What are Uses of thin airfoil and thick airfoil? I have heard that thin airfoils are used for fast fyling and thick airfoil a are used for more lift. Is it true? Can you explain specific uses of thick and thin airfoils ?

Thick, cambered airfoils such as the Gottingen, Clarke Y and Davis are the most efficient airfoils up to speeds where Mach number becomes critical.

Lift is determined by the lift formula:

$Lift$ =  1/2 × Coefficient of Lift × Air density × Wing Area × Velocity$^2$

Coefficient of Lift can be further broken down into Angle of Attack and Airfoil type.

So a smart designer picks the most efficient airfoil and the least amount of area required
to lift a given weight.

Now time to choose.  By virtue of Bernoulli, the highest lift to drag ratios are achieved by accelerating air over the top of the wing.  This is where thick wings shine, much to the amazement of aircraft designers 100 years ago.  Thick wings also have gentle stalling characteristics, helping make the Fokker D.VII a real hit in 1918.  The thickness also enabled internal cantilever construction instead of bracing wires, paving the way for higher speeds until...

Higher airflow velocity above the wing approached trans sonic speeds, which lead to an increase in drag.  This forced designers to give up the top lift and use bottom lift, where the airflow is slower. That lead to the "supercritical" wing for high subsonic cruise.  Super critical wings feature a very "flat" upper surface.  Note airliners drasticly increase camber for slower phases of flight with retractable slats and flaps.

Supersonic aircraft wings are thin to avoid drag, but can create lift with a swept leading edge, much like a paper air plane.  This type of lift is called vortex lift.  As seen from the formula above, Velocity is squared.  This means that at high speeds, the wing need not be as efficient to generate the desired lift, as seen in designs such as the Concorde.

Of course, with very little weight, like a paper airplane, it works too.


Can lift occur if air is flowing over an object, but not under it? Can lift occur if air is flowing over a fixed-shape object, but no air is flowing under any part of the object? Maybe the object is the top half of a wing that's glued to the ground, or the object is dome-shaped, or some other shape. Maybe the object doesn’t actually rise or move, but when air flows over it, it exerts less weight on the ground.

Yes.  This jar of paint weighs 34.8 g, but only 32.6 g to 33.4 g when a fan blows at it.  The reduction of weight persisted when I repeated the experiment with a hodgepodge of baffles made from CD jewel cases and Post-Its to prevent any air leakage under the scale's sensor plate.  Others can easily replicate these results.
Per FlanMan's comment, without the fan, I also tried pushing the jar horizontally with a very sharp pencil, like a pool cue to avoid imparting vertical force.
No matter where I put the jar on the scale,
no matter which way I pushed relative to the scale,
no matter how high the pencil tip touched the jar,
by the time the jar moved, the scale's report changed no more than -0.1 g to +0.2 g, and usually increased: it "got heavier" instead of "producing lift."
(The fan wasn't nearly strong enough to move the jar.  High volume, low pressure, in paint sprayer parlance.)
So the horizontal component of the fan's force accounts for at most 1/7 to 1/10 of the effect.  One could throw some statistics at this, or a milligram-accurate jeweler's scale, but more confidence would come from trying this in a proper wind tunnel whose load cell is quite independent of the air flow.
With baffles, with fan, without jar, the scale reported -0.5 g to +0.5 g, tending towards the negative (i.e., lift), depending on how I placed the baffles and the fan.  However one might label that effect, at least some of the time this flat plate also counted as an example that answers yes to the question.
The scale is a DigiWeigh DW-500BS, unsurprisingly out of production, capacity 500 g, claimed repeatability 0.1 g which I've confirmed every few months with the calibration weights from a triple beam balance.


Why does the Boeing 737 have a pitot tube on the tail? From this link:

Can anyone explain why the Boeing 737 has a pitot tube for the elevators? Wouldn't the airspeed be measured the same from the front of the airplane and the tail? Is the airspeed measured at the tail view-able from the cockpit?

There are five sets of pitot tubes on the 737, organized into two groups, the pitot tubes on the nose are used for airspeed measurements, independent for the pilot and copilot and one as a backup. There are two pitot tubes on the tail that are for the "elevator feel and centering unit" (pdf; page 8).

  The elevator feel computer provides simulated aerodynamic forces using airspeed (from the elevator pitot system) and stabilizer position. Feel is transmitted to the control columns by the elevator feel and centering unit.

The airspeed may not be the same due to shielding in high angle-of-attack situations. I do not believe that the airspeed from the pitot tubes in the tail is visible (or useful) to the pilots. There is a warning that appears in the cockpit when the sensor fails.


Can instrument training be done in IMC with an instrument-rated safety pilot? I'd like to fly some hours towards an instrument rating by flying with a safety pilot.  I have a safety pilot who is instrument-rated and current (he's working on building time towards a commercial rating).  Since he's instrument-rated, can we fly in IMC on an instrument flight plan?  If we do, can I be the manipulator of the controls, and can I log PIC time for the time that I'm the sole manipulator of the controls in IMC?  And if we're in IMC on a instrument flight plan, do I need to be under the hood by using a view-limiting device, or is just being in IMC good enough to count as instrument time?

If you are the sole manipulator of the controls then yes you can log PIC time in this scenario, however you cannot act as PIC. The FAA discusses this in at least two letters of interpretation Speranza 2009 and Walker 2011. Both which lay out effectively the same situation (I would advise reading them) but in short they go into the fact that acting as PIC is not the same as being the sole manipulator of the controls and thus logging time as PIC.
Interestingly the IFR Rated pilot cannot log the PIC time while you are the sole manipulator of the controls as there is no prevision for that in the safety pilot regulations. The FAA words it as follows:

However, Pilot A may not log PIC time for the portion of the flight during which Pilot B is the sole manipulator of the controls, and is logging PIC time, because there is no provision for this logging in 61.51(e)
...
Although Pilot B [non IFR Rate Pilot] properly may log PIC time for a
portion of the flight as discussed previously, Pilot B could not act
as PIC and was not a required flight crew member for any portion of
the flight under the aircraft's type certificate or the regulations
under which the flight was conducted.
Speranza 2009

If you are in IMC then you do not need a hood as you would log that as actual IMC
The Herman Interpretation (2009) may also be relevant reading r.e. logging PIC vs Being PIC


What is this abandoned plane? Does anyone knows what type of plane is this and its model?

(Source: YouTube channel AssombradO.com.br)

Source: dailymail.co.uk

It's a crashed Super DC-3 (C-117D) in Iceland.

  On Nov 24 1973, the US Navy airplane - a Douglas Super DC-3 - was forced to crash land on Sólheimasandur beach
  The crew all survived the impact, but the plane was abandoned rather than recovered - and lies there still
  All that's left is the plane's fuselage amid rumours a local farmer stole the tail to mysteriously sell it on

It started its life as a C-47A with construction number (CN) 12554, the 2,554th in sequence. It was for the USAF (tail-number 42-92722). It was then converted to become an R4D-5 (Super DC-3 / C-117D) for the US Navy, and was given the new CN 43309 (tail-number 17171).*

First flight was 1944. It crash landed after encountering icing conditions en route on a supply mission to a radar station (aviation-safety.net).

* rzjets.net and abcdlist.nl


Why are ethanol piston engines apparently cheaper to maintain than avgas piston engines? In the course of hunting for examples of 100% ethanol-powered piston engines, I found the Embraer EMB 202 from Brazil that is modifiable to be that way.

But the article also says "The alcohol fuelled Ipanema engine also has 20% lower maintenance and operational costs." (no citation there, howerver.)

I also found this PDF that says similar things:

  Avgas is slightly cheaper on a per-kilometre basis than ethanol, but the lower maintenance costs associated with the use of ethanol ultimately make it a cheaper fuel option for pilots.

Why is this? What exactly are the modifications made to normal engines that somehow reduces operation costs? And why aren't these modifications made to normal engines that just keep using avgas?

(Please: let's not get into an off-topic argument about whether ethanol should/will replace petrol products. I already know avgas is about 40 MJ/kg and ethanol more like 28. This question is not about that and I hope it doesn't become another casualty in that flame war.)

Apparently a study was run for the FAA at Baylor University and it found that Ethanol caused less wear on the engines. You can trace the bibliography for quotes on the study. They concluded after testing for the O-235 engine

  The relatively low wear and general engine cleanliness indicates that
  the engine can operate on ethanol fuel for longer time periods than on
  100LL avgas. An additional test is planned in which the engine will
  run according to the schedule met in this test, except for 300 hours
  rather than 150 hours, to justify an increase in recommended TBO for
  the engine operating on ethanol fuel.

The FAA also ran a wear test on E-85 that you can find here. Their summary is mainly about whether or not it works which it seems to, they don't touch very much on wear. Still worth taking a look at on the topic.

It does seem that ethanol can increase engine life as has been seen in current testing.


Why aren't flights continued after losing a tire on rotation? I have read about a lot of incidents where airplanes lose one of the tires on rotation, yet they cancel the flights and go back for an emergency landing at the departure airport, risking overweight landing or burn fuel for 2 hours until they are within the MLM.

My question is why don't they choose to continue the flight to the destination since they already airborne?

Fixing tire out of station seems much cheaper than burning or dumping fuel to me.

Blown tire - can't raise gear, shredded tire will not fit in the bay.    ->

Gear down - much lower maximum speed and lots of additional drag ->

Both of those mean very poor fuel economy ->

Very poor fuel economy means not enough fuel to reach destination

So that's off the table.  Given that, there's no question that it's better to follow the safest course... And you had an extremely high pressure tire just explode, a couple feet away from the airplane's skin.

Especially when that brings you back to a field that is in fact an operating base for that airline, and is likely to have inspection and repair services, and may have a spare airplane.  I could see a Southwest flight out of SJC diverting to nearby OAK, since OAK is a main operating base for Southwest and will have an easier time fixing or replacing the aircraft.


What is this little strake on Cessna 182 vertical tail? can somebody explain what is the reason of mounting this little horizontal strake on a Cessna 182 vertical tail?

It's one half of a VOR receiver antenna. There will be a corresponding element on the otherside of the tail.

The complete unit looks like this:

Source

Disclaimer: I have no affiliation with the the company from whom I borrowed the image - it was just a good image to illustrate the point.


How does maneuvering affect radar detection of stealth aircraft? Stealth aircraft like the F-22 and F-35 have very low radar cross sections. These planes have angles of their fuselage set-up in a way to deflect radar signals. As far as I understand another stealth property is that the aircraft itself is relatively flat. When it comes to maneuvering and for example rolling the plane do these features still apply, as the "flat" effect is pretty much gone? So how does maneuvering affect radar detection?

RCS depends on the aspect presented to the radar source, so an aircraft can offer a low radar return from one angle and a large one from another. How much the RCS changes by depends on how much work went into optimizing the airframe from all sides.

Since reduced RCS tends to be at odds with aerodynamic performance, not all sides of an aircraft will be optimized, which will affect mission planning: with proper intel, an aircraft can plan its ingress and egress vectors in such a way to present its "best profile" to any hostile radar installations.

As an example, compare the F-22 with the Su-57. Both airframes have had some low observability built into them, but to different degrees. In particular, the rear aspect of the Su-57 is pretty conventional, suggesting that for whichever reason (funding, time limits, serial production costs, contract requirements, etc) it was deemed acceptable to have a larger RCS from behind.


How does a linear acuator in flight control work? I see that the linear actuator moves the rudder but how? Thanks!

There are several types of linear actuators. The main three types are Pneumatic, Lead Screw Driven, and Hydraulic Linear actuators. In addition there are also solenoids and stuff but the three main types are used in heavy mechanical applications.
The one in the picture looks like a Hydraulic Linear actuator. The way hydraulics work is that the pressurized non-compressible liquid is forced into a cylinder using a pump. The pressurized liquid then moves a sliding wall inside of the cylinder. The sliding wall is also connected to the main shaft. When the liquid is pushed from one side, the sliding wall inside of the cylinder moves as the liquid pushes onto it, thus moving the main rod in and outside depending on the configuration.
Hydraulic linear actuators are used for mechanisms that reqiuire precision and high force / torque. İn this case, we want the rudder to move on a hinge that is placed on the root of the rudder. The linear actuator is placed on an lever that is offset compared to the hinge. Then, when the linear actuator moves, the linear movement converts to rotational movement and rotates the rudder surface.
Here is an image to help.


Can I legally land an airplane in in Badwater Basin, CA? Would it be legal to land a light plane in the Badwater Basin salt flats in Death Valley National Park?

Let me add that you can obviously land in the park, as you have runways at both Furnace Creek and Stovepipe Wells. So another way of asking is simply: do you have to have an official runway to land? Obviously, you would need to stay >= 500 feet from people, but people don't typically wander too far from the parking lot and that's a big area. Staying away from people wouldn't be too difficult, especially in July or August when there's basically no one in Death Valley.

https://www.nps.gov/deva/learn/management/rules-and-regulations.htm says you would have to have special permission.

  36 CFR §2.17 – AIRCRAFT and AIR DELIVERY

  (a)(1) The use of aircraft on lands or waters within the park other than at locations designated by special regulations is prohibited.
  Death Valley’s special regulations at 36 CFR 7.26(e) authorizes the
  use of aircraft only at the following locations:

  Death Valley Airport, latitude N 36°27’50”, longitude W 116°52’50”, commonly known as the Furnace Creek Airport.
  Stovepipe Wells Airport, latitude N 36°36’15”, longitude W 117°09’30”.
  The Saline Valley Warm Springs Airfield, latitude N 36° 48.41”, longitude W 117° 46.90”, commonly known as the Chicken Strip, is open
  pending special regulation.

  The landing of aircraft at locations other than those listed above,
  such as for wildlife research, is permitted only under conditions of a
  permit issued by the superintendent.

  Aircraft have a significant potential for impacting park resources and
  human safety. By requiring permits for these activities, the
  superintendent can manage these activities and mitigate negative
  impacts.


How many axes are used in weight and balance calculations? When doing weight and balance calculations on fixed wing aircraft is the transverse axis (wingtip to wingtip direction) used?  Or is only longitudinal axis considered?

@Ron Beyer's comment is correct. Here's a little more information.

Generally speaking, in narrow-body aircraft, it's sufficient to calculate the CG only along the longitudinal axis. There may be limits along the lateral axis but which would only be exceeded with severely imbalanced wing fuel loads, and that is often handled by specifying a maximum fuel load imbalance.

Wide body aircraft typically require looking at the lateral CG as well as the longitudinal CG. However, the allowed lateral moment about the longitudinal axis may be high enough that the limitation comes into play only when carrying cargo. That's the case with 747-100/200/400 aircraft. As I remember, the max lateral imbalance for a 747-400 is 10,000,000 inch-lbs.

There are some fixed-wing aircraft that require looking at the vertical CG. The 767-300 does. I don't know about other 767 models. It's not an issue for 747s.

If you're interested in playing with CG movement on 747-400 and 767-300 aircraft, you can go to 747.terryliittschwager.com and look at worked examples or do your own.


Are the replacement satellites for Iridium going to provide surveillance coverage? Iridium satellites are to be replaced using SpaceX rockets.

Will the new LEO satellites be used for continuous tracking of commercial flights?

The next generation of Iridium satellites will have ADS-B receivers installed on them which will be used to track properly equipped aircraft. The company providing this service is Aireon.

Most airliners are already equipped with ADS-B transmitters. From 2020 it is manadatory on the USA and in Europe to have ADS-B.

The first satellite were to be launched earlier this year, but so far nothing has been launched.


How much is air mass flow rate of bleed air used for environmental control system in civil aircraft? I'm looking for a general use-case formula or reference that I can use to calculate the total mass flow rate of bleed air use in the environmental control system of any aircraft. I'm not looking for information on a specific aircraft - simply a general reference that can applies to all civil aircraft.

EASA and FAA engine type certificates often identify the total bleed mass flow, as a % of the engine's high pressure compressor exit flow. For example, see section 10, "Maximum permissible air bleed extraction" in this for the CFM56-7B, which shows 7 to 13%. If you assume in a civil aircraft that all the bleed air from the engine if for the ECS, you get an answer. (Or maybe you where actually looking for the total bleed mass flow).

If you can get this data from enough references, you could develop a general guideline.


What was the value proposition of the 767-400ER over the 777? Aside from visible differences, what are some differences in the usage and design of the 767-400ER as compared to the 777-200/200ER (or 300/ER if you feel makes more sense)?

What was the value proposition of the 767-400ER when compared to the 777, as both came into service within a decade of each other?

The -400ER was a strategic response to the Airbus A330-200
... but it didn't have the range and ultimately didn't sell well.
If you can get the customers who want to replace their DC-10s and L-1011s with the more comparable A330-200 (which entered service in 1998) in talks about the 767-400ER, you have a shot at selling the bigger 777 as well. But if you only have the bigger 777, the negotiations will be harder.
Flight noted the following in 1999:

Anyone seeing the 61.4m (201ft)-long aircraft emerge for the first time could be forgiven for experiencing a certain deja vu. Not only is the -400ER a mere 3.6m shorter than the 777-200, but almost exactly 10 years ago, a similar "767-X" design was being proposed to airlines. This soon evolved into the all-new 777, and for a while the stretched 767 plans were shelved.
(...)
Delta's TriStar replacement competition therefore provided the expected battleground for yet another Airbus versus Boeing fight. Airbus pushed hard to get Delta aboard the A330-200 as part of a wider fleet deal involving widebodies and narrowbodies. Boeing was equally desperate for victory, as much to keep Airbus from gaining a foothold in Delta territory as to launch the strategically vital 767-400X.

I've also put together this derivative work showing the similar length noted by Flight above.


How does humidity's effects on density altitude get factored into Takeoff Distance Performance Charts? Given that density altitude is a function of Pressure, Temperature, and Humidity, and most takeoff performance charts adjust takeoff distance as a function of pressure altitude and temperature, how is the humidity effect on density altitude taken into account?

Add 10% to your computed takeoff distance if it is too humid.

  Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient temperatures, the atmosphere can retain a high water vapor content. For example, at 96°F, the water vapor content of the air can be eight (8) times as great as it is at 42°F. High density altitude and high humidity do not always go hand in hand. If high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.

Source: Density Altitude, FAA, 2008, faasafety.gov

It's worth noting that Airbus makes no mention of humidity in their performance guide, Getting to Grips with Aircraft Performance. So simply follow the FAA's advice, and any correction factors mentioned in your aircraft's flight manual.


What are the hinged(?) structures in front of the F-4 Phantom's intakes? What are the large flat structures on the sides of the F-4 Phantom directly in front of the engine air intakes? They are more or less square, and may be hinged vertically.

From Wikipedia:

  A splitter plate is a component in some jet aircraft, used to control the airflow into the engine. Where the engine air intake is mounted partway back along the fuselage or under the wing, the splitter plate diverts the boundary layer away from the engine intake. It is a form of boundary layer control.

From Youtube: an F4


What is the occasional crackling sound on an A330? On a recent long-haul flight on an A330, at various points during the flight a strange crackling-type noise could be heard through the aircraft. It first happened whilst taxiing for takeoff and then subsequently every two hours or so.

If I had to guess, I'd say it sounded like ice-crystals or something being blown down the air-conditioning ducts. I'm no aviation expert (by a long shot) but I've never head this sound before and wondered what it could be?!

I have recently flown on a China Eastern A330-200 from Shanghai to Beijing and that noise was heard around 10.000 feet both on the way up and on the way down. It sounds awful! Everybody was rather worried, including myself who, despite being a frequent flyer, had never heard that noise. At first I thought it was some fatigue issue of the aircraft skin while "inflating" as internal pressure increased.
But a few days later I flew on a Cathay Pacific A330-300 from Hong Kong to Jakarta, and after hearing that noise again, I asked a flight attendant who told me it was likely ice crystals in the air conditioning tubes that form when the outside air is very humid. She said she heard it many times and it's considered normal on A330's.


Is this paragraph about the dihedral effect in the FAA's Glider Flying Handbook correct? I've been reading the FAA's Glider Flying Handbook, 2013 (FAA-H-8083-13A). In chapter 3 ("Aerodynamics of Flight"), the book is discussing stability. On page 3-12, it states:

  Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane. As a glider flies along and encounters turbulence, the dihedral provides positive lateral stability by providing more lift for the lower wing and reducing the lift on the raised wing. As one wing lowers, it becomes closer to perpendicular to the surface and level. Because it is closer to level and perpendicular to the weight force, the lift produced directly opposes the force of weight. This must be instantly compared to the higher and now more canted wing referenced to the force of weight. The higher wing's lift relative to the force of weight is now less because of the vector angle. This imbalance of lift causes the lower wing to rise as the higher descends until lift equalizes, resulting in level flight.

That doesn't sound right to me.

This paragraph says that there's more lift on the lowered wing and less lift on the raised wing. That's not true, is it? The amount of lift only depends on a wing's airspeed and angle of attack, not on the wing's bank angle.

The paragraph then explains that the lowered wing produces more upward lift than the raised wing. I think this is true, but it's not relevant, because upward lift isn't the only lift which contributes to rolling moment. Rolling moment depends on the total lift (and the direction of the lift).

All in all, I think that dihedral can't produce a stabilizing rolling moment in the way that the book says that it does. Any stabilizing effect must come from a difference in airspeed or angle of attack between the two wings.

Is the book's description of dihedral correct?

No, the explanation is not correct.

Tanner, you are right when you say that lift does not depend on bank angle. Lift is caused by a pressure difference, and pressure can only act perpendicularly to a surface. Therefore, the lift on each wing and its lever arm to the center of gravity won't change with bank and no "correcting" rolling moment is created.

Instead, what does happen is a side force from the wing's bank angle which will accelerate the airplane sideways. This in turn will result in a sideslip condition, and only now will a dihedral effect show up: By changing the angle of attack on each side differently, the sideslip in combination with dihedral will create a correcting rolling moment.

As you correctly observe, a stabilising effect can only come from a difference in angle of attack between both wings, and that only occurs in a sideslip.


How to calculate service ceiling and absolute ceiling in PA-44 Seminole? I am hard to understand to calculate service ceiling and absolute ceiling in PA-44 Seminole. Is there anyone know how to calculate it in the condition(T/O weight: 3599.24 L/D weight: 3354.44 PA:2329 Temp:20)

You can use the given chart to define (single engine) ceilings. There should be separate graph for both engines running case.

Note: it is important to understand that the definition absolute ceiling is absolute: zero climb rate while service ceiling is defined by local regulations.

In Europe, in commercial operations for class B aircraft (which Seminole is) service ceiling is the altitude where both engines running aircraft is able to climb at 300 ft/min at present gross weight and local OAT. A single engine service ceiling is defined as "positive gradient" so it is the first usable altitude below single engine absolute ceiling.

To define required altitude enter the graph from the right from the desired vertical speed, go upwards until you meet your current (or estimated) gross weight. From there draw a horizontal line all the way across the left side of the graph. The point where to local OAT (at an altitude) and your horizontal lines cross is the ceiling according to the selected climb requirement.

When interpolating between the altitude lines make sure to go perpendicular between altitude lines - not up and down along the grid.


What are the guidelines for call signs for aircraft dealing with US ATC? Suppose a general aviation aircraft is operating VFR under part 91. When they call ATC they'd normally use their tail number as the call sign. Something like "November 1-2-3 Foxtrot Uniform." What if they called up as "little piggy 27?" Are there guidelines or regulations about this? Does a custom call sign require anything or can you just make one up?

According to JO 7210.3Y, Air Traffic Organisation Policy, section 4-4-2 there are several requirements from both the FAA and FCC. The FAA requirements are (paraphrased from the JO):

Comply with FCC regulations (remember that the FCC is responsible for radio communication technical regulations, not the FAA)
Get FAA approval "to avoid possible duplication or conflict
with air−ground call signs assigned on a national
basis to other aircraft operators"

The FCC requirements are in 47 CFR 87.107 (not 87.115 as the JO says):

  (a) Aircraft station. Identify by one of the following means:

  (1) Aircraft radio station call sign.

  (2) The type of aircraft followed by the characters of the
  registration marking (“N” number) of the aircraft, omitting the prefix
  letter “N.” When communication is initiated by a ground station, an
  aircraft station may use the type of aircraft followed by the last
  three characters of the registration marking. Notwithstanding any
  other provision of this section, an aircraft being moved by
  maintenance personnel from one location in an airport to another
  location in that airport may be identified by a station identification
  consisting of the name of the company owning or operating the
  aircraft, followed by the word “Maintenance” and additional
  alphanumeric characters of the licensee's choosing.

  (3) The FAA assigned radiotelephony designator of the aircraft
  operating organization followed by the flight identification number.

  (4) An aircraft identification approved by the FAA for use by aircraft
  stations participating in an organized flying activity of short
  duration.

In other words, your call sign must be approved in advance by the FAA, otherwise you can't legally use it per FCC regulations. In reality, if you call up ATC as Little Piggy 27 you'll either get a laugh and a "no, seriously?" or - in the worst case - you'll just annoy them.

There are also military call signs in use but I know nothing about them. I do remember being in the pattern while an Air National Guard aircraft out of Memphis, TN was doing touch and goes using the call sign Elvis 1. But I have no idea how that call sign was set up between the ANG and FAA.


Can a Private pilot fly a company aircraft for business purposes if he is not being paid? I understand that if you are carrying passengers and getting paid, then you have to have a commercial license.

What about this scenario? I have a PPL (Private Pilot License).  My friend owns a company that owns a plane.  If I fly him and/or his employees around, and am not being paid for the time and just doing out of the goodness of my heart, is that okay?

I'd say you'd be on thin ice with this. The principle is that private pilots cannot be compensated for their flying, with very few exceptions. That doesn't necessarily mean getting paid to fly, it can mean having the expenses of the flight given to you in some way.

Say you take 3 friends on a trip, and your friends pay all the costs. You'd be in violation of FAR 61.113 because you haven't paid your share, in other words you've received compensation. You need to pay 1/4 of the costs of the trip.

In another case you are going on a business trip and decide to fly yourself and your employer pays all the expenses. This is legit because flying is incidental and not the purpose of the trip.

In the case you are stating you have no reason to be flying the airplane other than the flight itself, therefore flying for free means you are receiving compensation and a violation of FAR 61.113.

Note other countries may vary, the UK is extremely similar to the US on this, as are most European countries, I can't speak for anywhere else.


Why did the FAA ban all supersonic flights over the United States, rather than only supersonic flight in densely-populated areas? A major part of why the Concorde was far less of a hit than predicted, and of why the many 1980s and 1990s supersonic transport projects never got off the drawing board, was that the FAA had banned all commercial supersonic flights over the United States in 1973 (two years after the original U.S. SST effort collapsed) due to concern over sonic booms, locking SSTs out of the lucrative U.S. transcontinental routes.

Given that transcontinental supersonic flights could have been flown with minimal disruption on the ground by routing them mainly over sparsely-populated areas and using brief periods of subsonic flight over major population centers (here is a map of some possible routes), why did the FAA impose a blanket ban on commercial supersonic flights, rather than merely banning supersonic flights over or near densely-populated areas?

The short answer is a bad PR fallout following the Oklahoma City sonic boom tests, even though the tests were generally positive.

[The National Opinion Research Center] reported that 73% of subjects in the study said that they could live indefinitely with eight sonic booms per day.
(...)
The FAA's poor handling of claims and its payout of only $123,000 led to a class action lawsuit against the U.S. government. On March 8, 1969, the government lost its appeal. The negative publicity associated with the tests partially influenced the 1971 cancellation of the Boeing 2707 project and led to the United States' complete withdrawal from SST design.

The tested 96 Pa (2 psf) overpressure -- what an SST produces and is far lower than military jets -- according to concordesst.com is:

(...) equivalent to the pressure one would feel when putting a hand out of the window of a car travelling at 30mph.

A 1973 article by The New York Times, "Supersonic Civilian Flights Over U.S. Are Outlawed", does not give reasons, only that it was expected.

In a new order, the Federal Aviation Administration virtually outlawed today supersonic, flight over the United States by civilian aircraft, effective April 27.
The rule had been expected for three years, since the F.A.A. first proposed it in April, 1970, in a notice inviting comment for or against the action.
Accordingly, it came as no surprise to manufacturers of supersonic transport planes such as the British‐French Concorde and the Soviet TU‐144.
(...)
Congress canceled the Boeing‐General Electric program for an American SST twe years ago, for budgetary reasons.

The New York Times also reported in 1970 about the supersonic trials over populated areas in "West Scotland, Wales and England" with no reports of any damage.
Open questions
Was it politically motivated? Were there really no solutions such as the proposed corridors? Those and @CptReynolds' commentary that [the US] banned it to "prevent a European aircraft from capturing 100% [of the SST] market share with US airlines" are worthy of posts on Skeptics or Politics.SE. Over there they'd know where to dig deeper.
One of the things I found for example was a ban on Concorde before it set a tire in New York. So it was a ban that came before any evaluation for its subsonic noise. The Supreme Court eventually intervened and lifted the ban (The New York Times, 1977).
Another find was a quotation from Ohio Senator Clarence J Brown, saying:

I see no reason to oppose the SST [meaning Concorde] just because we were stupid enough to get out of the SST business ourselves (concordesst.com).


Why would helicopters not have black boxes? Is there a legitimate argument against having a black box on helicopters? I'm assuming there must be reasons beyond financial ones?

Airline and commuter type aircraft are required to have cockpit voice recorders and flight data recorders. There are no requirements for any other type of aircraft to have them.

Most helicopters are actually small or light aircraft. The helicopter in which Kobe Bryant and others died would be considered a medium sized helicopter, but a small aircraft.

You will find VERY few non-airliner or non-commuter aircraft have any type of “black boxes” as they’re commonly (and erroneously) called. They are not required to, either. Expense, weight, complexity, and maintenance are all reasons. The smaller number of lives at risk might be another. After all, they carry the same number of people as between a sports car and a passenger van. School and passenger buses carry more people.

Saying that, technology has become cheaper and more compact in order to provide de facto recorders. Some modern avionics have that function built in. With the proper app, iPhones and iPads have that function. Some pilots will go the extra step and buy their own sensors to provide more accurate data on Attitude, Heading, Position, Altitude, Ground Speed, Carbon Monoxide presence, and Traffic Data. Add-on apps will even record voice transmissions picked up by the headsets and audio system. This is not mandated by the FAA. It is more for the pilots Situational Awareness and review for training purposes. ForeFlight, CloudAhoy, Stratus, Sentry, and Lightspeed are some common devices and apps used to record data.


How to know if an airport has PAPI lights? Is there any way to identify whether an airport has PAPI lights (2-light or 4 light system, doesn't matter)?

I am working on some map generation and using aerial images to identify PAPI lights for airports. I was wondering if there is any quicker way to identify visual whether PAPI lights exisit on airport runways.

Whether an airport has PAPI lights or not is indicated on the approach charts. For instance on this chart, there is a schematic of the airport at the bottom of page 3 that shows you where the PAPI is on the airfield.

If you are looking for airports in France, you can find all visual approach charts on the government website.
Otherwise, this wiki has various links to approach chart ressources.

Edit: If there is not indication concerning the location of the PAPI on the approach chart, you can refer to page 40 of this FAA document concerning visual aids. The schematic here describes the standard location of the PAPI relatively to the strip.


Do thrust reversers make more noise than engine at same RPM? I'm talking about general aviation jet engines here. I live near an airport and I rarely listen engine noise on takeoff but always hear one during landing.
Would a jet engine at the same RPM make more noise when using thrust reverser than not using one?

Yes. Most of the noise you hear from aircraft is not mechanical noise from engines but disturbances in airflow around the airframe and engine outlets and inlets.
The airflow from reversing jet engine is much more turbulent than when the engine is forward pushing and thus more noisy.


Is a Control Law Degradation in Airbus Planes displayed on the ECAM? I would like to know if there is any warning displayed on the ECAM, when control laws get degraded from, let' say normal to direct law, other than the small text on the PFD.

Also if there are any sounds played in the cockpit.

This is because I am at the moment intensively researching myself on XL Airways Germany Flight 888T.
Have there been any software updates, that tell the pilot if the laws are getting degraded since then?

Maybe even because of that crash.

Since you are trying to understand how the crew is aware of law reconfiguration, it's important to look at this event in its context, why it does happen and what are the consequences for the control of the aircraft. You may skip this overview and go to the direct answer to your question which is in the last section of this post.

Flight control system and computers

The crew and the FMGS (autopilot) send commands to redundant computers:

2x ELAC (elevator aileron computer) for elevators/THS and ailerons,
3x SEC (spoilers elevator computer) for elevators/THS and spoilers,
2x FAC (flight augmentation computer) for rudder,

and computers send commands to actuators.

Source.

When the system is fully functional, all computers work using a normal law and it provides protection against:

excessive pitch and roll attitude,
high-angle-of-attack,
high-speed and low energy,
excessive load factor.

This can be done by receiving aircraft dynamic data from multiple sensors visible in the diagram.

If a flight control computer fails (or stop receiving required data), the crew receives an indication on the overhead panel:

Source.

When one or several of those computers fails, the role of the remaining ones are redistributed.

Reconfiguration of the computers

The failure of an element of the flight control system needs that other available elements compensate for the missing computer or peripheral. This reconfiguration allows available computers to control an additional axis using other laws. However departing from the normal law for an axis means losing one or more of the protections available under the normal law.

Degraded laws are:

alternate law (two levels: With and without reduced protections),
direct law (no protection),
mechanical (mechanical linkage between controls and control surfaces).

The mechanical mode is a backup mode to control the tail surfaces from the pedals and the pitch trim wheels in case something prevent actuators to work (e.g. loss of all electric power sources before the RAT is deployed).

Source: Airbus A320 FCOM

Alternate and direct control laws modify the relation between the stick movement and the flight control movement (linearity and amplitude).

Effect of pitch axis alternate law in different phases of flight

After the flight control system has transitioned to an alternate law, some protection may be available, but law may also transition again according to events representative of the different phases of the flight. Here is what happens around alternate law on pitch axis, which a change to direct law as the landing gear is extended:

Source: Airbus A320 FCOM

A very degraded flight control system: Direct law

While alternate law on a given axis still maintains (under some conditions) some level of flight envelope protection, this protection is lost when degrading to direct law. According to the axis, the direct law impacts are:

Pitch: The stick commands linearly elevators movement, automatic pitch trim is unavailable, all pitch protections are lost as well as alpha-floor.
Roll: The stick commands linearly surface movement, but overall amplitude is different according to slats/flaps status, only ailerons and spoilers 4/5 are used (all spoilers if ailerons fail).
Yaw: Mechanical control using the pedals, yaw dumping and turn coordination unavailable.

Permanent indications on PFD

Green $\sf \scriptstyle \color {green} {\text {=}}$ indicate active protections: Bank angle ± 67°, pitch angle at 15° nose down and 30° nose up. On the speed scale, speed limits for alpha protection.

Loss of protections after the degradation from normal law is indicated by amber $\sf \color {orange} {\text {x}}$.

Source: Airbus A320 FCOM

When the direct law is in effect for pitch the amber warning $\sf \scriptstyle \color {orange} {\text {USE MAN PITCH TRIM}}$ is displayed below the lines of the flight mode annunciator.

In addition if the law transitions to mechanical backup (which is supposed to be a transitory state), a red $\sf \scriptstyle \color {red} {\text {MAN PITCH TRIM ONLY}}$ replaces the amber indication.

General alerts

Audio chimes are triggered at the time of a component failure, when the mode changes. Alerts also activate either of the overall alerting lights (master warning and/or master caution).

ECAM messages

Failures and law activation trigger messages on the ECAM, with suggestions for crew actions.

Source.

However, and this was noted by investigators on AF447 accident, ECAM messages can be shown or hidden in the scrollable list according to messages priorities, due to the high number of messages that may appear in a short period of time during such sequences of events:

Source.


What happens when you break the rules while flying? In a car, when you speed, you get pulled over, probably get a ticket, and you might get points added to your license.

How are air traffic rules enforced? Are there patrol planes that make sure no one is going over the speed limit? I have always been curious about that.

When you break the rules while flying, it falls into one of two categories:

Only you know about it
The FAA knows about it

In either case, the very first thing you do is file a NASA ASRS form.  You might ask, why does NASA care about airplanes?  They are acting as an uninterested 3rd party to your actions, and collate statistics from this information.  If the FAA becomes aware of your actions, the presence of an ASRS form in many cases limits the action the FAA can take against you.  It is always worth filling out and I have filled out a few myself.

In the case the FAA knows about it, how do they know about it?  This typically happens when you are in radar control and do something wrong or are on the ground and do something wrong.

Some examples of this are:

Flight plan in the FMS is incorrect and you make a turn off of your cleared course.  The controller will see this.
You taxi incorrectly
Runway incursion
Climb / descend through your cleared altitude

In most of these cases, the controller will just correct you.  In cases that result in a loss of separation or a runway incursion, you may get a phone number from the controller to call when you land.

Less commonly, something you do may be witnessed by an FAA inspector -- you buzzed the tower when he was on the ramp watching you.  Or perhaps he watched you get into a plane and fly off without performing a pre-flight inspection.  Perhaps you were ramp checked and something wasn't right with you or the airplane.

In all of the above cases, if you are able to file an ASRS form, do it.  If you do something that may be severe and are a member of AOPA, call their legal team and discuss the situation.

If the FAA responds to something they know about, it can range from a controller talking to you over the phone up to certificate action (suspension, revoked) to the infamous "709 ride", in which the FSDO administers a checkride and decides which, if any, ratings you keep and which you lose.


How many minutes between jetway (air bridge) detachment and take-off? Is there a standard policy / guideline that dictates how many minutes a jetway or air bridge must be detached from a commercial aircraft before take-off?

while I don't think such a regulatory minimum would make sense, I'm still curious about whether it exists.

No, there is no such regulatory minimum.

On average, it's between 10 and 20 minutes from pushback to takeoff, although there are a couple of places I can think of (very short taxi from gate to runway) that I've seen it done in under five minutes.  That's moving pretty fast, though -- almost nonstop checklists, other than during the time the engines are starting.  But whatever you can do safely, is fine as far as regulations are concerned.  If you're ready to go, no regulation makes you wait.

Depending on the aircraft, there may be an engine warmup requirement or recommendation.  That's typically much more restrictive for the first flight of the day, and that would be a limiting factor in the very short taxi cases.


Is there always turbulence over China? I'm not sure if this is appropriate to Aviation.SE, so apologies in advance if not.

I have taken several domestic flights over mainland China (as in, the People's Republic thereof) and, so far, it has always been a bumpy ride. I'm more familiar with flying (as a passenger on a commercial airline) around Europe and, to a lesser extent, the US and don't recall it ever being that bad. Likewise, the trip from Europe to China is reasonably stable.

Is there something about the weather or the terrain in that part of the world that causes this? (This, anecdotally, appears to apply both to flights across China, over the mountainous regions, as well as along the relatively flat eastern seaboard.) Or have I just been unlucky?

China is well known for having very strict air traffic control, who often are reluctant to give pilots the flexibility to deviate around bad weather or try a different - smoother - flight level (as I've experienced first-hand). That can lead to aircraft in Chinese airspace encountering more turbulence than in other parts of the world.

Secondary to that, China can get some nasty weather: Particularly in the South. Compared to Europe they'll have more thunderstorms, for example, and that can lead to increased turbulence. This image here gives an overview of the number of lightning strikes around the globe. Aside from a few patches in Italy, China has more lightning - and hence more turbulence due to thunderstorms - than anywhere in Europe. As you mention, the mountainous terrain in some parts of the country also has an effect, although you get that in Europe too.


How does an aircraft designer increase dynamic stability? This question explains that an aircraft can be statically stable (it will seek to return to equilibrium) but dynamically unstable (the amplitude of the oscillations increase) if there isn't enough damping in the stability equation.
That's fine from a mathematical point of view, but what practical change would increase the damping and dynamic stability?
I've seen quite a few radio-control trainers that are dynamically unstable, and would love to know how to fix the problem so that they're easier for the student pilot to fly.
These planes typically pull out of a dive on their own, but then climb excessively and stall, leading to another dive. Each subsequent stall and dive is more dramatic than the last.

Damping is produced by drag and by large induced speeds at the tail surfaces from a given disturbance. This can be caused by long lever arms of these surfaces or by high air density.
More on the topic can be found here:

What is aerodynamic damping?
Will control surfaces on a plane be less efficient at a higher altitude?. The lower density at high altitude reduces the vertical or sideways speed component caused by a rotation relative to flight speed, thus reducing dynamic stability.
Another trick is to use a conventional tail instead of a V-tail, as discussed here.
What also helps is to shift the eigenfrequencies of the plane away from the response delay of the pilot in order to avoid PIOs. An unusual example would be the Wright Flyer.

These planes typically pull out of a dive on their own, but then climb excessively and stall …

This is the classic long period mode in longitudinal stability. Since rotation rates are low, pitch damping also is low and the most important damping contribution is from drag. A low L/D reduces the tendency to overshoot, a high trim speed reduces the tendency to stall (and shifts the motion to higher speeds with lower L/D). Reducing static stability will make the period longer such that it becomes easier for the pilot to react. However, lower stability will make the pitch response more sensitive which increases the risk of too large control inputs.


What is the difference between Part 121, Part 135, and Part 91 operators? What are the important differences between Part 121, Part 135, and Part 91 operators?

So loose guidelines to match the loose question...

Pt 91 - generally private flying.  That is, no flying an airplane with paying customers unless those customers are government officials on government business.  This rare exception is called "public use"...it gets more complicated but let's leave it at this for now.

A rich guy who owns a jet to fly around in (or a company that owns a plane for business use) can be pt 91 as long as they don't sell seats or cargo space to the general public.

Pt 121 - scheduled commercial air service with paying customers.  Planes are flown on defined routes as often as the operator wants.  This is where companies like Delta and American operate.

Pt 135 - on-demand and scheduled charter.  When you call a company and say "take me (or my freight) fom LA to NY" you are likely talking about a pt 135 unscheduled charter. "Scheduled" charter is allowed but is limited to a few days a week (four, I think).


Was the introduction of the A320 flight control system controversial? There were multiple severe accidents briefly after the entry into service of the A320, many involving to varying degree the augmented control system as a cause. In my eyes the A320 seems to have a very aggressive manipulation of the aircraft controls historically early (1987).

Conversely, the introduction of composites of the Boeing 787 generated a lot of stir and questions about safety, repairs, and lifespan.

Was there ever any questioning of the Airbus augmented control prior to the introduction to service?

Summary

No. The aspects that are talked about now are not the ones talked about then.

My (hazy) recollection is of concerns that the A320 lacked a system of cables and pulleys of the sort used by the Wright brothers. Perhaps pilots of the time had more focused concerns but I don't recall them making the front pages of newspapers.

In the 1980s there was discussion of Airbus flight control systems but the discussion focused on factors that were not involved in the most notable A320 crashes in the late '80s and early 90's.

Several Airbus accidents seem to be at least partly the result of pilots lack of familiarity with sophisticated aircraft systems in general rather than with features that are not found in other aircraft.

The Airbus control system

The main innovations by Airbus were the application to civil airliners of designs that had previously only been used on other types of aircraft.

Fly by wire. No mechanical cables, pulleys and associated friction.
Sidesticks. To provide clearer view of displays and greater pilot comfort.
Flight envelope protection. To prevent excessive inputs stalling or damaging the airframe.

Transfer of control

Airbus spent some time comparing and investigating support for transfer of control between pilots, including in emergency situations. They investigated pilot preferences for several approaches to this. They revisited this aspect over the years and it has become a point of focus since the AF447 report. However it doesn't seem to have been a particular point of controversy in 1987. Perhaps few people in the 1980s fully realized the extent to which pilots can operate independently and antagonistically in severe emergencies and the degree to which teamwork and communication disappears.

  There were multiple severe accidents briefly after the entry into service of the A320

You mean these:

26 June 1988, Air France 296. Pilots flew aircraft into trees during unrehearsed demonstration flight at unfamiliar aerodrome after mistaking trees for grass on approach.
14 February 1990 India Air 605. Idle/open descent mode engaged and pilots failed to act as aircraft descended into terrain near airport.
20 January 1992 Air Inter 148. Aircraft flown into terrain after autopilot incorrectly put into vertical speed mode and entered numbers appropriate for flight-angle mode. Air Inter had chosen not to equip aircraft with ground proximity warning system.

The total number of hull-loss accidents for the 320 doesn't seem high compared to other aircraft, but it's hard to compare the numbers if you don't take into account fleet size, cycles flown etc.

  the A320 seems to have a very aggressive manipulation of the aircraft controls

I'm not sure what you mean by "aggressive manipulation" but that seems an extreme view. The Airbus systems are a progressive development of existing trends. The accidents were largely the result of pilots setting inappropriate modes in systems like the autopilot. The accidents apparently didn't arise because of, for example, the way the control sticks operate.

  Was there ever any questioning of the Airbus augmented control prior to the introduction to service?

By "augmented control" I guess you mean features like the flight envelope protection that prevents pilot inputs from overloading the airframe. That doesn't seem to have been controversial.

The only controversy was over two items that had been used in military aircraft but not in commercial passenger aircraft

fly by wire
sidesticks

These don't seem to have been a large factor in the specific accidents mentioned above.

Further reading:

Indian AIr 605 - A320 Special Final Conditions - FAA
Pilots in the loop?
Electrical Flight Controls
Fly by Wire
AF447


What do I do if I wish to move my software from DAL Level A to DAL Level C? If I wish to downgrade my software's design assurance level (DAL) from Level A to Level C, what do I need to do? What are the benefits and drawbacks of doing this?

If I wish to downgrade my software's design assurance level (DAL) from Level A to Level C, what do I need to do?

DAL level A software is software which, if it fails, may have "catastrophic" results, defined as "Failure may cause multiple fatalities, usually with loss of airplane."

DAL level C, "Failure significantly reduces the safety margin or significantly increases crew workload. May result in passenger discomfort (or even minor injuries)."

So, to move your software from DAL level A to DAL level C, you'll have to reduce the responsibility of your software.  Your software's operation or failure to operate cannot:

result in passenger major injury or fatality
produce a large negative impact on safety or performance of the aircraft
reduce the ability of the crew to operate the aircraft due to physical distress or higher workload (though significant increases in workload is acceptable at Level C)

At most, your software's operation or failure to operate may:

Significantly reduce safety margins
Significantly increase crew workload
Result in passenger discomfort or minor injury

If you can change your software, or change your customer's use of the software (ie, cannot be used or installed as part of a safety critical control system), then the certifying authorities may allow your software to be certified at Level C.

  What are the benefits and drawbacks of doing this?

There are only 62 objectives that have to be met, and of those only 8 have to be verified by someone independent of the person who implemented the code that falls under that objective. While the number of objectives is only reduced a little (71 vs 62), the number requiring independent verification is cut by over 75% from 33 to 8. This significantly reduces the amount of work required to meet certification requirements, mostly in traceability.

The major drawback is that your software cannot be used or incorporated into systems which require a higher level of compliance.

So if your software is designed to control actuators and is currently certified to Level A, you might find that it's being used to control flight surfaces on some planes.

The move to Level C may, in that case, not require any software changes at all.  If you stop performing the full traceability, tests, and meeting objectives with independence at Level A, and only perform those necessary for Level C, then your release under this new system will necessarily only be certified under Level C.  The software can no longer be used to control flight surfaces, but if a new plane comes out with electronically adjustable air nozzles in the passenger compartment, it may be eligible for inclusion there where failure would, at worst, cause passenger discomfort and increase crew workload managing complaints about air vents being misdirected or opened/closed when the opposite is desired.

If your software is an autopilot, though, then it will never be used if it doesn't meet Level A.  In this case, there's no way to change your software or process to a lower level and expect it to be used - the usage of the software defines the level to which it must comply.

In most cases, the usage will drive which level the software must be certified to, and there's no real way (or reason) for you to change your level.  It would cease to be useful software at the wrong level.


What happens to the spinning wheels of the landing gear after takeoff, before retraction? What happens to the spinning landing gear right after V2 on takeoff? Are they automatically or manually braked to a halt before gear retraction? Or are they allowed to spin to a halt in the wheel wells?

I am asking this question in the context of common passenger planes, such as the Boeing 737-800 or the Airbus A320.

In most common passenger aircraft brakes are automatically applied when the gear is retracted. The wheels stop spinning before they enter the wheel well. This is particularly important because a spinning damaged tire may cause damage to hydraulic and fuel lines which are usually routed near the wheel bay.

The nose wheels have no brakes, so they are spinning while they are being retracted. Most aircraft have rubber brake pads (snubbers) fitted in the nose gear bay that stop the spinning of the gear once the wheels are retracted.


What is the difference between LNAV/VNAV and LPV minimums on an RNAV approach? This approach shows two sets of minimums that both have vertical guidance with a Decision Altitude (DA).  What is the difference between LNAV/VNAV and LPV, and why does LPV have lower minimums?

There's an FAA paper on RNAV approaches that explains the differences between LP, LPV, LNAV and LNAV/RNAV approaches. I made a table for my own reference but since StackExchange doesn't allow tables (AFAIK) here's a summary:

LP: no vertical guidance; WAAS required; MDA for minimums
LPV: vertical guidance; WAAS required; DA for minimums
LNAV: no vertical guidance; WAAS not required; MDA for minimums; requires RAIM integrity if WAAS is not available
LNAV/VNAV: vertical guidance; WAAS or baro-VNAV required; DA for minimums; requires RAIM integrity if WAAS is not available (i.e. if using baro-VNAV); possible temperature restrictions if using baro-VNAV

The difference between LPV and LNAV/VNAV is that although they both have vertical guidance, LPV was intentionally designed to be very similar to an ILS approach with an increasingly sensitive glideslope whereas LNAV/VNAV was not. Other answers have additional comments about LNAV/VNAV having been designed earlier and for different equipment which would certainly make sense.


What are the various thrust modes in the Airbus A320? I was going through A320 AFM and came across Thrust modes. I was not able to understand why thrust is in thrust mode during climb and descent because we climb or descent with a constant speed. So according to me it should be in constant speed mode.

A normal climb is performed with a constant thrust1 and a constant speed. The autothrust system will take care of the constant thrust (THR CLB) and the autopilot (or flight director for manual flight) will take care of the constant speed. This means you maintain the target speed by pitching up or down. If the speed is below target, pitch down to increase speed, and vice versa.
The same is true during a descent. The autothrust will maintain idle thrust (THR IDLE) and the autopilot or flight director will maintain the target speed by pitching down.
During cruise, where the altitude remains constant, the target speed needs to be achieved by setting the correct amount of thrust. Now the autothrust system is responsible for maintaining speed and will show SPEED or MACH on the FMA (Flight Mode Annunciator).

The A/THR modes are selected automatically in conjunction with the AP/FD modes (except ALPHA FLOOR).

(Airbus A320 FCOM - Autoflight - Flight Guidance - A/THR Modes)

1The actual thrust produced by the engine is not constant, it depends on altitude. The FADEC will compute a thrust rating limit depending on the selected thrust lever position:

The FADEC computes the thrust rating limit for each thrust lever position [...]. If the thrust lever is set in a detent, the FADEC selects the rating limit corresponding to this detent.

(Airbus A320 FCOM - Powerplant - Thrust Control System - Thrust Rating Limit)


What is the aircraft model used in the film "American Made"? In American Made, what is the model of the white airplane that the CIA presents to Barry Seal (played by Tom Cruise)? He says something along the lines of "the fastest piston twin". Are those continental engines?

If you are talking about the one prevalent in this trailer its an Piper Aerostar 600. The tail number is legitimate and the full registration details for 164HH can be found here.

Some of the movie may also have utilized a Beech Queen Air Model 88


Looking for techniques to model resistance to roll rotation in a very simple, force-based flight model I'm new to flight dynamics programming and am curious to know how people model an aircraft's resistance to rolling in very simple force-based flight models (not CFD, etc.).  For example, after a full deflection of the ailerons, the stick is returned to center and the aircraft will stop rolling.
I assume I should be looking at factors including dynamic pressure, wing area, and a center of pressure for the wing.  I'm just not sure if I should just be modeling the "wing pushing against the air that it is rolling into" or if there are other more significant physical principles at play.
Essentially I am trying to create a model in which I can tune maximum roll rates against an aircraft's tendency to want to stop rolling.
Thanks, and sorry in advance if this is too vague, but any info will certainly be appreciated and will help steer me.

The way this is commonly done in simulators is using linearized stability derivatives. For your example, one could calculate the rolling moment on the aircraft as the sum of such terms as:  rolling moment due to aileron deflection angle, rolling moment due to roll rate (roll damping), roll moment due to side slip angle, roll moment due to yaw rate, roll moment due to rudder.  For doing simulation each of these coefficients can be precalculated based on flight testing or CFD and then during simulation lookup tables can be used to find the current value.
My suggestion is that you take a look at JSBSim which is a cross platform opensource flight dynamics model that is used by FlightGear for example.  JSBSim allows one to build an aircraft model by defining the values in an XML file.  The website includes documentation, some links to papers and books, and example aircraft that are very useful.


Would atmosphere floating inside an airship be counted towards the mass of the airship? Airships are filled with hydrogen.  However as pointed out by Peter Kampf in the answer to the question below when an airship is on the ground only a portion of its hydrogen cells are filled with hydrogen.
During WW1 Zeppelins began their missions with Hydrogen cells at 33% full. How did this provide enough lift to get off the ground?
This chart details the hydrogen density in a given airship before takeoff.

LZ104's cells are only filled to 42% at sea level.  Does this mean that below the hydrogen cells sits an empty volume of air equal to roughly 58% of the volume of the cells?  If so is this air counted towards the mass of the airship?
I ask because water in a ship's hull makes it heavy and sink.  Why wouldn't air in an airship make it heavy and fall to earth?  Or would it?

Would atmosphere floating inside an airship be counted towards the mass of the airship?

Yes, but not for buoyancy.

Does this mean that below the hydrogen cells sits an empty volume of air equal to roughly 58% of the volume of the cells?

Yes, exactly.
When calculating the lift produced by the interior of the airship, the gas with lower density compared to outside air (hydrogen) produces lift while the internal air of equal density to outside air has no influence. It simply cancels out. While hydrogen displaces air of higher density, creating lift in the process, this internal air displaces outer air of equal density. So for lift that internal air can be disregarded.
However, when maneuvering this air does count. It increases the inertia of the ship, both when changing direction and when accelerating or decelerating. To this air the airship designers even counted the outside air which is influenced by the motion of the ship. An airship in motion continuously displaces air at its bow and lets it flow together again at the stern. The air is thus given an impulse which - as the product of mass and speed - can be described as an additional mass of the airship at this speed. In the case of an elongated body, this additional mass in the direction of travel is small in relation to the mass of the displaced air, but is of the same order of magnitude in case of lateral or vertical movements. In case of rotational movements, the inertial forces and moments of the gases within the airship envelope are added. To make it easier to calculate the forces on the airship hull, the concept of virtual mass was introduced. This virtual mass depends on temperature, altitude and speed. Although this technique allows a simple mathematical description of the effect near an operating point, it can only be generally applied with additional effort. If you are interested in the actual values: NACA-Report 184 contains tables and correction factors for the calculation of virtual mass.

I ask because water in a ship's hull makes it heavy and sink.

Because this water would displace air which is lighter than water and makes the ship float. This comparison only works when air would be replacing hydrogen in the gas bags. See the empty volume in the airship as akin to an external tank which is added to a ship and submerged. Its weight would be carried by the water. The air-filled volume in an airship is needed to have space for the gas bags to expand into without disturbing the aerodynamic shape of the hull.

Why wouldn't air in an airship make it heavy and fall to earth? Or would it?

Because this air displaces air of equal density. It cancels out in the buoyancy calculation.


Why does a steeper approach cause an increased landing rollout? A pilot of a high-performance airplane should be aware that flying a steeper-than-normal VASI glide slope angle may result in:

A) a hard landing

B) increased landing rollout

C) landing short of the runway threshold

This is an ATP question and the answer is B. Could anybody explain why the answer is B, not A?

I'll just formalize my comment here as an answer:

Landing is all about managing the aircraft's energy. When you are coming in for a landing with a steep approach it usually means that you will have more energy (speed) as you approach the ground. A high ground speed as you approach the runway usually means floating and/or, just like in a car, a longer distance to slow down. That extended distance you would need to bring the plane to a stop or slow enough to taxi off the runway would be the extended roll-out. Therefore B is the best answer.

A is incorrect because a steep approach does not necessarily mean a hard landing. The smoothness of a landing is dependent on the pilot knowing the appropriate moment to flare.

C is incorrect because unless the pilot is trying to land short of the threshold a steep approach can be done anywhere over any part of the runway/approach.


Precision Approach systems: Why is ILS used more frequently than PAR and MLS? I have studied that the main three systems for performing Precision Approaches (PA) are the ILS (Instrumental Landing System), PAR (Precision Approach Radar) and MLS (Microwave Landing System).

However, in most of the airports the ILS is implemented as the main PA aid; while the others are often developed, but not fully implemented or unused.

Questions:

What's the reason that the ILS is the most popular PA system?

What are the benefits and disadvantages of the other systems for not being as much considered?

How do they compare regarding approach performances?

At least in the U.S., ILS is popular because it is popular. It's relatively cheap to install and operate, the receivers share technology with VOR, and it is already installed everywhere. Momentum (and cost of replacement) has a lot to do with the failure and success of navigation systems.

MLS requires new hardware on the ground and in aircraft. The FAA tried to roll it out in the U.S. in the 80s, with a goal of replacing ILS by 2010, and utterly failed. The equipment was expensive for aircraft and ground facilities, and the cost of developing the approach procedures was high, as well as each approach needs tailored antenna programming (ILS is reasonably standardized). The adoption rate was low and the benefits (better arrival area guidance, better accuracy in some cases) were duplicated by the much cheaper GPS.

Precision GPS (LPV or LNAV/VNAV approaches) only appeared with the development of the Space Based Augmentation System (SBAS), also known as the Wide Area Augmentation System (WAAS). Augmented GPS has higher vertical precision and allows a single navigation system to be used for enroute, terminal, and precision approach guidance, a huge cost savings. The relatively new Ground Based Augmentation System (GBAS, originally called LAAS) adds high precision approach capability bringing GPS approaches closer to ILS and MLS.

PAR exists, but you may misunderstand what it is: it is a controller staring at a radar scope and issuing pilots instructions. It's stressful for pilots and controllers and very time consuming. Outside of military facilities you would be hard pressed to find a place to practice a PAR these days, at least in my experience.

In order of precision, high to low:

MLS
GPS with GBAS (uncommon)
ILS
GPS with WAAS (common, aka LPV)
PAR

In order of (estimated) total cost, high to low:

MLS (very expensive install + unique approach design + maintenance)
PAR (relatively cheap install + costly workforce; educated guess on my part, could be cheaper than ILS?)
ILS (expensive? install + simple approach design + maintenance)
GPS with * (no install, simple approach design, no obvious maintenance)


Can a paraglider rescue chute and its pilot handle terminal velocity? Imagine a paraglider cuts his main chute and reaches terminal velocity. Can he still land safely when he deploys hes rescue chute?

The european standards require that paragliding rescue chutes can handle opening shocks as high as 60 m/s at full load. Google tells me that termial velocity is around 53 m/s for a human skydiver. Is that requirement enough to safely say it can handle terminal velocity? Note that a paraglider can have equipment up to 15kg. Is it possible one could hurt himself when opening paraglider rescue chute because they can open to fast?

European Standard EN12491

EDIT: I've found this post where somebody talks about how he deployed a hangglider reserve while he was falling at terminal velocity. He talks about how he survived and how he has seen another similar case where it went the other way.

I have been told in person by the owner of High Energy Sports, a well-known manufacturer of hang gliding / paragliding emergency parachutes, that their products are not designed to be safely deployed at the terminal velocity of a falling human being.  Regardless of what some particular standard might suggest, I doubt that any hang gliding or paragliding emergency parachute is really designed to have a robust safety factor in the context of deployment in a true terminal-velocity freefall.  The greatest concern would seem to be catastrophic failure of the parachute and/or harness, not injury to the user due to opening shock, but the latter would seem likely to be a possibility as well.

Keep in mind that a hang glider or paraglider pilot does not actually cut away from the rest of their equipment (e.g. the main canopy of a paraglider) before deploying their emergency parachute.  There is no way to easily do so.  And that is why the reserve parachute is not designed for a terminal velocity freefall.  (Edit: see other answer for an exception-- apparently some paragliding rigs are designed to allow for a cutaway.)

Nonetheless, in an article in "Hang Gliding" magazine from well over ten years ago, well-known test pilot Mark Stucky described an incident from his younger days when he went skydiving using his hang gliding emergency parachute.  I don't recall whether the article specified how long he waited before opening the parachute and whether or not he might have been anywhere near the maximum terminal velocity of a falling human being-- one would tend to assume not.  He did survive without injury.

See also answers to related question Whats the difference between a paragliding rescue chute and a parachute?


What is the easiest way to update sectional charts with bulletin changes between printings? I am a student pilot and just purchased my first sectional chart a couple months ago. While the chart itself is within the effective time frame, there are Aeronautical Chart Bulletins that contain updates to the printed chart. I am supposing that I should notate these changes on my paper chart so I have the most current information, but I am trying to find the best way to do this.

My initial thought was that I could use something like http://vfrmap.com/ to see what those changes look like on an actual chart, and notate them on my chart to match. -But the changes are not reflected on vfrmap, or any other online source that I have found.

Of course I could just create them manually, based on the information listed in the bulletin, but it would be a lot easier to copy them from an existing online chart.

For example, here is the change listed in the bulletin:

But here is the airport on the vfrmap.com:

Maybe I am going overboard here, but I would love some input from more experienced pilots on a practical way to keep my chart current between (6-month) printings.

...or any other online source that I have found.

Sites like VFR map, Foreflight and the such get the maps from the freely available digital downloads the FAA produces. These downloads coincide with the physical printed versions and update along with them.

The FAA (at least partially) explains what to do right on the link

  ...why we publish the Aeronautical Chart Bulletin in the back of
  each Airport/Facility Directory (AFD) every 56 days and why it is
  important that you consult the Notices to Airmen (NOTAMs) prior to
  each flight. FAA regulations state that a pilot must be familiar with
  all available information concerning a flight. This includes having
  up-to-date charts and keeping them current...

The main point here is to check the available supplements and make sure you have all the up to date information relevant to your flight.

But lets talk about this in practice a bit because its important to understand what changes and what you should mark. NOTAMS often inform you of things like VOR outages and runway closures. In some cases these are permanent but in many cases they are temporary due to construction, maintenance or calibration. You should not go crossing out VOR's or Airports on your chart if they are only going to be down for potentially the duration of your flight that day (as relevant to you). With that in mind you should notate them as appropriate to your flight.

In many cases if the the runway closure or VOR outage is not along your route or a nav aid you plan to use it is of no relevance for you for that flight anyway. When going VFR you may even hear the briefer (if you call in) ask "would you like to hear about nav aid outages in your area". You can easily respond no as you may not be using VOR's to navigate that day.

The key is to make sure you have all the information on your destination and your posible alternate fields along the route and any potential deviation plans. You don't need to keep the whole span of the chart up to date, thats why they reprint them.

If your flight school allows you may want to look into something like foreflight or garmin pilot which makes sure your stuff is always up to date.

FWIW when i used paper charts I used to write all over them and often replace them before they expired because they were written all over.


Do they wait until all aircraft have landed before winding up an airline? This BBC article suggests that any announcement about winding up Thomas Cook will wait until late at night and all its planes are on the ground (I guess they don't fly long haul distances?) before an announcement is made about bankruptcy.

What is the significance of this? Surely no airport would turn away an aircraft that needed to land?

Is it an insurance thing?

How do airlines which never have all their planes on the ground get wound up?

The article speculates based on a single precedent:

  It's likely (looking at the precedent of Monarch's collapse) that any announcement about that will be made late at night, once all planes are on the ground.

But evidently that's not entirely the case. Thomas Cook ceased trading at 0109 UTC of the 23rd, with some of its planes still in the sky:

(Flightradar24) Showing only TCX callsigns.

As you can see, some of them are long-haul returning from USA. I also could not correlate last departure time with the announcement. The latest I found was at 1131 UTC (MT2643 from Orlando to Manchester).


What's the purpose of depiction of visual flight track on some Jeppesen approach charts? 29 is the visual flight track in this chart. Why is it depicted from the MAP to the threshold of the runway? Does it signify some alternative procedures pilots are expected to perform if they decide not to perform a missed approach at the MAP?

From the Jeppesen chart legend PDF:

Visual flight track is shown when the missed approach point is prior to the runway threshold.

So this depiction is when the MAP is before (and not at) the threshold.

FAA style:
(airnav)
It's quite hard finding an early MAP, but here it is above as depicted by the FAA. It means to expect a visual segment as shown above when/if continuation below the visual descent point is not approved.
I also found this very nice read: Where To Start The Missed Approach


Can briefly explain the principle behind the inversion on this power required curve? I’ve noticed on several graphs in the “Advanced Pilot’s Flight Manual”, that power available is lower for any given speed at 10,000 feet as compared to sea level flight. The power required curves intersect at the sea level best rate-of-climb (BROC) speed. Can someone briefly explain why more power is required at 10,000 feet when below this BROC speed and less power is required at 10,000 feet when above it?

Power required is drag times speed. In essence, you ask why drag at the same lift and in 10,000 ft is growing either side of the best climb speed at sea level.
As @Kolom rightly points out, the coincidence of best climb speed at sea level being equal to lowest drag speed at 10,000 ft is just that, a coincidence. There is no causal relation.
The power required curve is shifted right due to lower density at 10,000 ft which requires the aircraft to fly faster for the same dynamic pressure. The X-axis of the plot is True Airspeed, so all polar points will shift right as altitude increases. Next, you will also note a small upward shift. This is caused by the lower Reynolds number at higher altitude which increases zero-lift drag at the same indicated speed.
The best rate of climb speed is where the local slopes of the power required and the power available curves match. Then the distance between both power curves  is greatest and leaves the highest specific excess power after the power required for steady flight has been subtracted from the available power. If the curves would be plotted over Indicated Airspeed, they would simply shift slightly up for the power required and down for the power available as density drops with increasing altitude. The author of your book chose to plot them over True Airspeed, so it might seem that the best ROC at sea level has some significance for the drag at 10,000 ft. It hasn't.
Please make sure you read the linked answers, too, because they should help to give more background. I have kept the answer short, so some things are not fully explained. For example, why there is a minimum drag point at all is explained at the target of the first link.


Why would different crossing runways at one airport be on different radio frequencies? In this answer the following image was included which shows that control of two crossing runways are on different radio frequencies.

Source: AIP Denmark EK AD 2 - EKCH - AREA OF RESPONSIBILITY

It seems to me that having crossing runways on different tower frequencies could lead to a runway incursion or worse, an accident, if there is any loss of controller-to-controller communication within the tower. One of the key tenants for pilots is situational awareness, and being on a different frequency means that the pilot of an aircraft receiving clearance for take-off on 12 won't know about a plane receiving clearance for take off on 22L at the same time and thus, couldn't question the clearance.

Now, I realize that it's unlikely for both 12 and 22L to be in use at the same time, but if the winds are fairly calm and to the North, it could be possible, especially if traffic was high. However, I seem to recall some discussion here about an (near?) accident that happened on crossing runways where the pilots were unable to see each other as they began their their take-off rolls.

My question is: Why would different, crossing runways be assigned to different tower frequencies since this can lead to a lack of situational awareness and potentially cause incidents?

That division is applicable only when the parallels (4's or 22's) are in use, which is the preferential system (see title of chart).

As for rwy 12/30, from the eAIP (PDF; link may die; homepage):

Between 6 AM and 11 PM local:

  RWY 12 and RWY 30 may be used when one or both of the preferential runways cannot be used (...)

Between 11 PM and 6 AM local:

  RWY 12 and RWY 30 are closed for take-off and landing, however, RWY 30 may be used for landings when the crosswind component on the preferential runways exceeds 15 KT or the preferential runways are not available due to disabled aircraft, snow clearance, work on the runways, etc.

Once 12/30 is in use, that chart does not apply. Further:

  During periods with low traffic intensity one or two positions may be responsible for all three area, but apron service will be provided on three separated frequencies simultaneously. The frequencies will be combined by ATC. [emphasis added]


Which fighter jet is the safest outside of combat? I've heard that the Bloodhound SSC project decided to use a Eurofighter engine, because this was "safe". This got me wondering if there are any numbers on which fighter jets are safest. Something like a non-combat fatalities to flight hours ratio?

There are a number of barriers to labeling a single fighter as the "safest" - your proposed method is probably the best overall picture, but there are some barriers to even that.

Differences in use and training

Fighter aircraft are extremely complex and are asked to performed complex missions, even in training, where there may be little room for error. Due to the United States' huge military budget and wide-reaching foreign operations, US military pilots tend to have high flight time compared to many smaller countries, including other ones that fly US-produced aircraft. (Russia's modernization has led to increased flight time there, though still far below US or Cold War levels. China's modernization has pushed their training levels much closer to the US, as noted on p. 10 in this 2016 study ).

As pilot experience leads to decreased accident rate (to a point, at which accident rate goes up again even higher than inexperienced pilots), some of the air forces which fly very little likely have some contribution to a possible higher accident rate, irrespective of the safety of the aircraft they fly.

In addition, differences in philosophy of use from one country to another may lead to more dangerous employment - high speed low level flight is inherently more dangerous than flying high-altitude caps - and two air forces may utilize the same aircraft in such vastly different roles. This may lead to a fatal accident from a malfunction that at altitude, would be comparatively benign.

Lack of records

This is the biggest barrier, I think. Unlike commercial airliners, which worldwide predominately are produced by only two major manufacturers (Airbus/Boeing) and a handful of smaller regional-class producers, fighter aircraft are produced by a number of companies:

USA - Boeing, General Dynamics, Lockeed Martin
Sweden - Saab
Britain - BAE
Europe (multiple contributors) - Eurofighter GmbH
France - Dassault
Russia - UAC (combination of former companies Mikoyan, Sukhoi, Yakovlev, and others)
China - CAC, XAC, HAIC, Shenyang
India - HAL

While some of these aircraft are unique to a single country, many are used by multiple countries (e.g. for US-produced aircraft, numbers may be available for the US accident rates, but not for other countries). The F-16 has had 4,500+ aircraft produced, and is operated by 26 different countries on 5 continents. One difficulty with this, other than the difference in records is the number of different variants in service. As Mazura noted in a comment below, the F-35 has seen no fatal incidents in US service, but other countries have had incidents. 9 countries currently operate a F-35 variant, with at least 4 more countries planning to acquire the type. The MIG-29 has at least 24 current and 10 former operators spanning 5 continents as well. So great difficulty exists in retrieving fleet-wide numbers (in addition, as political situations change, it may make spare parts acquisition or maintenance more difficult, leading to major variances in reliability).

In addition, countries with smaller militaries that have traditionally relied on importing their military aircraft such as Japan, Israel, Turkey, Pakistan and Taiwan have produced or are developing indigenous fighters.

Due to the diverse nature of these countries that produce the aircraft - some of which are produced by somewhat state-owned companies, it makes the retrieval of records difficult.

Additionally, since Airbus and Boeing need private corporations to purchase their aircraft, and safety is a huge selling point (or rather, poor safety record is a major turn off) in selling tickets, then the easy retrieval of accident histories for airliner class aircraft is much more crucial than high-performance military aircraft.

Age of Fleets

Looking at probably the best kept public record producers,  The U.S. Air Force Safety Center has year-by-year and lifetime mishap histories for all current USAF fighters, but it doesn't separate out combat and non-combat incidents/fatalities. But when you compare the various US fighters:

A-10: 51 fatalities, 5.65 million hours (0.90/100,000 hours)
F-15: 44 fatalities, 6.80 million hours (0.65/100,000 hours)
F-16: 86 fatalities, 11.28 million hours (0.76/100,000 hours)
F-22: 1 fatality, 355 thousand hours (0.28/100,000 hours)
F-35: 0 fatalities, 96 thousand hours (0.0/100,000 hours)

But the F-22 has only been flying since FY2003, and the F-35 since FY2012, so comparing those to aging aircraft that have been around since the early 1970s doesn't paint an accurate picture. You can look at the 5-year and 10-year trends, however to see a more recent summary. For the USAF, the best rate would be the A-10, with no fatal accidents in the last 10 years, averaging 88,000 flight hours a year. Not unique to the USAF (indeed, all aviation sections of the US Military deal with this), many other countries have dealt with aging fleets as well, often due to the less frequent production of new designs since the cold war. (ex: Russia, Iran).

One note is that many combat aircraft in service with even the most modern air forces are no longer in production - the A-10 was last produced in 1984 and the MIG-31 in 1994, while some aircraft (like the MIG-29 and F-16) are still in production today.

Conclusion

Due to the barriers listed above, I think objectively calling a single fighter the "Safest" is likely an impossible task. But from the various characteristics, the safest record probably belongs to an aircraft that is flown by a country fairly regularly, leading to good experience in its pilots, possibly is employed predominately in an air-to-air role more regularly, and is produced more recently, yet long enough ago that the major design flaws in a new aircraft are worked through. I would say the best candidates would be the F-15E or the Eurofighter (which I found an article indicating 2 deaths in 240,000 flight hours across a diverse group of countries - Wikipedia indicates another 4 fatal crashes since then in an unknown amount of flight hours).


Can I work on my own airplane without formal mechanic's training? Suppose I am not planning to be a career A&P mechanics, but would like to get some hands-on skills for airplane maintenance, for reasons of both cost and hobby. Say my basic goal is to perform tasks from as simple as tire replacement, instrument replacement to avionics installation and troubleshooting. My goals may be progressive. The aircraft for maintenance is self-owned or owned by share. The ownership and operation of the aircraft is within US.

Can I work on my own airplane? Since I'm not certified for mechanics work would it be OK that a certified A&P endorse my work is standard-compliant?

FAR Part 43 contains the information of what you can and cannot do to an aircraft and what requirements you must have.

If you are a certificated Private Pilot (as in not a sport pilot), there are quite a few things you can do to an aircraft you own, that is not used for commercial purposes. Like:

Replace wheel bearings
Remove/repair/reinstall wheels
Replace safety wiring and cotter keys
Service landing gear struts by adding oil/air
Lubrication not requiring disassembly of non-structural items like cover plates, cowlings, and fairings
Simple fabric patches not requiring rib stitching or removing structural parts or control surfaces
Replenishing hydraulic fluid
Refinishing decorative coating of fuselage and other parts
Repairing upholstery without disassembly
Simple repairs to fairings, etc
Replacing side windows
Replacing safety belts
Replacing seats or seat parts with approved replacements
Troubleshooting and repairing broken circuits in landing light wing circuits
Replacing bulbs, reflectors, and lenses of position and landing lights
Replacing wheels and skis where no weight and balance computation is required
Replacing cowling not requiring removing the propeller or disconnecting flight controls
Replacing or cleaning spark plugs and setting gap clearance
Replacing any hose connection except hydraulic connections
Replacing prefabricated fuel lines
Cleaning or replacing fuel and oil strainers or filter elements
Replacing and servicing batteries

And others Source

However, note that it has to be your airplane, and it can't be used commercially (which I think includes offering it for rent). There is very little you can do in the way of avionics (read #31 and #32 in the reference I gave).

The fun thing to note is that although a PPL is required, the PPL training does not give any special attention to any of these items, and therefore really somebody off the street is technically just as qualified as a private pilot, but the private pilot is the only one authorized to do the work (provided they own the plane). When I got my PPL, nobody walked me through replacing cotter pins or anti-back-out devices...

Edit
These rules though may be different when you get into experimental category aircraft but I can't source it now and will do more research and edit when I can clarify.


What is the maximum angle between an airplane and runway centerline when touching down at a crab angle in a crosswind landing? What is the maximum angle between the airplane's heading to the runway when an airplane is being landing sideway due to crosswind?

A B777 (as in your photo) would generally have a maximum crab angle of about 16 degrees when approaching and landing at the Maximum Demonstrated Crosswind Component of 38 knots.

A 38 knot crosswind is not limiting and can be exceeded provided you apply a sideslip or crab correction at touchdown. It is however limiting if you plan to touchdown maintaining the crab.

E6B Heading, Ground Speed, And Wind Correction Angle

By using a sideslip or crab correction at touchdown, Airliners will sometimes approach and land when the wind exceeds the Maximum Demonstrated Crosswind Component. If the crosswind component was 50 knots and the aircraft was had a lower approach speed (due to light weight), the crab angle during approach could be as much as 25 degrees.


How prevalent are anti-explosion bladders in fuel tanks? Some aircraft fuel tanks are fitted or retro-fitted with internal bladders, to prevent fuel evaporating inside the tank. I've heard of this being a requirement for at least one type, perhaps because of a poor fuel tank design, but how commonplace is it? How many aircraft have these bladders?

According to this wikipedia article :

Most jet fighters and all US military rotary wing aircraft have some type of self-sealing tanks. Military rotary wing fuel tanks have the
additional feature of being crashworthy. High altitudes require the
tanks to be pressurized, making self-sealing difficult. Newer
technologies have brought advances like inert foam-filled tanks to
prevent detonation. This foam is an open cell foam that effectively
divides the gas space above the remaining fuel into thousands of small
spaces, none of which contain sufficient vapour to support combustion.
This foam also serves to reduce fuel slosh. Major manufacturers of
this technology include Hutchinson, Amfuel (Zodiac) (formerly
Firestone), Meggitt (formerly Goodyear), Robertson Fuel Systems, GKN
USA and FPT Industries. FPT is now part of GKN. For military use,
tanks are qualified to MIL-DTL-27422 (includes crashworthiness
requirements) or MIL-DTL-5578 (non-crashworthy). An aircraft fuel tank
sometimes consists of several interconnected fuel cells. The
interconnecting hoses are typically also self-sealing.
In addition to fighter aircraft some military patrol vehicles and
armoured VIP limousines feature self-sealing fuel tanks.
Self-sealing fuel tanks using military technology are also required in some motorsport categories.

Given the fact anti explosion tanks were invented in 1917, and are in use since then, and have undergone many technological advances since then, they are being used quite commonly and in most of the planes where pressurization is required.