And this one:

  A thorough hand search of the sea bed was then carried out the Lebanese Navy divers who finally succeeded in retrieving the CSMU and handing it over to the D/ICC in the presence of members from the IC on 16 February. The CSMU was immediately packed in water to prevent/delay the onset of corrosion and transported under the custody of the D/IIC and an Ethiopian member of the IC to the BEA laboratory in Paris France, for readout on 16 February, 2010.

While this could be inaccurate, it's much more likely that the reconstruction missed this point.


Can hours accumulated in a glider count towards the 1500 hours for an ATP certificate?

According to my reading of 14 CFR 61.159, glider hours do count toward the 1,500 hour total time requirement, but not several of the other requirements. From 14 CFR 61.159 (parts in bold are where I've highlighted those requirements that must be met in an airplane category, i.e. not a glider).

Presumably you're also not logging much instrument flight time in gliders, so I didn't highlight that.

  § 61.159 Aeronautical experience: Airplane category rating.

  (a) Except as provided in paragraphs (b), (c), and (d) of this section, a person who is applying for an airline transport pilot certificate with an airplane category and class rating must have at least 1,500 hours of total time as a pilot that includes at least:

    (1) 500 hours of cross-country flight time.

    (2) 100 hours of night flight time.

    (3) 50 hours of flight time in the class of airplane for the rating sought. A maximum of 25 hours of training in a full flight simulator representing the class of airplane for the rating sought may be credited toward the flight time requirement of this paragraph if the training was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter. A flight training device or aviation training device may not be used to satisfy this requirement.

    (4) 75 hours of instrument flight time, in actual or simulated instrument conditions, subject to the following:

      (i) Except as provided in paragraph (a)(4)(ii) of this section, an applicant may not receive credit for more than a total of 25 hours of simulated instrument time in a flight simulator or flight training device.

      (ii) A maximum of 50 hours of training in a flight simulator or flight training device may be credited toward the instrument flight time requirements of paragraph (a)(4) of this section if the training was accomplished in a course conducted by a training center certificated under part 142 of this chapter.

      (iii) Training in a flight simulator or flight training device must be accomplished in a flight simulator or flight training device, representing an airplane.

    (5) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties of pilot in command while under the supervision of a pilot in command, or any combination thereof, which includes at least -

      (i) 100 hours of cross-country flight time; and

      (ii) 25 hours of night flight time.

      (6) Not more than 100 hours of the total aeronautical experience requirements of paragraph (a) of this section or § 61.160 may be obtained in a full flight simulator or flight training device provided the device represents an airplane and the aeronautical experience was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter.

  (b) A person who has performed at least 20 night takeoffs and landings to a full stop may substitute each additional night takeoff and landing to a full stop for 1 hour of night flight time to satisfy the requirements of paragraph (a)(2) of this section; however, not more than 25 hours of night flight time may be credited in this manner.

  (c) A commercial pilot may credit the following second-in-command flight time or flight-engineer flight time toward the 1,500 hours of total time as a pilot required by paragraph (a) of this section:

    (1) Second-in-command time, provided the time is acquired in an airplane -

      (i) Required to have more than one pilot flight crewmember by the airplane's flight manual, type certificate, or the regulations under which the flight is being conducted;

      (ii) Engaged in operations under subpart K of part 91, part 121, or part 135 of this chapter for which a second in command is required; or

      (iii) That is required by the operating rules of this chapter to have more than one pilot flight crewmember.

    (2) Flight-engineer time, provided the time -

      (i) Is acquired in an airplane required to have a flight engineer by the airplane's flight manual or type certificate;

      (ii) Is acquired while engaged in operations under part 121 of this chapter for which a flight engineer is required;

      (iii) Is acquired while the person is participating in a pilot training program approved under part 121 of this chapter; and

      (iv) Does not exceed more than 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

    (3) Flight-engineer time, provided the flight time -

      (i) Is acquired as a U.S. Armed Forces' flight engineer crewmember in an airplane that requires a flight engineer crewmember by the flight manual;

      (ii) Is acquired while the person is participating in a flight engineer crewmember training program for the U.S. Armed Forces; and

      (iii) Does not exceed 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

  (d) An applicant is issued an airline transport pilot certificate with the limitation, “Holder does not meet the pilot in command aeronautical experience requirements of ICAO,” as prescribed under Article 39 of the Convention on International Civil Aviation, if the applicant does not meet the ICAO requirements contained in Annex 1 “Personnel Licensing” to the Convention on International Civil Aviation, but otherwise meets the aeronautical experience requirements of this section.

  (e) An applicant is entitled to an airline transport pilot certificate without the ICAO limitation specified under paragraph (d) of this section when the applicant presents satisfactory evidence of having met the ICAO requirements under paragraph (d) of this section and otherwise meets the aeronautical experience requirements of this section.


The energy variometer gives the rate change of total energy of the aircraft expressed in vertical knots.

A traditional variometer gives the vertical speed of the aircraft derived from the change in barometric pressure. This is very useful for gliding, especially when looking for thermals that lift you up. The problem with ordinary variometers is that they do not indicate why the glider is climbing; is it due to a thermal / ridge wind or is it because the pilot pulled up and the aircraft is losing speed.

When the aircraft is climbing in a thermal or ridge wind, the total energy is increasing. When the aircraft is climbing while it is exchanging speed for altitude the total energy is constant (neglecting drag).

Energy equations

Total energy is the sum of kinetic energy and potential energy:

$E_{tot} = E_{kin} + E_{pot}$

Kinetic energy is proportional to the velocity squared:

$E_{kin} = \tfrac{1}{2}mV^2 $

Potential energy is proportional to the height:

$E_{pot} = mgh$

So a change in total energy is the result of the changes in kinetic and potential energy.

$\Delta E_{tot} =\Delta  E_{kin} +\Delta  E_{pot}= \tfrac{1}{2}m\Delta(V^2)+mg\Delta h$

Differentiating with respect to time gives:

$\frac{\textrm{d}E_{tot}}{\textrm{d}t} =mV\frac{\textrm{d}V}{\textrm{d}t}+mg\frac{\textrm{d}h}{\textrm{d}t}$

The climbrate ($\frac{\textrm{d}h}{\textrm{d}t}$) resulting from exchange of kinetic energy to potential energy can be expressed by:

$\frac{\textrm{d}h}{\textrm{d}t} = -\frac{V}{g}\frac{\textrm{d}V}{\textrm{d}t}$

with $-\frac{\textrm{d}V}{\textrm{d}t}$ being the deceleration.

This is the correction that is applied to the ordinary variometer to obtain the energy variometer. This can be done in an electronic way but amazingly it can also be done by mechanical pressure measurement.

Total energy and pressure measurements

When we look at pressure measurements that are used to calculate the airspeed, there is a striking similarity to the energy equation.

A pitot tube facing the incoming airflow measures total pressure which is the sum of static pressure $P_s$ and the dynamic pressure $q_c$

The dynamics pressure, which is the measure for airspeed, is given by

$P_t =P_s+q_c =P_s + \tfrac{1}{2}\rho V^2 $

Note that $q_c$ scales proportionally with kinetic energy. The static pressure changes with height but it decrease while potential energy increases.

When mounting the pitot tube backwards with the total pressure port facing backwards, the total pressure measurement would change to:

$P_t =P_s-q_c =P_s - \tfrac{1}{2}\rho V^2 $

When the aircraft trades kinetic energy for height, the speed drops so the term

$-\tfrac{1}{2}\rho V^2$ would increase in value (becomes less negative).

At the same time, the static pressure $P_s$ decreases with the same amount due to the climb. The result is that the total pressure measurement from this aft facing port remains unchanged when the total energy of the aircraft is constant. That is why an aft facing pitot port is called a Total Energy probe (TE probe).

The rate of change of the $P_t$ is a direct measure of the rate of change of total energy. So while an ordinary variometer measures rate of change of static pressure, the energy variometer measures rate of change of the total pressure of an aft facing pitot port.

To work best, a TE probe must be in undisturbed air. This is difficult as the probe itself is ahead of the pressure port so there is always some turbulence affecting the measurement. A common approach is to have the probe stick out in front the vertical stabilizer.

The pressure port is at the end of the tube, facing the vertical tail plane:


Why does the Boeing 787 engine nacelle exhaust have such an unusual shape?

Well, it does look pretty, doesn't it?

Boeing wanted to reduce the noise generated because of jet blast of the engines. Many airports around the world are implementing new noise regulations.

As mentioned by Boeing:

  To combat the sound of jet-blast from the rear of the engine, Boeing,
  General Electric, and NASA developed serrated edges called chevrons
  for the back of the nacelle and the engine exhaust nozzle. The
  chevrons reduce jet blast noise by controlling the way the air mixes
  after passing through and around the engine.

  The acoustic liners and chevrons are such effective noise suppressors
  that several hundred pounds of sound insulation may be eliminated from
  the fuselage.

As NASA was involved, they state (my paraphrasing):

  Chevrons are the sawtooth pattern seen on the trailing edges of some
  jet engine nozzles. As hot air from the engine core mixes with cooler
  air blowing through the engine fan, the shaped edges serve to smooth
  the mixing, which reduces turbulence that creates noise.

  ... The new Boeing 787 is among the most modern jets relying on
  chevrons to reduce engine noise levels. It sports chevrons on the
  nacelles, or fan housings.


Why are the pitot tubes on supersonic combat aircraft so long?

The length is not needed for supersonic flight, but for subsonic flight at high angles of attack. Due to the high wing sweep, a high angle of attack capability just happens to coincide with supersonic configurations.

In subsonic flow the air ahead of the aircraft is influenced by the aircraft's pressure field, and at high angles of attack and high wing loadings this reaches out quite a bit. The pitot tube can only measure total pressure when it points into the flow direction. Ahead of the aircraft, the local flow angle increases the closer you are to the aircraft, and this increases measurement errors, because now the pitot tube sits at an oblique angle to the airflow. A longer pitot tube reaches farther out into still relatively undisturbed flow, so less compensation is needed to arrive at good values for total and static pressure. In early flight test, the pitot tube is much longer again, because the compensation factors are not yet established.


Why do the Diamond DA-42 and DA-62 have a minimum fuel temperature requirement?

From the DA-42 TDI Approved Flight Manual:

Fuel filter is not heated and can clog if the Diesel fuel flocculates. Jet fuel does not pose same limitations.


Why some planes are scrapped way sooner than others?

In general, the retirement age of aircraft is around 25 years and most of the aircraft fleet in operation is well over 15 years old.

Source: airfinancejournal.com

There are various reasons for scrapping the aircrafts rather than selling them to other airlines (or other buyers, in general).

Availability of Buyers- People should be ready to buy the aircraft on offer. In this case, the aircrafts were probably offered for sale at a time when tn the economy was in a bad shape and the airlines would've found it difficult to sell them.
Condition of the aircraft- Though the aircraft may be new, it has to be seen if it is properly maintained. In case the aircraft are not maintained properly or had the parts been cannibalized for spares, it would be difficult to find buyers as it takes a lot of money to put the aircraft back into operation.
Sum of Parts- In some cases, the sum of parts of the aircraft may be more valuable than the aircraft itself. Cost of operating an aircraft is quite high and the aircraft parts like engines etc are quite valuable. It would've made more economic sense to scrap the aircraft and sell the parts rather than (trying to) sell the aircraft itself.


What causes noise when a plane is on the ground?

That would be an Auxiliary Power Unit, a small jet engine that is used to provide power on the ground when the engines are off and when there is an emergency in the air.


What is the Relation between Critical AoA with Chordline?

It is a bit unclear, but I think what he is trying to say is if you have longer chord, then separation of the flow will happen at lower AOA than with a shorter chord, which is plausible as the boundary layer grows thicker and thicker with longer distance. And stall=flow separation basically.