Can 'neutral static stability' only be achieved by constantly augmenting the control surfaces?

I know this is old, but for the people who are still looking for information regarding this subject, this explanation is not complete and partially inaccurate.
First, there are 2 types of stability. Static stability and dynamic stability. It is true that stability comes from the current situation attitude (angle of attack), but it has different effects for different aircraft. And it is possible to have positive static stability but with dynamic instability. Case in point, 727's deep stall where it's attitude effectively locks the tail in a 'wind shadow.' It causes oscillating moments that don't exist outside of this flight profile.
Static stability has to do with the short period moments of the aircraft. An aircraft with positive static stability is said to have a forces acting to try and return the aircraft to trimmed flight at its trimmed alpha. A neutrally static aircraft will desire to hold the current alpha even after disturbed. An aircraft with relaxed static or unstable static stability will continue to increase its alpha in the direction opposite of the flight path. In other words, neutral static stability will not remain at 2 degrees of pitch attitude (theta) once the stick is released. Instead, it will maintain the pitch rate (theta dot) that it had when the stick was released. Static stability will never have a description of oscillating moments, instead it will describe what happens right here, right now if a momentary force acts upon the aircraft.
Dynamic stability is the stability of the aircraft in the long term. It describes what happens after a certain amount of time. A dynamically stable aircraft will have no or little oscillating moments and will dampen oscillations over time. It will follow a trend that is predictable. An aircraft with neutral dynamic stability will maintain its oscillations until acted upon, and then after, continue whatever oscillations remain. But an aircraft with negative dynamic stability will continue to have diverging oscillations, increasing in amplitude from its original orientation.

The following 9 conditions describe, by example, what will occur with static and dynamic stability are both taken into consideration:

Positive static stability with positive dynamic stability: Aircraft will immediately attempt to return to its trim condition and
will have oscillations that dampen out as the forces return to
equilibrium.

Neutral static stability with positive dynamic stability: Aircraft will hold its current alpha and its pitch rate will remain
constant with regard to its current alpha and the oscillations from
the disturbance will slowly dampen out and return to equillibrium.

Negative static stability with positive dynamic stability (bit of a misnomer, explained below):
Aircraft will continue to
increase alpha and its pitch rate will increase. The misnomer is that
there will be no oscillations because there are no forces acting to
return the aircraft to its previous state in the long term. So this
technically can't exist. Airspeed will continue to decrease and drag
will continue to increase until it is more of a brick than a plane.
This is undesirable in fighter aircraft as if it enters a spin, any oscillations that could help exit the spin will quickly
dampen out and recovery will be nearly impossible. (with regards to
the lateral axis only)

Positive static stability with neutral dynamic stability: Aircraft will want to return to normal flight, its AoA will decrease,
and its pitch rate will reverse. But the aircraft will continue this
oscillation indefinitely if no corrective measures are taken.

Neutral static stability with neutral dynamic stability: Aircraft will want to continue to maintain it's current angle of
attack and any disturbances and oscillations for correcting forces
will continue to oscillate as they did before. It have epitrochoidal
motion.

Negative static stability with neutral dynamic stability (yes, this exists, and is what is common in some fighter aircraft):
Without corrective inputs in a timely manner, this will have an
oscillating period and will also continue to diverge from normal flight. This can be a good thing when trying to recover a
fighter from a spin as the oscillations will continue and the aircraft can be "rocked out." The F-16's deep stall behavior is very much described like
this, and can be described as a falling leaf. I have a video on this,
from a simulator called DCS World, here. In fact, in the video, you can also see that the longitudinal axis also suffers from neutral dynamic stability in this flight envelope. The switch I press at 1 min and 45 seconds overrides the computers maximum control surface deflection window and I can then "rock the plane out" by increasing the amplitude of the oscillations until the airflow over the control surfaces allows the computer to recover from the deep stall.

Positive static stability with negative dynamic stability: This is what you want when you want to make passenger planes into
vomit planes. The corrective forces acting on the plane cause it to
overshoot and the oscillations increase in amplitude. But the plane
is still trying to right itself in the short term.

Neutral static stability with negative dynamic stability: At this point, I don't know if there is a point in describing the
behavior because I think you might be able to get the gist. The plane will want to maintain its current alpha, but will then reverse, causing larger amplitudes of oscillation on each reversal. It might have 1 or 2 cycles before it completely
diverges from controllable flight. Not the characteristics you really
want to have in an aircraft.

Negative static stability with negative dynamic stability: Just pure chaos. Basically any orientation along the lateral
axis will have a 'butterfly effect' response and unpredictable
behavior. Forces put in to try and correct it will have an
oscillatory response that will increase in amplitude on the next
cycle, causing the need for a larger input to correct and the pattern
repeats.

Currently studying flight dynamics as a hobbyist and with 2 years of engineering school. Current project is for model aircraft with a flight control computer to emulate an aircraft with negative static stability and neutral dynamic stability.
And, for references sake, so you can figure this out on your own in a sort of mental simulation:
https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/


What does 'laterally aligned' mean in this description?

The only meaning that I can conceive of is that "laterally aligned with the centerline" is a fancy way of saying "on the centerline"—or, at least, keeping the distance from the centerline under control, so that you don't drift off of the runway.

The paragraph seems to be describing the following technique for lateral control during a crosswind takeoff:

Use the rudder to keep the heading (the direction the nose is pointing) parallel to the runway centerline.
Use the ailerons to control the bank angle so that the airplane stays approximately over the runway centerline.

The reason the Airplane Flying Handbook uses the word "laterally" here is that the ailerons are being used to control the airplane's lateral (left-to-right) motion.

Some comments speculate that the paragraph is really saying that the ailerons should be used to keep the wings level. That doesn't make sense to me; in a crosswind, if you're using rudder to keep the nose parallel to the centerline, and you're keeping the wings level, then you'll drift downwind off the runway.


Do the Mt. Everest rescue helicopters have modified engines to operate at high altitudes?

No, the helicopters are standard production versions. The Eurocopter AS350 is a common model used for these operations.

In 2005, Didier Delsalle landed a Eurocopter AS350 B3 on the summit of Mt. Everest at 29,029 feet (8848m) (twice). The only changes he made to the standard version were removing a few things like extra seats to reduce the weight, and of course flying solo. It also took careful planning and the use of updrafts to get there.

The B3 version of the AS350 (now called the H125) is marketed as high performance, designed for operation in hot and high conditions. It also holds the record for the highest long-line rescue mission at 25,590 feet (7800m) (in the Himalayas).

Although the official ceiling for hovering out of ground effect is 11150 feet (3400m), the flight ceiling is 23,000 feet (7000m), and rescues at 20,000 feet (6000m) are typically limited to 2 passengers rather than the full capacity of 6.

The Safran Arriel 2D engine used in the B3 version is rated up to 952 Shp on takeoff and operation up to 29,500 feet (9000m).


Who pays for the compliance with Airworthiness Directives (AD)?

It depends on the agreements between the operator/owner of the aircraft on one side and the manufacturer of the aircraft and its suppliers on the other side.

Often there will be some form of compensation involved if an AD is issued when that AD is addressing non-compliance with regulations / standard in effect when the aircraft was produced. Normally there is some time limited warranty covering the costs of ADs at least partially.


What is the difference in VTOL technology between British Aerospace Harrier II and F-35B?

the F35 has a ton of electronics that the Harrier could only dream about, but I will focus on propulsion differences.

The biggest is that the F35B uses a separate lift fan in front of the engine, oriented to provide vertical lift that balances the vectored thrust from the rear of the engine. The lift fan is disconnected when flight speed is obtained and covers closed. Roll control at low speed comes from bleed air from the front fan in the engine.

The Harrier uses a much larger version of a bleed system from a single engine to provide both forward located vertical lift/thrust and a similar total diversion system for rearward lift/thrust through four rotating nozzles. All nozzles are working all the time and rotated to a lift/thrust position. A separate bleed system provides roll and pitch control at low speed.


Can you help me identifying this WW1 German or Austrian airplane from my photo collection?

@DalamaniSingh is correct, but the people who built and flew this plane never called it "Otto pusher".

Gustav Otto was one of the seven children of Nikolaus Otto, who designed the first practical internal combustion engine and after whom the Otto cycle is named. He started an aircraft factory in 1909 near Munich and later relocated north to Oberschleissheim. The first product of this factory was simply called "Otto Doppeldecker" (Otto biplane). It was produced from 1912 to 1915, but was obsolete by the time WWI started. The plane was mostly used for training. The factory had to close in 1915 and the historic building now houses a collection of aircraft and has become a museum. In the attached restoration workshop, a replica was built recently.


What are these grilles in the passenger cabin of this 787?

The answers to this question indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.

The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.

US patents # 6129312 and # 5137231 claim systems for combining the cabin air circulation vents with the depressurization relief vents. From the '9312 patent:

  An air return grille mounted below the passenger cabin sidewall
  comprises openings to allow return airflow from the air conditioning
  system. The grille openings are covered by a single thin baffle with
  cutouts within its periphery that slip over and are held in place by
  fasteners on the molded grille. During normal operation, the thin
  baffle restricts flow through a small opening in the grille. During a
  sudden decompression of the cargo compartment, the baffle is dislodged
  from the fasteners. This allows the airflow to pass through all of the
  molded grille openings in order to achieve rapid pressure
  equalization.


What is the width of seat tracks on airliners?

The attachment rail/track width (gauge) is not fixed. It differs from plane to plane, and the chosen seat manufacturer obliges.

21" or 55 cm seems the average though.

Source: stanford.edu

Above image shows B717 cross section, you can notice the 3-abreast and 2-abreast both are on two rails. No high-res available.

Different plane models:

Some are wall mounted.


Why shouldn't you use HF radio while refueling?

The safest way to refuel an aircraft is with everything turned off and the airframe grounded (or bonded) to the fueling system.  This is often not practical (particularly for airliners), so a number of other rules exist to make refueling as safe as possible -- not using HF radio while refueling is one of them.

So why is the HF radio singled out? Power, combined with the antenna design factors you mentioned.

With small radios you can easily light a fluorescent lightbulb (incidentally this is why radio geeks are such great fun at parties). HF radios can put out several hundred watts of power - this makes them more likely to generate a spark that can jump the gap between say a refueling nozzle and the refueling port/tank, igniting fuel vapors and ruining everyone's day.
HF antenna designs which use the aircraft itself to radiate part of the energy can further increase the chance of a spark.


The reason not many drones use ducted fans is efficiency.
Ducted fans are great for fast forward flight but not for static lift.

Explanation: Yes, the 'duct' reduces losses at the blade tips, However, adds significant losses at the intake lip and exit where the adjacent air is pulled into the flow. For ducted fans installed in aircraft, These losses are reduced at high forward velocities and increased at low forward velocity. For drones, there is situation with perpendicular intake flow, when drone is travelling sideways and fan pointing up, there is even greater losses at front intake lip.

The most efficient blade design for static lift is to have the blade as long as possible (like a helicopter) and travelling as slow as possible. Least efficient to have short blades travelling at high velocity.

It is easier to imagine what is going on if you increase the viscosity and imagine the fluid is water instead of air. Very large blades would move water only a small about, but over a large area. however, very tiny blades would be require to move vast amounts of water through a small area to produce the same thrust. moving that much water increases drag as the blades have to suck in water at the intake and the exiting water would be slowed very quickly by the surrounding water at the exit.


How does the spoiler actuation mechanism work on the Airbus A340?

Most aircraft utilize a weight on wheels switch. When the landing strut is compressed when ground contact is made, the weight on wheels switch is activated. The signal is distributed to various systems in the aircraft, including the spoiler system.
The spoiler is activated as soon as the aircraft touches the ground to "scrub off" any lift and to make sure the aircraft doesn't become airborne as the jet engines spool down.


Why do we always use circular nozzles for commercial aircraft engines?

Simple geometry: Circles are strong, and provide a large area for the amount of material used. That means you get more intake/nozzle for the cost of the materials, while giving you a strong and (relatively) light engine.

A circle also helps to keep the airflow smooth, because it's (again, relatively) uniform: no weird edges, corners, or angled things to cause turbulence, vortices, or otherwise disturb or make the airflow "complicated". This is ideal at subsonic speeds.

Overall, this makes a circular engine the most efficient, and when it comes to commercial aircraft efficiency is king: the lighter and simpler you can make a safe engine, the better.

In some aircraft we will see "off-circle" (eg 737 engines, which have a not-quite-circular intake) or oval intake, but they're basically just circles where the dimensions have been adjusted to take a slight efficiency hit for the purpose of fitting the shape of the aircraft better.

Military aircraft which use rectangular air intakes tend to do so for two different reasons, rather than because it's actually any more efficient.

Stealth. Circles scatter radar waves in a "bad" way for stealth. Angles are much better for stealthy aircraft
Because that air intake is actually an Intake Ramp designed to slow the airflow into the engine by deliberately creating a shockwave to slow the air down. This is because jet engines need subsonic air to work properly: this is why you tend to see square intakes on supersonic aircraft. Commercial aircraft don't usually fly supersonic, though, with the notable exceptions of Concorde and the Tu-144 which both had square intakes for this reasons.

If you don't care about subsonic efficiency (Concorde/Tu-144, which were both designed to spend most of their working lives supersonic), or if you just want to be able to go supersonic at all, a square intake can make sense - but for those of us living life under Mach 1, and for airlines who care about fuel efficiency above pretty much anything else, a circle is best.


How can I calculate the "stability state" of air from a given set of data?

You should calculate the environmental lapse rate (ELR), not use a fixed value. In the table you presented, the ELR is:
$$ \frac{T_{alt_1} - T_{alt_2}}{alt_2-alt_1} \cdot 1000$$
Compare this calculated value to dry and moist (or saturated) adiabatic lapse rates.

If the value falls between them, the air is conditionally unstable.
If the value is larger than the dry adiabatic lapse rate, the air is
absolutely unstable.

For accurate results, you must calculate SALR, as it varies strongly with altitude. Formula is in Wikipedia article.
See Wikipedia: Lapse rate and  Skybrary: Lapse Rate for more in-depth explanation.
Caveat, the aforementioned sites explain absolute unstability differently, I have no recolletion which it actually is, and my study books are buried in the garage...


Why does a turbulent boundary layer cause more friction drag than a laminar boundary layer?

No, a higher Reynolds number only signifies a lower ratio of viscous to inertial forces. Since you increase the Reynolds number by either increasing speed or length, both will drive up the inertial forces, making the friction forces relatively lower.

Let's do a Gedankenexperiment: Consider an infinitely thin flat plate which moves through air in its lengthwise direction, so there is no pressure gradient along the flow path. The friction force caused by the boundary layer is F. Now add some length at the trailing edge of the plate. The added surface will add some more drag F', so the total drag will be F + F', both being positive. Since the length has increased, the Reynolds number of the flow has increased in the same way, but so has the friction force.

What has decreased is the coefficient of friction, however, because the thicker boundary layer towards the end of the flat plate will cause less friction force per unit of length. The coefficient is calculated by dividing the force by the dynamic pressure and the surface area, and since the surface area has increased by more than the drag, the coefficient of friction is lower.

You can repeat the experiment with a flow speed increase, and now the friction force will go up, too. So both ways to increase the Reynolds number will increase friction drag. Again, the drag increase is lower relative to the increase in dynamic pressure because the boundary layer at the higher speed is thinner, and again only the coefficient of friction is lower.

Only now, when that is out of the way, I want to answer your question directly: If you compare fully turbulent boundary layers, an increase in the Reynolds number will lower the coefficient of drag but will increase friction drag as outlined above.

Now compare two similar fully laminar flows at two different Reynolds numbers, and again the flow with the higher Reynolds number will experience more friction drag. The different speed profiles of their respective boundary layers will only be significant when you compare a laminar flow with a turbulent flow at the same or similar Reynolds number. In the range of Reynolds numbers typical for aircraft the turbulent flow will cause more friction drag.

If you now repeat the initial Gedankenexperiment with a laminar boundary layer and add some length at a point where the local Reynolds number is safely above 500,000, the local flow will already have transitioned to turbulent and the added length will add a fully turbulent portion. Now it can happen that the added surface will experience more drag per unit of area than the forward, mostly laminar surface. The result is both a higher friction drag and a higher coefficient of friction.

If you allow me to copy the illustration from this answer here, the effect can be easily seen:

Friction drag coefficient over chord for an E502mod airfoil at 3° AoA. Blue: Top surface, Red: Bottom surface.

Now this plot uses a real airfoil with a changing pressure coefficient over length, but with a flat plate the plot would look similar. The much higher local speed gradient at the surface of the boundary layer in the rear part will add much more drag per length and the total drag will increase disproportionately. However, a drag decrease with increasing Reynolds numbers is nowhere to be seen.


What factors lead to mounting engines towards the rear of aircraft

I am just a Flight Sim enthusiast so everything I say should be heavily scrutinized, but there is a flightsim that features the A-10C which has an engine configuration just like you describe as you can see in the picture.

The (ingame) flight manual has the following to say about the engine configuration:

  Engines
  All versions of the A-10 have been powered by twin TF-34-GE-100A engines that are mounted high
  on the rear fuselage between the wings and the rear stabilizers. The unusual placement of the
  engines provides several distinct advantages:

  The high mounting reduces the likelihood of the engines ingesting Foreign Object Debris
  (FOD) when operating from rough, forward bases in war-time.
  Engines can remain running when aircraft is being rearmed and refueled. This leads to
  faster mission turn-around
  Ease of servicing the engines
  Reduced IR signature from below due to the shielding of the horizontal stabilizer.

Another related thing I found was how they combine with the leading edge slats, it says.

  Added leading edge slats for improved airflow to engines at higher angles of attack

As most of these reasons are in a military setting I cannot comment on the advantages for civilian aviation.


Recurrent training is required annually by the FAA for both pilots and cabin crews, by 14 CFR 121.427.  I am only familiar with the content of recurrent training for flight attendants so this answer is limited to them only.  In a previous answer I wrote:

  Annually we would be required to attend jet recurrent training where we reviewed NTSB accident reports with an emphasis on cabin crew responses to emergencies, review of first aid and CPR techniques, re-testing of our knowledge of the location and how to use all the safety equipment on board, including life preservers, life rafts, and emergency slide procedures. We were also briefed on any changes to FAA mandated procedures and tested on everything.

The purpose of the training is to refresh our knowledge of required safety procedures that might have gotten rusty and to stay up to date on possible changes to FAA required procedures.  Emergency procedures are almost never needed and yet it's vital to be able to recall them when needed.  One day a year was devoted to recurrent training and you had to pass all the tests to stay current.  For this reason, we took it very seriously.


How to balance a propeller aircraft for left turning tendencies?

There are several ways, and the most effective vary their own yawing moment contribution with engine thrust:

A trim tab on the rudder which forces it into a correcting deflection. This trim tab can either be adjusted in flight or only on the ground. The flight-adjustable trim tab is set according to flight speed and is used on airplanes with a wide speed range.
Mounting the engine at a slight angle to the airplane centerline. This places the propeller at an offset which produces its own yawing moment.
Camber on the vertical tail. Since the tail is in the propeller slipstream, it experiences a relatively higher dynamic pressure at low speed and high engine settings, just when the p-factor is biggest.


It's typically stored in the cargo bin with the checked luggage.  If it is to be returned to the jetway, it will be tagged with a distinctive tag so that it's clear that it has been screened already (i.e. it came in through security, so it can go back out into the terminal with you at your destination).  In many cases, though, once an item has been gate-checked, it will be delivered to baggage claim to be picked up there.

The exception to that is baby strollers -- they should be returned to the jetway since parents tend to need those to get the young ones around the airport.

Individual airline policies, of course, will vary.  With small jets (not a 737, but a regional jet or turboprop, perhaps), the carry-on space is so limited that all sorts of things that would be accepted as a carry-on for a larger aircraft can't go into the cabin, so those may go into a dedicated cargo hold & all returned as the passengers deplane.


What is this RAF biplane, and where was the photo taken (possibly in the 1920s)?

I believe the plane is an Airco DH-9A.

https://en.wikipedia.org/wiki/Airco_DH.9A#Interwar_RAF_service

The location may be Iraq. It looks like the RAF 55(B) Squadron: https://en.wikipedia.org/wiki/No._55_Squadron_RAF#Between_the_Wars.

On another picture I think I recognise the dog on the right in your picture (but now a bit older): https://55bomber.wordpress.com/2010/12/02/raf-55b-squadron-hinaidi-iraq/

Blotto:


Are there any widely applicable models for plausible aircraft auto-routing?

I am looking for routing algorithms whose output is the actual path that if set on an autopilot will bring the aircraft through an endpoint at a specific bearing, level and speed

There are indeed models for moving between waypoints: I remember siting on the flightdeck of an A320 and noticing the airspeed increase as we descended towards the airport. I asked the pilot and he responded that this was in preparation for a turn coming up as to not loose airspeed. Not a huge differences perhaps but will make a difference over a year with a lot of aircraft.

Most airlines have company routes detailing which waypoints to use to a destination. To the best of my knowledge, these are fairly standard (you might have  few) and not too weather dependant, unless there is a big storm for instance. One exception to this are transatlantic tracks that are changed daily depending on weather. At least for most flights, possible variations are few and will be in busy airways. Changing airways is normally a larger deviation so it is just good to bare with it, and the weather is unlikely to be considerably better for just a slight detour.

  As a secondary question, do ATC systems have some sort of auto-routing as an aid to operators at busy areas?

ATC have normally no real incentive to cut down routes, at least in cruise. They might help you skip some waypoints and get the aircraft down a little quicker if the airspace is calm during arrival. To also put things into perspective, I was on the flightdeck into a moderately large european airport where we were put first into a hold due to bad planning and then told to speed up after we left it to catch up again. Arlanda airport had something going on however with green approaches.

  I am looking for routing algorithms whose output is the actual path that if set on an autopilot will bring the aircraft through an endpoint at a specific bearing, level and speed.

The issue here is that there is no incentive for aircraft manufacturers to release such algorithms to the public. If Airbus uses algorithm A, and Boeing algorithm B, each with a different inputs, you neither would want the other to make algorithm C that works with both inputs and being more efficient. The bottom line however is that everybody remains constrained by the infrastructure already in place that is pretty rigid.


How hard is it to fly with night vision goggles?

All those answers are pretty wrong.  One eye..Can't read the instruments.. Aviation ANVS like the ANVS 9's are dual tube.  You focus them to your liking,individually for each eye!  I personally get 20/20 with them and you mount them high and simply look under them to read the instruments! I have over 7,000 hours of single pilot NVG.  They make things safer, easier, and it's just like flying during the day, only everything is green.  Things like towers and wires (powerlines) are actually MORE visible under NVG's.  It usually takes established pilots with zero NVG time only a few hours to adjust.  Nothing to it.


What is the clearance between a fan blade and the nacelle?

Can't say for sure without specific information from the manufacturer, which would be proprietary, but it's probably somewhere between 0.010" and 0.100". As you make the clearance tighter, you run the risk of a heavy rub between the blades and the case.  But as you make the clearance more open, you lose efficiency.  Somewhere in between there is a sweet spot where efficiency is fairly good and risk of a rub is low enough.  The exact clearance may vary significantly between different manufacturers and models, depending on what type of blades they have.

And yes, this distances changes significantly as you go from idle to takeoff to cruise.  Thermal expansion is a player, but most of it is mechanical loads (i.e. centrifugal force).

also, very minor point, but the clearance you are looking for is between the fan blades and the fan case.  The nacelle is a separate structure that is outside of the case.


Does the pilot in command always sit on the left seat?

No, the PIC does not always sit in the left seat, and there's no regulations about it. Most airplanes are configured so that the primary pilot would sit on the left, however there are military trainers (the Slingsby Firefly comes to mind) which have this on the right so that the pilot has the throttle on the left and stick on the right as single seat military jets have. Helicopters have the pilot on the right, not the left.

In any case the PIC is not necessarily the handling pilot, instructors usually sit on the right and are often the pilot in command.


What does ICAO database "engine count c" mean?

I don't know the exact ICAO phraseology, but it certainly seems to describe an aircraft that has two engines combined to a single driveshaft.

Soloy has trademarked the 'Dual Pac', a pair of turboprops driving a single shaft through a gearbox. All of the Soloy aircraft there use this powerplant.

The Fairey Gannet was a (super cool) anti-submarine aircraft designed in the late 40s. It was powered by the Armstrong Siddeley Double Mamba which you could argue was the direct predecessor of the Dual Pac, albeit using a presumably very different gearbox to drive the contra-rotating prop. One half of the Double Mamba could be shut down in flight, and both props would continue to be driven:

  The Double Mamba engine could be run with one Mamba stopped to conserve fuel and extend endurance for cruise flight. The contra-rotating propellers meant that when only half of the Double Mamba was running there were no thrust asymmetry problems.


What do the regulatory bodies say about landing in CAT IIIB?

The ability to perform autoland approaches requires specific approvals.  In the US, it is outlined in AC 120-28D Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. BTW, CAT I and CAT II approaches are covered by AC 120-29 Criteria for Approval of Category I and Category II Weather Minima for Approach.

The AC describes the necessary installed equipment, specific crew training requirements, and the need for operational procedures.  Once approval is granted, the crew training and operational procedures become part of the airline Op Spec.

If any hand flying is allowed, it will be spelled out in the Op Spec. Since Op Specs do vary from airline to airline, it's not possible to state definitively what is or is not allowed.

That said, it is physically possible to manually take over and land the aircraft. If the crew has the runway in sight at 200 feet, it's really no different that a CAT I approach. At 50 feet on an autoland approach, the autopilot will be starting the flare and de-crab maneuver, so manual intervention at this point may or may not be allowable depending on the specific aircraft performance and the airline's operational philosophy.


Why would aircraft take-off in the opposite direction as landing despite wind direction?

Apparently, only the 05 runway has precision approach markings and (Medium Intensity) Approach Lighting System (MALS). Runway 23 has non-precision approach markings and no Approach Light System.

So, the aircraft only land on runway 05.


Are there any photos or videos of the drones at Gatwick?

It is very likely that there were no drones, and this was a case of mass hysteria.

One actual crashed drone was discovered during the search, but that could very likely have been a hobbyist drone that got away, days, perhaps weeks earlier.  Lost toy drones happen all the time (even though it shouldn't happen near a major airport, it does...)

https://www.secretflying.com/posts/police-admit-it-is-a-possibility-there-was-never-a-drone-at-gatwick-airport/


Were civilian aircraft allowed to overfly the Soviet Union?

During the Cold War, Soviet airspace was very restricted to other countries. Flights from the US often had to follow polar routes to Asia and Europe that avoided entering Soviet airspace. The end of the Cold War allowed new routes to be used.

Source

Flights to the USSR were allowed on a limited basis, often in conjunction with service by Aeroflot. Air Canada had the first service (via Copenhagen) from North America in 1966. Some flights were allowed to cross the USSR (JAL started in 1985) but would often stop on Moscow. Otherwise, Anchorage was a typical stop on such a route. International relations may have improved since the Soviet era, but even today not every airline gets permission to overfly Russia.

The strict policies are the reason that flights such as Korean Air Lines flight 007 were shot down when deviating from these routes, and the flight of Mathias Rust to Moscow was a big deal.


What happens to manifold pressure reading following a reduction in both throttle and RPM?

I think you misread the question. Assuming you quote it properly,

  To set climb power, you reduce the throttle (manifold pressure)

… so the first step does reduce the manifold pressure and is therefore not subject of the question …

  and then the props (RPM lever). After setting the props you expect the manifold pressure to:

… only the second step is.

In the second step, the manifold pressure slightly increases, which

  On the other hand a reduction in RPM would mean less suction/vacuum so the MP reading would increase.

… you already understand.

So yes, the manifold pressure decreases first—when you retard the throttle—and then increases slightly as you reduce pitch. Just the question only targeted the second part.

The increase won't be back to the original value, because the throttle is still partially closed. With wide open throttle the manifold pressure is (for normally aspirated engine) almost ambient pressure and there is nowhere higher it can go.


As already done in this answer, we can calculate the True Air Temperature (TAT) that the aircraft's skin will feel from the Static Air Temperature (SAT) and the Mach number:
$$ \frac{\text{TAT}}{\text{SAT}} = 1 + \frac{\gamma - 1}{2} M^2 = 1 + \frac{1}{5} M^2 $$
(using $ \gamma = 1.4 $ for air). For different SATs the curves then look like this:

I added a dashed line at a TAT of 10°C because this is the limit defined for icing conditions. At higher TATs, the ice should definitely melt (although it could already start melting before). We can now obtain the intersection Mach number as a function of SAT by inverting the formula above:
$$ M = \sqrt{5 \left( \frac{\text{TAT}}{\text{SAT}} - 1 \right)} $$
The resulting curve looks like this:

As you can see, a Mach number between ~0.4 and ~1.3 will be required to melt ice, depending on the SAT.


This looks like an Aircraft Fuel Servicing Cart, which transfers fuel from an underground network to the aircraft. There are two types of vehicles that transfer fuel at larger airports with these fuel networks:

Hydrant trucks:

(image source: Wikimedia)

  Aircraft fuel servicing hydrant vehicles do not have tanks. These vehicles
  connect to a pressurized Airport Fueling System hydrant and transfer the
  fuel to the aircraft through a filter.

(ICAO - International and FAA Fuel Fire Safety)
Towed Carts:

(image source: aeroexpo.online)

  Aircraft fuel servicing carts are equipped with facilities to transfer fuel between a
  Airport Fueling System hydrant and an aircraft and are normally parked at a fixed
  location near a gate.

(ICAO - International and FAA Fuel Fire Safety)

The cart in your picture seems to be the latter of the two (a different model though). You can see the hose that will be connected to the aircraft on the metal staircase. The other hose on the ground connects to the hydrant (hidden behind the aircraft in your image). Your picture shows an Airbus A321, which has the fuel coupling typically located under the right wing (number 11 in the following image):

(image source: Airbus A321 Aircraft Characteristics and Airport and Maintenance Planning Document)

  11 − REFUEL/DEFUEL COUPLINGS (OPTIONAL−LH WING)

Your picture was taken at Charlotte (CLT), which according to this page is equipped with a Jet A Fuel Hydrant Facility. Airports without an underground fuel system will use tank trucks (self-contained fuel trucks) instead:

(image source: shell.com)


Why don't airliners use in-air refueling systems?

Fuel Quantity Unlike smaller fighter jets, you would need to offload a substantial quantity of fuel. For a B777, you're looking in the range of 60 tonnes of fuel for half a tank. The boom of a KC-135 (faster than a basket) can do around 3 tonnes a minute. The math comes to then 20 minutes of aerial refuelling. The KC-46 can do perhaps 180 tonnes, so you might squeeze out three refuelling operations from one flight.

Risk This is considerably more dangerous than the very conservative safety margins aircraft normally operate within. You would have one aircraft filled to the brim with fuel, and another aircraft with 300+ passengers. A quick search on YouTube is enough to propose this entire thing is very dangerous.

Furthermore, you need to consider that the aircraft needs safety margin to divert should refuelling not work, which cuts into the benefits you can expect.

Many intercontinental routes will be flown at night, making the manoeuvre more difficult.

Refuelling Area Looking at the route you proposed, the closest airfield would probably be in Alaska, at least 500km away. Guess another relevant area would be Iceland/Greenland. Even if you did reroute, you'd get away from the jetstreams that aircraft over the Pacific use lying considerably further south, reducing efficiency. The same scenario applies for eastbound Atlantic flights.

Expensive You would need to get another aircraft (~$200m for a KC-46) with special equipment and crew training. The receiving aircraft crew would also  need special training. The airframe modifications would be complicated and require certification. Furthermore, you would need to get everybody to agree on some common standard.

Logistics Planning is difficult. Each aircraft would have to be rerouted to intercept the tanker at a certain time and place. You want to refuel them perfectly one after an other, which is almost certainly not possible. There's just not the volume of aircraft movements feasible for this, especially for the ultra-long flights where it may have the greatest benefit.

Other Landing Benefits
Include changing crews and possibly offloading passengers.

Applicability
The number of flights where this can be practically implemented and used is very limited and for all intents and purposes, you can just land the plane itself and refuel it. Even for the route you propose, it's around 7,500 km which is not a lot for a B777.


What is the fuel burn rate for climb of a C-130J Hercules?

A rough estimation: for a mass of 60000 kg, the energy needed for that (1000 ft = 305 m) altitude change will be 60000 x 9,8 x 305 = 179 MJ. Jet fuel has a heating value of 44 MJ/kg, and if we assume a global conversion efficiency of –say– 7%, the fuel needed would be 179/(44 x 0,07) = 58 kg...


How do the linearised equations of motion work?

Um..it's a bit of a counterintuitive and hard to visualise field of aeronautics. There are six degrees of freedom (if we assume that the aircraft is rigid) that need to be transformed from aircraft axes into earth axes. And since we're looking at aerodynamic stability, we need to consider wind axes to introduce disturbances: if the aircraft moves as an effect of the disturbance, the relative wind changes.

Since we're working with transformation matrices, we can only solve these analytically if there are simple multipliers in the matrix, as soon as there are trigonometric terms such as sines & cosines introduced, we'll need to use numerical methods. So we linearise by looking at small disturbances only, and set $cos(\phi)$ = 1 and $sin(\phi)$ = $\phi$. Usually OK from a body axis perspective, since stall occurs at 15 degree which is a small enough angle. But..

..to make it all even worse, it is one of the few remaining bits of engineering where the dark art of quaternions are used, since when the nose is pointing straight up some of the Euler angles are undefined and we need a fourth dimension to provide continuity.

I reckon if you're interested in this book, the only prior knowledge you need to have is the realisation that it is going to be complex. What has always worked for me is just taking the results at face value, start to work with it to gain experience, then look it up again so that the context is much clearer.

The book does indeed seem to be a bit laborious to read. What they're saying with the statement on $\dot{w}$ is that in the end result, the linearisations and simplifications are justified, in that no big inaccuracies occur if effects of $\dot{u}$ and $\dot{v}$ are neglected. Effects of $\dot{w}$ are significant though and do need to be considered, which the authors do in the rest of the text.

Equation 4.32 is the most generic statement there is on moments - what is says is: the total moment is (how the moment changes with a variable) times the variable. It does not yet consider what the cause of the moment change is.


How does the Boeing 787 Dreamliner's glide ratio compare to other airliners?

As discussed here, glide ratio is related to the L/D (lift-to-drag) ratio of the plane. When talking about commercial aircraft, "glide ratio" is typically used to mean the ratio with engines inoperative. If the jet engines aren't running, they add a lot of drag, which will decrease the L/D ratio from the typical max value.

This analysis estimates the max L/D of the 787 at 20.8, while this site estimates 21.

Other sources estimate L/D of the A330 and A340 at 20. The 747 is estimated at about 18. So based on these estimates, the 787 is slightly better than other large airliners.

Most of the efficiency gains of the 787 are from the lower weight of composite materials (which allow bigger wings) and more efficient engines.


Are tripropellant jet engines a viable way to increase fuel efficiency for large commercial airplanes?

First of all: jets are not "n-propellant". Rockets are bipropellant. That's because rocket needs to carry fuel and oxidizer, while air-breathing jet gets the oxidizer (plus huge amounts of reaction mass) for free from the air. So jets carry only one propellant (which doesn't make them monopropellant either). The tripropellant rockets were imagined to overcome problems with specific impulse. The most complicated and impractical rocket engine could go as high as 542s - while average jet operates around 3000s easily. So specific impulse is the problem jets never had in the first place.
The biggest advantage of RD-701 was to use the kerosene-hydrogen mix on low altitude (where kerosene is more efficient) and then switch to pure hydrogen in space (where hydrogen is more efficient). They started with a hydrogen engine (RD-0120) and then added kerosene to increase efficiency by reducing the amount of hydrogen needed - so the exactly opposite reasoning. The main problem with your idea here is that jets never leave the kerosene zone.
The main problem of liquid hydrogen is that the fuel is lighter, but the tanks are so heavy it completely obliterates any savings. Rockets here get it easy - rocket working lifetime is measured in minutes (Space Shuttle burn: 8m30s Soyuz burn: bit over 10m50s). A plane on the other hand would need to store the hydrogen for a dozen of hours = even more weight. There is no weight-efficient way of keeping the cryogenic fluid on the go. All you can do is to let some of it "boil-off". Twice as heavy as kerosene tank is a huge understatement IMHO, as in modern jet liners the tanks - "wet wing" - are almost free (in terms of mass). Can't do that with hydrogen - you'll need to sacrifice cargo space for dedicated LH2 tank.
Another issue is that hydrogen burns in higher temperature. We're already having problems with making the turbine survive kerosene temperatures, adding hydrogen would make the engine design even more difficult. From the green perspective it's not all roses either, because the higher combustion temperature, the more NOx created.
I have to agree with others here - hydrogen planes won't happen unless we'll run out of easier options. Having 2 kinds of fuel is the most difficult of them all.


Which should I get a FAA glider licence or a Canadian glider licence?

This is heavily dependent on the local jurisdictions you plan to operate in and what their laws are.

First off you must find out if your desired countries of operation accept foreign certificates. This varies heavily all over the world and can range from full no questions asked acceptance, to you needing to simply take the test again (in that country) through any various case you can think of.

Second you need to find out what certificate you need. Here in the US to give rides (and receive compensation for such) you need a commercial certificate. This looks like it extends to gliders as well.

Hours.

The FAA requires a few more hours than the Canadian certificate will,

FAA

  Are at least 16 years of age; and
  Have logged at least 10 hours of
  flight time in a glider and that flight time must include at least 20
  total glider flights, and
  Have 2 hours of solo flight time in a
  glider, and
  Have passed the FAA written examination; and
  Have passed the flight exam with a FAA Examiner.

Canada

  A minimum of 6 hours of flight training under the direction and supervision of an instructor
  At least 2 hours of solo flight time, including a minimum of 20 takeoffs and 20 landings

Right off the bad Canada requires 4 less hours which, no matter how you spin it will be cheaper (since you have to pay for less time).

Here is the US if you are not a citizen you will need to pass an NTSB background check to train. I do not know if Canada has a similar requirement.

Travel, assuming you are in the Caribbean already it may be cheaper to travel to Florida (or the like) to train then go all the way up to Canada for your training.

Edit:

According to this document the ECCAA does issue glider ratings

  2.1.2.3 RATINGS ISSUED

  (a) The Authority may issue the following ratings for pilots
  (i) Category ratings in the following aircraft:

  (a) Aeroplane;
  (b) Rotorcraft;
  (c) Glider;
  (d) Lighter-than-air;
  (e) Power lift.


What is the working principle of tip jets?

Tip jets spin the rotor by blowing out compressed air. It's the same principle as a jet engine or a jet-ski impeller, just that the exhaust is pushed out through the rotors. Some designs even had jet engines on the rotor tips! The primary advantage of the design are that there is no torque created, so no need for anti-torque rotors or thrusters, the tip jets also increase the rotor's moment arm and mass, making autorotation safer in some cases, although the drag from the tip jets often slowed the rotors down in many cases. Some say it is easier to learn to fly and safer to fly a tip jet design as there's no torque.

There are very few systems which use tip rotors, the main disadvantages would seem to be a lack of efficiency and drag from the tip jets. The rotor design is complex as high-pressure air must be carried through them, causing many design and materials challenges. Also, lack of a tail rotor means there's no way to pivot in hover unless unless you put some sort of tail thruster on.


Can airlines recover costs if they dump fuel for a medical emergency?

You are overestimating (by asking) the cost of fuel in such a worst-case scenario diversion that requires fuel dumping.

With this week's jet fuel price, 50 tonnes (approx. half of what is loaded on a 787 bound for a long-haul) would set you back \$31,000.[a] But,

  A single flight diversion can cost Emirates anything from US\$50,000 to over US\$600,000, depending on the nature of the diversion which includes fuel, flight catering, landing and ground handling fees, air navigation cost, passenger rebooking costs and onward connection, as well as other associated costs to care for crew and passengers.

Since 50 tonnes can be considered an upper limit, and Emirates' upper limit is \$600,000, fuel is just 5% of the cost. Further,

  Adel Al Redha, Emirates' Executive Vice President and Chief Operating Officer, said: "We can never hope to recover the costs of a flight diversion, but the wellbeing of our customers is always our number one priority (...) [emphasis added]

Source: Emirates reveals the real cost of medical diversions for an airline, www.airlineratings.com, 2017

[a]: Also, for reference: An A380 returned to SFO not long into a flight due to a medical emergency, and dumped "40 tons" on the way back (avherald.com).

Related: How much compensation does an unruly passenger have to pay to an airline if the flight has to be diverted to remove them?


What is the relation between the Lift Coefficient and the Angle of Attack?

Normally the relation is linear within the practical range of angles of attack used in flight. However, the Mach number and the Reynolds number will cause shifts. For the Mach effect, use the Prandtl-Glauert rule in subsonic flow up to Mach 0.7: $$(\alpha - \alpha_0)_{Mach} = (\alpha - \alpha_0)_{incompressible}\cdot\sqrt{1-Mach^2}$$

A higher Reynolds number means a relatively thinner boundary layer, and since the boundary layer on the suction side of the airfoil grows in thickness with increasing angle of attack, the same airfoil will have less boundary layer growth at a higher Reynolds number. This translates into a higher effective angle of attack at the same geometrical angle, so the lift curve slope becomes steeper and will reach stall at a higher angle.

There are no standard conditions, but a proper lift curve graph should note both the Mach and Reynolds number for which it is valid.


Usually an APU provides electricity and bleed-air, just like the aircraft engines do. An all-electric APU only provides electricity. This can be done in the Boeing 787 because of the bleedless architecture where systems that would normally require hydraulic or pneumatic pressure are fully electrified.


What are the bars protruding from this C-130?

It is a civilian owned C-130 Hercules used for aerial spraying operations. It is owned by the International Air Response company based out of Mesa, AZ.

The aircraft is equipped with a RIDSS spray system for use in things like oil spill response (for spraying dispersants). You can read more about the system here.


Why isn't there a procedure for dual-engine failure in twin-engine airplanes?

There are.  The Metroliner (SA227) has a dual flameout checklist.  It's possible that the company I flew for developed their own and had it approved, but we DID have a checklist for that.

I think they're rare on pistons twins because there are a very limited number of environmental factors that can cause double engine failures.  It's also possible to simply run out of gas.*  A turbine can ingest water, ice, dust, sand, snow, etc (no filters) that can contribute to a flameout.  The factor that causes one to quit will likely effect the other in a similar way.

I do know a pilot who flamed out both engines by using engine inlet heat on both engines at the same time.  The ice melted off the inlets at about the same time, both engines ingested the ice, and they quit.  He managed to relight both of them and landed uneventfully.  That likely would not happen in a piston twin.  The alt air doors would open and keep the engines running.

*If you find yourself thinking "I'm going to need that double engine failure checklist in about 20mins," you should probably seriously reflect upon the decisions you made throughout the flight.


Do runways need some special kind of tar or protective layer?

Airport designers may use any type of material, based on construction research status and local meteorological conditions. Pavements may be flexible or rigid. There is no single technique to protect against wear and tear, and none specific to airports, this is similar to regular road construction.

Information on pavement types at Texas Dept of Transportation.
E.g. a rigid pavement:

FAA provides advisories to help designers. ICAO requires airports to report their pavement characteristics.

(source: Detroit Free Press)

FAA documentation related to pavement design:

Pavement Design & Construction
Airport Pavement Design and Evaluation

ICAO requires member states to report aerodrome-related aeronautical data, including pavement strength. The standardized method is known as the Pavement Classification Number (ACN-PCN) method.

As a critical characteristic of a pavement is the friction, this matter is refined in Measurement, Construction, and Maintenance of Skid-Resistant Airport Pavement Surfaces. This document includes data on rubber removal frequency, e.g.

Busy runway end:

(source: Bridgepoint Road Markings Ltd)

A technique for rubber removal:

(source: Waterblasting Technologies)

This thesis describes how to optimize runway pavements in order to increase friction and how to remove rubber deposit:

Optimizing Airport Runway Performance by Managing Pavement Infrastructure by Samantha Theresa Pinto. Note that:

  Rubber accumulation on a dry runway is generally not a problem since
  the rubber interaction between the aircraft tire and the rubber on the
  runway creates traction that may actually improve friction. However,
  in wet conditions, rubber accumulation leads to loss of friction on
  the runway by clogging the pavement texture and creating an especially
  slick surface with minimal drainage capability when the runway surface
  is wet. Additionally, rubber accumulation causing decreased friction
  values means pilots have less directional control of the aircraft than
  expected on a bare and dry runway.


How to use Rudder during takeoff in single engine propeller aircraft? e.g. Cessna 172

Is this common in real life flying? Are actual pilots consistently
  able to align the nose wheel to the runway centreline during takeoff
  in such single engine propeller plane or am I being too idealistic?

In my experience flying primarily light singles its not terribly difficult to stay aligned on the centerline during takeoff. Even my first solo was nicely centerline. However the more powerful the aircraft the more the tendency and the more control/care may be required.

  How is the operation of rudders in an actual aircraft like? Slow
  smooth gentle depression e.g. to the right pedal each time the
  aircraft swerve to the left or are the left and right legs constantly
  moving in quick succession to keep the aircraft aligned to the runway
  centreline?

All control inputs should be smooth and with care. I generally ease onto the right pedal as I apply throttle and ease off/back on to stay center line.

  Generally, how much of a push is required on the pedals during
  takeoff? It seems to be very very slight 1cm touches to my rudder
  pedal on the simulator. Is that realistic?

This is what most home sims are worst at. Real light aircraft rudder pedals take a fair bit of force to work an have far more than a 1cm throw. I would add a curve to your response so that the initial throw of the pedals is less sensitive.


Do I need to obtain a student pilot certificate to transition from Glider to Sport Pilot?

You can't get another student certificate because as a Private Pilot you don't need one (and can't get one). Rather, you're a Private Pilot adding to your existing certificate.

Private pilots already have sport privileges, but if the LSA is a new category/class for you then you still need an endorsement for it. The requirements are in 14 CFR 61.303 (and see AOPA's guide too). You can check the table yourself, but let's assume you're a Private Pilot and you have no medical but you do have a US driver's license. That means the sixth row in the table applies to you:

You must hold the endorsements required in 61.321 for the LSA's category and class
You must comply with the limitations in 61.315, except 61.315(c)(14) and, if a private pilot or higher, 61.315(c)(7)

61.321 requires a logbook endorsement from an instructor who's trained you in the areas listed in 61.309 and 61.311; a proficiency check from a different instructor; an application to the FAA; and a logbook endorsement from the check instructor. You also need a certain number of hours, depending on the category/class of the LSA; see 61.313.

I assume your glider experience already covers a lot of the knowledge and experience required, but your sport instructor will have to determine exactly what training you still need.

(This appears to be the opposite of this question, by the way.)


Is drift and sideslip angle the same in the accompanied figure?

The answer to the title question is no, sideslip angle and drift angle are not the same thing.  (in this figure, or any other...)
What the diagram shows is drift angle, and drift angle only.  If you are moving through a uniform airmass that is itself moving relative to the ground, your path across the ground (ground track, or course) will be affected by the movement of the air.  The difference between your heading (simply the direction the aircraft is pointing) and your actual ground track is the drift angle.  That is the teaching point, and purpose of this figure.
Regarding the numbered questions:

There is not enough information in the diagram to determine this.  Sideslip angle is the lateral angle between the relative wind and the longitudinal axis of the aircraft.  If the aircraft is in balanced flight, with the balance ball centered in the slip indicator, the sideslip angle will be zero.  It doesn’t matter what your heading is, it doesn’t matter which direction the wind is moving, or whether the aircraft may be drifting from its intended course.

The direction of the relative wind is always opposite to the direction the aircraft is traveling through the airmass.  In this example, presuming balanced flight, the relative wind is coming directly from the East.  If the aircraft was unbalanced, (balance ball out to one side or the other) there would be a sideslip, and relative wind would be striking against the side of the fuselage instead of head-on.  That angle is the sideslip angle.

There are different and better illustrations that show sideslip angle if you need more help understanding what it is.


Is hypersonic flight possible with a Busemann's Biplane?

Why Mach 10? Would Mach 3 or 4 not be enough? See here and here and here for the complications which arise at higher speeds.

The linked article is full of misrepresentations - of course will the Busemann biplane create lift at sub-supersonic speed (what is that, anyway? I read it as subsonic speed). Give it a proper subsonic airfoil and it will do so just like any other biplane, and even with the trapezoidal airfoil it will do so, albeit with more drag. It will also create lift and drag if operated at a proper angle of attack below its design speed (maybe that is meant by sub-supersonic speed, but the author never clarifies that).

Busemann biplane as shown in the linked article (source)

Did you note that the shock lines in the graph only occur between the two wings? That means that on the outer surfaces you will only see ambient pressure. The pressures internally cancel each other, and the result is that the Busemann biplane will not create any lift at its supersonic design speed in order to do its magic. As soon as it starts to create lift, it needs to create shock waves on the outside which will invariably be audible. That the researchers at Tohoku claim them to be 85% weaker is easy if they omit to mention to what they compare it.

Now look at the bulky passenger compartment on the top wing. This is clearly not optimised for supersonic travel (please compare it to the fuselages of the Concorde or the Tu-144 to get an idea what works).

No, this article is just utter nonsense.


What is the flight trajectory of a commercial airplane in regards to altitude v/s distance?

Your friend is not correct and you are somewhat correct. For now I will consider long-haul flights as the effect is more visible. As the aircraft takes off it climbs towards it cruising altitude, say 33,000ft. Then during the cruise the airplane uses fuel meaning that the aircraft is effectively losing weight. Since during a steady cruise, lift = weight the aircraft will have an excess lift if nothing but the weight changes. This will make the aircraft climb to higher altitudes were the air is thinner and thus more lift is required.

Thus on some long-haul flights you can see that the flight ends at 38,000ft. Then the pilot initiates the decent for landing. Depending on ATC the aircraft is allowed to gradually climb during cruise to higher altitudes. If that is not allowed the aircraft will climb once every hour or so to a higher flight level.


What is the safe speed to use highspeed turnoff?

Please be careful applying a blanket figure during high speed turnoff. There are many variables which you should consider. These are:

•   Exact angle of the taxiway(you're turning onto):  If it's a very shallow angle off the runway and therefore does not require much turning, high-speed turnoffs are easier and manageable, and vice versa.

•   Rate of deceleration

•   Side load limitations on the CRJ

•   Tolerance of the nose gear

•   Traffic situation on taxiway

Generally speaking you need both sets of eyes outside, especially if you are joining or crossing another Taxi Way.


What is the fastest unpressurized aircraft?

Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor.

  The operational ceiling was limited by what the pilot could endure for
  several minutes while breathing oxygen from a mask, without losing
  consciousness. Pilots underwent altitude chamber training to harden
  them.

Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.

The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h.

About 370 were built.


How much thrust is needed by an aircraft to have vertical takeoff?

First, let's agree on terminology: What you saw in airshows is a vertical flight path. Flying horizontally first, the airplane pitched up until the nose was pointing straight into the sky.

Surprisingly, no thrust is needed to perform this maneuver. Even gliders can do it. What happens is that kinetic energy is converted to potential energy, the rate of potential energy increase being proportional to flight speed and aircraft mass. If you start fast enough, this vertical flying can be maintained for several seconds, until the aircraft runs out of speed and stops in midair, followed by an uncontrolled drop. Skilled pilots orient the aircraft in the right direction by starting a rotation around the vertical axis at the top of the climb, so the following drop lets them pick up speed again with the correct nose-down attitude. Now potential energy is converted back into kinetic energy until speed is sufficient for a pullout. In aerobatics, this maneuver is called a stall turn or a hammerhead stall.

A few conditions apply, however. The airplane must be able to fly fast enough to have the needed potential energy to sustain the maneuver through the pitch-up phase. This is helped if its engines add energy, so the kinetic energy bleeds off more slowly. Also, at the top of the maneuver it is flying at zero g, and this requires at least that all items on board are securely fastened. Lastly, the pitch-up needs a load factor bigger than 1 g, and the higher the maximum load factor is, the tighter this pitch-up can be flown.

Now the question has been changed: The vertical flight path is flown right after take-off. This limits the entry speed for the maneuver, and gliders will not be able to do this. If we take the 737 from the question and fly it with no payload and little fuel, the flight mass $m$ of a 737-700 is 40 tons, and the installed thrust is about 200 kN (sea level static). Let's assume that the pilot accelerates after takeoff to a horizontal speed $v$ = 100 m/s (194 KTAS) while retracting the flaps, the kinetic energy ($0.5\cdot m\cdot v^2$) is equivalent to a potential energy ($m \cdot g \cdot h$) of an altitude gain h of $$h = \frac{v^2}{2\cdot g} = 510 m$$
The engines deliver less thrust with increasing speed; maybe 40% of the weight, so the airplane will still accelerate for the first 18° - 20° of the 90° flight path change. This will delay the point when speed has been bled off and add maybe 150 m to h. At 100 m/s a pull-up with a radius of 500 m will add a load factor of 2 g. The pilot needs to pull less first and harder at the end of the maneuver in order to stay within the maximum load factor of 2.5. When speed bleeds off, so will wing lift, and in the second half of the pull-up the wing will not create enough lift to change the flight path enough in order to reach the desired vertical attitude. Also, the aircraft will be very low for a safe recovery.

This makes it rather doubtful that an airliner can be pulled up to a vertical climb after takeoff. If the maneuver is started at a higher speed and with a little more distance from the ground, I see no reason why it should not safely be possible.


Is air in the toilet isolated, to prevent seeping into the cabin?

If someone can get chemicals on board a flight, they can also get a bomb on. Bombs are easier to make and more effective. Better air-conditioning won't help here, better security will.
It is possible to isolate the toilet... but the problem apparently wasn't just inside the toilet. The toilet isn't pressure sealed, so some smell will escape regardless. It's impossible to completely seal the toilet from the rest of the aircraft, and could be potentially dangerous if it was
The aircraft does introduce fresh air into the cabin, but the compressors to do so only have a fixed capacity. The capacity of these could be increased, but at the price of extra weight, fuel consumption and cost, it's not worthwhile for the 1 time in 100 million that a flight has to turn around for this reason. Even with higher capacities, it's not possible to just constantly flush the cabin air - and remember that all that air needs to be compressed (to maintain pressure) heated, and filtered. All of that costs more money, and adds more complexity, weight etc.


Is it practical to use wires to stiffen rotor arms?

I see little reason why small gauge wire couldn't be used to strengthen rotor arms on a multirotor. The only major issue I could see developing is that the wire on the top side of the arm might accidentally intersect the prop circle.

MS Paint sketch of a possible design idea:


Does the yoke move when trimming a THS like it does when using elevator trim?

When I was type rated in the CRJ200 with hydraulic controls and a THS, directly coming from light aircraft, the differences in technique are significant and it was definitely something new to learn.
With a THS, column neutral is always the same spot.  This means trimming to a speed is not a case of moving the control to location X and trim until it stays at location X. You have to apply a control input to achieve a speed/attitude, and trim until you can relax the control input and let it go back to its "permanent" neutral position while the airplane maintains the target trim speed.
So what you do is, say, you want to slow down from 200 kts to 180 kts, so you pull and hold to slow down. While you're holding the input, you start to blip the trim switch with your thumb as, at the same time you let the column move a bit back toward neutral.  The objective is to make the trim input as you relax the control input so that the stab incidence change is replacing the elevator deflection and the airplane holds a constant attitude and speed.
You keep doing this; blip-release-a-bit, blip-release-a-bit, blip-release-a-bit, until the column is back at neutral and the plane is holding your target speed with no input.  With practice you start to pull and blip the trim right away and it all just flows automatically.
It's murder on the trim screw jack drive systems, because they get many many start/stop cycles a flight from pilots going blip-blip-blip on the switch whenever they are hand flying.  The autopilot also does the same thing, sending numerous trim pulses to trim out speed changes to offload the AP drive servo.
Light planes with trimmable stabs, like the Cessna 180-185 family, are very similar, because the elevator just wants to "trail" when hands free, and the neutral position doesn't change the way it does with a tab, although it varies a little compared to a jet with hydraulic controls.


Why do gliders have bungee cords in the control systems and what do they do? Are they on all control surfaces? What about ultralights?

The bungee is just a bidirectional spring that tends to hold the elevator at position x, and if you move the stick you are stretching the spring in one direction or the other.   They are used in the elevator control circuit to provide an adjustable centering force for trim purposes, and on gliders with all flying tails, a measure of stick free static stability (you don't get a lot - many gliders are mildly divergent in pitch if you aren't holding the stick, but this isn't a big deal since you are almost never flying hands off).  The glider's pitch trim control adjusts the "at rest" or relaxed position of the spring so it will tend to fix the elevator at this position or that unless overcome by a control input.

Normally you would have a trim tab on an elevator/stabilizer tail or a trim/anti-servo tab on an all flying tail, as regular airplanes do to provide speed-seeking trim and strong stick-free stability, but gliders operate with much lower loading and forces on the tail and can get away with a bungee spring that acts like a little man under the floor pushing on the stick, this way or that to try to hold it at a given position, so you don't have to.  This does away with all the linkage, tab surface etc, which is a big deal when you are shooting for a 40:1 or more L/D.

You can put bungees in any control circuit if you want to be able to trim it with an adjustable centering force, or want to increase the forces.  Some light aircraft use bungee springs in the rudder control circuit to provide rudder trim.  The spring just replaces your foot by exerting a spring force so you don't have to.  The downside is the spring loads are present when you move the controls out of the trimmed position, adding to the perceived rudder forces when maneuvering.

I'm pretty sure there are ultralights out there that use elevator bungees for trim purposes instead of a tab.  However, driving the surface directly aerodynamically using a tab on the surface provides a more sensitive and robust solution and if I was designing an ultralight, where drag isn't that big a consideration, I'd use a tab for the elevator, and perhaps a bungee in the rudder and aileron circuits if I wanted rudder and roll trim.


Is there a minimum distance between a runway and a public road?

Yes, there is a minimum area that must be free of obstructions and closed to vehicles, but only for airports that have air carrier operations (see 14 CFR 139.309) or that receive FAA grants. AC 150/5300-13A - Airport Design has all the technical details; the actual distances are calculated from formulas, so there's no single answer. If you're really curious, you can read all the gory details in Chapter 3 and this FAA FAQ has a (very) simplified explanation:

  What is the size of the RSA [Runway Safety Area]?

  The RSA is a rectangular box surrounding
  the runway and is based on the runway design code. The dimensions
  range from 120 feet to 500 feet in width and 240 feet to 1000 feet in
  length beyond the departure end of the runway. (see FAA Advisory
  Circular 150/5300-13, Airport Design) Generally, on airports that
  serve air carrier aircraft, the RSA extends 250 feet either side of
  the runway centerline and 1000 feet beyond each end of the runway.

You didn't mention which airport you're asking about, but presumably it's either privately owned and/or doesn't receive FAA grant money.


Can a military pilot make his airplane invisible for the ATCs?

Radar systems have 2 aspects, the primary and secondary systems, or the Radar and the interrogator. The radar sends out a RF signal and receives everything that is returned on that same frequency, and performs calculations to determine the azimuth and range of everything it receives. The interrogator sends out radio pulses which transponders in aircraft receive and decode. The transponders then transmit data on a different frequency than what was received. The interrogator then receives that signal and decodes it and correlates it with the primary radar data to tell the operator that 'yes, that is definitely a valid target' and gives the info on the flight to the operator.

You can turn off the transponder, but all that will do is make it so that the ATC won't be able to see a correlated flight, (one that is definitely an aircraft and not just noise coming through the primary radar). The screen would still show a blip where the radar picks up a signature though. If the plane has radar absorbent material or is close enough to the ground, that would get filtered out as noise.

An ATC, who is used to only working with aircraft who want to be seen, and thus keep their transponders on, might be fooled by this. But a military radar operator might not be so easily fooled by this.


How can an aircraft owner/operator track unauthorized flights?

The owner could do regular checks of the aircraft's Hobbs meter, and if the times don't match what the log is saying, then there could be some red flags being thrown.

If someone it out to deceive, I'm sure they could get around this, but maybe then, the problem is giving access of expensive equipment to people who aren't trustworthy.

There are GPS tracking systems that could keep a movement log in memory.


How to calculate altitude in a flight simulation?

You can't directly calculate the altitude of an aircraft from any set of control settings. The altitude is dependent on the history of the aircraft. For example:

Fly level at full power for one minute, and pitch up suddenly. You will be near the ground, having quite some speed.
Climb at full power for a minute. You will be at high altitude, going quite slow.

In both situations, your aircraft is pitched up and at full power, but at very different altitudes. So, what are then the minimum variables needed?

This answer is quite difficult to give. If you're making a professional simulator, thousands. If you're making a platform game, just pitch and power, as well as some way of knowing the history of the aircraft. Usually, this is just a stepped simulation: at each moment in time, determine what your current pitch and power do to the attitude and altitude of the aircraft, and save it for the next time step. Of course, you need some model for relating these variables to actual accelerations of your airplane, and again, this model can be as complicated as you want or need it to be. See Koyovis' answer for more details on such a model.


How thoroughly can WINGS phases substitute for a flight review?

All Wings courses that qualify for a Flight Review include both a online knowledge activites as well as required flight activities that are verified by a CFI. These flight requirements generally end up being longer than the 1 hr minimum flight time required by §61.56.  But if you are completing a Wings course for the purpose of insurance discounts, a BRF would be a side benefit.

  To accomplish each Phase of WINGS, you can attend safety seminars,
  take online courses, and demonstrate flight proficiency.

Also, section 1.3 of the Wing Manual states:

  Note that pilots need not accomplish the flight review requirements of
  14 CFR part
  61.56, if he or she has satisfactorily completed any Phase in the WINGS - Pilot  Proficiency Program

So, the answer is yes, you can use Wings phases as a substitute for a flight review, but it might not be the most efficient way.


Is it difficult for a GA aircraft to escape a thunderstorm?

If you take off with a storm 20 minutes away from the airport, it won't have been "unforecast"; the NWS is much better than that. They can pretty accurately predict what's going to happen regarding various fronts at least 24 hours in advance, in turn giving you the advance notice to adjust your flight schedule to ensure you can get around the storm or well away from it.

But let's say someone goofed (probably you), and now you find yourself climbing into the air off the tarmac with the leading wall of a cumulonimbus looming over the horizon, which will be between you and your intended destination before you reach it.

First rule; never fly toward the storm. Cumulonimbus formations are deadly to small craft, and even commercial airliners prefer to stay at altitude in a holding pattern well above the storm and let it pass by before attempting to land. These "thunderheads" produce torrential rains, powerful and frequent lightning, strong updrafts, hailstones ranging from pebbles up to grapefruit, and turbulent cyclonic winds that can culminate in tornadoes. Your Cessna 172 stands no chance.

Flying away from the storm will, at worst, gain you the time between the storm's speed and yours that you can use to find an open airstrip and put down. As Michael Kjorling said in his answer, most Cessna 172 variants cruise at over 100kts, which would require the storm moving across the ground as fast as a Category 2 hurricane rotates, so in most scenarios even if you're flying in still air with the storm moving over the cooler air you're flying in, if you just put the squall line to your rudder you should gain ground. Ideally, the storm will be pushing a mass of air in front of it which will give you a tailwind, and thus you'll be able to put that much more distance between you and the storm front.

Second rule; unless your planned flight is a cross-country flight heading pretty directly away from the incoming front, get on the ground as quickly as you can do so safely, and find a shelter for your aircraft. Obviously, being at your home FBO where you rent your tiedown or hangar is best, but anywhere you can slip your aircraft under a roof or awning for a couple hours will do. Remember that unless you have your instrument rating, the conditions during and after most thunderstorms are IMC and thus illegal (and deadly) for you to fly in.

If you're flying a pressurized aircraft, and/or you have instrument privileges, you have more options, the best of which is to get above the storm and wait it out. However, your Cessna 172 is not pressurized, so unless you have an oxygen bottle aboard (and are endorsed to use it) you're limited to about 12,500 feet and only for a half-hour, well below the top layers of a thunderhead, so you'll have to outrun it, get around it, or put down and shelter in place while it blows over you.


Performance metrics of a propulsion system - optimising a propulsive fan system

Well, if all you care about is the fan performance by itself, then you've pretty much got it.  Maximize the component cycle efficiency and maximize the propulsive efficiency and that's it.  But if you want to start getting more real-world, then things to think about:

Off design conditions.  You can perfectly optimize efficiency at one condition (i.e. for a large commercial airliner this would be cruise: Mach 0.8 @ 40,000 ft), but the thing still has to have acceptable performance at other conditions (e.g. takeoff, Mach 0 @ 0 ft).  You may have to compromise on cruise efficiency to get acceptable takeoff performance.

Since this is an electric fan, it will be driven by some kind of a motor. That motor will have some speed where it is most efficient.  That may not be the speed at which your fan is most efficient.  You'll need to match the two of them to maximize overall aircraft efficiency.

If you want to go even one step further, that electric motor has to get power from somewhere. Presumably a battery.  To get peak efficiency from the motor might demand X current at Y voltage, but your battery might be optimized for less than X current.  Again, you may need to match the power source to the motor.

If you want to go even further, there are a number of real-world turbomachinery considerations to deal with including: Operability / stall margin, Inlet distortion tolerance (i.e. crosswind), Fan blade aeromechanics (i.e. flutter)


Is it harmful to operate an engine "over square"?

No it's an old pilots' spouse's tale.  Those are just two numbers.  Having said that, Brake Mean Effective Pressure in the cylinder is higher when manifold pressure is high and rpm is low, for a given horsepower output, than the opposite way around, so there is a bit more stress on the cylinder head and piston.  And you do want to avoid that condition at the extremes to avoid really excessive BMEP, which could result in detonation.

The key word is "extremes".  With wide open throttle with the propeller at min RPM, it could be damaging, so it is normal practice to always lead with rpm on the way up and throttle on the way down.

But in the normal operating range it's not a big deal.  Look at a supercharged engine like a R985.  Redline is 36" of manifold pressure at 2200 or 2300 rpm.  It's jugs should be blowing off left and right.  And you cruise it at maybe 28" and 1600.  Oversquare enough?

So on the one hand you do have lower BMEP running higher rpm/lower MP, so somewhat lower stress on the cylinder.  On the other hand, heat and wear from friction is higher because of the higher rpm itself (and very importantly, the higher piston speed), so that tends to favour higher MP/lower rpm, for the same horsepower output.  My own preference in a normally aspirated engine is to cruise at say 2000 rpm and 22" instead of the other way around, because there is more benefit from the lower piston speed (piston rings wear out after traveling a specific distance, so the lower the piston speed the better, within reason) than from marginally lower internal cylinder pressures.


What is the purpose of the ball and string attached to the tip of the vertical fin on test aircraft?

That is a static pressure line. The "shuttlecock" device on the end pulls the line out behind the aircraft, where it can get a more accurate pressure reading, away from the air flowing around the aircraft that affects the regular pitot-static system. It will also be more accurate during flight conditions that normal aircraft rarely experience, but need to be accurately analyzed during testing.

Cropped from this image

Static pressure has an effect on the performance of the aircraft, so during testing that is one value they want to measure accurately during flights. Once testing is complete there is no longer a need for that information, and the regular pitot-static system takes the measurements needed.

The line can be seen in this photo, and in this video, you can see the reel inside the rear of the aircraft that is used to pull in the line when it is not in use.


How do aerodynamic loads explain the Spaceship2 accident?

Aerodynamic load overpowered the actuators holding it in place, forcing it to open and causing SpaceShip2 to break up in flight. Although the rudders were unlocked early when SpaceShip2 was going at Mach 1.0, this action alone should not have been enough to pivot the tails upright, because neither pilot took the further step of turning the feather handle to actually move them. However, the tails rotated upward anyway, and the increase in drag at this point in the flight caused it to crash.

  How is possible that this configuration change, meant to increase drag, needs a "preventing" (for lack of a better word) mechanism?

The feather is ultimately there for re-entry. It acts as a hinge and can go up and down. This system increases atmospheric drag on the spaceship so that it naturally positions itself during re entry. To make sure the tail doesn’t move forward early, SpaceShip2 has a structural lock that holds feather in place. Around Mach 1.4, the lock is moved so that the feather is now mechanically free. It won’t move though because at Mach 1.4, aerodynamic forces keep it nailed back. In addition to the lock, the feather itself has a big actuator that drives it up and down. So just because it’s unlocked doesn’t mean it’s just flopping in the wind and it can go where it wants. In the (initial) boost phase, the aerodynamic forces can overcome that, which is why it is  locked .

  How do the aerodynamic loads make the drag naturally increase?

Aerodynamic forces are generated by the difference in velocity between the spacecraft and the air. There must be motion between the spacecraft and the air. If there is no relative motion, there is no lift and no drag. In order to maintain a constant airspeed, thrust and drag must remain equal, just as lift and weight must be equal to maintain a constant vertical speed. If in level flight, the engine power is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate until its airspeed is insufficient to support it in the air.

Calculations can be found here

Aerodynamic forces are used differently on a rocket than on an airplane. On an airplane, lift is used to overcome the weight of the aircraft, but on a rocket, thrust is used in opposition to weight. Because the center of pressure is not normally located at the center of gravity of the rocket, aerodynamic forces can cause the rocket to rotate in flight. The lift of a rocket is a side force used to stabilize and control the direction of flight. While most aircraft have a high lift to drag ratio, the drag of a rocket is usually much greater than the lift.

  Wouldn't the drag on the tail section make it go down again?

There are two conventional ways to transition from space back to Earth: One is to come in as a winged airplane, like NASA’s space shuttle. In this way the spacecraft can land where it wants, but hitting the perfect angle of attack is tricky. The second method is as a capsule, which is essentially dummy-proof, but has no control over where it lands and subjects its occupants to high G-forces. SpaceShip2 is a bit of both. Its tail folds up and down, changing the vehicle’s shape from something that falls like a capsule to one that flies like an airplane once it’s back in the atmosphere.

Each of the two tails acts as a rudder, and has a small, horizontal flap at the back, extending to the outside. When the plane is descending, both tails can pivot upright, together, from 0 to 90 degrees, so that they stand “vertically” behind the plane. In this configuration, the tails and flaps create drag. When pointed upward, the tails are at right angles to the direction of airflow, creating a huge amount of drag on the vehicle, which slows it down without overheating the spacecraft. This method works because SpaceShip2 is not coming back from orbit.If put in feathered configuration during powered flight, the drag vector and thrust vectors are horribly misaligned, resulting in extreme pitch rates and, since the aircraft wasn't built to withstand those stresses, it would break apart


Does the Boeing 737 MAX use electro-hydraulic actuators?

The Max will have hydraulic systems very similar to the NG airplanes, except for the spoilers; those will be "fly-by-wire" and electrically controlled, but still hydraulically actuated. (Edited to correct what I had wrong in the original post. The comments are correct.)

Discussion of the FBW spoilers' capabilities


How does an aircraft turn while taxiing?

Primarily steering is accomplished in two main ways:

Steerable nosewheels - Usually limited to tricycle undercarriage, and usually controlled using the same left/right pedals as control the rudder in flight
Using the rudder itself - Often associated with the traditional undercarriage (tail draggers to some). In this case the rudder works on the ground in much the same way as it does in the air - ie, deflecting the rudder pushes the tail in the opposite direction, thus causing the nose to pivot in the required direction.

In addition, steering can be augmented in two ways

Differential braking - often the left and right main undercarriage have their own deferentially controlled brakes. These are usually controlled by "hats" on the top of each rudder pedal. By applying greater braking pressure to, for example, the left wheel will cause the aircraft to pivot more around that wheel causing a left-turning tendency.
Differential thrust - in a similar vane to above, greater thrust applied to the left engine will cause a  right turning tendency.


Why are twin engine airliners more fuel efficient than three or four engine airliners?

Not a fully definitive answer, but a large part of the "efficiency" of an aircraft is drag and weight. Two (larger) round engines will have less surface causing drag than four smaller ones. In addition, each engine will add complexity to the aircraft (fire monitors, fuel piping, extra hydraulics) as well as necessitating duplication of things like thrust reverses, all of which add extra weight and sometimes drag.

So, all things being equal, one huge engine would be most "efficient". This would obviously be difficult, and impractical for most passenger jets, so when they will fit, without scraping the ground, two are used. If more thrust is needed, then they break it down into three or four.

(Note, the main reason 4 engines were used in the first place was for redundancy. At least one country's [US?] regulations stated that the aircraft must be capable of sustained flight, including climbing, with the loss of an engine. 2-1 = 50% thrust lost where as 4-1= only 25% loss, therefore requiring less reserve power)

*Edit note #2

"Twin engine airliners tend to have more excess power" they climb faster may be relevant in terms of efficiency, but part of the reason a twin will usually have more reserve thrust is due to the redundancy "keep flying" rules described above.


Gas Venting Stations: What's The Big Deal?

Gas venting stations sometimes release natural gas at extremely high pressures, so there is a risk of strong updrafts or turbulence. In any case if you fly directly over you may be going through a cloud of combustable gas which could possible ignite. It's a low risk, but given it's rarely a problem going around them it would be my advice to do so.

I've flown over one without realizing and it put me in a 1200ft climb, I had to cut throttle in a hurry and dive to stay out of the London TMA. After that I paid attention to them.


Do all Class B and Class C towers operate 24/7?

The short answer is no. It's easy to find examples of class C towers that aren't 24/7, by checking in the Chart Supplement. Two examples are KXNA (as mentioned in the comments) which operates 1130-0500Z and KHSV, which operates 1200-0600Z. Note that the airspace changes when the tower is closed: KXNA becomes class G, and KHSV becomes class E.

As for class B, I would also assume that they're 24/7. I checked a few (KATL, KBOS, KSEA) in the Chart Supplement and they are indeed 24/7 but I don't feel like checking them all :-)


Why does the ATIS say "read back all instructions"?

Due to the ongoing runway incursion epidemic, US ATC must hear every aircraft read back certain instructions, including callsign. If you don’t read them back correctly, they’re required to repeat the instruction until you do. No matter how many times it takes.
When this rule was first adopted (or at least enforced), a shocking number of pilots couldn’t give proper readbacks. This caused a lot of frequency congestion and other issues, such as making an aircraft on final go around or holding departures just because someone taxiing toward that runway wouldn’t properly confirm they’d hold short of it.
Things have gotten somewhat better as pilots have learned, but it is still common enough of an issue in practice that they won’t take the reminder off the ATIS. And since some other countries either don’t have or don’t enforce this rule, it may never go away.


How many total airframe cycles should a B737-400 be put through before it becomes too costly to maintain?

According to this chart from the FAA the B737-400 variant was designed for 75,000 cycles. You can find more general info in this question as well as this discussion and this one. However there are lots of other factors that may make a 737 too costly to maintain long before it reaches the cycle life time.

Just to give an idea of flight cycles to time comparisons. Lets say a 737 flys an average of a 2 hour flight and 30 minutes to turn the plane around. 75,000 * 2.5 = 187,500. There are 8760 hours in a year so if the plane were to fly continuous 2 hour legs with 30 minutes to turn the plane around it would take 21.4 years to hit the 75,000 cycle life of the fuselage. However we all know that planes need service and most airlines dont have 100% airframe utilization all the time so it will take well over 21.4 years to hit that cycle limit.


Are any flight schools or clubs in Canada using Pipistrel's Alpha Electro or Velis Electro?

The only Pipistrel Electro registered in Canada is an Alpha, C-IZAP, registered to a private owner in British Columbia.  So the answer is for now, no.
Pipistrels, including the Electros, are licenced in Canada as Advanced Ultralights.  I don't see any reason an Electro couldn't be used for training under the Advanced Ultralight regulations (not for Private Pilot however).  Probably cost is an obstacle.


A backlog of orders does not indicate a manufacturing problem. Quite opposite, it is good for the business as it adds stability. Airbus outpaced Boeing on every aircraft expect the A350/787 last year.

Producing all of the backlog in one year is not a good idea, as investments into the production line would be enormous and after a year, utilization of the line would be horrible. If all A380 were to produced this year, what should they do with the production line next year? A chance of another 150 orders per year coming up is unlikely.

A350 production started last year while 787 started in 2011. The ramp-up of production has already happened for the 787, but not for the A350. According to Wikipedia, by 2018 the production has ramped up to 10 per month, which would outpace the 787 at the current capacity.

https://en.wikipedia.org/wiki/Competition_between_Airbus_and_Boeing

https://en.wikipedia.org/wiki/Airbus_A350_XWB#Entry_into_service

In conclusion, I do not think they have a manufacturing problem. Keeping reasonable backlog is good for the business.


Tailless means no horizontal tail, but a vertical tail is still allowed. Examples are the Convair F-102 or Convair B-58.

Convair B-58 Hustler (picture source)

Compare this to a flying wing: Here even the vertical tail is left off. Since a classic delta would have too little lever arm for yaw control, this requires a higher aspect ratio wing. An example is the Horten IX:

Horten IX V1 prototype aircraft (picture source)