What is the simplest way to explain a sonic boom that is still technically correct? I'm looking for an explanation that preserves technical accuracy while remaining brief. Bonus points for including the double boom.

Try to avoid discussing laminar flow and such.

I will try to explain the principle, briefly and practically as you requested.

Imagine that we have an airplane flying in position 1. It is flying at supersonic speed.

Good, now this airplane moves forward. But it is not the only one moving forward, the perturbation that the airplane produces over the air moves as well, but at a different speed, the speed of sound. In the next figure you will see that the perturbation (or pressure wave) moves less distance than the airplane, which moves from 1 to 2.

The airplane keeps moving to position 3, and at position 2 there are some perturbations (pressure waves) that are also generated and also move as speed of sound. However, I have smartly selected a perturbation that reaches exactly the same point in space that the perturbation (pressure wave) that I selected is reaching as well. So both perturbations meet at the same point.

As you imagine these two are not the only perturbations (pressure waves) that are meeting at this point, there are several points between 1 and 3 that also generate perturbations that join. Actually, this is also not the only point in space where perturbations (pressure waves) join together: there is a line that envelopes the points where the pressure waves meet at each instant. Well, actually a surface in 3D.

OK, if you do the same with a better painting tool than me… you draw the same but also evolving with time like this:

Which is the image in Wikipedia. What is the effect that you experience?

So the orange observer that is located in advance/before the blue lines does not feel any perturbation due to the airplane, nor hear it, as the pressure wave have not reach him yet.

However, the green observer has already felt the airplane passing by and sees how the air is altered by the airplane (imagine that you are close enough).

Finally the brown observer is just in the middle, coming from a region which has not yet experienced the perturbations made by the airplane and finds suddenly a concentration of perturbations (pressure waves) as described above. That is a sudden change in air flow properties and is experienced as a sonic boom.

To give a real end to this, notice that the further you are from the airplane the less strength the pressure waves have when they arrive at the blue line, making the sonic boom less and less intense.


What are the pros and cons of using shielding as opposed to liquid cooling the skin of supersonic aircraft? What are the pros and cons of using shielding as opposed to liquid cooling the skin of supersonic aircraft? This is in light of the fact that most shielding materials used in aircraft, like ceramics, are very heavy. Could a radiator with 100 liters of water possibly weigh less?

I’m not aware of any supersonic aircraft which use ‘shielding’ to protect them from atmospheric friction heating, save for rockets, and a few experimental platforms like the X-15B which used an ablative coating, mostly of cork or rubber, which slowly burnt off during the flight, protecting the aircraft structure underneath.

As for cooling systems, I suspect that the weight of a network of tubes throughout the skin of the aircraft coupled with a heat exchanger and pump to circulate cooling fluid through them was too expensive and complex.  A more efficient method is simply to use the aircraft’s fuel as a heat sink and absorb atmospheric friction heat be slowly warming the fuel in the tanks.  It’s a lighter and simpler solution.

Ceramic protective layers, such as the tiles on the exterior of the space shuttle orbiter can be made fairly light when they are a matrix of nitrogen gas pockets inside of them.  They are quite effective a heat protection on a unit mass basis but can be difficult and expensive to maintain.

Since the 1970s there have been few aircraft programs venturing past Mach 2 so practical applications of heat protection for airframes has been limited.  We will have to see what new developments in military hypersonic vehicles, such as Lockheed’s SR-72 reconnaissance aircraft, are using to address atmospheric heating.


What emergencies or failures should I report to ATC? During a flight, what kinds of emergencies or failures should I report to ATC immediately? For what kind of emergencies or failures am I not required to report them?

What is the general rule? Could I have some examples?

You don't specify a country, so I'll answer for the US and hope that other countries are similar.

14 CFR 91.183(c) is the closest I can find to a "requirement" to report an emergency.  It only directly applies to IFR flights, but I can see no reason why VFR flights shouldn't do the same.

My rule of thumb is that if I wouldn't have taken off had I known about the problem then, it needs to be reported now.  Common examples might be low fuel, rough engine, weather approaching VFR limits or required instrument failure.

Note that while the FAA earned a bad rap for excessive enforcement actions in the 1980s, in the 1990s they adopted the stance that declaring an emergency is a sign of good decision making and risk management, and the stats since then show virtually no enforcement actions after emergencies, so when in doubt, I'd err on the side of declaring.  At worst, there may be a "counseling session" if they later decide that the situation didn't warrant it.


Do wing warmers exist? In moto racing, they use tire warmer to keep tires hot when the tire is not running.

In particular weather conditions (e.g. heavy snow), aircraft require de-icing operation after staying parked for few tenth of minutes.

Do any device to protect the wings and other parts of the aircraft requiring to be free from ice before take off from icing and possibly keeping those parts above icing temperature exist? It could save some deicing time (and de-icing fluid) by reducing the amount of ice accumulated while the aircraft is on the ground.

EDIT: When asking the question, I was thinking of airliners between two legs in winter in Canada (with jetbridge connected, only few tenth of minutes before next scheduled leg, international airport ground support available,... and snow and icing conditions). I don't think the question should be restricited to this case, but it should take it into account.

The standard solution for keeping aircraft parts "above icing temperature" when on the ground is, as was pointed out in the comments, a nice heated hangar.
This has the secondary benefit of keeping all the other components at a warmer temperature where they're more amenable to operating (gyros spin up better, wheel bearings rotate more easily, etc.)

Once outside of a hangar deicing fluid is used, and provides "anti-icing" protection for a specific period (the fluid's "holdover time") - we discussed that in more detail on this question.
Obviously once the aircraft has been exposed to icing conditions or snowfall for a period exceeding that holdover time it must be de-iced again as there is a chance for ice to begin accumulating.

There is also the potential to use the aircraft's ice protection systems (in particular electro-thermal systems such as used in the Boeing 787 which heat the wing in the manner you allude to in your question), though I'm not aware if any systems are currently approved to be used in this way.
One of the key challenges there would be verifying that the wing surface is indeed free of ice, snow, and frost - that's easy to do from the deicing truck (particularly since deicing fluid is tinted so you can see where it has been applied), but on a typical passenger jet the pilots can't see the wing from the cockpit, and poor visibility from blowing snow, fog, etc. may make the visual determination difficult from the cabin as well.


For large commercial aircraft, how are the tires changed? Something at least the size of a 737, all the way up to the A380. How are aircraft tires changed for large commercial aircraft?

Are they actually jacked up like with a car?

In general, the jack is put under the axle of the wheel to be changed. Thus the oleo still carries the weight of the airplane and stays at the same extension, and the jack just has to lift the wheel far enough to be able to get it off the axle. Unlike a car, you don't lift the body off the wheel, you lift the wheel off the ground.


What is the A320's procedure for dual engine failure at very low altitude? What is the procedure for a dual engine failure in an Airbus A320 (perhaps a large bird strike, since these engines only tolerate medium birds) at a very low altitude, shortly after takeoff (approx 1,000 ft)? Additionally, will any procedure realistically save an aircraft in such a scenario?

Every pilot is taught that in an emergency you fly the plane first and everything else is secondary. Aviate, then communicate. At 1,000 ft there's not going to be much you can do other than figure out how to try to get back down in as few pieces as possible. If you have done your pre-flight planning correctly, long before you line up on the runway you have already thought of what you will do if you find yourself in such a scenario, and discussed it with your FO so you are both on the same page.


Can anyone identify the aircraft in the background of this photo? This photo was found in some family belongings and we're trying to date it to identify the family member who may have been in his tri-service imaged. So, can anyone identify the plane in the background please?

It looks like a 1921 Blackburn Dart, British carrier-based torpedo bomber biplane.

(source)


Which models of the Boeing 737 are still in production? The BBC is reporting that the production of 737 aircraft has been reduced from 52 to 42 per month due to a loss of 737-Max orders.  It isn't clear in the report whether the Max is the only 737 being made now.

Does Boeing currently produce any other models of the 737 or is the Max the only one?

The variants in production are:

700
800
900ER
BBJ
C-40
737 AEW&C
P-8
MAX

In bold are the passenger variants. The BBJ is a business jet variant. The first three are 737 Next Generation. As of February 28, 2019, there is a total of 38 unfilled orders for that family (discounting the military P-8). Whereas there are 4,636 unfilled MAX orders as of the same date.


What is the maximum acceptable gap between the jet bridge and airplane door? I noticed on an American Airline flight a gap of about 4 inches between the floor of the bridge and the airplane. Wide enough that if the passenger is not paying attention to the floor the foot may slip in between or at  least get stuck.  The flight attendant said there's always a gap. I've seen a gap but not that wide. Is there a maximum acceptable gap?

In general, there is a gap between the jet bridge (also called Passenger Boarding Bridges or PBB) and the aircraft. This is due to various reasons like,

To prevent damage to the fuselage
The jet bridge may not fit all the aircraft similarly as their fuselage dimensions are different.

According to FAA Advisory Circular 150/5220-21C,

  The PBB should be equipped with sensors to safely prevent
  dangerous contact with the aircraft

The jet bridges at most airports are designed for larger passenger aircraft that are higher off the ground. When a small aircraft utilizes these jet bridges they must use a "ramp" to bridge the gap between the jet bridge and the aircraft.

According to the same Circular,

  The maximum horizontal gap allowable along the path to the aircraft can be no
  greater than 1.25 in (32 mm)

So, I guess that a 4 inch gap is not allowed.


Why isn't the wing span of a certain airliner longer with the longer fuselage versions? The Boeing 737 next generation models (700,800,900ER) all have the same wingspan at 117ft 5 in. Why doesn't the wingspan increase with bigger and longer models? With the 737-900 wouldn't you need a longer wingspan than the 737-700 because of the extra weight you would be carrying and you need more lift to support that extra weight.

There are two main reasons:

It is easier to plug in a fuselage section than a (swept back) wing section. If you notice, one thing that has been kept constant across models is the fuselage cross section. Basically, the manufacturer can make another fuselage cross section and add it to the exiting ones.
If a wing section is added, on the other hand, the aircraft characteristics will change appreciably, in addition to the difficulty in doing so. If you are adding wing section, quite a lot of additional modifications have to be made (like control surfaces, etc.) which will increase cost, eliminating any cost savings the manufacturer could've had by using common parts across models.

The next, perhaps more important reason for this from airline viewpoint is that it will change the aircraft design group. The following table is from the FAA AC 150/5300-13 Airport Design (The ICAO definitions are more or less the same):

Image from FAA AC 150/5300-13 Airport Design
For example, the Boeing 737 and Airbus A320 have wingspans at the limit of the design group III (B737 has 117 ft 10 in while A320 has 117 ft 5 in with sharklets). Any increase will change the ADG, which will restrict the number of airports that the aircraft can be operated from, which the airlines won't like.
For example, when the 747-8 wingspan was increased, the aircraft category was changed (from ICAO category E to F, equivalent to V to VI) and compatibility studies had to be carried out to permit operations in airports were 747-400s are already in operation.
The airlines are perfectly happy to use different operational procedures or sacrifice some other characteristics (like range- for e.g. B737 MAX range drops around 200 miles between MAX 7 and MAX 9; aircraft rarely operate those distances anyway) for having the same aircraft operate in a large number of airports and reduce maintenance burden by having common parts across models.


Why doesn't the aviation industry use SI units? This is a followup to What is the measurement system used in the aviation industry? and related to this question from History.

I can understand the arguments as to why adoption of SI units would not make sense for the general population, but aviation is a specialised business. All professionals are highly trained, and would (should) be well versed in both systems anyways, so the transition would be much simpler from the point of view of human factors. The technology would probably be much harder to shift but again, with more and more displays and documentation going digital in cockpits, this again seems to be a not so huge problem. Maintenance and manufacturing is again fairly specialised and restricted to a smaller number of companies as compared to the general case.

What are the historical factors that lead up to the adoption of imperial units in the industry? Why are they still being used when widely accepted scientific standards exist?

Non-SI is only used for altitude, distance and speed except in US and some other American countries.

Altitude is in feet because 1000 ft happens to be reasonable vertical separation and 1000 ft is easier to calculate with than the corresponding SI figure 300 m. Also the procedures for instrument flying were first developed in the USA using feet.
Distance is in nautical miles because it is related to the unit used for measuring latitude and longitude. 1 nautical mile corresponds to 1 minute latitude (and longitude on the equator), which makes it easier to calculate distances from navigational maps using the grid lines as scale (large area navigation maps need to preserve angles, so they can't have constant scale). If angles were converted to decimal, 1 km would be 1/100 gradian. Alas, angles and time were never converted to decimal.
Speed obviously based on the distance unit in use.

Nevertheless if it was not for the prevalence of US-built planes after WWII and more advanced state of aviation in the USA at that time, we would probably be using metric in Europe too as continental planes before WWII usually had instruments in metric.


Is air to air refuelling possible at "cruising altitude"? When an aircraft uses air to air refuelling are there constraints on the maximum altitude this can take place at?
For instance I'm guessing that a Boeing Globemaster when on a long journey would cruise at 8,000m or higher (Wikipedia says service ceiling is 14,000m / 45,000'). Is there any reason why air-to-air refuelling can't take place at that sort of height?
I'm wondering whether there's something about the lack of air pressure that makes it problematic to transfer fuel or the speeds that must be used for the transfer are not viable at very high altitudes.

The main limitation is that the receiver needs to have enough excess thrust available to be able to precisely maintain formation on the tanker. Near the service ceiling, you typically don't have a lot of excess thrust, so refueling happens somewhat lower than that.  Since fighter generally have more excess thrust at a given altitude than a transport will, they can refuel at somewhat higher altitudes.
Other than wanting to have enough excess thrust available to accelerate forward in the event of a "breakaway" call, tankers don't have particular limitations that I'm aware of, though I've not flown those. (I'd assume being right at the limits of IAS / TAS isn't particularly comfortable for a tanker, so at least something below their service ceiling, but I don't know what that limit is.)
Pumps, rather than differential pressure, move the fuel, so that system is unlikely to drive any altitude constraints.
It's common to hear refueling happening in the mid-20's over the U.S.  How much that's driven by airspace availability vs by performance, I don't know.


Would a "fuel leak" at the gate be a reportable incident? This news story shows video of fuel being expelled from a wingtip of a 767, while at the gate. The engines were apparently running during this event. After the cockpit crew learned of what was going on, the engines were shut down and fire equipment was brought in.

Since this happened on the ground, is this an incident that must be reported to the FAA? The article says that the airline hasn't given any details of the incident.

Perhaps this belongs in another question, but what could have happened here?

There are a couple of places in the regualtions that require FAA or NTSB notification of certain events:

14 CFR 121.703 Service difficulty reports
14 CFR 121.705 Mechanical interruption summary report
49 CFR 830.5 Immediate notification (to the NTSB)

The closest thing to what you described would be the following in 121.703(a):

  (11) A fuel or fuel-dumping system that affects fuel flow or causes hazardous leakage during flight;

Note that it says during flight though, so this wouldn't apply and they wouldn't be required to report it.


How do commercial flights get overbooked? The other week my flight was overbooked and so the airline looked for volunteers to take another flight the following morning. I can't stop thinking about this since the number of seats available and sold should've been known to the airline for quite some time.

Someone claimed that airlines expect a number of passengers to rebook or not show up and thus with purpose overbook sometimes. Can this really be true? Or is it a data consistency thing where availability for ticket booking is prioritized? Or was it perhaps due to an unplanned change of aircraft model? I'm certain there might be other answers and that reasons may vary from occasion to occasion and different airlines, but please enlighten me here!

Airlines have lots of historical data for any given flight segment that they can use to give statistical probabilities of people missing flights due to various factors such as misconnection or just not showing up.  An airline is a profit driven corporation and to maximize revenue they want full airplanes.

If they know for a flight that 5% +/- 2% of passengers fail to board they can pretty safely sell around 103% of the seats to maximize passengers and minimize risk of overboarding.  Statistics are averages, however, and if you sell 103% of your capacity it is entirely possible for a given instance of that flight to be overbooked.

In most cases of overbooking, volunteers can be found that will give up seats if the plane becomes full.  US airlines generally top out at around $800 compensation for volunteers, so if you are willing to do so it can be advantageous to wait for them to get desperate for volunteers.  If no volunteers can be found it is possible to be involuntarily bumped off the flight and you should consult with the specific airline for any rights you may have in that case.


Fine for operating not single-pilot certified jet with one pilot Are there any clear-cut fines for flying an aircraft like a Global 5000 which is not single-pilot certified by a single pilot?

And what would be the probability of being caught and how would they catch you?

According to FAA Order 2150-3B in Appendix B (penalty guidelines) the recommended penalty for "Failure to comply with operating limitation" is a 30 to 90 day license suspension. That is what I would expect would happen in real life if the FAA investigator thought that the pilot had no good reason for operating without a co-pilot. If there was some good reason and the pilot's record was clean otherwise, they might change it to a warning.

I disagree with the other answers that claim it would be quickly reported. Unless an informed passenger noticed it and reported it, I doubt it would get reported by anyone else. The fuel guys have no idea what operating limitations are or who is operating the plane. In many cases, crew and passengers are long gone by the time the fuel truck pulls up. For the fuel truck to deal with just one guy or to fuel an unoccupied aircraft is commonplace. For all the fuel truck guy knows, there could be crew inside the aircraft doing stuff. He has absolutely no idea what the operating situation of the aircraft is.


How do you calculate coefficient of drag at an angle? I am trying to make a simple 2d drag simulator for an object in freefall with an x and y component of wind. If I knew the coefficient of drag on the bottom and side of the object could I use theta to get the approximate CD at an angle?

Probably and oversimplified function but something like:

$CD_{\theta} = CD_{bottom} + (CD_{bottom} - CD_{side})*\frac{\theta}{\pi/2}$

Let's try to derive something. Assume the x-axis points to the right and the y-axis points up. I'm gonna use the subscripts b and s for bottom and side. The incoming velocity vector is decomposed in two components along the axes, so $v_s=v\sin\theta$.

Writing the total drag force as a vector:

$\vec D=\begin{bmatrix}-D_s\\D_b\end{bmatrix}=\begin{bmatrix}-C_{D_s}\frac{1}{2}\rho S v_s^2\\C_{D_b}\frac{1}{2}\rho S v_b^2\end{bmatrix}$

Taking the magnitude of the vector:

$D=C_D\frac{1}{2}\rho S v^2 = \sqrt{{C_{D_s}}^2\left(\frac{1}{2}\rho S\right)^2 v_s^4+ {C_{D_b}}^2\left(\frac{1}{2}\rho S\right)^2 v_b^4}$

This can be further simplified to:
$C_D = \sqrt{{C_{D_s}}^2\sin^4\theta+{C_{D_b}}^2\cos^4\theta}$

I don't think this can be simplified further.

Edit: S in this case is of course just the area of the rectangle, I got confused before


Why do electronics have to be off before engine startup/shut down on a Cessna 172? Can someone explain in simple words why we make sure electronics are off before we start up/ shut down our Cessna172 engine?

You shut the components off to protect the sensitive electronics from electrical surges that may occur during start up of the engine.
Circuit breakers are designed to trip open if too much current in amps is drawn from the power source by the electrical components or a short circuit downstream of the circuit breaker. They do not necessarily protect the components from too much amperage and/or voltage generated and pushed through the circuit breaker from upstream of the circuit breaker.
To be safe, disconnecting the components from the power source via a physical switch that can be opened Is prudent to mitigate the risk.


What is the exhaust port on the tail of jetliners for? Jetliners usually have their engines mounted on the wings, so what is the exhaust port on the tail for?

That's the exhaust port for the APU (Auxillary Power Unit).

The APU is usually used to power on-board electronics when at the gate, or used to start the main engines. They may or may not be running while in-flight.


How do I factor sweep and dihedral in the lift equation? I'm guessing that you add cos(angle of dihedral) to calculate lift loss by dihedral, but I'm not sure how to factor in the lift loss by the wing swept. Could someone help me include lift loss due to dihedral and sweep? Thank you
Note: I mean without factoring it into the lift coefficient.

Snorri Gudmundsson's "General Aviation Aircraft Design"'s equation 9-57 allows you to estimate the lift coefficient curve's linear slope for trapezoidal/swept wings:

Note: the equation is in radians, not degrees

As for the dihedral, Equation 9-33 estimates the impact on angle of attack and Equation 9-35 estimates the resulting lift force:

Let me know if it's not what you wanted, though, I didn't quite get the "without factoring it into the lift coefficient." part - isn't the question's title about how it impacts the lift coefficient?


What was the mass of the Hindenburg? What was the mass of a fully fueled Hindenburg (discounting any cargo other than fuel/gaz) ?

I do mean to exclude any lift provided by buoyancy.

According to airships.net, LZ-129 Hindenburg had a dead weight of 118,000 kg and carried 58,880 kg of fuel. So that comes to 176,880 kg. Here are the full details:

Actual payload from a Hindenburg flight across the South Atlantic using hydrogen; table from airships.net


What makes an airplane fly? How does conventional flight work?

Explain what factors are in play in order to get a plane airborne and stay there.

Four Fundamental Forces

Weight

Weight is the force of gravity acting upon an object's mass.  The mass of an airplane is the sum of all parts that make up the airplane, including fuel and lubricating oils, crew/passengers, and cargo.  In a normal flight attitude, this force acts in the down direction, and it will always act towards the earth's center of gravity.

The airplane's center of gravity, or CG, is the location where the aircraft's mass/weight is evenly distributed in all directions.  Optimally, the CG should be as close as possible to the airplane's center of lift.

Lift

In a normal flight attitude, lift is the force opposing weight, acting in the up direction.  It is produced by the interaction of the wings and fuselage with the surrounding air as the aircraft travels through.

There are three scientific principles at work in the production of lift as it applies to aircraft.

Bernoulli's Principle: As air travels faster, its pressure decreases.  A typical airplane wing is designed like a foil, where the upper surface is comparatively rounder than the lower surface.  Air flows faster and at lower pressure over the top of the wing than air flowing underneath it, actually arriving somewhat earlier at the trailing edge than the air traveling underneath the wing.  This pressure difference accounts for most of the lift, but not all.
The Coandă Effect: Airflow is attracted to nearby surfaces.  Air "hugs" the wing as the wing passes through it.  Additionally, since the airflow over the wing is faster than that below the wing, the net result after the wing has passed through is downward air movement.  As the wing angle relative to the initial airflow (the angle of attack) increases, the trailing airflow is increasingly deflected downward.  (However, too much angle of attack will result in disrupted airflow on the leeward side of the wing, causing the wing to stall, resulting in a loss of lift.)
Newton's Third Law of Motion: A force in one direction caused by an object is countered by an equal force upon that object in the opposite direction.  Air deflected downward off the trailing edge of the wing results in a simultaneous upward force on the wing itself.

Drag

Drag is the force of all friction acting upon the airplane, operating in the backward direction.

Drag comes in many varieties.  Skin friction is the resistance caused by moving a solid object (the plane itself) through the air.  Induced drag occurs at the wingtips, where the high pressure air below the wing flows into the low pressure region above the wing, creating inwardly rotating swirls of air (vortices) behind the airplane.  Additional drag is encountered as an airplane's speed approaches the speed of sound and beyond.

Thrust

Thrust is the force causing the aircraft to move forward through the air.  In most airplanes, thrust is produced by a propeller or jet engine.  This is Newton's Third Law in effect as well: as air is pushed backward, the aircraft moves forward.  This forward motion creates a flow of air over the wing, producing lift.

When an airplane's thrust produced equals its drag, the airplane remains in level flight (barring any pilot control inputs).  When thrust exceeds the drag, the increased airflow over the wing creates additional lift.  When thrust is less than drag, the decreased airflow results in less lift being generated.


Under FAR Part 121, is it mandatory to execute a missed approach when unable to land within the touchdown zone? If the crew of an aircraft operating under FAR Part 121 and conducting an instrument approach determines, as they begin their descent from the MDA/DA/DH, that they will be unable to touchdown within the touchdown zone of the runway is a missed approach mandatory?

This is spelled out in 91.175(c), in particular

  Operation below DA/DH or MDA. Except as provided in paragraph (l) of this section or § 91.176 of this chapter, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH unless

...

  The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing


What should a pilot do when flying in the vicinity of parachute jumpers? There are several airfields nearby where parachuting activities routinely take place. How far away is considered to be a safe distance from the airfield when flying past it en-route to another destination? Are there any particular frequencies to monitor besides the airfield frequency? How would I know parachuting is happening besides any NOTAMs?

Qualification: I worked at a busy drop zone for 10 years.

Just announce yourself on the airfield frequency about 5-10 minutes out and they'll tell you what's going on, including which runway is active and how high they are jumping from - standard altitudes are 3000ft and 12,000ft AGL. At our place it was really simple: left-hand for 34, right-hand for 16. We landed everyone on the east side, and if we are dropping at 3000 all day you can just pop up to 4000 and be completely out of the way.

Under no circumstances make a cross-field pass as that's usually right in the middle of everyone else's landing pattern, and we don't carry radios.

If you are just passing by a 2km buffer is lots - no one is likely to be in the air that far away.

No reply usually means nothing in the air.


What data file format is used by FDRs? For analysing the FDR, data is downloaded to the local computer from the FDR and feed into the Analysing Software. I would like to know what is the file extension of the data stored on the FDR.
I came to the conclusion that it must be csv for storing all the data about Height, longitude, latitude and so on. Then this .csv file is feed into the Analysing Software for analysing. But I am not sure if I am correct.

The builders of an FDR must comply with DO-178C (the older DO-178B may also be of interest here) as per AC 20-141B issued by the FAA. These documents do not specifically define a data format but provide requirements for what it must be resilient against. Here is an interesting research paper that covers some of the topics you are asking about. Since most avionics use ARNIC 429 as their standard of communication some FDRs provide ARNIC 429 inputs to record the avionics instructions at the time. I would assume these inputs dump the bus data right to the storage medium (most likely solid state memory these days). Since the formats the device uses may be proprietary as well as very low level (and memory efficient) I am sure they make software to dump it to other formats. For example L3 makes a full unit for their FDRs.

After some more searching I found what may be the most comprehensive answer from this Boeing summary document

  What Is a Data Frame?

  A flight data recorder (FDR) data frame is the
  order of the words that are transmitted from the digital flight data
  acquisition unit (DFDAU) to the digital FDR (DFDR) each second over
  many seconds (see ARINC 717 for additional information). Most FDR
  system data frames are made up of four subframes within one
  superframe. For a 64-words-per-second (wps) FDR system, a DFDAU will
  output 64 12-bit words to the FDR each second, where each word
  typically contains the value of an analog parameter. The order of the
  words (for example, word number 12 of the 64 words) within a subframe,
  as well as the order of the subframes, define an FDR data frame. This
  order is important to understand in order to decode the data recorded
  in the DFDR.

It should also be noted although they may have all been phased out by now, some older cockpit voice recorders and FDRs recorded on analog mediums and may still be inservice doing so.


Why did Boeing remove the engine chevrons on the 777-X? From the most recent renderings on Boeing's site, it looks like the engine chevrons found on the 787, 747-8 and 737-Max have been removed from the 777-X. Is there a good reason for this?

The 777X is still in development and it will be powered by the General Electric GE9X (also still in development). The current talk is that GE is working on a proprietary method (has been for 20 years) that will make the chevrons obsolete. Namely the use of an oxide-oxide ceramic matrix composite, which they have unveiled already (October 2013) for the smaller Passport 20 engine.

It's a new material that can withstand very high temperatures, make the elaborate cooling methods a thing of the past, and bring back the mixer seen on old jets (since it can withstand the hotter exhaust of the new engines).

(compositesworld.com)

  The oxide-oxide CMC material replaces what would have been graphite-epoxy components in the Passport 20 – namely, the four-piece panel enclosing the engine core and a nine-piece component called a mixer, says Judd Tressler, director of Bombardier programmes for GE.

Mixing produces a more uniform less turbulent stream, turbulent stream is a lost thrust. Mixer aside, using the CMC in the turbine and combustor will save on wasted bleed air and provide better combustion respectively, both save on fuel (noise) for same thrust. With the noise level brought down, there is no need for the drag-adding chevrons.

  Likewise, GE is also looking to expand the use of oxide-oxide CMCs to other commercial turbofan engines – possibly including the GE9X, in similar regions of the engine as the Passport.


Can noise from the passenger cabin be picked up by the cockpit voice recorder? How sensitive is the cockpit voice recorder?  Can it pick up sounds from the passenger cabin?

It depends a bit. One of the microphones for the cockpit voice recorder is an "area microphone," which is designed to record whatever someone standing in the cockpit would hear (the others record from crew headsets and possibly the PA system). This area mic is continuously recording; it does not just turn on when there's more than some threshold of noise. If the noise in the cabin is loud enough to have been heard in the cockpit, it will be heard on that mic. If not, it won't.


How many VHF system can be in MEL for an A340? How many VHF (Voice Communication) system can be in MEL for A340 aircraft?

What are the guidelines to decide that and is it different for aircraft operating under FAR 121, 125 and 135?

There are three VHF systems on A340 aircraft, as described here:

  Item: VHF Systems
  Number Installed: 3
  Number Required for Dispatch: -
  Remarks or Exceptions: Any in excess of those required by FAR may be inoperative provided it is not powered by an Essential Bus and not
  required for emergency procedures.

MMEL is required by FAA:

  A minimum equipment list is required in the United States by the
  Federal Aviation Administration:

  When operating any turbine-powered aircraft such as jets or turboprops.
  When operating under part 135 (Commuter and on-demand operations)
  When operating under part 125 (Non-airline large aircraft operations)

MEL for FAR 121 is discussed here.


What configuration of autothrottle is required when pilots are landing an Airbus? Do Airbus pilots normally disengage the Auto Throttle on a manual landing?

Do pilots (Airbus or Boeing) normally prefer a manual landing to a A/P landing?

Do Airbus pilots normally disengage the Auto Throttle on a manual landing ?

Depends on what you mean.

During the initial part of the approach, no, they do not deactivate the autothrust (not autothrottle, in this case):

But the autothrust is not designed for landing and just before touchdown there is an automatic callout in the cockpit: Retard. Retard. (it can be heard in this video 4 times).

The callout is needed because of the A/THR inner working: while in SPEED mode it can apply any throttle between 0 and the current throttle lever position; i.e., if for example the throttle is at the 80% setting, the autothrust can apply any throttle between 0% and 80% (based on the required airspeed). At the Retard callout the pilots will move the levers to the IDLE position, effectively deactivating the autothrust:


What is the sequence of ATC frequencies for final approach at JFK? Recently I was listening to liveATC.net and tried to understand the sequence of operations for the voice communications.

However, it seems that there are a lot of frequencies for the approach procedure, and I got confused by their names and jobs. For example, the JFK terminal has two approaches (CAMRN 128.125 and ROBER 125.700), one Final approach (Final 132.400) that I don't really know what it is, and one two Tower frequencies (123.9 and 119.1). So the question is, (1) I think the tower will be covering the final approach but what is the "Final 132.400" frequency for? (2) when and where (e.g., distance from the airport) the aircraft will be handed over from one frequency to another? Does the tower frequencies only cover up to 5 nautical miles from the airport?

"Final 132.400" is final vectors (sometimes called 'final sector'), an intermediate stage between approach and tower. Whichever route an aircraft arrives, the relevant approach controller will hand it over to 'Final' for final vectoring onto the relevant final approach. Once the aircraft is established on that final approach (with proper spacing), it is handed over to tower.

I would recommend listening to Final first, whilst simultaneously watching a certain well-known flight radar website. It may take a few minutes to get 'in sync' with what's going on there, but you should soon get the gist of it. Then listen-in on the approach sector(s).


How much does a typical private pilot pay for a non-training flight? I realize one can expect to pay upwards of $10k to receive their Private Pilot certificate. Understood entirely.

However, my question revolves around the everyday expense of your average Joe pilot who wants to go out to the local airport and take up, let’s say, a Piper Arrow for a nice relaxing local flight around the area (we will say Kansas City Wheeler Downtown in this case). As a hobby. Maybe take a couple friends or family if they chipped in.

How much money could an average Joe pilot look to spend per flight, taking up a small craft, just because?

It depends on your location. It will vary based on cost for tie-down or hangar space, fuel costs, maintenance costs, etc. In order to compare apples to apples, let’s  boil it down to a wet-Hobbs-rate.

Wet-Hobbs-Rate is an all-in rate (minus tax) for the use of the aircraft to include everything except away-airport landing fees where applicable. That includes fuel, oil, home-airport landing fees, etc. Wet-Hobbs-Rate is charged on the time that the engine is actually running. Although some Hobbs timers run off of the time that the electrical system is on, most run off the oil pressure sensor of the engine.

Here are some real world examples of the Hobbs Rate costs of rental aircraft in my area.

Cessna 172S with G1000 and autopilot - $175/hr wet-Hobbs.
SportCruiser with Dynon and autopilot - $125/hr wet-Hobbs.
Piper Apache - $250-300/hr wet-Hobbs.
Piper Cherokee - $145-155/hr wet-Hobbs.
Piper Comanche - $225/hr wet-Hobbs.
Cessna 150/152 - $100-125/hr wet-Hobbs.
Cessna 172E-N - $125-175/hr wet-Hobbs.
Robinson R22 - $250-300/hr wet-Hobbs.
Beechcraft Duchess - $250-500/hr wet-Hobbs.
Piper Aztec - $400-600/hr wet-Hobbs.

Wet-Tach-Rate is another way to measure aircraft cost. Wet-Tach-Rate is similar to wet-Hobbs-rate. Except that wet-tach-rate is more like an odometer that calculates time by the amount of engine revolutions. The less RPMs at which you run the engine, the slower the clock moves. More RPMs mean more time calculated. Typically, this means that you can cut your costs down significantly. Your calculated time can be around 75% of your actual time running the engine or less with good engine management.

Most of the flying clubs in my area use Tach Rate as the basis of their costs. In order to rent aircraft from them, you have to be a member of the club. Membership ranges from 30 to 100 dollars a month.

Here are some real world examples of the Tach Rate costs of rental aircraft in my area.

Cessna 150/152 - $50-100/hr wet-tach.
Piper Archer - $75-$150/hr wet-tach.
Piper Cherokee - $50-150/hr wet-tach.
Piper Arrow - $125-150/hr wet-tach.
Cessna 172 - $50-150/hr wet-tach.
Cessna 182 - $75-175/hr wet-tach.
Piper Cherokee Six - $75-150/hr wet-tach.
Beechcraft Bonanza - $150-250/hr wet-tach.

One thing to consider when deciding to join a club or not is their overnight policies. Most rental airplanes are run several hours a day, every day. They do not want to have their airplanes sitting on the ground. During slow season you can rent overnight as long as you pay for a minimum number of hours Hobbs-time each day usually starting at three hours each day.

Most clubs have enough downtime for their aircraft that they will allow you to have the their airplanes for multiple days. Their per day hourly minimum can be as low as one or two hours per day. If you time it right, you can be the last person to sign the plane out after hours on day one. Fly the plane the club’s minimum number of hours the next day. Then sign the plane in the following day before working hours. That would conceivably be 40 hours of use with only a one day minimum to meet.

Something else to consider is that you will have to factor in a minimum of 30 minutes of non-flying time each flight for which you will have to pay. That will be engine or electrical time doing tasks like:

Starting your engine
Getting your ATIS and clearance
Taxiing to the runway of use
Run-up of the engine
Taxiing back to the hangar after landing
Shutting down the engine

You will pay less in Tach-time than you will in Hobbs-time because the engine RPMs will be averaging a much slower rate.

As an aircraft owner, your variable hourly operating costs could be similar to the lower end of the club rates. This would include your fuel, oil, maintenance and other costs. In addition to that, You would have fixed costs for the aircraft of:

Hangar space ($200+/month);
Or Tie-downs ($50-100/month);
Inspections ($1000-2000/year);
Major Overhauls ($10k-$20k at any time)

Lastly, insurance for a renter can be anywhere between 100 and 500 dollars a year depending on coverage. And, more for helicopters. Electronic Flight Bag subscriptions range from 100 to 500 dollars a year. You will need at least two hours of flight instruction every two years to meet flight currency requirements. And, you will need an FAA AME flight physical (time varies by age and privileges desired) unless you want to fly under Basic Med Class III reform or Light Sport Aircraft only. A Medical Certificate will cost around 100 dollars.

You can cut costs dramatically by flying gliders and ultralights. The glider club in my area has a 30 dollar per month membership with its lowest cost glider renting for 10 dollars an hour. You will have to research Ultralight costs yourself.


Does the cannon of the A-10 deliver more thrust than the engines do? The A-10 Warthog is built around a 30 mm cannon that fires depleted uranium rounds, 65 of them per second.

I heard from someone who worked on the flight controls a long time ago, that when the cannon is fired it provides more thrust than the engines. Is this true? What are the effects of this?

Update

The original question was answered by @Gypaets: thrust of the cannon is comparable to thrust of one engine. The wikipedia article on the GAU-8 cannon mentions that effects of thrust of the cannon are well accounted for by placing the firing barrel at the centre line of the air frame. Ron Gordon's answer reveals some interesting side effects from spooling the cannon up and down when operating - the aircraft bucks. It's a big cannon.

Image source

The A-10 uses a GAU-8/A. According to Wikipedia:

  The average recoil force of the GAU-8/A is 10,000 pounds-force (45
  kN), which is slightly more than the output of one of the A-10's two
  TF34 engines (9,065 lbf / 40.3 kN each). While this recoil force is
  significant, in practice a cannon fire burst only slows the aircraft a
  few miles per hour in level flight

Your statement is only partially true: two engines have more, but one engine has about 10% less thrust than the recoil force of the cannon.

The recoil force of MiG-27's GSh-6-30 was even larger: 60 kN.

As pointed out in a comment by @tj1000 the recoil of the GAU-8/A wasn't such a problem as the smoke from its shell propellant. If you're looking for the video mentioned in the comment (test pilot ejection due to engines out after firing ammunition with a new flash suppressor) this is the link (12:08-13:59).


Can a pilot take off at his own discretion? [I'm referring to an ATC conversation during the military coup in Turkey with an MD83 EP-TAS (ATA TBZ5402).
Can a pilot take off at his own discretion?

To summarize the link: a pilot is begging the tower to be allowed to depart and repeatedly denied due to what the controller describes as a "military operation" or "people in the tower". The pilot indicates the situation is dangerous and even reports seeing gunfire on the ground at one point. The pilot then repeatedly asks for permission to depart "at my own discretion". In the end, it appears that he does not depart.

Would there be any consequences for the pilot? Is he allowed to take off if the situation on the ground is more threatening than in the air? Are there any international regulations in this case?

Generally speaking a pilot can (physically) do whatever they want: ATC doesn't have a remote control. If you push the throttles forward and pull back on the yoke you will take off, no matter what mean things the folks in the tower are shouting at you over the radio.

Practically speaking doing so can get you into trouble: You may be met in the air by planes with more guns than yours, or projectiles from the ground may be fired in such a way as to intersect your flight path ("You might get intercepted or shot down by very annoyed military types.") -- even if that doesn't happen you may face administrative or civil action for ignoring ATC instructions in the jurisdiction where you did this.

I'm not sure what the ATC policies are in Turkey, but the US FAA has a phrase specifically for this: UNABLE TO ISSUE DEPARTURE CLEARANCE. DEPARTURE WILL BE AT YOUR OWN RISK. (you'll find it in JO 7110.65W, on page 3-3-1).

The emergency authority of a pilot is also recognized worldwide (at least in ICAO states - you'll find it in Annex 2, 2.3.1, conveniently quoted on Wikipedia), so if the pilot in command deems that remaining on the ground constitutes an emergency they can violate regulations to the extent necessary to meet the emergency (i.e. forego getting a clearance and take off even if ATC is saying not to).


Was this bumpy Boeing 777-300 landing caused by windshear? I was watching a Video of a Korean Air 777-300 landing at Narita International Airport (IATA: NRT). The Comments on the YouTube Video are again unprofessional.

I want to ask here, what caused this very bumpy landing?
Some comments state this was a wind-shear, is that correct?
Is required by rule to make a maintenance check after such a landing?

Some comments state this was a wind-shear, is that correct?

Looking at the windsock which appears in the right bottom corner at 0:12 and remains visible for about 4 seconds, you can tell there is little wind, not what you would associate with wind shear.

  I want to ask here, what caused this very bumpy landing?

It is always hard to judge the actions of the pilots without knowing what was going on in the cockpit, and what control inputs were given. But by looking at the position of the control surfaces I am fairly sure that this is a case of Pilot Induced Oscillations. I.e. the pilot tries to correct a disturbance but his efforts cause the aircraft to overshoot to the other side of the desired attitude, leading to an increasing series of corrections in opposite directions.

It seems to me that the flare is a tad early, causing the aircraft to float over the runway. At the same time several cycles of alternating roll inputs are given, causing increasing oscillations. I could distinguish at least 3 full cycles before touchdown. Every cycle is a bit stronger than the previous one.

The last right-wind-down command before touchdown is strong enough for all the starboard spoiler to deflect. This, combined with the low speed due to early flare causes the right wing to loose most of its lift. The aircraft touches down hard on its right gear first. At the moment of touchdown, the roll command is strongly left wing down as can be seen from the spoiler extension on the left wing. With almost no lift on the left side and standing on the right gear, the aircraft rapidly rolls to the left.

The left roll is then counteracted again with right ailerons up but too late; the right gear lifts of again. When then right gear touches for the second time, still a right wing down command is given, causing the right roll to continue.

This process of chasing the roll is continued and mixed with similar behaviour in pitch.

Command right wing down, slight spoiler deflection

Command left wing down, slight spoiler deflection

Command right wing down, slight spoiler deflection

Command left wing down, slight spoiler deflection

Command right wing down, right spoiler deflection.

Touch right gear down,  left wing down, left spoiler deflection. This causes a rapid left roll.

Touch left gear down, right gear looses contact, right side spoiler deflection. This causes a rapid right roll.

Halfway the roll to the right, all main wheels are on the ground. Instead of damping the roll by left deflection, a right roll is still commanded.

This continues down the runway, also combined with pitch oscillations. Every time the pilot corrects, he is essentially too late to damp the oscillations.

  Is required by rule to make a maintenance check after such a landing?

If a hard landing is made, the aircraft will detect that by itself and will send a report to maintenance if a hard landing check is required. See this published patent application.


Is it legal to drop/shoot paintballs from an airplane? My brother and I are big into paint balling and aviation, and we've often wondered if it would be legal/possible to use an aircraft in a large scale paintball match?  Mainly to drop paintballs, or perhaps even bags of paint, onto participants to simulate air support.

A few questions then, to cover this fully:

Is it even legal to begin with?
Assuming it is legal, what is the
recommended way to do this?  Plane size? Configuration considerations?
Lastly, is it legal to shoot back at the
plane?  Or is that just wildly unsafe?

I know, it's kind of a broader question, but I was hoping to get the entire answer in one place for ease of access to the answer.

Greg covered this very well in his answer.
The "miss by a surprising distance" bit is especially important here in the context of FAR 91.15 -- you would have to have a VERY large-scale paintball match for this to be viable, and some specific rules of engagement for the aircraft (always firing "into" the arena area as you don't want to miss and send a paintball into someone's car or kitchen window as that would get some distinctly unfriendly attention from the FAA, and probably the local constabulary).

Operationally, if we're doing this with an airplane high-wing aircraft would obviously be the order of the day (Cessna 152), with a pilot (whose sole job is to fly the airplane) and a gunner/bombardier (who is responsible for hitting targets on the ground). You really don't want the pilot distracted by the game.
Your weapons guy is going to have to be able to account for the aircraft's speed and altitude in targeting (which is pretty hard with an adjusted scope/bomb sight - doing it without one is going to be tough).

As for shooting back at the plane, if we're talking about a "regular" airplane (something manned, like a Cessna 152) I would advise against allowing players to target the plane. At safe operating altitudes they're unlikely to hit the plane, but if they do aircraft skin is very thin & dent repair is very expensive: If someone is lucky enough to score a hit it could be a costly "death".

Now, as a paintball player myself I will tell you that I find the idea of death from above when I can't shoot back at the aircraft to be rather unsporting!

What I would suggest instead is to deploy quadcopter drones. These can be had pretty cheaply and with some basic electronics skills they can be modified to carry/drop paint grenades (think Squadbusters) on your enemy.
If you want to get really fancy add a GoPro or some other remote camera to help with targeting, or add a paintball gun to the thing like these folks did.

The FAA's rules for "unmanned aerial systems" are still being written, but if you keep to the general rules for model aircraft (below 400 feet, and not charging anyone money for the drone's services) I think you'll be OK (when the final rules come out you'll have to review your drone for compliance).
As a bonus if the drone isn't too large and you have an indoor arena with liberally-minded management you may even be able to use your drone during indoor games.

(Kill rules for the drone and its operator are left as an exercise for the players.)


How do I calculate density altitude by hand? I've seen the following equation for calculating density altitude on a dozen websites:

[ (OAT - ISA) * 120 ] + Pressure Altitude.

But it was never very clear what the units of measurement are supposed to be.

Question 1: Are OAT and ISA in Celsius?

Question 2:
So for example, if current true altitude is 6000 feet above sea level then:
ISA = 15 + (-2C * 6) because for each 1000 feet we lose 2 Celsius. Does this mean to make this calculation I just substitute field elevation for true altitude? So ISA is equal to 3.

If the outside air temp is 10C, then we have 10 - 3 = 7 multiplied by 120 = 840.

Question 3:
Pressure altitude confuses me a little bit, I read that it is the altimeter reading when the Kollsman window is set to 29.92" hg. So let's say I'm sitting in my airplane on the runway, I set my altimeter to 29.92 and it reads 6200 feet. In this case my final equation would be: 840 + 6200 = 7040 feet density altitude.

What happens if I don't have access to my plane's barometer, is there another way to calculate pressure altitude?

Question 4:
How does this all change if I'm trying to figure out density altitude while flying the plane? OAT is the air around the plane. What about ISA? Do I calculate pressure altitude by resetting my Kollsman window in the middle of the flight to 29.92" hg? Is it normal to try to figure out density altitude while up in the air?

Question 5:
Does this entire equation change or become meaningless above a certain altitude threshold, such as 10,000 feet above sea level?

Density altitude is a value that directly correlates aircraft performance to atmospheric conditions.

  Density altitude is pressure altitude corrected for non-standard temperature.

Almost all temperatures in aviation is Celsius including this formula.

You will notice that all aircraft performance calculations require pressure altitude and temperature and in effect is calculating density altitude.

To calculate pressure altitude you will calculate
$$ \left( 29.92 - \frac{\mathrm{current~altimeter~setting}}{\mathrm{inHg}} \right) \cdot 1000 \, \mathrm{ft} + \mathrm{current~field~elevation}$$

For example if the current altimeter is $30.12 \, \mathrm{inHg}$ and you are $ 6,400 \, \mathrm{ft} $ MSL, the pressure altitude is
$$ \left( 29.92 - 30.12 \right) \cdot 1000 \, \mathrm{ft} + 6,400 \, \mathrm{ft} = 6,200 \, \mathrm{ft}$$
pressure altitude.

Your formula to calculate density altitude is correct but is never required to be known to calculate aircraft performance as the "calculation" is being done in the background of the performance charts.

To calculate density altitude, you must first calculate pressure altitude and then apply it to your formula.

If you are at 10°C at $6400 \, \mathrm{ft}$ MSL with an altimeter setting of $30.12 \, \mathrm{inHg}$:
$$(10-2.6) \cdot 120 \, \mathrm{ft} + 6,200 \, \mathrm{ft} = 7,088 \, \mathrm{ft}$$ density altitude

I have never calculated density altitude once I got into the air.  I have calculated density altitude while determining if its a good idea to takeoff.  In higher elevation airports on a warm day, the airplane may not get out of ground effect due to the density altitude.

The formula for density altitude above is correct for any altitude, albeit a more simplistic formula.  Humidity also has an effect of density altitude and there are formulas that take humidity into account when calculating density altitude.


What is the metal plating on the back of the A-6 for? What is the perforated metal plating on the back of the A-6/EA-6 for?

Source: Wikipedia

It is a speed brake, a plate hinged at the forward edge which can be hydraulically extended to increase aerodynamic drag. It had been used on early models, and the speed brakes were mostly replaced by firmly riveted plates later, because flight test revealed handling problems. The wake of the speed brake disturbed the flow around the horizontal tail especially during approach and landing. They were replaced by wing tip brakes, and from the 304th aircraft on were completely omitted.

See here for further details.

Below is a picture of an A-6 with the wing tip speed brakes (circled red) extended:

(Source: navy.mil)


Is moving payload masses during flight to restore mass centering, or to avoid having to trim, a useful approach? Note: Title was previously "Movable big missiles/bombs/tanks under fighter jets: conceivable?"

Example: lateral weight distribution in fighters

Fighter-bombers jets are small and lightweight in proportion to the under wings load (including payload but also external tanks) they carry which means most stuff can't be placed right below the body of plane and heavy payload (very big missiles and bombs) ends up relatively very far from the center of mass.

(Although some designs have a bay to hide payload, the bay is small compared to the maximum possible load of other fighter and they still have the option to carry under the wings.)

So here is a (possibly unworkable) idea:

Is it conceivable to mount heavy load on movable arms that can move weight left or right to adjust mass centering:

at under wing load installing time?
during fight?

The heavy load, including external tanks, would be automatically moved to preserve mass balance.

Example: longitudinal balance for cargo planes

The idea here is to move the fret of a cargo plane in the longitudinal axis, to adjust balance and minimize trim.

While I don't know of any airplanes that move load in flight for CG purposes, there are definitely airplanes that have various fuel tanks that can be used to trim the airplane during flight. Some of these systems will move the CG aft after takeoff for better performance in cruise, and then move the CG forward before landing for a more stable approach and landing.


Has there ever been a successful 'bail-out' during an airliner test flight? Has there ever been an instance of a crew having to 'bail out' during a test flight of a commercial style aircraft (I.e Airbus, Boeing etc)?

I know that test flight crews often wear orange suits, parachutes during preliminary flight in case there is an issue.

In this experiment to observe a deliberate airliner crash

http://en.wikipedia.org/wiki/2012_Boeing_727_crash_experiment

the crew exited in flight down the tail stairs...


What does the phrase "electrically controlled and pneumatically operated" mean? While studying the air-conditioning of the A320 from the FCOM I came across the information that the pack flow valves are "electrically controlled and pneumatically operated". I can not understand what it means.

It means that an electrical switching signal is used to drive the pneumatic or air valves in the air conditioning system.

Think about it like this: We want to use our garden hose to push open a cat flap but we want electricity to control the pressure in the hose. We could apply an electrical signal to a motor which opens the tap and lets water come out. This water pressure could then be used to control how wide the cat flap opens. The only difference in your airbus situation is that it isn't water (which would be hydraulics), it's air that comes out of the tap (Pneumatic means air). So this system would be called an electrically controlled, hydraulically operated system. Substitute the water in the hose for compressed air and you have your answer.

The way electrically operated valves work is generally based on the principles of electromagnetic force by utilising  some sort of coil. This setup turns a piece of wire into an electromagnet, which is attached to or interacts with the movable part of the valve. This then ports air pressure to one side or the other, to balance open the valve to a set amount in proportion to the original electrical signal.

So we use electricity >>> this changes the balance of air >>> the air opens or closes the valve.

The valve can be fully open, fully closed or anywhere in between.


What systems, in the cabin ceiling, when damaged could cause the masks to deploy? I feel guilty when I chuckle at this incident.

Does anyone know what system(s) might be contained in the cabin ceiling which when struck by a high speed champagne cork could lead to deployment of the passenger oxygen masks?

The scenario where ALL the masks drop, which the article somewhat implies but doesn't outright say, sounds fanciful to me.  A cork striking an individual mask unit might cause that individual unit to drop its masks (so if the unit was for one side of a 3/3 seating arrangement, the 4 masks it contains would all drop), but the masks in Boeing airplanes at least are electrically activated, so physical damage to the cabin ceiling seems unlikely to cause a mass deployment.

Now, if the cork happened to hit the switch in the COCKPIT, then perhaps all the masks could deploy then.  Normally the masks drop only when the system senses a cabin altitude in the range of 15,000' MSL, but there is a switch the pilot can use to drop the masks if you get the rapid depressurization up high and the automatic system failed.  Typically this switch is safety wired shut for obvious reasons, but if some operator didn't use safety wire and the cork hit the switch JUST RIGHT, then this scenario might work.  (We're talking a one-in-a-million shot here, but it might be possible.)

There are stories of hard landing sometimes dropping one or a few rows of masks -- this happens when the latch on the container isn't quite as secure as it is supposed to be, and as long as nobody reaches up & activates the flow of O2, it's a pretty simple maintenance procedure to repack and restow the masks.  (If somebody DOES pull down & activate the O2, then the oxygen generator canister for that unit has to be replaced, and that's a more lengthy process -- especially if the generator isn't on hand at that airport!)  So a physical shock from a flying champagne cork being enough to drop the masks from ONE unit isn't too far fetched, but unless you're talking about a really old design where the signal to all units went out mechanically instead of electrically, I have a hard time believing this scenario. (Please note, I'm not claiming that even in the oldest designs a mechanical linkage connecting all the units would necessarily be present -- just that modern designs are NOT mechanical until you get to the latch on an individual unit.)

And noting that the plane isn't allowed to fly with masks hanging sounds right... but 7 hours just for repacking one unit seems rather lengthy.  I'd suspect that the paper took the facts of ONE unit deploying, and artfully crafted it to sound like lots or all of the masks dropped.  Sensational writing?


Is the EASA RT License valid internationally? I am planning to get a PPL license in Czech. As part of the PPL license I should get a RT License (in English, not Czech).
Is that license is international? I mean, if I want to rent an Airplane in another country in Europe (like Hungary), should I get Hungarian RT license? or my Czech RT license is valid for all EASA countries?
In addition, is the PPL EASA license (with valid RT license) sufficient to rent aircraft in Europe?
Or do I have to pass some test for each country?

The answer saying that "Each country has their own governing rules regarding qualification for Private Pilot Licenses" is of course wrong. The OP has specifically asked about EASA. The privileges of an EASA licence may be exercised by the holder in any EASA member country by virtue of § 11.1 of the Basic Regulation – Regulation (EU) 2018/1139.

if I want to rent an Airplane in another country in Europe (like Hungary), should I get Hungarian RT license? or my Czech RT license is valid for all EASA countries?

The latter.

is the PPL EASA license (with valid RT license) sufficient to rent aircraft in Europe? Or do I have to pass some test for each country?

The former (don't forget about the medical!). Of course, nobody in their right mind are going to let you fly their aircraft without having you checked out first, but that's a club / school / owner policy not something State-mandated.
In any event, ask your flying school in the first instance and the CAA in the second. Only a complete fool would rely on random stuff seen on the internet!


Do I need to be acting PIC when doing takeoffs and landings for my 90-day currency? The 90-day currency requirement means that to act as PIC of an aircraft carrying passengers you must complete the three takeoffs and landings. Does this include an aircraft where you are logging PIC time but not the acting PIC?

Could you technically then log the landings/TO's if you've blown your 90 days if you have a friend as acting PIC? The FAR also states this is required if you're operating an aircraft certified for two or more crew operations. Does this mean even without passengers, if you are a required crew member on a two crew ship, you need the 90 day currency?

The regulation does not address whether or not you have to be PIC to establish currency.  It instead states that you have to do "three takeoffs and three landings as the sole manipulator of the controls" and in the "same category, class, and type (if a type rating is required)":

  Except as provided in paragraph (e) of this section, no person may act
  as a pilot in command of an aircraft carrying passengers or of an
  aircraft certificated for more than one pilot flight crewmember unless
  that person has made at least three takeoffs and three landings within
  the preceding 90 days, and -

  (i) The person acted as the sole manipulator of the flight controls;
  and

  (ii) The required takeoffs and landings were performed in an aircraft
  of the same category, class, and type (if a type rating is required),
  and, if the aircraft to be flown is an airplane with a tailwheel, the
  takeoffs and landings must have been made to a full stop in an
  airplane with a tailwheel.


Is there a radio frequency that is sure to be quiet? When flying in beautiful VFR weather, I often search for a quiet frequency so I don't listen to a lot of chatter.

Guard (121.5) sometimes works, but sometimes has "Guard"-ians on it.
("Yer on gaaard")

Many frequencies end up being a distant CTAF that have garbled transmissions.

Short of turning my radio completely off, is there a frequency within the US (or even an ARTCC) that will always be silent, but can easily swap back to a working frequency?

Some have suggested just turning the volume down and back up.
That is functional, but not quite as easy as pressing the XFER button, since the volume may be too loud or too soft after adjusting it.

I like the idea of trying the low-power frequencies that I'm less likely to hear; but I am ultimately hoping to learn of a frequency marked "do not transmit on".

From the document referenced by mongo,

  Frequencies (MHz): 121.425 - 121.475Use: Band Protection for 121.5
  Frequencies (MHz): 121.525 - 121.575Use: Band Protection for 121.5

It seems that these frequencies exist only to be silent space around Guard/121.5.

No one should transmit on them, and you should probably hear total silence there.

I think these are an excellent place to look for quiet space.


How do I convert true heading to magnetic heading or the other way around? Is there a method by which the conversions can be achieved on the fly by using a mathematical formulae and not using specific area charts?

Short answer: Yes there is, but it doesn't always work, it involves a LOT of math, and you're probably not gonna like it.

Medium Answer: Most flight planning software incorporates a True/Magnetic heading conversion system (ForeFlight for example).
Using an electronic flight planning tool like this is probably your best bet if you want an on-the-fly determination made for you.

Long answer:
International Association of Geomagnetism and Aeronomy (now there's a mouthful!) periodically publishes what's known as "International Geomagnetic Reference Field (IGRF)".  It's a mathematical model of Earth's geomagnetic field, which you can work from some exceptionally complicated trigonometry (or just look up the coefficients in a table which is what most sane folks do).
If reading text like "Mathematically, the IGRF model consists of the Gauss coefficients which define a spherical harmonic expansion of the magnetic scalar potential." makes your eyes roll back into your head (I was a math nerd and it makes MINE roll back!) using the model is probably not for you.
If your eyes are still focused on this text however, click on through to that link way back at the top of my answer and you'll be rewarded with a formula you can plug into your trusty calculator.


Does the Airbus A320 or Boeing 737 have better resolution screens in the cockpit? I know this is kind of a specific question so if nobody knows it's fine. Does anybody know if the Airbus A320 or Boeing 737 has better resolution cockpit screens like PFD and ND? ( For the Airbus A320 I also mean the new screens like some A320 have more circular cockpit screens and the new ones are more square and look more defined

Here's the new screens, well it's not new but it's better

Here is the more circular screen

B737NG DUs (Display Unit) have a resolution of 1152x1152, as per the maintenance manual. The display size is 18x18cm, making it around 163 PPI.

I have found a source that claims A320 PFD has a resolution of 768x768 and the display size is 158mmx158mm, making it around 123.4 PPI.

I imagine B737 MAX is using the same 15.1" display as the B787. In this case, the resolution is 1400x1050, and the PPI would be around 116 PPI.


What does mean 'GS' in RNAV (GPS) Chart? Hello.

I have a question about 'GS' in my RNAV(GPS) Approach Chart.

Usually, (Including PAPI, VASI whatever...) Normal Glide Descent Angle means 3.00 degree.

but some 'GS' says 3.07, 3.04, 3.09 even 7.75 in KSBS airport. (Picture above)

In this case, What does mean 'GS' exact ?

As far as I know, it's 'GS' means draw an imaginary line extend from Touch-Down Foot marker (1,000ft marker) to FAF for Non-Precision Approach.
(Even do I Step-Down Approach ? I also wonder.)

but someone says, it's a line extend from 1,000ft marker to VDP,

and another one says, it's a Normal Descent Rate when PAPI indicates 2white-2red. (Exact Center-line of PAPI Glide Slope. I know approximately 0.4 of marginal has exist on PAPI for 2White-2Red (2.81-3.19)

If the last one's word is true, an airport have a 7.75 GS, like a KSBS Airport, then the PAPI of KSBS airport indicates 2white-2red when I follow the 7.75 GS ?

Waiting for your answers. Thank you.

BEST

Are you asking about the specific term 'GS'?  I don't see that on the chart in your question.

The angle symbol and the 7.75 are explained as a Visual Descent Angle in the Chart Users Guide.

  A VDA and TCH may be published on non-precision approaches. For Copter approach procedures, a Heliport Crossing
  Height (HCH) will be depicted in place of the TCH. The VDA is strictly advisory and provides a means to establish a stabilized descent to the MDA.

So it is an aid for maintaining a stabilized approach that should put you at the MDA at the correct point.

  If the last one's word is true, an airport have a 7.75 GS, like a KSBS Airport, then the PAPI of KSBS airport indicates 2white-2red when I follow the 7.75 GS ?

No.  In fact it's called out on the chart: "VGSI (Visual Glide Slope Indicator) and descent angles not coincident".  So they do not match.  You can look in the chart supplement for the airport and see what the actual angle is.  For KSBS runway 32, the Glide Angle (GA) is 4 degrees.

  RWY 14–32: H4452X100 (ASPH–GRVD) S–50, D–60 HIRL

  RWY 14: Rgt tfc.

  RWY 32: REIL. PAPI(P2L)—GA 4.0º TCH 40´. Thld dsplcd 600´.


Where do I place pressure ports on an airfoil? What are the criteria for placing pressure ports, i.e. is there any procedure for determining where to locate them? I have a NACA 0012 airfoil with a chord length of 150mm and I want to add pressure ports to it. How should I proceed?

Depends on what you want to measure.

Normally, pressure ports are bunched close to the leading edge to give sufficient resolution of the steep pressure gradients (which result from the high local curvature). So scale the local pressure port distance with local curvature.

But maybe you want to have a high resolution of the beginning flow separation near the trailing edge? Then you need to place the pressure ports close to the trailing edge.

Or maybe you want to study the laminar to turbulent boundary layer transition at mid-chord? Then any pressure ports in the laminar region will cause a local transition and show a 15° cone of turbulent flow which will mess up all other measurement behind them. Now you need to restrict the number of ports in the forward part of the airfoil and need to give them enough lateral separation to get what you want.


How was the 747 in the China Airlines Flight 006 affected by the wing damage it sustained? The passengers on China Airlines Flight 006 had a very memorable flight when the crew lost control and the aeroplane went into a high-speed dive.

Various pieces of the aircraft were torn off. The plane was repaired and returned to service, with its wings bent permanently upwards by 5cm.

How did this permanent damage affect it afterwards? Did it fly or perform noticeably differently? Did it require a special inspection regime for the rest of its operating life?

As you know the aircraft manufacturers keep tracking their aircrafts because any accident of these would damage their reputation. All automatic data transmissions are traced by the manufacturer whether the operator has a tracking contract with the manufacturer or not. Should it had an asymmetrical behavior causing any dangerous consequences the manufacturer would have made what is necessary to ground it.

In the website you refer to, it is written « After repairs were made to the plane, it returned to service on 25 April 1985. It continued in service for nearly 12 years »:

I may assume this aircraft has landed in Europe, or in USA, should it have been dangerous, it would have been grounded.

As far economical performances are concerned, that’s secondary compared to safety, it is the operator decision to evaluate to make it fly or not if it becomes less economical.


What does "share a type rating" mean? The Boeing 787 shares its type rating with the 777, but what exactly does that imply?

Surely it cannot mean any 777 pilot can just jump into a 787 and fly it. While being similar, the 787 has some different systems, most notably bleed-less engines.

I assume it means something like a reduced course to switch to the 787 instead of a full type rating. But then again, Airbus advertises how easy it is to switch between different Airbus aircraft (see "Benefits for Pilots" section in https://www.airbus.com/aircraft/passenger-aircraft/commonality.html), so shouldn't they all share the same type rating then?

So what are the limits for "same type rating"?

If two aircraft (let's call them Aircraft A and Aircraft B, since this isn't specific to 777/787) share a type rating (technically known as a "common type rating"), it means that, if a pilot is already certified to fly Aircraft A, and wants to get certified to fly Aircraft B, they only have to take a course on the parts of Aircraft B that are different from Aircraft A.  If Aircraft A and Aircraft B didn't have a common type rating, an Aircraft A pilot wanting a certification for Aircraft B would have to train for every part of Aircraft B, even if many, or even most, of those parts were exactly the same as on Aircraft A.

As you can imagine, getting two (or more) aircraft types a common type rating can save quite a lot of training time and money for pilots switching from one to the other.

Grossly-oversimplified example

Our aircraft:

Aircraft A is composed of parts A, B, C, D, E, and F.
Aircraft B is composed of parts A, B, C, G, E, H, and I.

Our pilots:

Grace is certified to fly Aircraft A.  She wants to become certified to fly Aircraft B.
Tedd is certified to fly Aircraft B.  He wants to become certified to fly Aircraft A.

Scenario 1: Aircraft A and Aircraft B do not have a common type rating

To gain certification for Aircraft B, Grace has to take courses on parts A, B, C, G, E, H, and I.
To gain certification for Aircraft A, Tedd has to take courses on parts A, B, C, D, E, and F.

Scenario 2: Aircraft A and Aircraft B share a common type rating

To gain certification for Aircraft B, Grace only has to take courses on parts G, H, and I.  She doesn't have to take courses on parts A, B, C, or E, since those parts are the same as on Aircraft A, which she's already certified on.
To gain certification for Aircraft A, Tedd only has to take courses on parts D and F.  He doesn't have to take courses on parts A, B, C, or E, since those parts are the same as on Aircraft B, which he's already certified on.

Real-world example

The A330 and A350 share a common type rating (but the A340 doesn't, because different number of engines); as a result, whereas (say) a 767 or A300 pilot wishing to gain A350 certification needs 23 days of comprehensive training on every aspect of the A350, an A330 pilot has to take an eight-day course to become an A350 pilot.  This eight-day course, covering only the parts of the A350 that differ from their counterparts on the A330, is, appropriately enough, termed "differences training".  (Source)


Why are general aviation planes so expensive? My question simply is why are GA planes so expensive?

I can understand that an airliner is an incredible money sink simple because of the size, complexity, development cost and especially the certification process.

But I do not completely understand why GA planes are so expensive,
because:

most GA planes are not luxurious
they have considerably less features than modern cars
there shouldn't be a lot of development cost to cover because a lot of GA plane designs are quite old and "simple"

Possible reasons I can see are:

there are no cheap regulations/certifications for privately used
planes
the overall production amount is not enough to profit from the
economy of scale

Further clarification:

I'm interested in the reasons for purchase and operating cost.
To narrow down the scope of the question I mean planes in the class
of a Cessna 172
location of interest is Europe/Germany but a more general answer would be appreciated to give the question/answer more weight

Simple: short production numbers. If light airplanes were as common as motor cars, manufactured by the millions, they would be much cheaper...


What are the US regulations governing experimental aircraft? There are more experimental class aircraft in the USA than any other.  That being said, what type of requirements does an experimental aircraft have to meet?

Also, are there ways around certain regulations?  For example, the guy in the picture below was battling with the FAA so they would let him fly his jet pack in the USA over the grand canyon, which they originally said no to, but they eventually let him - is this because he is "famous" and therefore has less rules he needs to follow?

Your first question

  There are more experimental class aircraft in the USA than any other. That being said, what type of requirements does an experimental aircraft have to meet.

is very broad and should probably be its own question.

As to your second question

  Also, are there ways around certain regulations?

The short answer is "Yes".  You can apply to the FAA for a waiver of specific regulations.  It's commonly done for aerobatic contests to permit aerobatics below 1500', within 4 nm of an airway, etc.  The waiver is of extremely limited duration and calls out the regulations to be waived.  Air shows also have them.

I haven't personally requested such a waiver but I've seen them.

The FAA probably permitted the overflight because the pilot was able to show it was safe, would do no harm, and wouldn't cause them any problems.


Is the visual artifact seen over the wing from different air densities, a reflection or something else? I have noticed a strange visual artifact over the wing of a Boeing yesterday. You can see two manifestations of the artifact in the pictures below.

The first shows it at the front edge of the wing. There were actually many of those, but I only managed to capture one. It looks very similar to a scratch in the window, but moving my head in front of the window didn't move the artifact. So it must have been in a distance somewhere above the wing.
The second pictures shows something similar which looked a bit like there was some kind of reflection on the wing, but the sun was on the other side of the plane, so I wasn't sure what caused it. An odd thing was that the stripe moved kind of in sync with the turbulences

Both of those things moved during the flight and were not always on the same position on the wing. Is it possible that those were caused by pressure/density differences of the air or is it just some reflection (or even something totally different)?

That is the visual effect produced by a shockwave.

The air above the wing is accelerated and will reach Mach 1 (the speed of sound) locally.

It is usually best visible when the sun is right behind you (from the viewers perspective, so on the 3 or 9 o'clock position from the aircraft) and high above the horizon.

Here is a video of shockwave on an A320 on youtube (thanks to @mins for finding it).

Thanks for sharing those really nice pictures! I have tried to make some in the past but I couldn't make out the shock effect from all the dirt on the window.


When is pre-heating a piston engine important, and how does one pre-heat correctly? As temperatures here in New York plunge toward (and below) freezing it's time for those of us without nice heated hangars to start thinking about pre-heating our aircraft before we turn the key.

Lycoming SI 1505 tells me that "The use of pre-heat will facilitate starting during cold weather, and is required when the engine has been allowed to drop to temperatures below +10°F/-12°C (+20°F/-6°C for –76 series engine models)", but the general recommendations I've been given are to preheat if it's been below freezing in the last 4 hours.

At what temperature does it start to become beneficial to preheat a piston aircraft engine prior to starting, and how does one ensure that the engine is thoroughly and evenly heated?
I'm pre-heating on the ramp, using a forced-hot-air heater.

I'm going to be referring to an old AvWeb article by Mike Busch with my links in this post.
As with most of what Mike writes, it's worth a good read.
Why is Pre-Heating Important?
Pre-heating your engine in the winter is important for a bunch of reasons, but there are two main ones that should get your attention - the first is preventing metal-on-metal wear by ensuring all parts have adequate clearances to function, and the second is improving lubrication by making sure your oil is ready to flow, and the spaces it's going to flow in to are opened up enough to let the oil in.
Metal-on-Metal Wear
Starting a piston engine is responsible for most of the wear the engine sees in its life - surfaces which have long since had their protective coatings of oil drip off are banging together while you crank the engine, and for a few seconds after startup until the oil pump can build up pressure and splash lubrication in the crank case can thoroughly coat the other moving parts.
In the case of a truly cold start there's an additional complication: like 2NinerRomeo points out, different metals in your engine have different coefficients of thermal expansion, and you have steel (probably several alloys), aluminum, brass, and a few others in your powerplant. When cold the clearances between these metals can be so tight that the additional wear is substantial, and at a certain point you may even damage the engine by attempting a start, shaving or spinning bearings and scuffing cylinder walls with the rapidly expanding pistons once the engine fires.
Lubrication
The other factor to consider in a cold start is lubrication - both the oil doing the lubricating, and the spaces into which it needs to flow.
Take the dipstick out of a cold engine and you will marvel at how the oil just clings there in a thick layer (this is especially true of heavy single-grade oil like 100W, but even 15W50 starts getting very sticky down in the 30°F-35°F range).  Oil that has thickened up from the cold doesn't pump very well - your engine may take a longer than normal time to develop oil pressure, and narrow spaces may get little to no lubrication until the oil has heated (and thinned) enough to get through the narrower oil galleys and tight spaces between some components.
Those tight spaces, as I mentioned under Metal-on-Metal Wear, tend to be tighter when it's cold, which means it's even harder for oil to work its way in and build up a good film of fluid. Until that film is built up and doing its job your engine is really inadequately lubricated, even if it's running. (Check out the webinar video in 2NinerRomeo's answer where Mike Busch goes into a lot of detail on how oil works, he has a nice explanation of the fluid film lubrication process.)

How do I make sure I do my preheat right?
The best preheat possible is spending the night in a nice, warm, heated hangar. It basically guarantees that every part of your aircraft has been brought up to the same temperature, and even affords you a nice place to preflight as opposed to doing it out on the ramp in the 6-but-feels-like-60 knot winter winds.
A hangar preheat isn't just the best for your engine -- your gyro instruments get brought up to a nice warm temperature (remember, mechanical gyros have to spin up, and they have the same issues as your engine did in terms of clearances being reduced by the cold), and the ship's battery gets warmed up too so it's ready to provide its best possible cranking current.
I don't have a hangar you insensitive clod!
Unfortunately some of us (like ME) don't have hangars - heated or otherwise - or our local FBO charges a hefty sum to spend the night in the warm. Those of us without hangars have to make due with a ramp pre-heat of some kind.  I'm going to assume you're in the same boat as me (no hangar, no electricity), and have to use some other technique.
Most ramp pre-heats where you have neither a hangar nor electricity take the form of a "forced hot air" system - either one wheeled around by the FBO, or portable ones powered by propane. There are even ones built around a camp stove which are deceptively good at the job (and in a pinch can burn the avgas you drained checking your tanks for water).
The general procedure for a forced-hot-air preheat is to point the heat into the cowl's cooling inlets, letting the warm air flow over and around the engine and out the cooling outlets. Start the heater up before your preflight, and with a little luck by the time you're done with the walk-around your engine will be ready to start.
If it's extremely cold you may want to block part of the inlets and put a blanket over the cowling during the pre-heat to help keep the heat trapped on the engine, and as 2NinerRomeo points out, when using a high-intensity heat source like a propane heater make sure you don't pump so much heat into the cowling that you blister your paint.  A gradual warm-up is what you want - we're slow-roasting the engine, not broiling it.
How do I know the preheat is "done"?
There are three common ways to tell if your preheat has had enough time to adequately warm your engine - each technique works pretty well, and you can use whichever makes the most sense for your situation.
Testing by feel
This is classic if you have a cowling that can be easily opened like most Piper aircraft, just feel all of your cylinders and the crankcase. They should, if your preheat was thorough enough, all be slightly warm to the touch. It's low-tech, but it works well if you can use it.
As a practical matter it's usually easier to tell if the "top end" (cylinders) are ready this way than it is the "bottom end" crank case / crankshaft / bearings), but with some cowlings (many Cessnas, or my Cherokee 180) you're simply not getting your hand in there to feel the engine: you can reach the front cylinders and a bit of the case, but the aft half of the engine is basically inaccessible without taking the cowling apart (and losing all that trapped heat).
Testing with the dipstick
Another good indicator is that when you take the dipstick out of the engine the oil flows off nicely -- compare with how thick and syrupy the oil was in the cold engine and is should be obvious when it's "ready".
This works for pretty much every engine, even if the only access you have is the little oil door, but it's also one of the slowest & most conservative indicators: pretty much all the thermal mass of the engine needs to heat up before the oil in the sump is going to get warmed up and start to loosen up.
Checking with your instruments
The final way to check the status of your preheat is to check your engine instruments: If your cylinder head temperatures are all reading something reasonable (say 60-70F) and your oil temperature is at least "off the peg" your engine has been thoroughly heated.
This technique is a little instrumentation-dependent though -- some engines may not show an oil temperature indication until you get your oil up into the 90 degree range (which would be great for start-up, but lousy for sitting on the ramp waiting). For those engines you probably want to use the dip stick test to check if the oil is "ready".
This technique also works best if you have individual cylinder probes, as you can determine if your engine has been evenly heated (this is especially true of longer engines, like a 6-cylinder powerplant: if the front cylinders have been getting blasted with warm air and are in the 90s, but the rear cylinders are still reading 30 degrees you may need to adjust your preheat procedure to warm those rear cylinders!)

Parting Thoughts - Stuff outside the engine compartment?
I've concentrated mostly on pre-heating the engine, which is what most people think about when they think about pre-heating aircraft, but as I mentioned briefly it's worth noting that other parts of the aircraft benefit from pre-heating too.
For example, a warm battery (60-70°F) will crank better than a cold one. If your battery is in the engine compartment it will get warmed up during your preheat, otherwise you should be aware that cold starting puts a pretty heavy demand on the battery, and your battery will appreciate if you limit cranking as much as practical.
Similarly, your gyro instruments (turn & bank, directional gyro, attitude indicator) will all need to spin up - that happens in a hurry (as soon as you hit the master for the electric ones, as soon as the engine catches and the vacuum pump starts really spinning for the vacuum driven ones).
If you can direct some warm air into the cabin your gyro instruments will thank you (and you'll probably thank yourself - those control yokes are COLD!). The gyros take less of a beating than your engine, and certainly cost less to replace when they wear out, but keeping them happy is worth a little effort.


Why do many fighter jets have double vertical stabilizers? Many fighter jets have double vertical stabilizers, e.g. F-14, F-15, F/A-18, F-22:

On the contrary, most general aviation planes and commercial airliners do not. While there are certainly exceptions in both categories, I think it is fair to say that this design is, comparatively, far more common in fighter jets.

What is the advantage of double vertical stabilizers? Why is it common in fighter jets?

There are a few reasons many fighter/bomber designs have 2 vertical stabilizers:

one large tail produced more shear force at the root, 2 smaller tails mean less force at the vertical stabilizer root. This allows for a lighter structure
redundancy if one tail is damaged. The US navy philosophy is 2 is better than one, this makes sense given that its aircraft spend much of the time over oceans where there's few alternative landing sites
twin tails are shorter than one big tail, and vertical height is a consideration on carrier aircraft
twin engines spaced apart make for a wide fuselage back which can create a blanking effect with a single tail in the middle, 2 tails mean that there's always one vertical stabilizer that is effective
Twin tails create less of a radar cross section, and allow the tails to be angled to deflect radar signals away from airborne radar sources


How to realistically model propeller static thrust? I am graphing static thrust vs prop diameter for a 600 kW motor and a prop diameter up to 5 m. I have seen the post 'What is the equation for calculating static thrust?' and using its formula and fixed prop efficiency I get a pretty linear graph. At 5 m the thrust is 19 kN.

Structural factors become increasingly important at larger diameters, but what other aerodynamic factors (perhaps prop efficiency changes with diameter) need to be considered to make the graph more realistic?

Update - I assume the prop pitch can adjust to maximize static thrust, but the twist is optimized for high speed (200mph) flight. The engines full power is also available at the best speed for the size of prop being tested. This Equation to bind velocity, thrust and power simplified by assuming unitary engine efficiency is the same as what I have been using.

$$T_0 = \sqrt[\LARGE{3\:}]{P^2\cdot\eta_{Prop}^2\cdot\pi\cdot \frac{d_P^2}{2}\cdot\rho}$$

This basically depends on the prop loading and RPM. If you are not satisfied with the momentum disk theory based results then the next step would be to use blade element theory based calculation.

If I were you I would certainly run few test cases with a known propeller geometry by scaling the propeller. One of the easiest way I can think of doing this is to use QPROP from Prof. Mark Drela at MIT. This is a free software and on top of this it is a command line program which is very well suited for this kind of an activity because all the parameters could be changed on the go, preferably via a bash or python script.

HTH