How safe is the Certified Rotax 912 versus the Continental O-200?

As far as I can find out, no one knows. There's no good data available on engine reliability from the manufacturers - although they usually don't share that information anyway - and nothing substantial that I could find from any third parties.

There is some information online, although it's mostly opinions and there's no real, consistent data to support it. This 2008 article compared LSA engine options including the Rotax and said that there were more incidents involving Rotaxes than Jabiru engines (excluding non-engine related causes), but there were also many more Rotaxes in service so the comparison isn't meaningful.

In fact, the article said that any comparison is difficult because the Rotax was (is?) primarily used in experimental aircraft:

  The great problem in identifying engine-specific problems is that in
  the majority of installations, the engines power
  experimental/amateur-built category aircraft, where differences in
  mounting, accessory placement, cooling and vibration variations can
  instill issues that vary from one installation to another

And it also made the point that there are plenty of engine-related incidents involving Continentals:

  And accidents involving Continental O-200-powered craft were as
  numerous as you'd expect for the length and depth of its presence in
  general aviation aircraft. Here again, many occurred through no fault
  of the engine. Still, numerous reports involved engine stoppages,
  heat-related power loss and other engine problems.

The article's conclusion seems to be that a Rotax engine is not (was not) significantly different from any other engine:

  To paraphrase the words of one long-time Rotax service-center
  operator, the main problems involving the engines stem from many of
  the same types of problems—poor judgment, misuse, mismanagement and
  poor maintenance—befalling all aircraft.

I found various other forum discussions about engine noise (no conclusion), fuel efficiency (Rotax is better), maintenance costs (some preference for Rotax) and so on, but nothing authoritative. I suspect that a detailed, data-based comparison simply doesn't exist; finding anything for Lycoming vs Continental is difficult, even with the much longer history and larger installed base.

In general, engine comparisons seem to be mostly based on opinion and anecdote (as described here). That doesn't mean the information is useless, but you do have to spend some time sifting and weighing the opinions to reach your own conclusions.


What is this helicopter (and probe) in the film "Act of Valor"?

By U.S. Navy photo by Mass Communication Specialist 3rd Class Robyn Gerstenslager [Public domain], via Wikimedia Commons

Yes, it looks to be a Boeing CH-47 Chinook. Above is a photo from Wikipedia showing an Army Chinook with a Navy SWCC boat attached to the external cargo hook, looking almost exactly like in the movie.

And yes, the probe is the refueling probe. It needs to stick out from under the rotors, to prevent the rotors from accidentally cutting the fuel hose. (Although it doesn't prevent this from happening.)


Does fly-by-wire permit smaller tail fins (area and thickness) for jetliners?

After much google-fu, for the 777 it's an astounding yes (that someone was right after all).
From a 1996 ICAS paper (see PDF pages 2, 4, and 5) about the 777's innovations, the use of relaxed stability and FBW laws indeed helped reduce the tail area and its weight:

The paper mentions a wheel/rudder crosstie. The wheel here being the yoke. Before the 777 Boeing sized the tail based on controlling an in-flight engine-out using only the yoke. This digital crosstie allowed the smaller fin:

Vertical Stabilizer. Conventional sizing of the vertical stabilizer is based primarily on meeting requirements associated with static directional stability. Sideslip characteristics and the pilot’s ability to control the effects of in-air engine failure using wheel only have sized the vertical stabilizers on previous Boeing airplanes. With the wheel/rudder crosstie, the same desired sideslip and engine-out control characteristics can be achieved with the smaller vertical stabilizer. The rudder input due to the crosstie compensates for the reduced area.

The even smaller fin of the quad-A345/6 remains open.
It turns out my guess had merit. The A340 does use the roll surfaces to supplement the yaw:

The flexibility of fly-by-wire was used to optimize the VMCG speeds. In fact, the rudder efficiency was increased on the ground by fully and asymmetrically deploying the inner and outer ailerons on the side of the pedal action as a function of the rudder travel: the inner aileron is commanded downwards and the outer aileron complemented by one spoiler is commanded upwards.$^1$

And watching the engine-out test video again, this becomes clearly visible:

Note: either the author mixed inner with outer, or Airbus updated it since or flipped them for the -5/600, either way, it doesn't change the answer.

$^1$: C. FAVRE (1994) Fly-by-wire for commercial aircraft: the Airbus experience, International Journal of Control, 59:1, 139-157, https://doi.org/10.1080/00207179408923072 p. 153


If everybody jumps in an airliner, would the pilots notice?

30000 lbs of people leaving and landing on the floors at the same time?  They'd notice something.

Physically, the weight of the passengers on the floors would increase a bit as they pushed off and accelerated vertically.  Maybe 30000 lbs would increase to 35 or 40000 lbs for a split second, then once everybody's feet are clear, all the passengers are basically in a ballistic arc, so their weight is removed from the floors and the airplane is now 30000 lb lighter for a split second, then they would land and their vertical acceleration would apply maybe 40000 lbs to the floors for another split second, before returning to the steady state 30000 lbs.  So I would say it would feel to the flight crew like they hit a very small bump of turbulence, and there were be some kind of dull thud coming from behind from 300 feet landing at once.


What is minimum information required to identify if two people will be on the same flight?

Yes, Departure City + Flight Number + Departure Date will identify a single flight.

Where you may have a problem is if flight 123 goes from AAA to BBB to CCC to DDD on a particular day, and passenger 1 gets on flight 123 at AAA and flies all three segments, getting off at DDD, while passenger 2 gets on flight 123 at BBB and off at CCC.  To identify that those two are together on flight 123 departing BBB, you'd need to either have passenger 1 enter all the segments of his flight, even though he doesn't change planes at (nor even really much care about) BBB and CCC, or you'd need information about all the flights from some source, so that you know that "flight 123 from AAA to DDD" goes to BBB and CCC in between.


What are the limitations to adding wings with engines to a helicopter?

A very promising British design from the 1950s used both, a rotor at low speed and a conventional wing at high speed for lift creation. This was the Fairey Rotodyne, a prototype for a flying bus to connect remote locations with a VTOL aircraft. Its rotor was tip-jet-powered and burned a mixture of fuel and compressed air bled from two wing-mounted Napier Eland turboprops. After take-off, power was switched to the propellers and the rotor was unloaded to minimize its drag at high speed. This way, both lift-creating means could be optimized for their speed domain.

You will notice that the wing is unusually  small while the rotor is unusually large. This allowed both to operate at higher efficiency than wings or rotors which are designed to work over the full speed range. However, the combination of the two weighs more than either a rotor or a wing, which reduced the possible payload of the Rotodyne. Neither was meant to fold away because that would had increased weight even more.

Fairey Rotodyne (picture source Ed Coates collection)

In the end, it was wavering British government support rather than any technical issues which limited the Rotodyne to a single prototype.

Today, the most promising solution seems to be a coaxial rotor with a separate source of thrust; either a propeller or jet engines. The coaxial rotor is slowed down at high flight speed in order to keep tip Mach numbers below 0.9 and the lift asymmetry of conventional helicopters is no problem for a coaxial rotor design, allowing much higher flight speeds.

Folding a wing would not help much: When the wing is not needed (which is at low speed), its friction drag is low. Folding the rotor, on the other hand, makes more sense but so far is limited to a helicopter on the ground in order to reduce its storage space on aircraft carriers. Folding in flight has only been studied conceptually.

Illustration 1B from US patent application 2015/0474290 A1 by Bell Helicopter Textron Inc.


What are the standard ATC light signals and emergency messages?

Those signals are regulated by ICAO Annex 2 rule of the air:
https://www.icao.int/Meetings/anconf12/Document%20Archive/an02_cons%5B1%5D.pdf

Therefore international.Some other ground signals are indicated there but as far as light signals the ones you showed are the only ones.


What inspection packages commercial airliners follow which make them more reliable than a C-130?

As mentioned, factors like a different mission are much more likely to be a reason for any difference in reliability. Military aircraft often fly a harsh mission, the pilots don't fly to extend life, they use max power on every takeoff, fly at a high power setting, and run the engine closer to its temp limit. They aim to get there fast, and the C-130 flies into dirt strips, where the dirt gets ingested into the engines. In comparison, commercial airlines operate with a very close eye on the bottom (financial) line. They take-off derated if they can, and fly with a very simple cruise profile. They do not perform high speed tactical descents that a military C-130 would, into an airport possibly surrounded by hostile forces with air to ground weapons.


What are the Single European Sky (SES) and Functional Airspace Blocks (FABs)?

SES = Single European Sky Legal Framework. It started in 2004 when the EU Parliament and Council adopted Reg 549/2004 (framework regulation), 550/2004 , 551/2004 and 552/2004 (interoperability regulation). Also, a very important regulation was 219 which established EASA! (All these legislation is called SES I... amendments later came in place along a myriad of Implementing Regulations) which dealt mainly with What and How the Air Navigation Providers (ANSPs) must provide ANS services. This legislation aims at Uniting the National FIRs (Flight Information Regions) into BIGGER FIRs (Called FABs - Functional Airspace Blocks). Most of the operational advantages of FABs: Direct Routes, lower CO2 emissions and lower fuel burns for air companies are True and EU citizens will gain overall.

Since FABs will include the airspace of more sovereign nations, from a legal point of view, FABs need to be created with a Multilateral Agreement in the adjacent States.


Do mechanical attitude indicators have rotation limits?

They do. When an attitude gyro is rolled or pitched beyond its limits, the gyro gimbals hit their stops and the instrument is said to "tumble".
Tumble resistance is more important in military craft, which traditionally use a gimbaling mechanism that can withstand any amount of roll without tumbling- which differs from those used in GA aircraft.
These two types of attitude indicators have names, and I invite the experts on this site to furnish them.


What are these lines on the nose of a Boeing 737?

I'm no expert on B737 radomes, but I was able to find that these are called Lightning Diversion Strips or Lightning Diverter Strips.

Since radomes are generally built with composites, they do not transmit static electricity in the same way that aluminum skin would.

The B737-200 maintenance manual states that the radome is constructed of fiberglass and is provided with lightning diverter strips on the outside surface—though it states that these are not necessarily provided on all airplanes. It further states that these diverter strips are constructed of thin gauge aluminum.

From b737.org.uk:

  The radome (RAdar DOME) is an aerodynamic faring that houses the weather radar and ILS localiser and glideslope antennas. Unlike the rest of the fuselage it is made of fibreglass to allow the RF signals through.

  Fibreglass is non-conductive which would allow the build up of P-static (static due to the motion of the aircraft through precipitation). This would in turn cause static interference on the antenna within so the radome is fitted with six conductive diverter strips on the outside to dissipate P-static into the airframe.

And from this article:

  Another important characteristic of the radome is its ability to bleed
  off static electricity. This is done through the use of
  anti-static/anti-erosion coatings and through the lightning diverter
  strips extending from the frontal area of the radome to the airframe.
  "These diverter strips, says Bauer, continuously collect static
  build-up from the surface of the radome and conduct it to the airframe
  without severe sparking or arcing."

Here is a video that explains some of the principles of lighting diversion (note that the particular strips discussed may differ from those found on the B737). The diverter strips are discussed particularly starting here at 2:20:


What is the relation between drag and weight?

If you allow to make a few simplifications, the answer is easy:

Friction drag is not affected by the angle of attack change. That means no onset of flow separation on the heavier aircraft.
The wing's optimum L/D is reached with the same flap setting in both cases.
Lift changes linearly with angle of attack, so the lift coefficient $c_L$ can be expressed by the product of lift curve slope $c_{L\alpha}$ and angle of attack $\alpha$.
We neglect the changed lift contribution of engine thrust when the angle of attack is increased.

Now the drag $D$ can be expressed by this equation:
$$D = \frac{\rho\cdot v^2}{2}\cdot S\cdot\left(c_{D0}+\frac{(c_{L\alpha}\cdot\alpha)^2}{\pi\cdot AR\cdot\epsilon}\right)$$
Per the definition above the term $c_{D0}$ is constant, so the change in drag between the lighter aircraft (index 1) and the heavier aircraft (index 2) will be
$$\Delta D = \frac{\rho\cdot v^2}{2}\cdot S\cdot\frac{c_{L\alpha}^2\cdot\left(\alpha_2^2-\alpha_1^2\right)}{\pi\cdot AR\cdot\epsilon}$$
In order to express this drag difference $\Delta D$ in terms of the aircraft's mass, write the lift coefficient $c_L$ as $\frac{2\cdot m\cdot g}{\rho\cdot v^2\cdot S}$:
$$\Delta D = \frac{g\cdot\left(m_2^2-m_1^2\right)}{\pi\cdot AR\cdot\epsilon}$$

The other symbols are:
$\kern{5mm} \rho\:\:\:\:\:$ air density
$\kern{5mm} v\:\:\:\:\:$ velocity
$\kern{5mm} S\:\:\:\:\:$ wing surface area
$\kern{5mm} c_{D0} \:$  zero-lift drag coefficient
$\kern{5mm} \pi \:\:\:\:\:$  3.14159$\dots$
$\kern{5mm} AR \:\:$  aspect ratio of the wing
$\kern{5mm} \epsilon \:\:\:\:\:\:$  the wing's Oswald factor
$\kern{5mm} g \:\:\:\:\;$  gravitational acceleration

Now we can answer your questions:

  How much more drag will it be?

The drag will increase with the square of the mass increase. The gradient of that increase depends on the span loading of the aircraft.

  what would be the exponent?

2

  Is the aerodynamics in the design process optimised for "half-loaded" aircraft?

No, always for the fully loaded aircraft, as lighter loads can be tolerated much better than higher loads. However, since fuel burn will cause a change in aircraft mass over time, the aerodynamics must work over a range of altitudes.

  how does this translate to fuel consumption, say on 40000 feet with the usual speeds?

Drag is compensated by thrust, so you need more thrust to overcome the higher drag. Fuel consumption goes up linearly, but since cruise at 40.000 ft means that the aircraft is light and the engines are running at close to their maximum sustained thrust, a mass increase by 30% is impossible. For a practical result, the heavier aircraft would fly at the same angle of attack and speed but at a lower altitude where both the increased lift and the increased engine thrust can be provided by the higher air density.

If you need to calculate the trip fuel at different weights: We had a similar question before, so please follow the link for an explanation.


How unusual were MH370's final ATC exchanges?

I have not heard the actual audio in context; I've only read the transcripts.

However, from the transcripts, I see the timing between some of their calls as a bit confusing and unusual.

01:01:14 | MH370 | Malaysian 3 7 0 maintaining level 3 5 0
This is slightly odd, because 11 minutes earlier they were told to climb to 350.   I do not think it is common to report when you have finished your climb.  ATC can see you leveling off at your assigned altitude.

Because of this unexpected, slightly odd call, ATC responds with:01:01:19 |  ATC  | Malaysian 3 7 0
("You called me.  ATC is here; I hear you loud-and-clear; Go ahead: What is your request?")

There is no response for 6 and a half minutes. This could reasonably be because MH370 has no request, and doesn't need anything from ATC, or perhaps they are having radio difficulty and did not hear ATC's call?  The most extreme interpretation would be that there is currently something happening in the cockpit that is distracting them.  Perhaps a lack of oxygen? Or some other distraction?

After 6 and half minutes, MH370 makes the same call again:
01:07:55 | MH370 | Malaysian...3 7 0 maintaining level 3 5 0

Again, this reads like the preface to a request.  Perhaps they didn't hear ATC last time, and are calling in again with a request? Perhaps they're suffering from hypoxia, and don't even remember that they already reported in?

ATC quickly responds the same way again:
01:08:00 |  ATC  | Malaysian 3 7 0
(I'm still here, I still hear you.  Go ahead with your request.)

Silence again for 11 minutes, before ATC gives them a normal instruction to change to the next frequency.  MH370 responds normally by acknowledging the instruction. At this point, we have confidence that their radio is working, and can probably rule out technical problems for the earlier lack of a request.

Technically, they should read back the frequency, but it is not at all unusual for a pilot to just acknowledge when he is confident he copied it right, such as a crystal clear transmission, or a route he's flown many times and a frequency he knows well.  Or, the lack of confirmation could indicate hypoxia or another mental impairment. Or that he has no intention of copying the frequency or changing to it.

My biggest point of concern and confusion is that MH370 twice reports their altitude, which sounds like they're calling ATC to make a request, and then says nothing more for 6 and 11 minutes.  The significance of this is not at all clear.  Either hypoxia or radio/electrical problems would explain this.  A cockpit struggle does not seem apparent, but some sort of distraction in the cockpit could cause them to start a conversation but never complete it.

My personal opinion, without much supporting evidence, is mental impairment, potentially due to failed pressurization, carbon monoxide, or other contamination like engine-oil in bleed air, which has been known to cause neurological damage.


Why is the rotor system of an aircraft and the rotor system of a helicopter different?

The aircraft hydraulic pitch control system is much simpler and has fewer moving parts than a helicopter's main rotor pitch control system.

A helicopter's main rotor runs at between 500 to 800RPM, but (for example) in the world of general aviation using piston engines an airplane's propeller runs at between 2400 and 2600RPM. If built to withstand the dynamic loads at 2500RPM, a helicopter main rotor would be far more massive.

Airplanes have no need of the cyclic pitch control system that helicopters require in order to be controllable.

All these things mean that mounting a collective-and-cyclic blade pitch mechanism on the crankshaft of an airplane engine would be a meaningless exercise.


Is there a definition of light/medium/heavy aircraft?

Section 4.9.1.1 of ICAO PANS-ATM-Doc 4444 Air Traffic Management defines LIGHT, MEDIUM & HEAVY aircraft for the purpose of wake separation:

4.9.1 Wake turbulence categories of aircraft
4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certified take-off mass as follows:
a) HEAVY (H) — all aircraft types of 136,000kg or more;
b) MEDIUM (M) — all aircraft types less than 136,000kg but more than 7,000kg; and
c) LIGHT (L) — aircraft types of 7,000kg or less.
4.9.1.2 Helicopters should be kept well clear of light aircraft when hovering or while air taxiing.
Note 1.— Helicopters produce vortices when in flight and there is some evidence that, per kilogram of gross mass, their vortices are more intense than those of fixed-wing aircraft.
Note 2.— The provisions governing wake turbulence minima are set forth in Chapter 5, Section 5.8, and Chapter 8, Section 8.7.3.


How can compartment bay loading informating be derived from the IATA load message (LDM)?

Paranee,
There is no such detail in the LDM, it is just a WEIGHT value. There maybe specifics for certain SL/DG (review AHM again for those) as some SL/DG are required to show location and some location and weight - explained in AHM.

BUT - LDM is just an overview, not very specific as it does not need to be. For a through flight A>B>C, where A is OFFLINE to your system, if you have received teletype LDM for processing and you are doing the sector B>C then any load B>C is just shown (know by the system) as TRA (transit) load - nothing more. No more detail.

However for unitized aircraft, the CPM complements the LDM to show the detail required by bay as opposed to HOLD and/or COMPARTMENT.


Does training adolescents require a different approach with regard to the level of neurological development?

No, that article doesn't suggest a different training approach with teenagers.

What it does suggest is that the teenage brain has potentially more neuroplasticity, and that "the capacity of a person to learn will never be greater than during adolescence." It does point out a possible factor to watch out for in young trainees: sleep deprivation, but it doesn't say that the effects of sleep deprivation are different than in older trainees, only that they may be more likely to stay up late.

The article mentions "the importance of creating an environment in which teens can explore and experiment while helping them avoid behavior that is destructive to themselves and others". Flight instruction seems to me to be exactly such an environment.

Instructors already should be adjusting their training approach to the individual. (Transport Canada - Flight Instructor Guide: Individual Differences). The advice given is based on symptoms, and there is no indication that a teenage slow learner (for example) should be treated differently than an adult slow learner. There is a column in that guide relating to "immature" trainees, which might apply more frequently to adolescents, but unless a student is showing immaturity, there is no need to treat them so.


Should I use given resolution or calculate resolution based on the max range and number of sig bits?

It's important to recognize the the resolution provided in the ARINC 429 spec is an approximation so you can't necessarily use that exactly as it is.
From the commentary in section 2.1.6 Data Standards

For convenience, all binary word ranges in
Attachment 2 are shown as whole binary numbers
rather than such numbers less one least significant
bit value. Also, the resolutions shown are
approximate only. Accurate resolutions can be
determined, if required, by reference to the range
values and numbers of significant bits for the words of
interest.

In order to provide a range of 1.5 to 20, the label will have an actual range of 0-32, which with 11 significant bits, will provide an actual resolution of $\frac{32}{2048} = 0.015625$ (approximated to 0.016 in the ARINC 429 spec)


Why are/were there almost no tricycle-gear biplanes?

It is a bit more than a coincidence. You might also ask why there are so few biplanes with retractable landing gear. The reason is similar.

Biplanes are propeller-driven (except for this one), and in almost all cases the propeller is in the front. Efficient propellers are large and require a longer landing gear when a tricycle gear is used. Granted, the plane will lift its tail during the ground run so the prop comes down a bit, but that happens over a flat and clean area, so lower tolerances can be afforded during the ground run than during all other ground operations.
Tricycle gears produce substantially more drag. The nose gear sits directly behind the propeller and in its slipstream, so its drag contribution is typically 40% of the whole gear drag. Tricycle gears became less of a drag (pun intended) when gear retraction became standard.
Biplanes are built light and the heavy engine cannot have much distance to the center of gravity. Where do you mount the nose gear? There is preciously little room for another piece of structure. And once that nosegear is crafted on the plane, its distance from the main gear is rather small. Any bump on the field will produce strong pitch oscillations when hit by the nose wheel. Better to have a tailwheel or skid at some distance from the main wheels.
Tricycle gears became a necessity with higher wing loadings and higher landing speeds. Biplanes don't need powerful brakes. On the contrary, on a grass field a taildragger is easier to handle. And with their short ground runs, biplanes often operate from grass fields.

Not that tricycle gears weren't tried. See the picture of a Breguet 6 below (source), conveniently proving that pusher biplanes were also designed. Gun synchronisation removed the last reason not to place the propeller in front.


Why does a Phugoid occur? How can it be eliminated?

Move the center of gravity forward

What you experience looks almost like a phygoid motion (well, most of it is a phygoid), but involves stalling at the low-speed part of the cycle, so it is not the classical Eigenmode.

Rather, your aircraft's trim point is beyond its stall angle of attack. It will, therefore, stall when not actively steered towards a lower angle of attack and then, when lift on the wing diminishes and its center moves backwards while lift on the tail is still linear, the aircraft will pitch down and pick up speed again.

Maybe it will be already enough to change the setting on your elevator to a few degrees more trailing-edge-down deflection. What certainly will help is to move the center of gravity forward because this will increase static stability and require more trailing-edge-up elevator deflection to maintain the same trim point.

From your sketch I would also guess that your tail volume* is on the low end. Move the tail further back by lengthening the fuselage: This will increase stability and, especially, damping, so the pitch motion after stall becomes less violent.

*  This is the area of the horizontal tail surface, multiplied with it's lever arm (distance between center of gravity and the tail's quarter chord point).


What does an "X flagged" VOR mean on an en-route IFR chart?

The X in a flag for a intersection, waypoint or fix regardless of if it is a NavAid means there is a Minimum Crossing Altitude. An MCA present means you must be at or above a certain altitude when passing over the fix in the direction specified. This would provide obstacle clearance for that particular Victor airway. The MCA usually indicates a transition from one leg or section of the airway to another with a higher Minimum En Route Altitude.
In the case of the LLC VORTAC noted above, that altitude is 8500 feet MSL when traveling Northeast on the V6 Victor Airway. However, it does not necessarily apply to the other airways. In this case, it only applies to aircraft continuing on V6 or joining V6 going Northeast. The MEA for the upcoming airway section is 12000 feet MSL. Where as some joining airways such as V32 have an MEA as low as 8000.
This differs from the Minimum Reception Altitude. The MRA is used to ensure proper signal reception of the NavAid used to identify a fix. The fix will be on an airway. But the cross referencing NavAid used to identify the fix does not have to have an airway associated with it.


What is dual mode adaptive landing gear system?

From Testing and analysis of dual-mode adaptive landing gear, taxi mode test system for YF-12A the purpose was to obtain a very progressive rate to keep the gear from bottoming out on landing, but then move to a flatter rate to minimize ground shock when taxiing.

Two air reservoirs of differing capacity and pressure were valved to the hydraulic struts. The smaller reservoir was used for landing as it reached higher pressure faster with a given amount of fluid displaced by the compressing strut. After landing the strut was switched to the larger reservoir.
My reference provides a description of the test configuration as being similar to the actual construction, which it says is in its Reference 1 which I was unable to locate.

In any case, it is simply an additional air (now nitrogen) cylinder connected to the service port of the strut, a somewhat higher pressure is used and there is no valve since they were only testing a single mode, taxiing. The smaller reservoir is likely just the self contained strut air reservoir as in the unmodified design.
Unmodified strut

Automotive shock with reservoir to show test configuration (this was not used, it is not even a strut)


Why Do They Continue To Abbreviate Weather Products? Can We As A Community Change This?

The language of aviation is terse, compressed and intended to leave minimum room for interpretation or misapprehension.

It makes heavy use of formulas, themselves intended to to be unambiguous, but which are not always obviously and immediately meaningful to those who are unfamiliar with the language.

This is the norm for potentially critical communication in all fields that I'm aware of - the military, medicine and throughout the transportation industry.

The users of this language are trained in it; being familiar with it is an essential part of their competence.

Language, interfaces, procedures and tools in aviation don't need to be user-friendly, they need to be utterly precise, be usable in difficult circumstances, and on every occasion they're used have exactly the same outcome.

It's possible that an aviator has failed to understand a weather forecast and died as a result, but a weather reports that allowed themselves the liberty of free expression would kill a lot more than that.


What kind of container should be used for storing a disassembled Cessna 172?

A "dry van" (nonrefrigerated) 53 foot trailer, commonly towed as a "tractor trailer" sports an extra 3 inches of inside diameter (8 foot 3 inches) to go with 9 feet 3 inches in height.

You could also build a custom crate either onsite (storage shed) or portable (on a flatbed).
Transporting it to a local barn or unused building is also possible, better than leaving it to the elements.

I would look into local storage options, with the wing removed, there should be many.


How does the Angle of attack vary from the root to the tip of a propeller for a fixed pitch prop?

You are absolutely correct. The "relative wind" at any point on the propeller is a combination of the aircraft's forward motion plus the angular rotation speed of the prop, which is greater at the tips.  This is why, to get the best Angle of Attack, the prop (airfoil) is designed with a twist or "washout" towards the tip.

Notice, with variable pitch props, as the planes forward speed increases, the entire prop AOA
is turned forward to account for change in relative wind.

Fixed pitch props try to find a happy medium between low speed performance and cruise efficiency.  For example, a 10 x 6 (finer pitch) gives better acceleration for take-off, but a 10 x 7 pitch uses less fuel at cruise.


How do larger aircraft compensate for reduced visibility from the cockpit, e.g. less visibility of the rear of the aircraft?

Appearances can be deceiving. The view "behind" a Piper Cherokee looks something like the image below: no rear window so you can't see what's behind you, but that really doesn't matter since there's no way to go "backward" in this aircraft using the flight controls.

Visibility aside, ground handling is often a a complex production, particularly for larger aircraft or when you have a lot of aircraft in proximity to each other or obstacles.

Moving small aircraft around typically requires at least 3 people. When you're moving forward you will have the person operating the tug and two "wing walkers" who stand a few feet off the wingtips and walk along to make sure the wings aren't going to hit anything. When pushing an aircraft back in a tightly packed hangar there's sometimes a fourth person walking ahead of the tail to make sure you don't back the plane into something.
Since there's not usually much for the pilot to do inside the plane at this point, they usually serve as one of the walkers.

Larger aircraft observe similar procedures, but for push-backs or power-backs it's known that the area behind the aircraft is clear (this is usually coordinated over the radio), and as long as the tug driver or pilot keeps the nosewheel on the appropriate pavement markings there will be wingtip clearance.

Line personnel are also often stationed to guide aircraft to and from parking in close quarters where positioning is critical (e.g. when pulling an airliner up to a gate, or parking a small aircraft on a busy ramp).

The other half of the picture is taxiing under your own power and control.

When taxiing, it's generally assumed that the pilot knows their aircraft's dimensions and will avoid hitting things. This usually works out well, but that is not always the case.

Airport taxiways are marked so an aircraft with a specific maximum wingspan will be clear of any obstacles while taxiing along the centerline, and as you're generally only going forward that's where visibility is most important (you can assume if the area your wingtip was going to hit was clear 15 seconds ago when you last saw it and nothing was heading into that area it's still clear now, and you're relying on the pilot of the aircraft behind you to be paying attention and not taxi into your tail).


How could it be possible to fully prevent wing tip vortex formation?

Wingtip vortices are an unavoidable side effect of producing lift with an airfoil of finite length (thus having "wingtips"). The wing is designed first to have relatively high pressure underneath the wing and lower pressure above it. Second, air passing over the top of the airfoil is directed downwards as it leaves the wing (thus "pulling" the wing upwards by Newton's Third Law). These two characteristics of an airfoil combine to make air around the wingtip want to "spin"; high-pressure air below the wing will "leak" upwards over the wingtip, then be directed back downwards along with the rest of the air, creating the wingtip vortex:

Reducing the pressure differential between air above and below the wing will by definition reduce lift, because it is this pressure differential producing lift. By the same token, reducing the "downwash" off the back of the wing will reduce lift because the force creating the downwash is also lifting the wing. Therefore, conventional wings will produce wingtip vortices by definition.

Theoretical ways to reduce this vortex include:

Make it harder for air to "leak" upward over the wingtip. This is the principle behind the "winglets" you see on many airliners; they prevent air iunderneath the wing being able to move over the top of it, thus reducing the severity of the rotation of the vortex (which reduces drag, which saves the airlines fuel):

Even small aircraft, like this PiperSport light sport, often have little blended winglets to reduce vortex drag, saving as much available power as possible from the limited-horsepower engines of this class:

The A-10's wingtip has a similar idea implemented the opposite way; a slight downward curve to the wingtip holds the "cushion" of higher-pressure air under the wing, reducing the amount of leakage around the wingtip, which lowers the wing's stall speed and also reduces tip vortices:

Don't have a wingtip. "Nonplanar" wing designs, including ring wings, reduce (but do not eliminate) vortex drag compared to conventional wing planforms simply by not providing a single border between lifting and nonlifting surfaces:

This is a concept drawing, but the idea of a tipless closed-loop wing has been incorporated into production small planes, like this Belarussian small single:

Vortex drag is not totally eliminated because there is still a downwash behind the wings relative to the undisturbed air to either side of the plane, but the intensity of the vortex (and thus the energy lost producing it which counts as drag) is reduced considerably.
Reduce the pressure differential at the wingtip. This is a major way that swept, delta and double-delta wing planforms reduce tip vortices. Very simply, most modern wings on high-speed or high-performance jets have a smaller chord length and a thinner cross-section at the wingtip than the wing root. This provides less lift at the wingtip (and the wingtip stalls more easily and thus tends to stall first) than a straight wing with a relatively constant cross-section, but the reduction in pressure differential and downwash right at the wingtip reduces the two primary contributors to wingtip vortices:


Why does the Trent XWB have intermediate compressor stages?

Jet engine designers have to find compromises all the time, and the one that counts here is between lower complexity and higher efficiency.

In the 1960s, Rolls-Royce fell behind in large commercial jet engines as competitors introduced large bypass ratio fans such as the GE CF6 and the P&W JT9D, which powered the new generation of wide body jets being introduced at that time. All efforts on the British side came to nought, partially for lack of funding (source: Wikipedia):

  The company [Rolls-Royce] went ahead with the project and under the
  leadership of (Adrian) Lombard built the twin spool demonstrator.
  Overall development costs of the RB.178 was £2.6 million. As a
  comparison, the G.E. and P&W companies were awarded nearly $20 million
  by the U.S. DoD ( Department of Defense) to develop and build the TF39
  and STF200 technology demonstrators.

or for the preference of US aircraft manufacturers for US engines. Wikipedia again:

  There are perhaps many reasons why the RB.178 failed to take off and
  chief among them was the refusal of Boeing to accept the Rolls-Royce
  engine on the transatlantic 747.

Rolls-Royce realised that they had no chance to catch up by designing yet another two-spool fan and risked the company on a three-spool design, which would later become the RB 211. Wikipedia again:

  Meanwhile, Rolls-Royce was also working on a series of triple-spool
  designs as replacements for the Conway, which promised to deliver
  higher efficiencies. In this configuration, three groups of turbines
  spin three separate concentric shafts to power three sections of the
  compressor area running at different speeds. In addition to allowing
  each stage of the compressor to run at its optimal speed, the
  triple-spool design is also more compact and rigid, although more
  complex to build and maintain.

The advantages of using three spools is that the speed of each stage can be more finely optimized, so each stage operates at a higher efficiency. Another promised advantage was the use of carbon fiber for the first stage fan, which allowed considerable weight savings and offered Lockheed a distinct advantage in its L-1011 TriStar over the very similar Douglas DC-10.

However, the carbon fiber fan failed the bird strike test and the engine did not meet its development targets. In the end, Rolls-Royce had to be taken over by the British government and the TriStar entered the market with a one-year delay. But the RB211 went into production, albeit with a delay. The gamble did not pay off commercially, but technically it placed Rolls-Royce instantly ahead of its competitors. When a higher-performance version of the RB211 was added as an engine option to the Boeing 747, the direct comparison with the two-spool designs made the efficiency gains of the three-spool design obvious. FlightGlobal reported in 1980:

  The importance placed on fuel saving by airlines is emphasised by
  Qantas' adoption of RB.211-524 power for its new Boeing 747s - the
  only aircraft on which all big three fans are available. Qantas found
  that British Airways' Boeing 747s fitted with RB.211s burnt roughly 7
  per cent less fuel than its JT9D-equipped fleet, a saving of about $1
  million a year at today's prices.

The second oil crisis in 1979-1980 made those efficiency gains very relevant to the airline industry. But the three-spool design offers another advantage: By tailoring the number of stages and their diameter to the desired performance, the same basic design allowed Rolls-Royce to cover a much wider thrust spectrum with one basic engine design. The RR Trent is a direct development of the RB211 and covers a thrust range from 240 to 430 kN without compromising on efficiency. This enables Rolls-Royce to adapt the engine optimally to stretched or shortened versions of most airliners, so it is in an ideal position to offer the most efficient engine for the job. In fact, the successful development of a three-spool fan engine in combination with a better business model helped Rolls-Royce to gain an overall market share of 40%. Wikipedia again:

  Sales of the Trent family of engines have made Rolls-Royce the second
  biggest supplier of large civil turbofans after General Electric,
  relegating rival Pratt & Whitney to third position.

That is the reason for the intermediate compressor on the A350: It helps to give it the most efficient engine for the lowest development cost and risk. And yes, the fan is all what is left of the low pressure compressor stage on the Trent 1000. This way it can be run at a lower speed while the intermediate compressor can be built smaller since it runs at a higher speed.


How is unusable fuel taken into account in weight and balance calculations?

Since you've used the word "required", you may need a current specialist on the subject to get a truly valid answer.

That said, here's what I can tell you based on having written weight & balance software that was FAA approved for use at three different freight carriers in the period from 1988 through 2016 for B727, B747, DC-9, DC-8, and L-1011 aircraft.

That software made no provision for unusable fuel, as that was included in the basic operating weight. Also, the manual systems the software replaced had no provision for unusable fuel.

However, as I remember, the forms that came to me from the last weighing of the aircraft when adding an airplane to the program sometimes listed the unusable fuel, and sometimes the undrainable fuel as well, depending.

In other words, forms or software involved in the everyday calculation of weight & balance typically do not show unusable fuel. Forms involved in aircraft reweighing, whether due to maintenance or regulation, oft times do.

Edit - Seeing @Anilv's answer, I realized I may have misunderstood the question. I took "unusable fuel" to mean fuel that can not be physically delivered to the engines as opposed to fuel that can be delivered to the engines but should not be. Ballast fuel fits into that second definition. It physically can be delivered to the engines, but should not be. Using that second definition the answer to the question:

  Is there any time unusable fuel would be required to be listed either added or subtracted on a weight and balance form?

is: Yes, there was an entry field for use for ballast fuel if such might be required to meet weight & balance requirements. On 747 freighters, for example, ballast fuel was on occasion used to move the zero fuel weight c.g. forward to be within limits. This was especially true for 747s that had been pax aircraft and had been converted to freighters. The addition of a cargo door well aft of the wing root produced a configuration susceptible to a c.g. that was aft of the aftmost zero fuel weight c.g. limit. The solution, if you couldn't arrange the cargo to take care of the problem, was to to put ballast fuel in the fuselage tank, which was the most forward tank.

While you were not supposed to burn that fuel, it was comforting to know that it could be used if you really needed it.


What is the maximal altitude at which an emergency slide can be deployed?

In the US, TSO-C69c details the FAA's technical requirements and testing protocols for evacuation slides. I can't find anything in it that talks about a maximum altitude or minimum air pressure. The closest thing I can see is this requirement in 4.2 to function correctly in a given temperature range:

The device, including its inflation system, must be demonstrated to be
capable of fully functioning when subjected to temperatures from -40
to +160 degrees F. If the device is intended for installation outside
the pressurized cabin, the device must be capable of functioning after
being stowed at -65 degrees F. The function of the device must be
demonstrated in accordance with the hot and cold soak test procedures
described in paragraph
5.9 of this appendix.

Sections 4.11 on inflation operation and 4.15 on the inflation system don't mention anything about ambient atmospheric pressure.
And the testing protocols in section 5 of the document only mention temperature, not pressure. For example, section 5.9.3 is part of the cold soak test protocol:

Deploy the device into ambient temperature conditions (typically
defined as between 65 and 85 degrees F) from the appropriate airplane
door or a suitable airplane door mock-up or module, within 10 minutes
after removal from the conditioning chamber.

4.15.2 has the only reference in the document to an aspirator system and it only requires a filter to avoid ingesting small objects.
So, unless I've missed something in the TSO - which I might have! - maximum altitude/elevation or minimum ambient atmospheric pressure aren't part of the required certification, at least in the US.


How do engine manufacturers derive varying amounts of thrust from a single engine model?

The CF34-3and CF34-10E are quite different engines. The -10 produces more thrust for a number of reasons:

It has a higher mass flow. The CF34-3 has a 44 inch fan, while the -10E has a 54 inch fan. So, the mass flow will be higher in the -10E, and thrust is related to mass flow.

The bypass ratio is different, the -3 is 6.2 while the -10E is 5.4.  This is also due to the different fan diameters, and will cause a difference in thrust (even if the mass flow was the same.)

They are significantly different engines. The -3 weighs 700kg, while the -10E weighs 1700 kg. So, it's a lot more than just a tweak of the blades, or a change in fuel scheduling, neither of which will change the weight. With such a weight difference, you would actually expect the -10E to be about twice as powerful, so a question would actually exist if it wasn't considerably higher than the -3.

The turbine temperatures are also different, due to the large difference in technology levels of each engine. The -3 was certified in 1995, in comparison to the -10E in 2013.The EASA type certificate EASA for the -10E shows the max allowed EGT at takeoff is 983 Celsius. The -3 max EGT for takeoff is only 871 Celsius, in comparison.

With regards to overhaul intervals, this info is generally proprietary, as it's commercially sensitive. So, you won't find much info. Suffice to say, airlines don't make money if the engines need to be overhauled often, so later versions with better high temperature material capability are likely to have longer overhaul intervals, than earlier generation engines made in a time when shorter periods was the norm, and acceptable then, but not now.


Theoretically anything can be certified if resources are allocated to meet certification requirements. It may not be practical though:

The plane must have at least two engines (obviously)
It must have the range to fly to an alternate on one engine only
Engine failure should not increase cockpit workload too much. Mostly this means some automation system that can operate with asymmetrical thrust.

(2) rules out small twin-engine planes that only have a few hours of endurance on a full tank, and (3) rules out anything without a glass cockpit and avionics suite.

Also, ETOPS is not just about aircraft design; it deals with aircraft maintenance as well. The manufacturer and operator need to come up with a maintenance program that reduces in-flight failures by monitoring various metrics and taking preventive actions before a component fault happens.

Therefore, on some aircrafts, it may not be worth the effort to obtain ETOPS.


Can I log instrument flight time for currency purposes in an Advanced Aviation Training Device (AATD)?

From what I can tell, the Advisory Circular is outdated. It was issued July 14, 2008, while the most recent update to 14 CFR 61.57 was effective Nov 15, 2013.
When the AC was issued, 61.57(c) did not contain the wording specific to Aviation Training Devices (effective 11/17/2003). That language was added in the version effective 10/20/2009, and the AC apparently has not been revised since that point.
At this point the regulations are more specific (and regulatory) than the Advisory Circular. You should be able to log recent flight instrument experience in a ATD based on 61.57(c)(3), or the subsequent subsections that pertain to mixed currency.

(3) Use of an aviation training device for maintaining instrument experience. Within the 2 calendar months preceding the month of the flight, that person performed and logged at least the following tasks, iterations, and time [emphasis mine] in an aviation training device and has performed the following--

(i) Three hours of instrument experience.
(ii) Holding procedures and tasks.
(iii) Six instrument approaches.
(iv) Two unusual attitude recoveries while in a descending, Vne airspeed condition and two unusual attitude recoveries while in an ascending, stall speed condition.
(v) Interception and tracking courses through the use of navigational electronic systems.


Why do pilot and GPWS approach callouts say “minimums” instead of “minima”?

Minimums is an acceptable plural of Minimum (along with minima).

As for why its used as a call out is likely because it is the preferred pluralization used by the FAA and in my experience the term generally used when talking about "approach minimums" or "weather minimums".

Its worth noting that the FAA uses both minima and minimums in their handbook.


Why in helicopter autorotation phase the opposing torque is eliminated?

Because the engine is not applying power to the rotors. No engine power means there will be no torque pushing the airframe in the opposite direction.

In auto, the main rotor essentially becomes a pinwheel, kept moving by the air rushing past it...

Auto doesn't really power the rotors, it keeps them moving so that the spinning rotor creates air resistance and slows the descent of the helicopter, and maintains enough inertia in the blades to flare and stop the descent.

Of course, that inertia is only sufficient for one attempt at flaring and stopping descent.


Why do passenger aircraft have the little airjets over each seat?

Why do airliners have them? Because passengers like them.

When you're sitting in the penalty box waiting for a departure slot and the air conditioning system is barely running the extra cool air you can get from those vents may be the only thing keeping passengers from strangling the screaming child kicking the seat.

Also a common "cure" for airsickness is fresh air -- blowing cool air over someone's face will often help with symptoms of motion sickness, and may prevent a nauseous passenger from actually vomiting. Opening these overhead "gasper vents" and directing them at the passenger's face provides a quick way of doing this.


DC-9 (probably a DC-9-10 but not 100% sure), BAC-111, and Sud Aviation Caravelle, from top to bottom.


Can I get an EASA Class 1 medical with high astigmatism?

There isn't an equivalent of FAA SODA under EASA regulations.
The final decision is up to the ophtalmologist (in agreement with the AME). You should bring to the medical examinations all supporting material to help him take an informed decision.
In case you won't pass the medical examination, you will be given right to appleal. For the appealing procedure you will have to prepare a dossier to persuade that you are fit for the EASA Class 1 medical.
Good luck!


Is anyone able to identify this vintage aircraft please?

It appears to be a Spartan 7W Executive.


Does Buoyant Lift Force scale with Load Factor?

It would probably be easier to think of it as a 50,000kg aircraft with a "normal" wing.  This untruth will make calculating accelerations and turn rate incorrect (because the actual mass will make it more sluggish).  But the wing at all times has to provide 50,000kg of vertical lift for the craft to not accelerate downward.

Then just like a regular plane, the wing has to double the (aerodynamic) lift at 60 degrees.  This allows it to have 50,000kg vertically and the craft doesn't descend.

  If I go into, lets say, a 60 degree bank maneuver and induce a 2G load factor to the airplane

I don't prefer this language.  The load factor is the result of the lift.  If you had a wing that couldn't provide increased lift (it was already at the point of stalling), then banking wouldn't increase the load factor.  It would either stall the wing or the plane would start accelerating downward.  Saying the load factor has increased suggests to me that the lift has already been accomplished.

  does the buoyant lift force scale in some manner with the load factor, or is the lift force completely static and continue to provide 50,000 kg worth of positive lift regardless of the induced load factor?

The buoyant force is constant and always vertical.  Instead of coming from the relative wind, it comes from the vertical pressure gradient in the atmosphere.  So in the simple analysis, it isn't affected by banking.  But since the gravitational pull on the airplane is also constant and always vertical, you can just combine them to understand what the aerodynamic wing is required to do.


Do propeller planes give contrails like jets do?

Yes they can. That image from wikipedia showing B-17s is more descriptive

Contrails originate from condensation of vapor coming from internal combustion engines, regardless of being turboprop, turbojet/turbofan or cylinder engine. Vapor is a byproduct of the hydrocarbon (fuel) being combusted with the atmospheric oxygen.


Do airplanes have sensors to detect landing gear fires?

There are fire detection heat sensing loops in the gear well.  If a gear was on fire it would trigger a "GEAR OVERHEAT" message and warning when the gear was up.  There are no retraction prevention systems that are able to detect fires at the gear.  Nationair was a unique series of clusterflarks leading to the accident, on top of the fact that the magnesium wheels started to burn.  Modern magnesium alloys don't do that.


Can a drone travel beyond the range of its remote control signal?

If the controlling software allows it then yes. There's at least one drone available where autonomous flight is possible; the Vision 2 FAQ says that the drone will continue its "mission" even if you lose the signal to it, although the overall length of the flight path is limited to 5km and the drone will not fly close to airports. Other drones stop and hover or land if they lose the connection to the controller.

Whether doing this is actually a good idea (or legal in whichever jurisdiction you want to operate in) or not is a whole other question.


Why is the altimeter unwinding unevenely during a spin?

The altimeter works by measurements of the pressure in undisturbed air, also called static pressure. A static port is placed, usually on the side of the hull, perpendicular to the airflow.

During a spin, the airflow will not flow evenly past the static port, but will occasionally flow directly onto the static port. This will cause a rise in measured pressure, leading the altimeter to believe it is lower than it actually is. This cause fluctuations in the indicated altitude.


Can an engine that is extinguished by foam be returned to service?

A foamed healthy engine
(Original)

Inaugural Virgin Atlantic flight to Atlanta cancelled after 'water cannon salute' is swapped for foam and clogs engines
The de-clogging work was carried out as the pilot and flight crew went on a diplomatic mission to explain exactly what had gone wrong.

If it's only foam, the non-faulty engine can be de-clogged, checked, and returned to service.

For QF32, all the engines were replaced. There were issues with the Rolls-Royce Trent 900 engines. The rest of the Qantas fleet either received new engines or modifications.

Video for an engine inspection.
Video for an engine wash.


Which single pilot jet has the largest cabin and longest range?

According to Wikipedia's list of light jets, the longest ranges (for single pilot certified jets) are:

SyberJet SJ30: 2205 NM
Pilatus PC-24: 2035 NM
Embraer Phenom 300: 1936 NM
Cessna Citation CJ4: 1927 NM
Cessna Citation CJ3+: 1825 NM

Sorting by max. number of passengers, we get:

Pilatus PC-24: 8-11
Embraer Phenom 300: 7-10
Cessna Citation: 8-9
SyberJet SJ30: 5-6

Considering you are already rated for the Phenom 300, this is likely your best option. Anything larger will always require a minimum crew of 2.


Why were the speed records set by seaplanes in the early thirties?

The answer: Fixed-pitch propellers.

Until the 1930s propellers were fixed pitch only. From Wikipedia:

  The first practical controllable-pitch propeller for aircraft was
  introduced in 1932. French firm Ratier pioneered variable-pitch
  propellers of various designs from 1928 onwards, relying on a special
  ball bearing helicoïdal ramp at the root of the blades for easy
  operation.

This means all speed records were flown with fixed-pitch propellers, which needed to be optimised to the top design speed to coax as much thrust from the engines as possible. In turn, this means that most of the propeller blade was stalled at low speed, so take-off thrust was low. To achieve high speed, a high wing loading was needed and, in combination with low thrust, the take-off length of such an aircraft was beyond of the length of airfields back then.

On water, however, the maximum field length is close to infinity. Yes, drag with floats is especially high when transitioning from buoyancy to dynamic lift (the "hump"), but that can be overcome when that hump occurs at high enough speed so the propeller delivers enough thrust.


Are turn rate indicators calibrated to a certain speed? If so, what speed?

They are not calibrated for a certain speed.

If by "calibrated", you mean they deflect together / to the same side, then you're using the instruments wrong. The two instruments have are not linked to each other, they operate independently and they operate on different principles. They are located next to each other in the cockpit out of convenience.

The "slip indicator" is a ball in a U-shape tube. It tells you whether gravity (or experienced acceleration) is pointed down, or to the side. Period. It is as simple as that.

The "turn coordinator" (little white plane / white needle) indicates your yaw rate. It operates by using a spinning gyro; the precession effect of yawing causes the gyro assembly to roll.

The roll is restricted by a spring, which is calibrated to indicate standard rate turn when the yaw rate is 3 degrees per second. The turn coordinator works at any airspeed (part of the preflight check is to turn the plane while taxing and verify the turn coordinator works).


What's the appropriate course of action if an airliner bounces 5-10 feet on touchdown?

Well, "5 to 10 feet" is not "slightly."  One or two feet is "slightly," and in that case you can maintain your landing attitude or pitch very slightly nose-down -- really a matter of the slightest change in pressure, not really a perceptible change in pitch -- and you'll land soon after that.

However, for a larger bounce, the concern, especially on the longer 737's, the -800 and -900 and -900ER, is a tail-strike. The tail skid protects the tail from a strike on takeoff, but on landing the geometry is different and the skid won't be what contacts the runway; it will be the fuselage.  And while tail strikes on takeoff are more common, it's the tail strikes on landing that are far more damaging & expensive. So avoiding one of those is a major concern.  Thus, the usual guidance in the event of a bounced landing in the 737-800 is not to attempt to salvage the landing, but rather to go around.  You may touch down again during the go-around, and that's okay, but you're better off aborting the landing & taking it around to try again, than to risk damaging the aircraft by trying to recover from a high bounce.

One thing that complicates recovering from a bounce is the fact that your spoilers may have extended when you touched down the first time -- and the last ten feet to the runway with spoilers fully extended & power already at idle, won't look nearly the same as the usual last ten feet to the runway, with power coming off & no spoilers. You'll settle fast, and hard, and you won't have the elevator authority you're used to for adjusting the sink rate.  You can pull back, but the change in attitude probably won't arrest the sink rate the way you think it will -- you'll just hit harder & in a more nose-high attitude.  Thus the increased risk of hitting the tail.

On a shorter aircraft without the tail-strike concerns (say a 737-200,  -300, -500, -600, or -700), it might be possible to salvage the bounced landing, although 10 feet up is a pretty high bounce -- even if you had a lot of airspeed to play with and runway ahead of you, I'm not sure it would be wise trying to sort that situation out -- go around & try again.


Was there any kind of preventative collision training for dogfights during WWI and WWII?

No, there was no anti-collision training aside from "dodging the fireball" after enemy aircraft shootdown. The training resources and timeline didn't permit much training beyond safe employment of the aircraft gun.


Can a single seater airplane fly with 50 lbs of thrust

A key thing you need to provide is the ratio of L/D, which is also equivalent to the ratio of Cl/Cd, where Cl and Cd are the lift and drag coefficients.
Thrust required for level flight = D/L ratio * Weight.
Without knowing more information, including L/D ratio and Weight, the answer to your question can only be "maybe".
If the L/D ratio is 10:1 or better and the total weight is 500 pounds, then the answer is "yes".  Increase the weight or decrease the L/D ratio, without changing the other variables to compensate, and the answer becomes "no".
Also, naturally, we have to know the thrust available at the airspeed that we are actually going to fly at, which may, or may not, have been what you were referencing-- (I assumed it was, in the preceding paragraph.)
Edit: since you've now provided the total max weight at 400 pounds, the answer is "yes, as long as the L/D ratio is at least 8:1, and the full 50 pounds of thrust is actually available at the airspeed at which you are flying."  An 8:1 L/D ratio is certainly doable.  Of course there would be no excess margin to allow for climbing, if the thrust were exactly equal to drag (50 pounds in the case of an 8:1 L/D ratio) and no more.  If you can get the L/D up to, say, 15:1, the performance will be a lot better.  (I'll leave the actual calculation for achievable climb angle in that case up to you, but all the required information is present here.)


Why does a turbofan engine force bypass air into a smaller space?

It can't increase thrust, can it?

It does increase the thrust. This is how jet propulsion works. The decreasing space is essentially a nozzle, without one the jet engine is a mere gas turbine.

You could theorize an infinitely narrowing space and thrust would approach infinity.

Not really, a very tight opening will create a huge back pressure, initially and very momentarily propelling the vehicle the opposite direction, before the gases shoot out the front stalling the engine (gases building up).

In fluid dynamics that would be a choked flow state. The mass flow out the nozzle reaches the maximum, which means inside the engine the gases will start building up (pressure increasing). If the theoretical tiny nozzle can withstand this pressure, the gases will only have one way to go, which is out the front.

So instead the nozzle is designed to decrease the pressure (by increasing the velocity) to match the free stream pressure as much as possible.

  The nozzle of a jet engine is usually designed to make the exit pressure equal to free stream (NASA).


How are small airplanes steered on the ground if they don't have tillers?

Most small planes (there are exceptions!) have three wheels - two fixed wheels, one under each wing, and one under either the nose or the tail.

When the third wheel is under the nose ("tricycle" layout), that wheel is usually (but not always) steerable with the same pedals as the rudder, enabling accurate steering on the ground.

(Image Source: AvWeb - Author: Joseph Burnside)

The rudder itself will also add an element of steering moment depending on the airflow over the fuselage (taxiing speed is usually too low for this to have much effect, except that on single engine aircraft applying power will push air over the fuselage and tail, adding effectiveness to the rudder).

In addition, for sharp turning at low speeds, differential braking can be applied - press the brake pedal for the starboard main wheel only, and the plane will tend to pivot about that wheel - ie turn right.

When the third wheel is under the tail ("tail-dragger" layout), the tail wheel is not usually steerable (it usually pivots freely - but again there are exceptions!). In this case the other two techniques mentioned above - rudder and differential braking - are the only methods of steering on the ground. I've never flown a taildragger myself but I understand the taxiing technique takes some getting used to.

(Image Source: WikiMedia Commons - Author: RenioLinossi)


Apart from the C-130, are there other airlifter aircraft that are able to perform LAPES drops?

Just the C-130. The C-17 was intended to have that capability, but it was decided against.
That's according to a 1995 report to congressional committees (gao.gov; PDF):

C-17's Intended Role Has Changed
(...) the C-17's unique 60,000 pound low-altitude parachute extraction system (LAPES) capability is not needed, and the aircraft cannot meet original airdrop requirements.
Unique C-17 LAPES Capability No Longer Required
LAPES is a means of extracting equipment while an airlifter flies at low levels. At present, the C-130 is the only aircraft capable of LAPES operations, and it is limited to extracting 42,000 pounds of equipment. In 1981, the Army identified an "urgent need" to develop a LAPES capability up to 60,000 pounds. According to Army officials, this capability was needed to extract armored artillery pieces, ammunition, and towing vehicles from the same LAPES platform, which would minimize dispersion over the drop zone. The C-17 was intended to provide this unique capability. In March 1994, the Army acknowledged that " LAPES has been an expensive, unused, untrained capability and is potentially of limited battlefield use." The Army stated that the current C-130 42,000-pound LAPES capability "appears to more than adequately address foreseeable Army requirements." The Army intends neither to maintain the material systems and rigging required for the 60,000-pound LAPES platform nor to conduct C-17 LAPES training. Thus, the C-17 will not be used for this mission. However, AMC officials noted that testing of a 42,000-pound C-17 LAPES capability is currently underway.

While the report is from 1995, it is the only official mention of the matter that I've been able to find. That's probably why you haven't found videos for it either. The C-17 does parachute down equipment though from higher-than-LAPES altitudes.


How do paper airplanes create lift if their wings are flat?

Paper airplanes create lift just like any other airplane. Don't let the wing cross section confuse you; what the air "sees" is a different shape.

When a flat plate flows through air at a positive angle of attack, the stagnation point of the flow (where air splits into an upper and a lower flow path) sits slightly below the forward edge of the wing. Now the streamline just above the one which hits the stagnation point has to negotiate the sharp corner of the leading edge, which will cause it to separate from the surface. This creates a small separation bubble which to the streamlines further up looks like a round leading edge, and since the rest of the wing has no curvature, the flow will attach again soon. The bigger the angle of attack gets, the farther back the stagnation point will be and the bigger the separation bubble becomes. At some point, which is promoted by leading edge sweep, the separated flow will not reattach, but produce a vortex, like the vortex which provides lift on delta wings.

The picture above is from a simulation; see the full video here. Note the red color on the forward part of the upper side: This indicates low local pressure, which is needed to make the outer air negotiate the leading edge plus the separation bubble (see here for an explanation on the molecular level). In the separation bubble, low pressure is not accompanied by high flow speed since the separation turbulence has eaten up most of the kinetic energy.

The actual nose radius of the paper wing does not matter, and it might even be zero (like when you fit razor blades to the leading edge): The air will create its own airfoil shape by padding with separated air what cannot be reached with attached flow.

Paper airplanes have a low enough wing loading that their angle of attack is low and the separation does not become too big. All it needs is a center of gravity at the quarter chord of the wing; that is why you need to fold the piece of paper first.

It is interesting to watch a slightly tail-heavy, delta-shaped paper airplane fly: It will gradually reduce speed and will start to rock its wings when it approaches stall. Once one wing is going down, the sudden increase in its local AoA will increase lift so much that the rolling motion reverses, without noticeable yawing. The delta-shaped flat plate has effective roll damping right into the stall. The lack of yaw is mostly due to the high yaw inertia, but indicates also that drag buildup is moderate at best.

Of course, after a few periods of rocking the nose will drop and it will pick up speed like any well-behaved airplane.


What is the origin of the waypoint CLAWW and PAHTI(New England region)?

Read it as "wicked party" and Boston is famous for sea food, including crab claws.  Chowder, lobster and claws.


Why would an airliner make a 90° course change?

This answer assumes the manouevring was related to the Military training activities going on off-shore. While plausible, there is another plausible explanition in ymb1's answer.

That was a British Airways Boeing 747 on its way from London to Miami.

Just off the coast there is a military exercise going on (see the LJ35 doing circles in the screenshot from flightradar24.com below; probably a military contractor like the one in this question).

Therefore all flights that would normally fly in that area end up flying over the coast. It seems that when the peak traffic from Europe towards Miami was passing, ATC decided to put everybody onto their original route as soon as they passed the bottleneck in order to reduce the traffic density. The result is quite a few sharp turns.

A few hours later there were at least 4 Learjets playing around off the coast. Note that most military traffic is not shown on FR24, so it may have been quite a bit busier.


Has someone with no experience at all ever taken off in a plane? Could they?

The Aviation Safety Network has an article listing 11 airplanes that crashed after being stolen by a non-pilot. The most recent was the Horizon Air Dash 8 that was stolen by a ground service agent in August 2018.

There are more interesting stories such as:

The guy that tried to crash a Cessna 150 into the White House (with minimal experience and training)
Ground crew member that stole and crashed an F-16 in Norway
Mechanic that accidentally took off in an English Electric Lightning
Mechanic that took off in an F-86 Sabre
15-year-old that stole a PA-34 and took off with 4 passengers, later crashing

Whether these qualify as "amateurs" is a matter of opinion. Most of them were mechanics/engineers. It would be difficult (but not impossible) for a person not familiar at all with the aircraft to get it started and manage to take off.


Where can I find airspace maps for Turkey?

What you need would be named "ICAO 1/500,000 map", this is a VFR map with airspace class, based on a Lambert conformal conic projection. Example in the US (1/250,000):

The best I can find online is SkyVector, but you said it's not sufficient for you needs:

By searching carefully you may find things like that:

(source: University of Texas)

But these are 1/1,000,000th very old maps from US DoD.

An alternative would be to use the online GIS viewer from ICAO, selecting carefully layers and data to display, my trial at that:

It must be improved to suit your needs, but I don't expect it to be better than SkyVector. By clicking on a location, you can display all the information about relevant areas (description, limits, etc).

For enroute, Eurocontrol provide two maps:

High altitude
Low altitude.

Good for intersection points and routes.

On the airport plates side, the AIP for Turkey is not in public access, but several sites provide plates, e.g.:

Turkish Virtual Airlines (static)
RocketRoute (interactive)

AirportNavFinder provides link to other sites like SkyVector.

IVAO Turkey could be a good point of contact to discuss local documentation. They have a section Charts accessible to registered users.  - VACC turkey is another virtual network.

If you can provide additional requirements, me or someone else may provide better material.


What is the daytime visibility from within a cloud?

The answer is that visibility varies depending on the density of the cloud.  And it can vary significantly.
Flying in formation in IMC is very common in the military.  Sometimes it will be dense enough that all you will have is a wingtip or light.  In those cases it can be very disorienting and difficult to maintain position because you need more than a single point to discern relative motion.  The stress level can go up too since the denser the cloud the closer want you fly to keep sight.  But generally those sort of conditions are relatively brief, and within 10-15 seconds you will likely have the wing and fuselage back in sight.
For reference, in close formation the wing tip might be 10-20 feet from your canopy, with the lead's fuselage another 20 feet beyond that.  So, the worst visibility I have seen (when I lost sight) could have been anywhere from 0-20 feet.  If you see the lead clearly the visibility could be anywhere 50 feet to a mile or more.  You just don't know, all you see is another airplane and gray...  There is certainly no absolute or standard number.
Every formation flight brief should cover lost sight procedures in case it becomes dense enough to require a break away.  I have only ever lost sight and had to break away once.


Why are most runways laid out in a generally East-West direction?

While there are many exceptions to east-west runways, generally speaking prevailing winds (which blow from a single direction) blow east-west rather than north-south. It happens because of Earth's rotation generates Coriolis effect.

There are other wind patterns besides prevailing winds. Trade winds occur near the equator and flow from either the north or south towards the equator. They curve towards the west due to the spin of the Earth. Polar easterlies blow close to the north and south poles. They blow away from the poles and curve east to west.

Hence, most of the runways are Eastish-Westish (not exactly East-West).


What are the areas marked in green around the taxiways?

It's essentially "fake grass" -- a "no taxi" / "no drive" area, the same as real grass would be.  The concrete may very possibly be stressed to support the full weight of big aircraft, and the areas are painted that way simply as a matter of keeping traffic flow in the designated areas, although it's possible that some areas marked that way may not be in a condition that you'd want to taxi on them.

As far as emergency vehicles go, they have permission to do whatever they need to do when responding to an emergency, which includes driving on anything they need to.  And the people driving the fire trucks will be well aware of any parts of the tarmac that even they need to avoid -- but if it's even close to being able to support an aircraft, it'll support a fire truck just fine.

More than anything else, though, it's a matter of marking the ramp to designate the "don't taxi here" areas from those where you can taxi.  Often, separating a ramp (taxi more or less at will) from a taxiway (ATC clears you where to go, and it's obvious where aircraft will be entering/leaving the taxiway).  (As the overhead image shows, wingtips can & do extend over the green areas; adjacent taxiways separated by such areas would be laid out so that aircraft on respective centerlines would have whatever wingtip clearance is required.)


Why do anti-ice systems on airplanes with automatic ice detectors still need to be manually selected on?

Because most of the time the system has to be on even when there isn't detectable ice accumulation.  Because of what I like call "technical icing conditions" but which are just called "icing conditions":
ICING CONDITIONS:

TAT (total air temperature - accounts for temperature rise from compresibilty) less than 10C

Visible moisture (cloud, haze, anything that is water)

SAT above -40C

You also have to have WAI on for takeoffs when OAT is less than +5C with visible moisture (including visibility under 1 mile) regardless of whether there is actual ice formation (water streaming back on the wing on takeoff can freeze upon rotation due to the temperature drop from the low pressure over the wing).
When you get the wings deiced, you still have to have the WAI on at the start of the takeoff roll if departing in technical (or actual) icing.  You don't want to cook the thick type IV holdover fluid, so you leave WAI on selection the very last thing as you pull out to line up for the TO roll (the CRJ200 has an evaporative WAI system which meant the leading edges get VERY hot, about 105C, which could cook the fluid and make it crusty).
When flying, the only time you can leave the WAI off when in technical icing conditions is
above some speed (on the CRJs it was 230kt), unless you get an ICE caution from the ice detector.
The result is a system that automatically turns the WAI on when ice is detected would only cover about, oh, maybe 10 - 20% of the time you are legally required to have WAI on.
Cowls are similar, but have to be on even more often because there isn't the 230kt threshold for actual accumulations only. They have to be on all the time you are on the ground less than +10C with visible moisture (freezing around cowl inlets from velocity temperature drop).
Without the 230kt exclusion the wings have, you have to have the cowls on pretty much all the time you are in cloud, actual icing or not, so you are turning them on and off quite a lot if climbing/descending through layers (some pilots may just leave them on), whereas outside arrivals and departures the wings are only on when you get actual ice.
So you could automate the system like that, if you could figure out how to make the system reliably detect "visible moisture" outside somehow (probably possible, but very expensive).   Otherwise, the decision to turn it on is a crew decision based on the view outside, probably 80% of the time.


Yes. In different regions.

A fluid flowing through a constricted section of a tube undergoes a decrease in pressure, which is known as the Venturi effect. This is a consequence of Bernoulli's principle, which says that total pressure is constant. Basically,

$P_{1} - P{2} = \frac{\rho}{2} (v_{2}^{2} - v_{1}^{2})$

along a streamline. But here's the catch- it is applicable only for steady, inviscid, incompressible flows. This condition is satisfied when the flow speed is small (subsonic) and at a certain distance from the body (i.e. wing) and can be used to calculate the velocity or pressure fields at some distance from it.

However, in real world, the fluid (i.e. air) is viscous, though its effects are restricted to a certain region immediately near the body, as seen below:

By F l a n k e r - Own work, CC BY-SA 3.0, Link

You can see that in the immediate region of the immersed body, the fluid is slowed down- there is a velocity gradient between the different layers of the fluid as they are being slowed down. However, after some distance from the body, the fluid behaves as if it is inviscid- there is no velocity gradient.

In the end, we choose the situation that is more useful for us- in case of subsonic flow, for calculating lift etc. the flow can safely be assumed as inviscid, while the boundary layer is very important when we consider cases like heat transfer etc. There is nothing right or wrong here (the air follows the same laws, wherever it is)- it is simply the case of using the theory that makes more sense for easier understanding and computation.


What is the size of toilet or septic tanks in various planes?

The A380 has a total waste tank capacity of 2096L or 554 US gal, according to Airbus.

  B. Capacity
  There are four waste tanks, two upper deck tanks and two main deck tanks, see FIGURE
  5-4-10-991-003-A.
  (1) Upper Deck Waste-Tanks
  - 373 l (99 US gal).
  Each tank is precharged with 35 l (9 US gal) of chemical fluid.
  (2) Main Deck Waste-Tanks
  - 675 l (178 US gal).
  Each tank is precharged with 35 l (9 US gal) of chemical fluid.
  (3) Total Waste Tank Capacity
  - 2096 l (554 US gal).

The size will vary for each aircraft type, the Boeing 747 had a capacity of only 1000L.

Smaller General Aviation aircraft do not have lavatories, this depends on aircraft size and purpose. The average Cessna will not have lavatories either, pilots have to plan their stops properly or rely on accessory such as the TravelJohn. Larger General Aviation aircraft such as smaller jets and aircraft designed for more than 4 passengers are more likely to have lavatories on board.

(Image Source: Siebert Aero)

There is no dumping system for waste tanks, mostly for hygienic and environmental reasons, as the waste tanks are not only filled with fecal and thus biological matter, but also with chemicals.


When can a taxi clearance allow me to cross multiple runways?

The general FAA rule is indeed that ATC cannot issue a clearance to cross multiple runways.

However, waivers are available to airports with parallel runways close enough to make it impractical or even hazardous for aircraft to stop in between them and wait for a second instruction. They are essentially treating such a pair as a single runway for the purposes of this rule.

I'm not aware of any list of airports with such waivers or notification to pilots. However, one could predict whether it is likely by looking at airport diagrams. Consider the example of SFO/KSFO, which has such a waiver on both pairs; even a quick glance reveals why it's needed.

Also, I suspect the main concern behind the general rule is pilots misremembering which runway(s) they were cleared to cross when there is a substantial distance between them. The case of two very close parallels, along with the unusual (and thus memorable) multiple crossing instruction, does not seem to have the same safety concern.


Was Ludwig Wittgenstein's aircraft propeller ever built?

Ian Lemco of the Royal Society published a note on this in 2007. He described the invention more as a jet engine than anything, as it featured a central compressor feeding tip-jet nozzles. Burner position varied during development of the design, from central to blade tip. A patent dating from 1910 shows jet combustion at the tip. One central compressor was of the centrifugal type. Thus, it is seen in some ways as foreshadowing the Whittle jet engine.

Lemco found sources to be scarce, mostly anecdotal, and unreliable. Certainly Wittgenstein constructed various experimental devices and tested them, both in the lab and on a railway track. Some may have been rockets. One source remarks in passing on the engine that he constructed, but the reliability and context are far from clear. There is no confirmation that he ever built a complete jet engine.

It seems to have been fundamentally workable, as compressor-driven tipjet rotors were indeed later flown, but it was probably beyond the technology of Wittgenstein's day.

Ian Lemco; "Wittgenstein's Aeronautical Investigation", Notes and Records of the Royal Society of London, Vol. 61, No. 1 (Jan. 22, 2007), pp. 39-51. Stable URL: http://www.jstor.org/stable/20462605


What are those hatches on the engines of a B-2 that are open during takeoff?

These are secondary intake doors which allow more air to reach the engines. They are opened by the engine control computer when the main intakes are not providing enough air for the power setting of the engine. In order to minimize radar cross section, the engines of the B-2 are buried in the wings and the intake needs to be curved. Also, and for the same reason, the intake lips need to be sharp-edged. Both factors increase the drag of the intake flow and reduce the mass flow of air to the engines. Like on other low-RCS aircraft, secondary intake openings are needed to increase this mass flow.

In automatic mode they will close when the aircraft accelerates above Mach 0.4. Below 43F, they remain closed for taxi and are only opened for take-off. A take-off with closed doors is possible according to this thread, but needs a reduced power setting to 80% for the engines below 50 kt speed.


What is the maximum rate of descent in an instrument approach?

ICAO Doc 8168 stipulates that for a non-CDFA non-precision approach, the aircraft should not exceed 15% gradient when descending from FAF to MDA.

  1.7.4 Stepdown descent
  The third technique involves an expeditious descent and is described as “descend immediately to not below the
  minimum stepdown fix altitude/height or MDA/H, as appropriate”. This technique is acceptable as long as the achieved
  descent gradient remains less than 15 per cent and the missed approach is initiated at or before the MAPt. Careful
  attention to altitude control is required with this technique due to the high rates of descent before reaching the MDA/H
  and, thereafter, because of the increased time of exposure to obstacles at the minimum descent altitude.


Can an aircraft engine be started midair?

As long as there is engine (e.g., N1) rotation, no structural damage, no fire, etc., generally speaking an in-flight restart may be attempted.

There are certain guidelines/conditions that control whether the re-start attempt can be done with a windmilling engine or if a cross-bleed start (using air from the other engine to rotate the failed engine for starting purposes) needs to be done. (other conditions that must exist are shown in the in-flight restart procedure available to the crew).

The aircraft manufacturer typically provides an In-Flight restart procedure published in the QRH (Quick Reference Handbook) or Emergency Checklist.


The 'x' suffix is aviation speak in an abbreviated word called a contraction.  You can think of the 'x' as "cut short" shorthand.  Some are defined in manuals or advisory circulars.  For example, WX is weather as defined in an old advisory circular, AC00-45.
Official or not, a few others that come to mind are:

MX - maintenance
CX - cancel
TX - transmit
RX - receive

Maybe others will chime in the comments with more lore. The -x is also used outside of aviation, and is a common abbreviation marker, e.g. Dx ("diagnosis"), elex ("election"), and Hx ("history").


What are the eyesight requirements for LPPL/LAPL in the UK?

The CAA has a publication called Light aircraft pilot’s licence (LAPL) Medical certificate - A quick guide for Pilots. It says:

  Your GP

  Cannot perform the initial assessment if you have one or more of the pre-existing diagnoses that require additional assessment. See
  below:

  Decreased visual acuity in either eye below 6/9 despite any correction

In other words, if your corrected eyesight is worse than 6/9 (whatever that means) you need to go to an AME for the medical, otherwise the GP can issue it.

Unfortunately the guide is unclear (as far as I can see) on what exactly the AME will do:

  If your GP decides he/she cannot
  issue a LAPL medical certificate
  you will need to see an AME for
  assessment.

I don't know if that means the AME can issue an LAPL medical with limitations, or require you to get a class 2, or something else. Unless someone else can find more information, it may be easiest to call an AME and ask them.


Do the four engines of a BAe-146 make it efficient?

Do the four engines make the BAe efficient? No! Does it affect performance? Yes it does. The BAe 146 occupied a niche for Hot & High airfields, in that it can continue the takeoff when an engine fails: one failed engine out of four leaves 75% of full thrust. So take-off performance with a failed engine is better on a 4-engine aeroplane.

The engines themselves were quite clever designs: the Lycoming ALF 502 with a relatively high bypass ratio and a geared fan. The bypass ratio of 5.7:1 is higher than the 5.0:1 of the Trent 700 used for the A330. By comparison, the Fokker 100 used the Rolls Royce Tay, with a bypass ratio of 3.0:1 and the proven core of the RR Spey.

 Photo from the wiki.

But was the BAe 146 efficient because of the four engines? For an aircraft of a given weight and a given thrust requirement, the design considerations for number of engines would be:

Four engines are each smaller than two engines. Turbine engines do not scale down well. All else being equal, purchase price starts to be the limiting factor when reducing the size: half thrust engines are not half the price.
4-engined planes have worse fuel economy than an identical sized twin engine, due to decreased efficiency of a smaller turbine, and increased drag from more engine pods and a larger vertical tail area.
In cruise, a typical single aisle airliner uses a third of the take-off and climb thrust. So once in cruise, one out of two engines can provide all the thrust required to continue the flight, and have less yawing moment than a failed outboard engine of four. Picture and quote from this document:

  Specifically, current subsonic airliners require about three to five times more thrust to take off than they do to cruise, and the power produced by a high bypass ratio turbofan engine at constant throttle setting varies in much the same way.

Twin engine turbofans actually have a third engine on board: the APU. It can power the systems with full redundancy after an engine fail, and is normally shut off in flight, like the no.1 engine of a P-3 when loitering. Note that the propeller blades of the P-3 can be made to feather, providing much less asymmetrical drag than a windmilling turbofan.

So the advantages of four engines are mainly the consequences of an engine fail in the TO/climb phase: a four engined aircraft could opt to continue the flight, while the twin engine would have no more redundancy and a much harder time climbing to altitude. But all else being equal, four engines have twice the chance of failure than two engines...

The BAe 146 had a relatively wide and stubby fuselage, designed for 6-abreast seating but often configured for 5-abreast due to awkward headroom for the window passengers. This added to the drag penalty of the type over that of a comparable size aircraft like the Fokker 100, which is still a sought after aircraft for feeder airlines in Australia, the limiting and most expensive cost being engine overhaul. The BAe 146 has twice this cost...


Why do ADS-B outages occur without simultaneous RAIM outages?

RAIM (Receiver Autonomous Integrity Monitoring) is a function within an avaition GPS receiver. It uses redundant measurements for a consistency check to determine the intergrity of the position measurement.

Since RAIM requires redundant measurements, it is only available when there are sufficient measurement sources available. In practice, this mainly boils down to the number of functioning GPS satellites that can be seen from the receiver, and the geometry of the satellites positions relative to the receiver.

For normal RAIM prediction, the RNP integrity limit of 1 or 2 NM is used. This means that the RAIM function needs to be able to verify that the GPS receiver will not produce a lateral position error greater than 1 (or 2) NM with a probability exceeding 10-7 per hour.

The  online predition tool simulates the GPS constellation and calculates whether a GPS RAIM system would be able to reach that integrity limit with the (healthy) satellites in view of the receiver at the time of flight.

For ADS-B, the position transmitted needs to have a certain level of quality in terms of accuracy and integrity. The accuracy of ADS-B position (encoded as NACp) need to be 0.05 NM or below, and the integrity (encoded as NIC) needs to be 0.2NM or below.

To determine the integrity of the ADS-B position, the GPS receiver again uses RAIM. To qualify for ADS-B, not only needs RAIM to be available, it thus needs to determine the integrity at 0.2 NM or less, and at the same time an accuracy of 0.05 NM needs to be achievable.

Since the ADS-B Integrity requirements are much more stringent (0.2 NM vs 1 or 2 NM), the prediction of integrity for ADS-B shows more frequent outages.

Note that the prediction tool only provides a prediction. The actual values during flight may be different.

References:

RAIM prediction requirements FAA AC 20-138D section 5-2.3pdf
ADS-B requirements: 14 CFR 91.227 c) 1) i) & iii)


How is an on ground lightning strike test conducted?

You create your own lightning. You can do that with an impulse generator. Here's what a smaller one looks like:

I don't know if that's what is used for the certificates but it certainly one way to test against lightning strikes (provided that you can generate a high enough voltage).


Most likely the pilot is trimming the airplane to alleviate control pressures after he adjusts his airspeed in the pattern.  Ideally you want to trim out any kind of control pressure that you have to place on the yoke, or rudder pedals, first adjusting your speed as desired using the power levers, then trimming using elevator-rudder-aileron trimming procedure. The “coolie hat” switch on the left grip on the yoke provides both elevator and aileron trim.


Do Airbus aircraft have an "off" position for their landing gear?

I was working for Airbus before, never seen it on A300, A320, A330, A340, A380, A350 or A400M. Only Gear Up or Gear Down position.

For confirmation on A320, here the FCOM section:

  NORMAL OPERATION
  The flight crew normally operates the landing gear by means of the lever on the center instrument panel. The LGCIUs control the sequencing of gear and doors electrically. One LGCIU controls one complete gear cycle, then switches over automatically to the other LGCIU at the completion of the retraction cycle. It also switches over in case of failure. The green hydraulic system actuates all gear and doors. When the aircraft is flying faster than 260 kt, a safety valve automatically cuts off hydraulic supply to the landing gear  system. Below 260 kt, the hydraulic supply remains cut off as long as the landing gear lever is up.