Are nuclear bombers themselves damaged after dropping bombs? Shock waves resulting from low intensity explosions can easily reach 10,000m/s.  About 5% of the energy released in a nuclear air burst is in the form of ionizing radiation: neutrons, gamma rays, alpha particles and electrons moving at speeds up to the speed of light (thus, carrying a hell of a lot of energy).  With these stats it appears that for the bomber as well, a nuclear bomb is very bad. How are they protected?

Mostly, no.

The number one factor protecting the aircraft from the effects of the explosion is distance. The aircraft is traveling at a sufficient speed to be very, very far away from the bomb before it detonates. All effects of the explosion can be multiplied by the fraction: 1 / r, where r is the distance between the bomb and the bomber at donation (because circumference is directly proportional to radius). While the effects may be great at the target, they spread out as the circle grows.

Detailed Analysis:

As you emphasize the speed of light of radiation from nuclear reactions, I would like to write something about radiation and how it could influence the aircraft.

Radioactivity

Nuclear radiation consists of gamma rays, which are some sort of EM waves and particles like helium nuclei (alpha), electrons (beta) and neutrons (without Greek letter...). There is also some more stuff like positrons, protons or heavier fragments of the nuclei, but they behave similarly.

The particles indeed escape with a large fraction of the speed of light, but this has nothing to do with the nuke. They will escape with the same speed from a nuclear reactor or from tiny probes in a lab. The nuke just spills more particles than the probe, but does not give them more speed.

Alpha & beta radiation

You may know you can stop alpha particles by a piece of paper and beta particles by a 3mm sheet of aluminium. In air (ground, room temperature), alpha particles can travel not more than 10cm, and electrons not more than 10m. The exact range depends on the energy of the particles, but these are the maximum ranges for nuclear reactions. Also, the distances depend on the density of the air, so they travel farther in hot air / at high altitude.
In general, there is a high electric field inside atoms, and the charged particles.

Finally, these two particle types will never reach the aircraft throwing the bomb. Even if they could, they would be stuck in the hull and not do any damage.

Neutrons

While alphas and electrons are absorbed on a short range because their charge interacts with the strong fields inside the atoms they are traveling through, neutrons are neutral (really!), which makes them very special. They like to travel through heavy materials like lead, but are easily stopped by plastic or water (one reason for that large swimming pools in nuclear power plants, though swimming usually is prohibited). The reason is that neutrons are like ping pong balls, and lead atoms like basketballs. The basketball will not notice when it's hit by the ping pong ball, while the ping pong ball flies ahead with the same velocity, but different direction. The next hit(s) with other basketballs may put the ping pong ball into its original direction, still with the same speed.
Hydrogen on the other side is a atom as light as the neutron, hence it's like resting ping pong balls. When hit by the neutron, a hydrogen atom is punched back and absorbs a large fraction of the neutrons energy, causing it to slow down. If the hydrogen atom is bound in a molecule, it may be separated from it during this process. That's why plastic is a good shielding material, but nitrogen and oxygen in the air are already too heavy atoms, so air is a bad shielding material.

The metal the aircraft is made of is (nearly) not affected by neutrons. Unfortunately, almost every molecule of the human body contains hydrogen, especially the DNA. While the pilots do not suffer imminent injuries from radiation, this may increase the chance to get cancer.

However, Paul Tibbets, pilot of the Enola Gay (Hiroshima bomb) died in the age of 92.

One fact: "Neutron bombs" are nukes optimized to release a gigantic amount of neutrons. When ignited at high altitude, it kills all animals and humans, leaving the infrastructure (and almost all plants) intact. It's good that they are proscribed by all nations.

Gamma radiation

Coming to gamma radiation. People are most afraid of it, as it penetrates anything, and you know, the Hulk was created by it. Its nature is that all the photons it's made of have a tiny chance to interact with the atoms they are passing by. A few photons are stopped, the rest go ahead. This is why each centimeter of material reduces the intensity by a certain factor, but you will never shield gamma range completely (mathematically). Typical gamma rays need some 10cm of aluminium to reduce the intensity to 50%, or less than 10cm of lead. So, it will pass the aircraft's hull and also the pilots. Again, it will not influence the aircraft. And because it only reacts here and there with matter, there is a high chance the pilot's body will repair the damages, and the pilot will not turn green.

Other effects

Shock wave

As said in the other answers, the explosion causes a big shock wave which will definitely shatter and possibly damage the aircraft. Wikipedia says about the Tsar bomb that it was released from about 10km altitude and detonated at 4km. Using a parachute, the bomb was slowed down allowing the aircraft to travel 45km after release. Yet, the aircraft fell about 1km when it was overtaken by the shock wave.

Edit: The shock wave travels at the speed of sound, so the faster the aircraft, the farther away it is when it's overtaken by the shock wave, and the lower the intensity is. (And there are supersonic aircrafts... However, the Tsar shock wave traveled around the earth three times)

Heat

A nuclear bomb releases a lot of heat. It is said that in Hiroshima, trees 10km away from the explosion caught fire. But the aircraft was farther away and its hull is not only well cooled, but also much less inflammable than a dry tree in the summer. Nevertheless, the "Tsar-bomber" was painted with a special heat-reflecting paint according to the Wikipedia article.

EMP

Each nuclear bomb also releases a electromagnetic pulse, which can damage wide electric circuits like power grids as well as any semiconductors. However, the hull of an aircraft already provides some protection against this, and the protection can be improved more to make the aircraft EMP-hard. While today's bombers should be immune to EMP due to these protections, I guess the ones which actually have thrown a nuke were immune as they didn't use any semiconductors.

Summary

It turns out that radioactivity is not an issue because not all parts have the range to reach the aircraft, and those which do, have no influence on the aircraft.

The highest thread comes from the shock wave, which definitely damage the aircraft or at least bring it into an odd situation. I don't know what an issue heat is, but the Tsar bomber had some heat reflective painting, and an aircraft usually has a good cooling by its environment. EMP is not an issue as military aircrafts are usually already protected against them.

Remarks

Of course, radiation damage / effects depend on the time something was exposed to radiation as well as the intensity. So, when I wrote gamma rays will not harm the pilot that much, this already includes a larger distance and thus lower intensity. Near the detonation, the gamma radiation alone would kill you.

Being exposed to radiation does not automatically make an object radioactive itself. Gamma rays do not make anything radioactive, they ionize atoms and may destroy molecules. The particles may be captured by atoms forming new isotopes / elements. This isotopes / elements may be stable, or may not. If not, that atom is radioactive. However, for most materials, this induced radioactivity has a short decay time.

As said, alpha particles are not an issue as shielding against them is easy, they even can not penetrate the skin. Harmless? Body cells are most vulnerable to radiation damage during their separation, and your guts are covered with mucous membrane, where this happens very frequently. Swallow a piece of alpha emitting material, and getting cancer is more likely than just possible.

You see, radiation is a vast topic, and what I wrote about aircrafts and pilots may be correct for them, but not in general. Their main protection is: Distance is the best shielding, the farther away, the better.

Hmmm, this answer got a bit longish... sorry for that.


Would a random passenger be allowed to fix a problem with a commercial aircraft? I just read an article about a man (who is alledgedly an airplane mechanic) who was a passenger on an airplane that had some mechanical issues. Because there were no 'local' mechanics around, he decided to take matters into his own hands and fix the problem himself.

Here's the article I read (it's in Dutch though)

Here's one in English

This all seems nice but in this day of age where airplane security and regulation is so tight, I wonder if this is possible at all. It also seems to me that by the airplane company is taking a huge risk in letting this "random" person fix this problem. The article obviously oversimplifies a few things, but regardless of that, here's my question:

Is it possible for a random passenger, who also happens to be an airplane mechanic (working for airline X) to fix an issue on the airplane he happens to be on, given that there are no "local" mechanics around (however unlikely that might be)?

I happen to have personal knowledge of this particular case. I was in that airplane, too, seated two rows away from him. I'm also a former pilot and was naturally very interested in what happened, so I asked him some questions during the flight back.

He had worked as service mechanic for this company before, servicing their planes in Iceland. Their technical staff and the pilot of this plane already knew him, which made things easier.

Tools and parts were available at the airport in Alicante, but no qualified technician certified for this type of aircraft. The technical staff of the airline contacted his boss in Iceland by phone, who agreed to sub-contract him to the airline for the duration of the repair and pay him overtime for the work.
They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair.


Can (part of) the fuel in your tanks be considered "property carried for compensation or hire"? This question brings up that an experimental category aircraft cannot carry "persons or property for compensation or hire".  However, this raises a question regarding civil-registry refueling tankers (such as the venerable N707MQ).

Since the tanker is carrying fuel on behalf of another entity (the military unit they're contracted by to pass gas to) for compensation (contract payments), wouldn't the fuel that has been offloaded on-station be considered "property", backing N707MQ and its experimental (market survey) airworthiness certificate into a proverbial corner?

(Yes -- I know that contract tankers are considered Public Use aircraft when on-mission, which probably renders 91.319 moot, but it could easily be the case that a contract could say "operate IAW FARs with the exception of refueling operations", which'd inadvertently loop 91.319 back into the situtation.)

N707MQ actually has a special type of experimental certificate, according to its registration it is a "Market Survey" aircraft as you note.

The FAA defines this as

  Market surveys:  to conduct market surveys, sales demonstrations, and
  customer crew training for U.S. manufacturers of aircraft or engines.

This does not really provide us any special way to use the aircraft for commercial purpose but is worth noting.

Either way as you state 91.319 pretty clearly prohibits this. Interestingly enough 91.905 even prevents you from getting an FAA waiver to override 91.319 (as its not on the list of things you can have waved).

However 91.319 sub section H may provide us with a solution.

  (h) The FAA may issue deviation authority providing relief from the
  provisions of paragraph (a) of this section for the purpose of
  conducting flight training. The FAA will issue this deviation
  authority as a letter of deviation authority.

As far as I know these tankers are primarily used in our own airspace for training missions which would fall under "conducting flight training".

This article is pretty interesting and covers a similar scenario for using experimental for commercial purposes.


Why do some 2 seat fighters have side by side seating, while others have the pilots inline with each other? In some multi seat fighters, like the F-111 Aardvark the pilots sit next to each other, like so:

While in other multi seat fighters (most of them it seems) the pilots sit with one in front of the other, like this F-15E:

Why do some fighters have the pilots in a line like that?  Is there an aerodynamic or performance advantage that comes with having them in a line?  Or maybe it's so they can have a bubble canopy, which perhaps affords a better view? It seems it would be easier to work together when they are side by side.

So why do some multi seat fighters have side by side seating, and others are in a line?

The tandem arrangement (one behind the other) is used on aircraft that have single seat variants. The F-15, F-16, and F/A-18 are all primarily single seat aircraft. It's much easier to stretch the nose out a bit to add a second position than to make it wider. This arrangement is also more aerodynamic, requiring less frontal area.

Fighter aircraft designed for a crew of two will often choose the tandem arrangement for aerodynamics. If both seats need forward visibility, there are compromises available. Some aircraft like the F-15E can be flown from both seats even though there is less visibility from the back. It's possible to have a bubble canopy in side-by-side configuration (this is particularly seen in aircraft kits), though it will create more drag than a tandem canopy.

Having the crew sit next to each other can be helpful for crew interaction and sharing instrumentation. The F-111 is one such special case:

  The USAF wanted a tandem-seat aircraft for low-level penetration ground-attack, while the Navy wanted a shorter, high altitude interceptor with side-by-side seating to allow the pilot and RIO to share the radar display.

This may have also been easier to design the escape capsule. Of course the Navy changed their mind and went with the F-14 instead for better maneuverability.


Which civilian airport has the most runways (current and planned)? I just read Is the airport capacity related to number of runways? . While there are a host of factors, the idea of more runways is obviously so that more flights come in and out. Which is the biggest and baddest airport which has the largest number of people as well as aircraft coming in and out. Few days back I read that the Chinese have some ambitious plan for the Beijing Airport. I do not know how many runaways it would have (the newspaper article didn't tell/share that info.) but would be interested to know which Airport is the king?

Chicago O'Hare International Airport (ORD) today has 8 runways, which is more than any other current airport has. In 2015 14L/32R was closed and 10R/28L was opened, and further runway changes are foreseen.

Beijing Daxing International Airport will be Bejing's third airport (planned to open in September 2019). According to Centreforaviation it will have 9 runways. One runway will be reserved for military use. The ninth runway is not visible on public maps and illustrations at this moment.

Denver International Airport (DEN) currently has 6, but the long term master plan foresees 10 runways in total by 2030, and 12 beyond 2030. See Jonathan Walters' answer for images. (Fun fact: there are a plenty of conspiracy theories about the shape and size of DEN)
Additional runways are not necessarily used to increase (commercial) traffic. Runways maybe reserved for different wind directions, to separate different type of planes (military, general aviation), or for noise abatement. This was already discussed better in the other question that you mention.

As an example for the noise abatement,  this picture (from LVNL) shows a snapshot of active runways at Amsterdam Airport Schiphol. There are 6 runways, of which usually only 2 or 3 are used. The most distant runway (here in green) is almost always in use, causing very long taxi times in favour of reducing the noise exposure of nearby residential areas.


Why do some airfields not have a climb gradient for LPV? Can anyone advise why for some airfields there are minimum approach climb gradients for LNAV with none for LPV, while at other airfields there are minimum approach climb gradients for both LPV and LNAV?

This is an extract from the plate for Campbeltown - EGEC.

For other airfields, MPA gradients are also given for the LPV as well as LNAV such as Islay - EGPI, shown below.

When it is not stated, it means it is the nominal value of 2.5%, i.e., for the first example, the LPV climb gradient is 2.5%. When an additional OCA/H is published, then the OCA/H and gradient values are published (second example).
From PANS-OPS:

6.2.2.2 Climb gradient in the intermediate phase. The nominal climb gradient (tan Z) of the missed approach surface is 2.5 per cent.
6.2.2.3.2 The OCA/H for the nominal 2.5 per cent must always be published on the instrument approach chart. If additional gradients are specified in the construction of the missed approach procedure, they and their associated OCA/H values must be published as alternative options.


What airfoil is used on the Lazair ultralight? This is the airfoil of the famous Lazair ultralight. What is the name of this airfoil?

That looks close but I think there is too much camber in it as depicted above, and that is only the root airfoil.  There are significant changes along the span.

I used to jokingly call it my 'Lie Back' airfoil because I usually design things at night in bed :)

At some point I hope to get a 3D CAD model published of the whole Lazair series of aircraft but I can't promise when.

The Lazair is to be featured on a Canadian stamp this year and there is a Lazair in the Smithsonian :)

Dale Kramer

PS.  Peter Corley was killed in the Toucan of his own design and I never flew any aircraft with Peter hanging on my waist.


Are airport employees security checked? Is your average ramp or store employee at an international airport security checked before they are allowed to enter the gate side area or apron?

Many employees are not screened. This article, written in the aftermath of the LAX "dry ice bomb" incident, goes in detail about the levels of security employees face.

Ramp workers like 28-year-old Dicarlo Bennett, who was charged Thursday with two counts of possessing a destructive device in a public place, have access to just about anywhere in the airport. And though they must pass a 10-year background check before they’re hired, airport workers generally do not need to clear security when they arrive for their shifts.
...
Melendez said it’s not possible to check every employee when they arrive each day, especially when many of those workers have access to potentially dangerous or disruptive items once they’re on the job. (Dry ice is regularly used by caterers to keep food cold.)
...
Arif Alikhan, deputy executive director for homeland security at LAX, said the key is to have many layers of security. He said unannounced security checks are common, and inspectors might take a look at an employee’s lunch box or bags. Employees are asked to question who enters secure areas of the airport, and told to never hold doors open for each other.

After another incident in which employees smuggled firearms and drugs onto a Puerto-Rican bound flight, the Department of Homeland Security did a feasability study on 100% employee screening. Part of their findings were:

The concept of 100% airport employee screening is feasible. However, we do not believe it is realistic at this time. Regardless of the amount of funding, resources, and effort allocated toward this endeavor, vulnerabilities will persist. While the intelligence, screening, and law enforcement communities continue to make progress in addressing these vulnerabilities, 100% airport employee screening cannot entirely bridge existing gaps. As TSA changes its regulatory requirements, technology improves, and airports upgrade existing security measures, implementing this concept would be more realistic and should be revisited.

If you do a Google search on the subject, you will find numerous documents from large airports across the country detailing their procedures, most of which merely have to do with background checks and security badges.


Do military pilots have a phrase like "aviate, navigate, communicate"? The phrase "aviate, navigate, communicate" is commonplace among pilots, as a reminder to concentrate on flying the aircraft first, with navigation a secondary focus, and fitting in communication when it won't interfere with those other things.

Military pilots have other things to distract them from the primary focus on flying: tracking the target they want, keeping an eye out for other threats (not just collision threats), managing EM emitters or countermeasures, &c. Flying into a bad situation due to "target fixation" is not uncommon.

Do military pilots have any kind of drill, phrase, mnemonic, or teaching aid to set priorities for their flying, like "aviate, navigate, communicate"? To be clear, I expect they use that phrase for 'normal' flying, and I'm curious to know if there's an extra mnemonic about their combat-specific priorities; maybe something like "wingman, threats, targets".

I am a Naval Aviator and have commanded a flight training squadron.  We actually teach that very phrase.  Aviate, navigate, communicate was one of the first things I learned, and it has served me well over the years.

It applies no matter the situation.  The target fixation example you gave is a good one.  Getting an aircraft where it needs to be to put a bomb on target is clearly in the navigate realm.  It is superseded by not flying into the ground...which is in the aviate realm.


How many Angle of Attack sensor vanes should a commercial passenger airliner have? The ill fated MAX of recent news had 2 angle of attack vanes, with the MCAS computer picking the worst one.

It seems a "software patch"  may not be the only improvement option available.   Sensible engineering may suggest that a minimum of three, possibly 5, or even 7 vanes would enable writing a subroutine to reject "outliers" and use an Average AOA within an acceptable Standard Deviation.

Could more AOA sensor vanes, at various locations on the aircraft, be added to this design?

This depends very much on what the AOA sensors are used for.

Traditional airplanes

Traditionally, AOA sensors are only used for stall warning and stall barrier (i.e. stick pusher). In this case, you can get away with only two AOA sensors. A sensible design would trigger the pilot/copilot side stall warning with the onside (or opposite side) AOA sensor, which would confer the largest availability on stall warning. This is sensible because traditional stall warning does not effect flight controls and a nuissance stall warning can be inhibited manually by the flight crew.

Stick pusher, on the other hand, should require more prudence. By regulation, the activation of the stick pusher must be beyond the stall warning activation; and the airplane is designed to be stall-free if appropriate actions are taken from the recognition of stall warning activation. Therefore, the use of stick pusher is beyond the operational envelope of the airplane. A sensible design would be to require both AOA sensors to be functional and agree for the stick pusher to activate.

Fly-by-wire airplanes

For FBW or even partial FBW airplanes, AOA information is often directly in the loop of flight controls for gain scheduling and envelope protection (e.g. MCAS). This becomes a completely different problem. By regulation, continued safe flight and landing must be possible without exceptional pilot skills following any single point failure in any flight control related system.

This would require at least three AOA sensors for these airplanes. It's not enough to know that the AOA measurements disagree, but it is also necessary to detect which measurement is erroneous. With three sensors, if a single one fails and disagrees with the other two, you can shut off the failed sensor and continue flying as if nothing happened. Of course, the airplane would not be dispatchable until that failed sensor has been serviced once landed.

Some airplanes (e.g. A220) have four primary AOA sensors. But this is more for dispatch availability (MMEL) than for safety. Note that the A220 has two redundant AOA vanes on-top of the primary AOA sensors (SmartProbes) for Direct Mode operation.


How common is an airliner activating reverse while still in flight? Around 1977, I watched a DC-9 landing at Guacimeta airport, Lanzarote, Canary Islands, Spain, opening the flaps reversing thrust of both engines just over the airstrip end, but some 9 feet over the ground. Of course, it fell down that height as fast as a stone. Is this usual?

The system will allow deployment based on various criteria, and radio altitude height may be one of them, so even for a system not designed to operate "in flight", you still can deploy the TR's a few feet off the ground. Pretty ugly, rough touchdown when you do, though, and certainly not advised. How often that happens would be speculation, but probably very rare.


How much does a Black Box Underwater Locator Beacon (ULB) cost? I'm curious if anybody can provide a rough price estimate for a commercial aviation ULB like those used on Black Boxes.

Area of interest: Europe i.e. Euros, EASA CS-25.

(For instance the RJI ULB-362 or the Dukane DK-120, but any equivalent is good)

There is one listed here on Ebay for $750. Some of the makers like L-3 list contact info for getting pricing, all of the other sales sites I am finding look like you need to submit for a price quote.


What's the difference between TWEBs and TIBS? Everyone I know knows about DUATS and Flight Service weather briefers, but you almost never hear about TWEB and TIBS.  They have something to do with telephones and recorded weather information...are they the same thing?

What's the difference between TWEBs and TIBS?  When is it appropriate to use each?

NOAA has a great guide on their website called A Pilot's Guide to Aviation Weather Services which covers these (and more)!  They actually describe it better than I could.

The short version though is that TIBS is a telephone weather service while TWEB is a recorded route briefing that you can listen to while airborne.  Both are used for supplemental information and don't replace an official briefing.

  TIBS Telephone Information Briefing Service
  TWEB Transcribed Weather Enroute Broadcast
  PATWAS Pilot's Automatic Telephone Weather Answering Service

  TRANSCRIBED WEATHER BROADCASTs (TWEB) are continuous broadcasts of
  recorded NOTAM and weather information prepared for a 50-nautical mile
  wide zone along a route and for selected terminal areas. TWEBs are
  broadcast over selected NDB and VOR facilities and generally contain a
  weather synopsis, in-flight advisories, route and/or local vicinity
  forecasts. Winds Aloft Forecasts, current weather reports, NOTAMs, and
  special notices. TWEB outlets are listed below by state:

  PILOT'S AUTOMATIC TELEPHONE WEATHER ANSWERING SERVICE (PATWAS), and
  the TELEPHONE INFORMATION BRIEFING SERVICE (TIBS), provide continuous
  recordings of weather and aeronautical information. The information
  may include area and/or route briefings, airspace procedures, and
  special announcements. Telephone numbers for PATWAS and TIBS locations
  are found in the Airport/Facility Directory.

  TWEBs, PATWAS, and TIBS are for preflight or inflight planning and
  should not be considered a substitute for formal preflight briefings.


Why did the B-52 never have a JT4A-powered version? All versions of the B-52 use the Pratt & Whitney JT3C turbojet (except for the last version, the B-52H, which uses the JT3D low-bypass turbofan instead).

Problem: the JT3C is a horribly anemic engine, especially the early versions used on the A-E (producing a mere 44.5 kN per engine on the B-52A, increasing only to 46.7 kN/engine on the B-E models).  The engines used on the F and G did better, but still topped out at 61.2 kN each.  Due to this low engine thrust, the A through G models of the B-52 all required the use of water injection (with the associated weight and space penalties) to augment their engines' thrust for takeoff, without which the aircraft would have been incapable of becoming airborne.

For this reason, the other major users of the JT3C (the 707 and DC-8 airliners) promptly switched over to the more powerful JT4A turbojet when it (the JT4A) came on the market; the JT4A provided up to 77.8 kN of thrust, allowing the JT4A-engined versions of these aircraft to do away with the heavy and complicated water-injection system.

In contrast, the B-52 never switched over to the JT4A, even though, as a heavy bomber aircraft stuffed to the gills with fuel and ordnance, it would have benefited even more from the added engine power than the airliners did.

Why did the B-52 stick to the JT3C (until they finally jumped ship for the JT3D), and not go for the JT4A when it became available?

The military designation of the proposed JT4A is J75 (not to be confused with J57).
It was proposed for the G-variant of the B-52, but it would have slowed down the production:

The B-52G was proposed to extend the B-52's service life during delays in the B-58 Hustler program. At first, a radical redesign was envisioned with a completely new wing and Pratt & Whitney J75 engines. This was rejected to avoid slowdowns in production, although changes were implemented. The most significant of these was the brand new "wet" wing (...)
—livingwarbirds.com

Even a simple re-engine (as opposed to re-wing and re-engine) takes a lot of resources and time. The new engines are about 10% heavier, and they would need new nacelles, aerodynamic and flutter testing, etc. So, it was considered, but it wasn't the right time – fast production was needed for its role in the Cold War; 193 B-52Gs were delivered 1959–1961, making it the most produced variant.


How common are cancelled landing attempts? I recently traveled from Stansted, UK to Skavsta, Sweden with Ryanair, and was informed that depending on the weather (it was very foggy), we might end up landing at Arlanda (~150 km from Skavsta) instead.

The pilot later decided to try landing, went down to what felt like 20–50 meters (I could see the airport as clearly as one can see an airport in thick fog...) and suddenly rose sharply and then leveled out, causing some serious roller-coaster-like ~0g feelings.

The pilot then tried a second time and landed without much trouble.

How often does this occur? Was it dangerous? Is it according to standard procedure when approaching an airport in bad weather conditions? What might have caused the pilot to actually land the second time but not the first?

I could see the airport as clearly as one can see an airport in thick fog...

If you saw the runway by looking down from your passenger seat as the airplane passed over the airport on the missed approach, you were looking only through an amount of fog approximately equal to the altitude of the aircraft above the ground. For purposes of discussion, let's say that was 50 meters. The pilot, however, is looking through the front windshield trying to see the runway ahead to position himself for the landing. If he was on a standard 3 degree approach slope, that would mean he is having to look through nearly a kilometer (beware of old man doing math in his head) of fog.

Years ago when I flew for a commuter, I often had to make multiple missed approaches at Pullman, WA, USA in snow storms. My rule was that I would try 3 times before diverting. When we diverted we would often get complaints from passengers about not landing because they could see the airport as we passed over it. I could, too, but only through the cockpit side window, and with featureless snow covered rolling hills, there was no way to position for the landing without losing sight of the airport.

  What might have caused the pilot to actually land the second time but not the first?

The thickness of fog or how hard it's snowing or raining is constantly changing. Try again and you may be able to see what you couldn't before. If you can't, maybe try a third time and get lucky. Trying more that that would mean eating into your fuel reserve to get to your alternate, and, just as importantly in my view, pilot fatigue becomes a problem. Plus, you're beginning to seriously blow your schedule.


Why do many aircraft have both all-moving tailplanes and elevators? Many aircraft - particularly large commercial airliners - have both an all-moving tailplane (horizontal stabiliser) and elevators. (They also have a small third moving element on the trailing edge of the elevators too.)
As far as I can tell, all these surfaces do exactly the same thing, ie cause a change a pitch. Indeed this answer confirms that both stabiliser and elevators have the same effect, although hints there may be differences at transsonic speeds, something that doesn't apply to most airliners.
I understand trim is concern, but ultimately you're just trying to point the nose up or down. Especially with computers calculating optimum angles, having two control surfaces, which may counteract each other, seems to add a lot of complexity.
Of course there must be some advantage to having both, I'd like to know what that is?
 737 empennage (source)

Moving each part has different effect on drag:

Moving just the elevator increases camber, producing large force which is useful for manoeuvring, but it also produces more drag.

Moving the whole stabilizer, keeping the camber low, produces less drag for the same force, which is good for cruise efficiency.

That's why the elevator is used for control and the forward part of the stabilizer for trim.

The arrangement adds complexity, but it also adds redundancy. If one of the actuator breaks and jams, the aircraft is still controllable using the other part enough to make a safe landing.


How does the nose wheel lock on the Robin DR400 work? I recently learned that there is a locking mechanism on the nose wheel of a Robin DR-400. This mechanism locks the nose wheel in place when there is no weight on the wheel.
In particular, the wheel will be locked when the plane is in the air. And at landing, when there is weight on the wheel again, it unlocks.
But the nose wheel can also become locked if the center of gravity of the plane is too far back, or may fail to unlock upon landing. In this case, the aircraft becomes uncontrollable, and this can cause a runway excursion.

So I am wondering a few things:

How does this mechanism work?
What is gained by having the nose wheel lock itself? Rather than for instance a spring-like mechanism that just keep the wheel aligned with the plane when the plane is in the air.
Are there other planes with a similar locking mechanism?
Are there planes with a different mechanism to keep the nose wheel from moving in flight?

Here is what the landing gear looks like. It is not retractable and there are fairings on the wheels for better aerodynamics:

It looks like the "How do they work?" question has already been addressed pretty well, so I'll answer some of the other parts of the question:

  What is gained by having the nose wheel lock itself?

The nose gear is prevented from turning during flight when the rudder pedals are actuated. If the nose gear turned when the rudder was being used in flight, it would add a lot of form drag.
The nose gear remains straight until it touches down on landing, which prevents it from landing sideways in a crosswind landing where the rudder is being used to maintain runway alignment. This allows the pilot to continue to use the rudder inputs to maintain the centerline through the landing roll without damaging the tires or having a sudden yaw in the direction that the tire is pointing when the nose wheel touches down.

  Rather than for instance a spring-like mechanism that just keep the wheel aligned with the plane when the plane is in the air.

You mean like a Cessna-style spring linkage, where there is not a hard linkage between the rudder pedals and the nose gear? One advantage of not using the Cessna-style springs is that it's easier to taxi with direct linkages rather than spring linkages. Cessnas have a lag between rudder pedal input and the nose wheel actually turning because of the spring linkages, so you have to give directional inputs ahead of when you actually want to turn. With direct linkages, the nose wheel turns immediately when you push the pedals, which makes taxiing a little easier, especially when you're first learning.

  Are there planes with a different mechanism to keep the nose wheel from moving in flight?

Yes. See above for Cessnas. Also, in larger aircraft (e.g. passenger jets and similar,) there are no mechanical linkages to the nose wheel at all and the nose wheel is turned (mostly) by a separate control from the rudder. This control is called a tiller and it's moved by hand instead of by foot. It looks like this:


How much extra drag does landing gear incur? How much extra drag does landing gear incur?

I'm interested in large commercial airplanes. For a concrete example, let's say a 747 flying at 1,000 m above sea level at 800 km/h. Edit: Looks like 800 kmh is too high for lowered gear at 1,000 m. Let's say 400 kmh instead.

In those conditions, what is the drag with the gear lowered, and what is the drag with the gear fully raised?

Drag is a force measured in newtons.

Since drag equals thrust in level flight, we can work out an approximation for any aircraft that has a counterpart in MS Flight Simulator or similar software.

Before you fly away, real values would require renting a CFD / wind tunnel facility, or access to the 747 flight test data.

To compare the drag before and after, fly at the required altitude/speed, and notice the fuel flow with and without the gear extended.

Write down both fuel flow values, look up the jet engine model from jet-engine.net/civtfspec, note the number of engines, and use the SFC value (lb/lbf hr) to find out the force differential in lbf, then convert to Newtons.

I ran the test in a reputable 737-800 add-on, and with the values for the CFM56-7B27 engine (SFC 0.380), the difference was 67 kN at 250 knots IAS and 3000 feet MSL.

Source—CFD visualization of the turbulent structures around a landing gear.

Fuel flow figures (per engine) were 1.1 and 2.4 (x1000 kg/hr), angle of attack remained constant.

The drag (thrust) values were 57 kN and 124 kN. The landing gear more than doubled the drag at 250 knots (460 km/h).

In other words, the landing gear alone consumed 2600 kg/hr of fuel in those conditions.

That agrees with Terry's experience:

  It's enough drag that in flying a 747-100 gear down the 600 nautical miles from Yokota Air Force Base near Tokyo to Osan Air Force Base a bit south of Seoul, we burnt more than twice the projected fuel burn. Had we not been tankering fuel to avoid refueling at Osan, we would not have been able to do it when we couldn't get the gear up. It was all we could do initially to stagger up to FL190, and we never got higher than FL220.—Terry

A 747 has two bogies more than a 737, they're also bigger and tilted, so the drag penalty for a 747 is likely triple that of a 737, or even more.

At approach speeds the landing gear drag should be much, much lower.

  [The] drag depends linearly on the size of the object moving through the air.

  [The drag varies] with the square of the relative velocity between the object and the air.

  —NASA


Chicago O’Hare - Two Towers? Why does ORD have two towers? I flew in there for a connecting flight this last weekend and saw two towers standing, one obviously larger than the other.

Simply put, a lot of airports, especially ones that are Class B (very busy), will have multiple control towers. Most of the time, there is a single operational control tower focused on departures/arrivals into the airport environment with one or multiple smaller ground control towers focused on ground movement area coordination. I hope this helps!


What is the round thing on the tip of an MD-90 wing? I like to pretend I know about airplanes, but have always wondered what the round things on the tip of MD-90 wings are. Here is an image of what I am talking about:

To be clear, my question is what the round, hole-like item is in this photo. It is on the trailing edge of the wing facing backwards. My current best guess is that it is the hole that the smoke comes out of when Delta is using the planes for sky-writing and doing other cool loop-de-loop type tricks. I have yet to see the smoke come out...

This is a logo light. The MD90 as originally delivered to Delta didn't have these.

This is the wingtip light arrangement on an airplane without the logo light.


What is the cost savings of using electronic motors to taxi? Aircraft manufacturers are now planning to use High Torque electric motors on the nose wheel of the aircraft, which is powered from the APU, to taxi the aircraft from the gate to the runway without using its main engines. This is intended to reduce the fuel consumption by the plane while being taxied using the main engines, but also adds additional 150 kgs approximately to the weight of the aircraft.

How much fuel savings are we talking about here? Lets say we are considering an A320?

Jet engines are not very efficient at low power and speeds, so using an alternative method of propulsion is a good opportunity to save fuel.

An aircraft the size of an A320 will probably burn about 200 kg of fuel during taxi (based on this and this). Assuming fuel costs \$3 per gallon, this is about \$200 per flight.

The cost of the added weight from the electric motor will vary based on flight length, but assuming \$0.05 per pound, the extra cost would be \$16.50. Of course you are saving 200 kg of fuel, so this also helps to offset the weight. The electric motor is powered by the APU which uses much less fuel than the engines.

As KeithS points out, this adds up quickly. Even if United Airlines only equips its 150 A320 series planes with this equipment (which would certainly be a large upfront cost), and those planes make 5 flights each per day, that adds up to \$150,000 in savings every day. This will of course depend on fuel prices, which are actually well under $3/gal at the moment.

A simple way to save some fuel while taxiing is to only use 1 engine (or 2, for four engine planes) to taxi. This doesn't save as much fuel as the electric option, but also has much less upfront cost.


How did aircraft in WW2 navigate to the carrier? How did aircraft in WW2 navigate to the carrier after their mission? I was thinking of something like a beacon installed in the carrier but wouldn't it reveal the location of the carrier itself?

You are correct. US carriers did indeed have a homing beacon that enabled aircraft to find them.

The carrier transmitting units were called YE or YG, and the receiving units in the planes were designated ZB, and called "Zeebies". The transmitters consisted of rotating "superfrequency" beacons that broadcast different morse letters in different directions that could be decoded by the ZB units, but sounded like static to ordinary listeners. They were installed on all US carriers in WW2.

Here is an excellent summary of the system and its history. More information can be found here and here, and History SE already has a question about it.

The British also had a similar system, installed on a carrier in 1936 and used through the war.

Sources also indicate that the Japanese employed a radio homing system (Hirohito's War by Francis Pike), but I have been unable to locate any details.


Can Private Pilot training, funded by employer, count as company hours? My employer would like me to get my Private Pilot License, and is funding the remainder of my training. The intention is to have me do my training on company time.

I am an engineer, and sit in the copilot seat of aircraft we test while collecting data. A commercial pilot actually flies, but for safety's sake, they would like me to have my private license while doing testing.

Could this potentially breach the commercial limitations of a Private Pilot license, as I am "technically" receiving compensation (my salary) while flying on company time? Or, as this salary is already being paid to me for my actual job duties, would the salary not be considered a contributing factor?

(For example, plan is to instead of working 40 hours/week, I would work 30 hours/week, and fly for 10/hrs/week)

If this would be a violation, would the solution to instead be to continue working 40 hour weeks, and accomplish those 10 hours outside of company time?

No law against this. Nor is there any restriction on who pays for your flight training. You are referencing something which applies to your privileges as a student pilot. You may not act as Pilot In Command (PIC) for compensation or hire, nor carry passengers or cargo for compensation or hire - see §61.89. Having an employer pay for your flight training cannot be viewed this way. Again, these regulations are only applicable to time as PIC, so, as a student, this would be only solo flight time.
Now, if the employer attempts to have you perform flight duties in the furtherance of their business while you are a student pilot, i.e. they asked you to ferry an airplane for them somewhere, or perform some other task which is not related to your flight training, that does violate the law. However, the employer only paying for you to undergo flight training is not a violation of federal law.


What qualifies as "overwater"? I noticed that the saftey cards on a recent domestic US flight were the "overwater" version. Two questions about this:

What qualifies as "overwater"? Is there an official (i.e., FAA) definition?
Do airlines really have certain aircraft with booklets that aren't "overwater"? Are they prevented from flying certain routes without the right safety booklets?

I think that 14 CFR Section 91.205(b)(12) defines "overwater".

  (12) If the aircraft is operated for hire over water and beyond
  power-off gliding distance from shore, approved flotation gear readily
  available to each occupant and, unless the aircraft is operating under
  part 121 of this subchapter, at least one pyrotechnic signaling
  device. As used in this section, “shore” means that area of the land
  adjacent to the water which is above the high water mark and excludes
  land areas which are intermittently under water.

Then 14 CFR Section 121.573 outlines the passenger briefing requirements for flights with overwater operations:

  §121.573   Briefing passengers: Extended overwater operations.

  (a) In
  addition to the oral briefing required by §121.571

  (a), each
  certificate holder operating an airplane in extended overwater
  operations shall ensure that all passengers are orally briefed by the
  appropriate crewmember on the location and operation of life
  preservers, liferafts, and other flotation means, including a
  demonstration of the method of donning and inflating a life preserver.

  (b) The certificate holder shall describe in its manual the procedure
  to be followed in the briefing required by paragraph (a) of this
  section.

  (c) If the airplane proceeds directly over water after takeoff, the
  briefing required by paragraph (a) of this section must be done before
  takeoff.

  (d) If the airplane does not proceed directly over water after
  takeoff, no part of the briefing required by paragraph (a) of this
  section has to be given before takeoff, but the entire briefing must
  be given before reaching the overwater part of the flight.

Lastly, the requirement for printed safety booklets is from 14 CFR 121.571(b)  And to answer your last question, it needs to have only information pertinent to the flight, so if the airplane is not going to fly overwater at all, it doesn't need to have it.

  (b) Each certificate holder must carry on each passenger-carrying
  airplane, in convenient locations for use of each passenger, printed
  cards supplementing the oral briefing. Each card must contain
  information pertinent only to the type and model of airplane used for
  that flight, including—

  (1) Diagrams of, and methods of operating, the
  emergency exits;

  (2) Other instructions necessary for use of emergency
  equipment; and

  (3) No later than June 12, 2005, for Domestic and Flag
  scheduled passenger-carrying flights, the sentence, “Final assembly of
  this airplane was completed in [INSERT NAME OF COUNTRY].”


Beside payload size, what precise factors are considered to choose the fuselages shape? I am currently reading this very interesting book, which explains -I guess- most of the things you need to know in order to design aircraft.
Yet it is not clear to me besides the payload size what other factors are precisely used in order to determine the exact shape and size of the fuselage. I know that aerodynamics are considered, but that's very broad...

I read that the conic lofting method is usually used to determine the exact shape of the fuselage. But I would need some more explanation to get a better understanding of how this is determined.
In other words: I know my wing size, airfoil, payload size and would like to design the fuselage. How should I proceed?

Thanks

EDIT: Feel free to share the pragmatic approach

What bothers me is that I ended up finding a webpage with some interesting guidelines, but contains many constant values I have no I idea where they cone from: http://www.barnardmicrosystems.com/UAV/uav_design/guidelines.html

eg of such values: wingspan: 1.041, 0.382; step 3 length: 1.775

Assuming an engineering background, here’s a short list of factors affecting fuselage shape and size.

aerodynamic considerations
volume considerations
structural and elastic considerations
operational and interface considerations.

Aerodynamically, drag and stability are the major concerns.

Volume wise, payloads, subsystems, cargo, propulsion, etc must be considered. So it might take a few iterations for a preliminary design to converge at a feasible and optimum solution.

Structurally, elasticity of the fuselage is important, at least for weight efficiency, and general comfort. a bad shape can cause unwanted structural modes, which could risk failure in flight, or may cause difficulty in controls. If you were ever on a 747, you probably noticed how much it snakes towards the rear of the plane. The oval shape of the fuselage has parts to blame.

Finally, sometimes doors, openings, and structural interfaces dictate geometries of fuselages.

Additionally, geometric constraints such as tail strike prevention, or belly landing can be other reasons to shape the fuselage.


When is a transponder required below class B airspace? I just wanted to clear up questions I have about flying within the 30 mile veil of Class B. My understanding is that the Class B is the inverted wedding cake, having tiers as the airspace moves towards the center. If I remain outside of the published altitudes, yet within the 30 mile veil, am I required to have a Mode C transponder?

What are the requirements if I have no electrical system on my plane?

Short answer: mode C is required unless the aircraft was originally certificated without an electrical system; but ATC can always authorize you to operate within the veil anyway.

Your question isn't exactly about class B airspace itself, it's about the mode C veil; those are separate things although they do go together. (Note that from 1 Jan 2020, you'll require ADS-B out to operate within the veil so the name will be a bit outdated).

The AIM section 3-2-3 has a good explanation of the transponder requirements but the regulations are more precise and complete. They're in 14 CFR 91.215(b)(2) (and see 91.225(d)(2) for the ADS-B ones) and they apply within the veil, not within class B:

  All aircraft. In all airspace within 30 nautical miles of an airport
  listed in appendix D, section 1 of this part from the surface upward
  to 10,000 feet MSL;

That means that the shape of the class B airspace has nothing to do with the shape of the veil, which is always a cylinder 30nm across and 10,000ft high.

As for operating without an electrical system, that's covered in the same section. If the aircraft has no electrical system by design then you can operate within the veil but clear of class B:

  any aircraft which was not originally certificated with an
  engine-driven electrical system or which has not subsequently been
  certified with such a system installed, balloon or glider may conduct
  operations in the airspace within 30 nautical miles of an airport
  listed in appendix D, section 1 of this part provided such operations
  are conducted—

  (i) Outside any Class A, Class B, or Class C airspace area; and

  (ii) Below the altitude of the ceiling of a Class B or Class C
  airspace area designated for an airport or 10,000 feet MSL, whichever
  is lower; and

If the aircraft has an electrical system but it isn't working (i.e. you're taking the aircraft for maintenance) then you can request authorization from ATC to operate within the veil:

  (d) ATC authorized deviations. Requests for ATC authorized deviations
  must be made to the ATC facility having jurisdiction over the
  concerned airspace within the time periods specified as follows:

  [...]

  (2) For operation of an aircraft with an inoperative transponder to
  the airport of ultimate destination, including any intermediate stops,
  or to proceed to a place where suitable repairs can be made or both,
  the request may be made at any time.

You can also request ATC authorization if you don't have a transponder, or if it doesn't have mode C. But if you don't have a transponder, you need to request authorization at least one hour in advance, you can't request it "at any time" as you can in other cases (see 91.215(d)(3)).


Can I use a rental airplane to conduct air tours? Can a commercial pilot perform non-stop air tour operations under part 91 within 25SM of the airport using a rental airplane from the FBO?

I noticed that in the LOA application, the operation needs to provide:

  Type of aircraft, registration number(s), and make/model/series;

I wonder if this has to be the operator's own aircraft?

Beyond the aviation angle of this question there are legal liability and insurance concerns common to any business that ought to be addressed.  Any person renting equipment ostensibly for personal use, but in reality is using it to run a business on the side, ought to have a commercial lease agreement in place with the owner of the equipment.  Have you spoken with the FBO about your plans?

Regarding aviation specifics, these sort of questions have come up before and the CFRs don't offer much in the way of ownership requirements for aircraft involved in commercial operations.  Own vs lease just doesn't seem too critical from an operational standpoint as far as the Feds are concerned.  I won't start another "holding out" discussion here, but you should definitely talk to the FBO about your plans before moving forward with this idea.


What is the typical touchdown angle of a large airliner? Following up on my previous question about the typical vertical speed during the touchdown, I am now curious about the typical touchdown angle.

Airliners touch down with the main landing gear first, and only then the nose wheel. Hence there is some angle between the runway and the aircraft's body longitudinal axe. Let's assume that the runway is flat and that there is no slope in the runway.

Of course, the angle depends on many, many factors. It's theoretical maximum would be limited through a tailscrape event - and since aircraft are different in length, there is no general answer to this one. Theoretical minimum would be 0 degrees (or less, but then we have a nose wheel first landing). But, as highlighted in answers to my previous question, is there any information in operating manuals about typical angles, or can pilots provide some insight into which angle they are trying to achieve during touchdown?

Besides the typical angle, is there an optimum angle for any reason?

Again, I am not interested in the approach, only the touchdown after the flare.

(Source: 744 FCTM)

The relevant text reads:

  Airplane body attitudes are based upon typical landing weights, flaps 30,
  VREF 30 + 5 (approach) and VREF 30 + 0 (landing), and should be
  reduced by 1° for each 5 knots above this speed.

  Typical landing flare times range from 4 to 8 seconds and are a function of
  approach speed.

  Initiate the flare when the
  main gear is approximately 30 feet above the runway by increasing pitch attitude
  approximately 2° - 3°. This slows the rate of descent.

So for one of the largest, a delta of 2-3° is achieved in 4 to 8 seconds. As the text shows, the attitude varies based on speed, flaps, etc., so we can take the delta as both the optimum and typical value.

I checked the 777-200 FCTM, the values are different. And of course the smaller the plane, say a 737, the later the flare is initiated due to the smaller [and later] the ground effect.


Why might an altimeter read incorrectly by 4000ft after sitting on the ground for some time? My friend landed at Timaru Airport and when he put the aircraft into the hangar the altimeter reading was approximately 100ft [airport elevation is 89ft]. When he got back to go flying two weeks later the altimeter reading was over 4000ft.

He replaced the altimeter with a new one and now the same thing has happened again. Has anybody got any idea about what has happened?

This is how it works and I am surprised you switched the altimeter, unless I am missing something in your question. The altimeter reads the barometric pressure and should be set before you go flying. The airport will report the elevation and the knob on the altimeter can be turned till the altimeter is set to the correct altitude. You can confirm there is no blockage and the altimeter works correctly by getting the barometric pressure for the airport from the control tower or weather station, and it should match the barometric pressure numbers in the corner of the altimeter. Pressure changes on a daily basis and must be set each time. When we do a B check before flight, this is the exact procedure we do.

There is more information needed to properly diagnose this problem. With the lack of information I will continue on a couple of assumptions.

1) The altimeter has been bench tested and is serviceable
2) The altimeter system when installed in the plane does not match the local barometric pressure with the corresponding elevation

It was stated that the altimeter was replaced with a known good one. When this occurs the whole system must be pressure checked with a pitot-static test kit. This is a requirement since the system was opened up by replacing the altimeter. If this is not done the system is considered unserviceable. It appears this was not done.

To be complete I would check out the lowest point in the system for moisture and drain it if present.

Generally the procedures including connecting the tester to the pitot static system and taping (covering) all ports not connected. Then pressure is applied to the system, simulating different elevations. The airplane altimeter should match the testers simulated elevation within a certain + or -, for all specified elevations. Additionally, the rate of change of elevation is monitored for any abnormalities that may indicate a problem with any internal mechanisms.

Using the pitot-static tester is an absolute requirement in this situation.


What is Full Fuel load and how does it differ from Fuel Capacity? I've noticed that the fuel capacity is much higher than the full fuel load. For example in a Cessna citation 560XL
The Fuel Capacity is 6740 lbs
The Payload W/Full Fuel load is 960 lbs
Why is there such a difference?

The payload does not include the fuel itself. Payload means the total weight of all passengers, their baggage and all cargo on board. If you add this payload weight to your OEW (Operating Empty Weight), you get the ZFW (Zero Fuel Weight). On top of that you have to add the fuel.
The payload you quote is the maximum payload you can have if you also have full fuel on board. This is to make sure that the total weight of the aircraft remains below MTOW (Maximum Takeoff Weight) or possibly maximum taxi / ramp weight. If you fly with less than full fuel, you might be able to load more payload (as long as you stay below maximum ZFW).


When do you contact ATC when departing from a non-towered airport into class B controlled airspace? When do you contact ATC when departing from a non-towered airport into class B controlled airspace?

This is the situation where the uncontrolled airport sits under the Class B shelf.

I'm assuming you're VFR.

You contact ATC before you enter the Class B air space.

If necessary, you orbit outside, but you don't enter until after you hear the magic phrase "<your callsign>, cleared into the Class Bravo airspace" from ATC.

If it's a busy time and/or the weather is poor, you may NOT get that clearance.

See this blog for a more detailed explanation.

If you're flying IFR, the answer is the same except you would be in contact with ATC either before or right after take off. The Class B becomes more or less moot - they handle it for you.


Does a sudden drop of the velocity of the headwind affect the airspeed by lowering it? I had a discussion about air speeds with one of my instructors recently. He is claiming that there is a sudden drop in the wind velocity as you get closer to the ground (which is absolutely true, due to the wind friction with the ground). However, he also says that this will affect the airspeed by making it lower and this could be dangerous because it could lead to stall conditions.

For example: he says that, if my approach is with 90km/h airspeed, the headwind is 30km/h at a certain height and by the ground the headwind goes down to 10km/h, and the stall speed is 80 km/h, I will stall because the air speed will drop down to 70km/h.

Here is where I am confused, since I have read before that the wind does not affect the air speed in any way and it will only affect the ground speed by making it 20km/h higher.

Could you please try to make this clear for me?

Your airspeed does not remain constant because of inertia: it takes more time for the airplane to adapt to the new relative wind, compared to the time it takes for the wind to change.

Example One: you're flying 80 knots and the headwind is 20 knots. Over a time of 3 minutes, the headwind gradually reduces from 20 knots to 10 knots. Since the change is gradual, you will notice the ground speed smoothly increases from 60 knots to 70 knots.

Example Two: you're flying 80 knots and the headwind is 20 knots. In 2 seconds, the wind drops from 20 knots to 10 knots. At the start (t = 0), you are moving 60 knots over the ground. However, 2 seconds later, the plane has not yet accelerated 10 knots faster (maybe only 61.5 knots), but the wind is now 10 knots. Therefore you experience an airspeed drop in 2 seconds from 80 knots to 71.5 knots.

Given sufficient altitude, the plane will eventually speed up to 70 knots ground speed (airspeed 80 knots again). When you are landing, you may end up touching down short of the runway if corrective actions are not taken in a timely manner.


What is a 73W aircraft With all the issues surrounding the Ethiopia crash can you tell me what a 73W is?  Is it a 737 Max 8.  We're trying to figure out if our flights with Westjet on Friday might be effected.

Thanks

Frank N

According to https://www.airliners.net/forum/viewtopic.php?t=241063 (and others) B73W is a newer version of Boeing 737-700. A one with winglets installed.
So, it's not B737 MAX 8.

A more comprehensive list (which sadly does not include 73W) is here What is the difference between Boeing 73G, 73H and 737?.


Where did the "K" identifier prefix for US airports come from? Airports in the United States are (mostly) three letters, prefixed with a K. Where did this standard originate? Why K?

This is similar to "where did the U.S. N registration letter come from", but different!

The letter K was simply assigned to the contiguous US by ICAO, in order to have a system with unique identifiers for world-wide use, instead of trying to adapt local system to match. The IATA codes had been in use already and possible duplicates could not be excluded. The K in US ICAO codes is there to mark the region the codes is in, see the chart below.

A good summary of how many of the airport codes have been created can be found on www.skygod.com.

Airlines use the three-letter codes internationally in their own network, Sita, for messages such as passenger loads and departure times. World ATC and weather agencies use a separate teleprinter network, the Aeronautical Fixed Telecommunications Network (AFTN), which uses a four-letter "location indicator." Going from large area to actual airport, the first letter relates to the part of the world and the second letter the country. The third letter is a group of airports within that country. Most countries who use this particular convention use a letter to denote the FIR in which the airport is located. So F is Frankfurt FIR in Germany, M is Munich; P is Paris FIR, M is Marseilles. Other ways to use the third letter include identifying a group of airports with a common factor. For example, A was used in Germany for all Canadian and American air force bases. The last letter positively identifies a specific airport.
Thus Aberdeen, Scotland, has the International Civil Aviation Organization (ICAO) location indicator of EGPD—E for Northern Europe, G for United Kingdom, P for Scottish region, and D for Dyce field. Want to figure out LFPG? It's L for southern Europe, F for France, P for Paris FIR, and G for Charles de Gaulle airport. Easy!  Some are more easy to remember. The Dominican Republic codes all start with M (Central America) and D (Dominican) and then, well, see if you spot the code for international airports in Santo Domingo (MDSD), La Romana (MDLR), Puerto Plata (MDPP), Punta Cana (MDPC) and Santiago (MDST).
So if London Heathrow has two codes — and it does, LHR and EGLL — how come I've heard Chicago O'Hare only called ORD? The answer is unique to the United States and Canada. In the 48 contiguous States the ICAO code is formed simply by adding a "K" to the three letter code (or by adding a "C" in Canada). This explains why international flight plans refer to KORD, KMIA, KJFK, etc. A meeting of two rules is Key West, the FAA code is EYW (lose the 'K') and the IATA code is KEYW (add a 'K') which works great for KEY West.
Other airports within FAA jurisdiction have ICAO codes usually formed by taking two letters from the FAA name and prefixing them with PA for Alaska, PH for Hawaii, or PG for Guam. PT appears to be Pacific Trust Territories (Pohnpei, Yap, Chuuk, etc.), and there are various random other P codes like PWAK for Wake Island.
While most U.S. airports have the same three-letter location identifier for the FAA and IATA, note I didn't say all do. Scenic Sedona Airport in Arizona is assigned SEZ by the FAA and SDX by the IATA. Because for international airlines SEZ is Seychelles International Airport in Mah�, Seychelles. Another is the Hilton Head airport in South Carolina mentioned earlier. It's IATA code is HHH, but it's FAA three letter code is HXD.

Related question:
What is the methodology for assigning airport codes?


Why are highly cambered airfoils rarely used in propellers or wings? Highly cambered airfoils like the Seligs and Epplers are hardly ever used in propeller or wing sections. Why?

Does their performance reduce at high Re? Are they only adequate at low Mach, low Re?

The question only regards the aerodynamics. Ignore fuel storage and structural elements.

First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade. Of course you can pre-twist the blade so it will assume the correct shape in the desired operating point, but a propeller needs to work over a wide range of operating points, from take-off roll to high speed flight at altitude. In off-design points (i.e., most of the time) the propeller would have poor performance when compared with one which tolerates more diverse conditions.

Note that indeed thin, highly cambered airfoils are used on compressors and turbines in jet engines. Those are more stubby and enjoy much narrower variations in flow conditions, so the highly cambered, thin airfoil is indeed the best choice here.

Use on wings: Some aircraft do indeed use highly cambered airfoils. Those with low maximum speed like human powered or electric propulsion aircraft prefer those airfoils because they create the needed lift at the lowest possible speed, so the aircraft can fly with the limited installed power. As soon as the aircraft needs to cover a wider speed range, however, a lower camber is needed to keep drag low at high speed. This is similar to the use on propellers: A wider operating range requires to move away from the narrow optimum offered by those highly cambered airfoils.

Note that aircraft with a high wing loading use extensive and extensible high lift devices which turn their wings into thin, highly cambered structures for landing. Again, as soon as a lot of lift at low speed is needed, thin, highly cambered airfoils are the best choice. Adding slots between segments will make those work even better than a solid airfoil and allow to use more camber.


What is the purpose of this span wise "false spar" on Affordaplane wings? This is the wing of an Affordaplane Ultralight,what is the use of those spanwise foam boards between the foam ribs,I know it's not for aesthetics.

If you push up on the rib from below, it will eventually start to deform sideways, along its least stiff axis, and collapse.  The pieces in between them are just braces that prevent sideways deformation, and making the rib stronger.


Can I land on an abandoned runway in California? Can a private pilot land on an abandoned runway in southern California's remote areas?

There are a few unpaved runways near scenic hillsides, ideal for camping or swimming.

As noted in the comments it depends on who owns the land and what the local regulations are. With permission and proper operations it may be ok.

However if the runway was formerly active and is now no longer maintained its likely a bad idea to try and land on it. The FAA can even get you under their catch all 91.13 Careless or reckless operation.

There are procedures for closing a runway and marking it as such (you can find the AC here). Some times the "x" used to mark the closure is a physical structure that can make it dangerous to land on the runway. In other cases the runway may no longer be suitable for use or there may be active construction going on. While the runway may appear to be ok on the surface that does not mean it is safe to land.


How do I know my AGL altitude while in flight? I am about to start my PPL, still flying in my FSX.

I have a question, let's say I am flying from airport A to airport B. Both airports are in G airspace and I would like to stay in G airspace and don't go over 1200' AGL to class E. Let's say I would like to fly at 1100' AGL and keep this altitude from A to B. And let's assume we are in un-congested area, so we do not violate minimum safe altitude rule. But I do not understand if terrain in uneven, I mean its elevation is constantly changing, how do I keep my altitude 1100' AGL. I don't have radio altimeter like in Boeing. I understand that at the airport you can get your elevation, also you can have minimum safe altitude from sectional chart and also you can find any obstructions, but how do I know in a middle of my flight (if I need to).

If this question sound stupid and pilot don't fly in G airspace please let me know.

You can't know with 100% accuracy, even radio altimeters have limitations, and if you're flying on a PPL the aircraft probably wouldn't have one anyway. The VFR pilot flies with a map which shows the height of the highest terrain and noteworthy landmarks, but unless you are directly over one of these it is a matter of your experience in judging vertical distances.

I'm not sure if it's different wherever you come from, but where I fly in Australia, all airspace is measured in sea level altitudes or flight levels, never in reference to ground height, so we would never encounter your situation. Trying to stick to one height AGL might sound like a fun challenge to some but to me it sounds like less time to save my life when the engine quits.


Is extreme wind shear an official classification? Is it rare to land in 65kts headwind, gusting 95kts? I was a passenger in an A340 during a famous storm 65kts gusting 95kts. Resulted in a go around, but the plane sunk quite fast several times. Is this rare? Are there limits to headwind component? The captain said we experienced extreme wind shear, is that an official grade? Checked the metar for the kts.

Wind shear is also gusts, but the difference is that wind shear involves changes in speed and direction, where gusts are generally in the same direction. 65kt gusting 95kt is a very rare situation that no airline captain would want to tackle unless the situation demands it (low fuel, some other emergency) and the winds are straight as an arrow down the runway. You could land the airplane that way, but a 95 kt gust can certainly lift a wing or cause weathervaning when the aircraft is taxiing off the runway.

Now, if the report is for wind shear gusting that high, it would be crazy to try to land an A340 (or any other aircraft for that matter) unless the pilot had zero other choices (or just didn't know that there was low-altitude wind shear). The max crosswind component for landing on an A340 is around 37 kts.

From what you are describing, it seems they experienced wind shear, and not just gusts.


Would there be any benefit in placing the aircraft engines on the elevator (end-tips)? Looking at the HondaJet, we can see an interesting placement of the engines.

This leads me to wonder, for a slow flying aircraft how would it be affected with engines placed on the end tips of the elevator? (In other words, at the very edge of the left and right elevator wings.)

There are some issues with placing engines in the horizontal stabilizer tips (I'm assuming you mean putting the engines in stabilizer- elevators are the control surfaces):

The engines are heavy- putting them at the rear shifts the c.g aft. The associated change in the c.g requires a larger horizontal stabilizer.
Putting the engines in horizontal stabilizer tips mean that it has to be strengthened, increasing the weight
Also, the fuel lines etc. has to be routed through the elevator, which increases complexity and weight penalty (not to forget the control surfaces which have to be there, which places a lower limit on the horizontal stabilizer size).
Not only the engine weight has to considered, but also the fact that the thrust has to be transferred from the engine, which furthers the weight penalty.
Noise in the rear of the aircraft- though this can be reduced by placing them away from fuselage, this further increases the weight.
Another thing to consider is that the engine may be affected by the wing wake, especially at high angle of attack.


Why are my friends flying over Norway, Sweden and Denmark from Manchester to Vancouver? I'll start off by letting you all know that I am 100% blind, so I won't be able to read a map.

My friend Hudson came over to Canada with her friends last year, September 1 or August 31, 2016.

She likes to check where she's at once in a while when flying. She said it was an Air Transat Airbus 330-200. She doesn't remember the flight number, but it was probably TSC-285 or TSC-255.

Anyway, when she flew from Manchester (EGCC) to Vancouver (CYVR), she said "(laughs), Ann was out like a light when we flew over the Scandinavian countries." How on earth does this work?  I may be blind, but I do know that Vancouver is southwest of Manchester, and the Scandinavian countries are northeast of Manchester.

My question is, why would they fly over Norway, Sweden and Denmark when that is the opposite way to Vancouver? I'd love for an explanation please.

Note:

If you explain the route i.e. "she went north over x, then headed west and flew over by y" that would help, though it's not a requirement.  It is guessing after all considering she may have taken off northeast or southeast from EGCC.

(gcmap.com)

Above is the shortest route, it's called a great circle route. This is what airlines try to follow as much as possible.

The possibilities:

They've mistaken the icy parts of Greenland and northern Canada for the Scandinavian countries1
They've mistaken the western Scottish fjords for Scandinavian fjords
They need new maps :)

On a great circle route, like a taut thread over a ball, the bearing changes. Roughly it'll be something like this:

Initial heading of NW for 1,000 nautical miles, WNW for another 1,000, then West for 2,700. That's a very rough visualization, because again along the route the heading changes, just like the thread.

The route takes you along the western coast of Scotland, across the Atlantic, passing over Iceland, then Greenland, then Canada, where Vancouver is.

1 Denmark, Norway, and Sweden compose Scandinavia according to the normal definition.


How is engine thrust measured in flight? When aircraft engines are evaluated in a test flight, how is the thrust of the engine measured?

You would need accurate thrust numbers to calculate the real-world specific fuel consumption (i.e. not in a controlled environment like an engine on a test bed).

I'd assume it is nearly impossible to measure thrust indirectly with any accuracy since so many factors are involved (wing shape, wind, air density, lift, ...).

So I guess there should be a way to measure thrust directly, probably in the engine pylon.

So how is engine thrust measured in flight?

VectorVictor gives the right answer for jets.

However, for propeller aircraft this is really hard, and in the end only the difference between drag and thrust can be measured. By measuring the brake power of the static engine and the drag of the aircraft in a windtunnel, some data points can be acquired which help in calculating what the real thrust in flight could have been.

You are right, in the end it is impossible to produce a precise measurement of thrust. The most important part of the measurement is actually a precise definition of what thrust is: How do you account for the increased friction drag in the propeller slipstream? Is cooling drag part of the airframe drag, or should it reduce thrust? Measuring thrust is foremost an exercise in precise, meticulous bookkeeping.


How can an up-lifting tail have normal force direction and gradient when generating UP force? This article includes this statement about tail forces to validate the concept that the horizontal tail in conventionally tailed aircraft can be generating force DOWN or UP at different times and still have normal stability and control characteristics:

  I took a Cessna 172 Skyhawk and put a couple of large pilots in the
  front seats, with no luggage and no other passengers. That meant the
  center of mass was right at the front of the envelope, so the tail had
  to produce considerable negative lift in order to maintain
  equilibrium. There was lots and lots of angle of attack stability. I
  took the same Skyhawk and put a small pilot in the front seat, a
  moderately large mad scientist in the back seat, and 120 pounds of
  luggage in the rear cargo area. That put the center of mass right at
  the rear of the envelope, so the tail had to produce considerable
  positive lift in order to maintain equilibrium. The airplane still had
  plenty of stability. (As far as the pilot could tell, it was just as
  stable as it ever was.) The easiest way to determine whether the tail
  lift is positive or negative is to observe the direction of motion of
  the tip vortices, as discussed in section 3.14. To observe the
  vortices, I attached a streamer of yarn, about half a yard long, to
  each tip of the horizontal tail, at the trailing edge. The streamer
  gets caught in the vortex, so its unattached end flops around in a
  circle. When the tail is producing positive lift, the circular motion
  is in the direction shown by the green “circulation” arrows in figure
  3.29, i.e. downward on the inboard side. When the tail is producing negative lift, the direction of motion is the other way, i.e. upward
  on the inboard side.

Numerous posts on ASE, and too many comments to count, support this concept.  That's great.  There is a nagging problem though that prevents me from accepting the "consensus" just yet.

For the tail to generate downforce, local flow must be above the zero lift line of the tail airfoil.  The hinge moment imparted by air loads on the elevator is TE down, control stick forward, and aft stick force is required to counter it.  For trim, the tab must move TE down to apply a servo force UP to counteract air loads.
For the tail to generate upforce (lifting), the local flow must be below the zero lift line of the tail airfoil.  The hinge moment imparted by airloads on the elevator is now TE UP, control stick aft, and FORWARD stick force is required to counter it.  For trim, the tab must move TE UP to apply a local servo force DOWN to counter act air loads.

This is what you would call "stick force reversal" and I can't find any resources that say this is a good thing; actually quite the opposite, and I certainly wouldn't want to experience it in my own airplane.

So how exactly does that work?  How can a lifting tail still have normal control force direction and gradients, same as a downforce tail, and how could the trim tab work given that it now has to work in the opposite direction to provide the required servo forces for trim, on the same aircraft?

Let me break it down in sections. It's rather long I'm afraid.
1. Tail lift direction
Consider the following diagram of a wing-body and tail combination, with a non-pitching aircraft in steady-state. Let's assume that a true wingbody mean aerodynamic centre ($h_{n_{wb}}\overline{c}$) exists and therefore the pitching moment does not vary with AOA at that point.

Image ref: Etkins, Dynamics of Flight
The pitching moment for the whole aircraft about every point must be zero. Let's pick the point of CG ($h\overline{c}$) to sum our moments, since it also corresponds to the point of rotation for a free-body (this equation can be found in Etkins, Dynamics of Flight, but its derivation is simple and you should be able to deduce it from first principle):
$$C_m = C_{m_{ac_{wb}}} + C_L(h-h_{n_{wb}}) - \frac{\overline{l}_t S_t}{\overline{c} S} C_{L_t} = 0$$
where $C_m$ is the total pitching moment, $C_{m_{ac_{wb}}}$ is the wingbody pitching moment coefficient about the MAC, $C_L$ is the total lift coefficient (wingbody + tail), $C_{L_t}$ is the tail lift coefficient normalized over the tail surface area ($S_t$), $\overline{l}_t$ is the distance from the tail MAC to the wing MAC, and $\overline{c}$ is the wing reference chord.
The sign of the tail lift, $C_{L_t}$, to make the above hold can be either positive or negative, depending on the relative contribution of each of preceding terms. Note two facts:

For a positively cambered wing, $C_{m_{ac_{wb}}}$ is typically negative.
$C_L$ is positive for 1G flight.

Consider some extremes:

If the CG is way ahead of the wing MAC ($h \ll h_{n_{WB}}$), then the second term will be very negative, and $C_{L_t}$ must be negative to compensate. That is, negative tail lift.

If the CG is behind the wing MAC ($h > h_{n_{WB}}$), and $C_L$ is very large, then $C_{L_t}$ must be positive to compensate. That is, positive tail lift.
Furthermore, if the CG is behind the wing MAC, then there will always be an AOA above which the trimmed tail lift becomes positive, assuming stall does not occur.

2. Neutral point
Neutral point ($h_n\overline{c}$) is the longitudinal position at which the aircraft does not experience any pitching moment with a variation in AOA or $C_L$ (in another word, NP = MAC of the aircraft). If the CG is ahead of the NP, the aircraft will experience a pitch down moment with increasing lift (statically stable); and vice versa for unstable.
I will cite the following result, once again from Etkins (but simplified):
$$h_n = h_{n_{wb}} + \frac{a_1}{a} \frac{\overline{l}_t S_t}{\overline{c} S} \left ( 1 - \frac{\partial \epsilon}{\partial \alpha} \right )$$
where $a=\frac{\partial C_{L}}{\partial \alpha}$ is the total lift slope of the aircraft, $a_1=\frac{\partial C_{L_t}}{\partial \alpha_t}$ is the isolated lift slope of the tail, $\epsilon$ is downwash from the wing on the tail.
Note that the second term is always positive. Therefore, the CG can be behind the wing MAC, yet ahead of the NP.
3. Elevator hinge moment and stick force
For a reversible aircraft where the column is directly connected with an elevator, stick force gradient is directly conferred by the aerodynamic hinge moment on the elevator (let's assume there is no down-spring or bob-weight for simplicity).
For a non-cambered tail, there are two main contributions:

Hinge moment due to flow incidence ($b_1$)
Hinge moment due to elevator deflection ($b_2$)
Hinge moment due to trim tab ($b_3$)

You are right to point out that if a tail is lifting, and its flow incidence increases with aircraft AOA, then there will be increasingly TE up HM on the elevator, which would imply instability. However, static stability ensures that there is increasing TE up elevator needed to trim for increasing AOA, which will add TE down HM on the elevator.
The details are more nuanced than the above, and it will involve airspeed since HM is squared to airspeed, but it can be shown that the stick force reversal only happens when the CG is aft of the stick-free neutral point ($h_n^{'} \overline{c}$), and not at neutral point:
$$h_n^{'} = h_n - \frac{b_1}{b_2}K$$
where $K$ is a positive constant that is a function of the aircraft geometry (I've simplified from eqn 2.6.13 from Etkins). As you can see, if $b_2$ (HM due to elevator deflection) overpowers $b_1$ (HM due to flow incidence on tail), then we pretty much recover the NP. Down-spring and anti-servo tabs are some methods to increase the $b_2$ artificially.
More specifically, please consult the following figure, adapted from Bromfield, Factors affecting the apparent longitudinal stick-free static stability of a typical high-wing light aeroplane and Etkins, Dynamics of Flight:

In the graph, $\delta_t$ is the trim tab deflection.
Observations:

The first two curves are with the tab always set to 0 incidence against the elevator. The movement of the CG purely shifts the stick force vs. V curve up and down. When the CG is aft of the stick-free neutral point ($h>h_n^{'}$), any steady-state speed would require a pull force (despite the stick may be forward of neutral if the CG is still ahead of the neutral point!)

The last two curves are with the tab set for a trim speed ($V_{trim}$). Now, when the CG is aft of the stick-free neutral point, the stick force gradient also goes unstable! What effectively happens when the CG coincides with the stick-free neutral point is that the hinge moment contribution from the elevator+tab cancels that from the tail AOA; therefore, zero steady-state force is needed to hold any particular elevator/stick position.

Either way, whether the tail is lifting doesn't dictate the stability of the stick force vs. airspeed.


Why did the first Boeing B17 test flight crash on takeoff? Wikipedia says that the control surfaces were locked.

What does this mean, and if the control surfaces cannot be moved, how did the aircraft get into the air in the first place?

The prototype aircraft had small "blocks" (more like wedges) which were used to prevent the control surfaces moving in the wind and potentially damaging the control surfaces, or even moving the aircraft on the ground. A bit like putting chocks under the wheels to stop the aircraft rolling.

These were not removed when the aircraft was taken on a test flight, so the control surfaces could not move. Essentially, they were "fixed" in an approximately neutral position. Imagine if you couldn't move your steering wheel in your car because someone had put wedges next to your front wheels to prevent them turning: that's basically the situation the pilots were dealing with, but unlike you they can't just hit the brakes and stop once the aircraft is in the air.

The Boeing 299 / B-17 are "taildragger" aircraft.

As you can see in this photograph which I won't link directly due to the size, the aircraft's nose is pointing quite sharply "up" when on the ground.

Here's a B-17, which isn't identical to the Type 299 but is close enough that it makes no difference. Hopefully this gives you the visual to see why the aircraft will lift off at a high enough speed.

When accelerating the nose being pointed slightly up gives a positive angle of attack for the wings. This means that once the aircraft gets above stall speed, it will naturally lift off into the air. Essentially, the aircraft is already pitched slightly up before any control surfaces are moved.

That also means that when the aircraft takes off, it will climb - at least initially. With the locked elevators, there is no way to change the pitch angle of the aircraft, and the aircraft will either pitch up or down naturally - there is almost no chance that the aircraft would be perfectly trimmed by accident.

In this case, the position the elevators were locked caused the aircraft to continue pitching up until it stalled. The crew had no way to correct this and the aircraft stalled out, rolled and crashed into the ground.


Is there a niche market for A&P mechanics with an aerospace engineering degree? I've noticed a few people posting on this site who I would guess are aerospace engineers.

I am an A&P Mechanic, who is currently in the process of applying to college for an aerospace engineering degree.

My question is this:

Does a niche exist that would involve both skill sets, and will the A&P license/experience give me a competitive edge for hiring in some companies?

There is certainly a niche for both skill sets. Part of aerospace engineering is considering maintenance. There are multiple types of positions where having an A&P license would be a bonus.

During the design of an aircraft, maintenance must be taken into account. Projected maintenance costs are important to the success of a design. Engineers must address this during the design of new parts.
Manufacturing will have a lot of aspects in common with maintenance. Engineers must support manufacturing requirements in the design phase. Supporting emerging issues in manufacturing is also an important role.
Engineering is also needed to support regular maintenance tasks. Problems come up that aren't covered in the manual, and engineers will need to advise on a solution.

Based on these subjects, the following are a few types of companies will have some positions where an A&P license could help:

Designers of aerospace parts
Manufacturers of aerospace parts
Airline maintenance departments
Maintenance, Repair, and Overhaul (MRO) companies

This focuses mostly on the commercial side of things. You can also find either civilian or military jobs supporting the military equivalent of these functions as well.


What is an accessory gearbox? Many 737 engines have an accessory gearbox on the 9'o clock position, which is part of why the engine has a flat-bottomed shape. What exactly is an accessory gearbox?

The accessory gearbox is generally driven by the N2 shaft (connecting the high-pressure turbine to the high-pressure compressor) and in turn drives the engine accessories such as generators, hydraulic pumps, oil pumps, etc.  The air turbine starter is also part of this gearbox so during starting the accessory gearbox drives the high-pressure compressor.

This analogous to the serpentine belt on your car, which connects your crankshaft to your cars engine accessories (alternator, water pump, power steering, air conditioning, etc).


At what point can Approach mode be engaged in an A320? In my experience using Flightgear and other computer simulators, when flying an ILS approach on an A320, the approach mode can only be engaged at a certain point or later. What is this point, and why is it used?

The conditions for arming the ILS APPR mode are given in the FCOM:

Arming Conditions
The pilot arms the (ILS) APPR mode (LOC and G/S in blue on the FMA) by pushing the APPR
pushbutton on the FCU, provided that:

An ILS approach is selected,
The aircraft is above 400 feet RA,
The ILS and RA are available,
Go-around or takeoff or final mode is not engaged,
ILS frequency and course are identically set on both receivers.

(Airbus A320 FCOM - Autoflight - Flight Guidance)
These are just the technical limitations for when you can arm the APPR mode, not when you should. The FCTM has some more details on that:

INTERCEPTION OF FINAL APPROACH COURSE
When cleared for the ILS, the APPR pb should be pressed. This arms the approach
mode and LOC and GS are displayed in blue on the FMA. At this stage the second
AP, if available, should be selected.
If the ATC clears for a LOC capture only, the crew will press LOC p/b on the FCU.
If the ATC clears for approach at a significant distance, e.g. 30 nm, the crew should
be aware that the G/S may be perturbed and CAT 1 will be displayed on FMA till a
valid Radio Altimeter signal is received.
[...]
MISCELLANEOUS
The ILS or LS PB is to be checked pressed in the first stage of the approach. The
crew will check that

LOC and GS scales and deviations are displayed on PFD
IDENT is properly displayed on the PFD. If no or wrong ident displayed, the crew
will check the audio ident.

(Airbus A320 FCTM - Normal Operations - ILS Approach)


Could airliners' wings be thinner if no fuel were stored in them? I ask this question because wings are pretty thin overall, but I am curious if you could make them more thin.

No.

The wing thickness is primarily dictated by structural demands. It has to provide an aerodynamic fairing for the wing spar. Making it thinner would drive up the mass of the structure inside.

To simplify things, imagine the wing spar as an I-beam. It has two flanges which carry tension and compression loads, and a web between the flanges to transmit shear loads. The bending moment carried by the spar is the product of the forces in the flanges and the distance between them. If you reduce the distance, the forces need to increase to keep the bending moment constant. Bigger forces will result in bigger flanges to keep the stresses in the material below the yield stress.

Limiting the thickness is the desire to fly at a certain Mach number. A moving wing needs to push the air ahead of it aside, and thicker wings need to do more pushing. This results in a higher acceleration of the flow around the wing, and thicker wings have a lower critical Mach number (the flight Mach number when local flow reaches Mach 1). This drag penalty is especially severe in supersonic flight.

Thicker wings also help with creating higher maximum lift coefficients, up to a point. The plot below (taken from this website) shows that an airfoil thickness of 12% gives the best results, which helps to keep the wing area low. A thicker wing also makes it easier to integrate complex high-lift devices, which again helps to reduce wing area.

The resulting wing thickness is always a compromise, and the fuel volume is only the result of that compromise. If a wing needs to store more fuel, the designers will choose a lower aspect ratio, but will leave the relative thickness unchanged. If less fuel is required, the tanks will become smaller, and again the thickness will not change.