Why is the MD-80 magnetic compass behind the first officer's seat? I was watching this video about the MD-83 magnetic compass and I found it interesting that it's designed the way it is. I'm not sure which of the airplanes in this series featured this strange arrangement, but it was the case on at least the MD-80 and MD-83.

The magnetic compass on these machines was located behind the first officer's seat and a series of mirrors were used to make it visible to the pilots.

Why was it built this way? Were there any advantages to it? Did they go back to a more conventional design in later models?

In most aircraft, the standby magnetic compass is mounted well above the main instrument panel, usually above the central window post, in order to reduce interference. In the three piece window of the MD-80 (without a central window post), this is not possible, resulting in the different location for compass.

All the Douglas jet aircraft has this type of mounting. According to this thread, the compass was first mounted on the instrument panel, which resulted in severe interference, resulting in its relocation. From the thread:

  I understood that, early in the 60's, the compass was relocated in all DC-8 production due the three section type of the windshield (the compass has a lot of error/deviation at the center of the glareshield panel).

Note: I'm not able to find original documents supporting this. All the documents I can get simply instruct to check the compass using mirrors in the glare shield.

This design decision was used in all the later Douglas models, DC-9, MD-80/90 right upto Boeing 717 (which was developed as Dc-95); incidentally, all these models used a three piece front windshield devoid of center post.


Why are these windows on some airplanes red? There are some red windows on the 737 and the CS-300 as well. Why are they painted red?
Source

Source

A red/orange color is typically used to denote flight test instrumentation. Indeed, these windows do contain such instrumentation. Below you can see a similar window on a 747-8I flight test aircraft (note the 7th window from the nose):

And the corresponding inside view:


Why offload bags for missing passengers when flights are allowed to carry unaccompanied non-PAX air-cargo in holds? Several times, my flight has been delayed when passengers don't make it to the gate in time and then (after a long wait) the baggage-handlers have to go into the holds and manually find the bags to offload.

First, is this a legal requirement? Do all carriers follow this same practice?

If so, then why is this a security risk any greater than the same flight anyways carrying air-cargo in its hold? Flights do that, correct? Not all cargo space is filled with checked-in baggage?

Does anyone know the rationale behind this calculation? If the hold anyways contains unaccompanied cargo then why treat unaccompanied passenger baggage as a high risk?

Also, historically, is there a record of such offloaded bags (for missing passengers) actually turning out to be containing explosives?

Partial answer:

The rule about offloading passenger bags is known as Positive Passenger/Bag Matching. It was introduced after the Pan Am 103 bombing in 1988 where a bomb was placed in a suitcase that was checked through to New York without belonging to any passenger on the plane.

By the practices of the time, all bags tagged with the right flight number would be loaded onto the plane, without pausing to cross-reference them to passenger manifests. The investigation never found out for certain how the suitcase with the bomb had entered the baggage system (an inside job at Luqa airport was suspected but not proved), but the outcome was to require matching of all the bags to actual boarded passengers before departure.

So this is not necessarily a case where explosives were placed in the checked bag of a passenger who should have boarded but didn't -- but it is close enough to see that the risk is not pure fantasy.

The rule applies to all international passenger flights, but not to US domestic flights; the US authorities apparently have better trust in their ability to screen checked bags for explosives before loading them onto a plane.

I would assume that air cargo also undergoes similar screening and that is why it is not subject to PPBM rules -- even outside the US there is better time to screen cargo thoroughly than there is for bags that are only checked an hour or two before departure.

This Travel.SE question and its answers have more information about PPBM.


What are the limitations of the aerodynamics modelling used in flight simulators? I've asked around, and it seems all flight training programs use a combination of flight simulators and aircraft flights to train pilots. This works well for most pilots who fly airliners which don't spend much time in turbulent air or close to obstacles, and the flight simulator aerodynamics are very close to what is experienced in actual flight.

My question is where do the flight simulator aerodynamics models breakdown? I was wondering this in relation to rough air flying skills, where the aircraft or helicopter flow field interacts with surrounding environment (like inside a convective storm or near obstacles like trees and buildings) I would imagine these are situations that are hard to model since it depends on accurately modelling the interactions, which are difficult to run quickly enough for running a flight simulator. Fixing this by only doing flight training seems dangerous, especially for newbie pilots since the margin for error is very slim.

Personally, even though its more expensive than a flight simulator, a first person view drone would be a good intermediate step between the flight simulator and full scale training, with the response characteristics and control interface being the same as the full scale aircraft or helicopter.

That said, I don't know where the flight simulator aerodynamics begin to diverge from what is actually is experienced in real life, so the drone might be completely unnecessary. I'd love to see how this problem is resolved now.

Size matters here. Big aircraft have more inertia and take much longer to respond, but can equal out small-scale turbulence better. Your drone idea for training will not be representative of the big airplane at all.

Turbulence can be modeled quite faithfully - you just need to fly once through rough air, collect all the data and replay it in the simulator. The changes due to different pilot responses can be added on top without much loss in accuracy against a real flight.

Where models for airliner crew training break down is when the linear range of aerodynamics is left. Their post-stall and spin behavior is most likely not suitable for training, but that is not what they are designed for. Military aircraft simulators can even model the post-stall regime well, but need a lot of aerodynamic data, and most of that data is from wind tunnel models and not from real flight test.

What the simulation does is to plug the actual values of angle of attack, angle of sideslip, airspeed, power level angle and control deflections into matrices of coefficients and calculate the result from linear addition. That works well as long as the forces vary linearly of the flow angles, and correction factors will give you still good agreement with reality well into a stall. The coefficients are a mixture of CFD, wind tunnel data and flight test data. Gust loads can be modeled with Markov matrices which produce a realistic distribution of gust intensities, so even external, stochastic factors can be modeled faithfully.

I have less knowledge of helicopter simulators, and I would assume that flying near tall buildings can only be modeled in general terms, but not well enough to train a pilot for a specific location and weather condition.


How can a plane be cheaply owned by multiple people? How can multiple owners save on the cost of a plane that sits 1-4 people. If each nonprofessional person contributes equally in money, how would it contracted up to share all the costs are divided fairly. With at least 1 pilot and 1 A&P mechanic to contribute in labor, how many others could "time-share" on one plane before it too much to be worth it? Is there a method or scheme cost efficient and fair?

what you describe could be called a flying club or shared ownership. These sorts of arrangements are common enough that you should be able to find a template for setting up one which contains the details of how maintenance and overhead are covered and how the club's finances must be recorded and reported.
The cost effectiveness and fairness are determined by how money flows into the partnership and how it flows out. I would suggest you look into the Experimental Airplane Association's documentation on airplane ownership.


Help to understand a specific SNOWTAM My questions are:

According to the article ''H''; what about the other runway/runways when I want to add extra info other runway/runways too; there is just one runway in here?
According  to the article ''J''  it is indicated that critical snowbanks; but it is again just 1 runway I think so.
According to the article ''P'' 15 is the meter of this taxiway snowbank but in which direction is 15 mt.?

I did not fill any SNOWTAM format so; I want to ask in advance..

Reference: ICAO Annex 15, Appendix 2

  According to the article ''H''; what about the other runway/runways when I want to add extra info other runway/runways too; there is just one runway in here?

A SNOWTAM is for a single runway, if you want to involve multiple runways, you would release more than one SNOWTAM.

  According to the article ''J'' it is indicated that critical snowbanks; but it is again just 1 runway I think so.

Yes, this is just for the runway identified in C).

  According to the article ''P'' 15 is the meter of this taxiway snowbank but in which direction is 15 mt.?

This is the distance between snowbanks in meters. There is no direction there as it just identifies a relative distance between two (or more) objects.


Can foreign pilots get an FAA ATPL and work for a US-based commercial airline? Could a non-US citizen come to the US, receive flight training, earn an FAA CPL and be hired by a US-flagged airline? Are there any specific rules regarding the employment of foreigners as pilots for a US-based commercial airline? For the sake of example, an Iranian flying for a US-flagged carrier?

At the most basic level, the DHS/Dept.Of State regulations regarding work authorization have to be followed. There are essentially three categories you can be in, as far as employment is concerned:

A naturalized or natural-born U.S. Citizen. You're allowed to work for any U.S. employer.
An (in-status) permanent resident (green card holder). You're allowed to work for any U.S. employer.
A non-resident. You're allowed to work only with DHS authorization.

The first two are somewhat self-explanatory. Some positions may have special requirements - for example, you can't be a U.S. President without being natural-born. Some employers place limits on non-citizen employment due to various U.S. laws usually related to access to information of military nature or applicable to military needs. Thus, a lot of aerospace manufacturing is off-limits to non-U.S. citizens, simply because the company might also handle aerospace jobs and doesn't wish to sufficiently compartmentalize access. For example, most if not all SpaceX jobs are for U.S. citizens only. No such limitations apply to normal commercial transport pilots.

Permanent Resident Workers

Unless you're already a permanent resident, you may wonder how to become one. In most cases, someone else - a family member or an employer - must petition on your behalf, you can't apply yourself. There are also strict yearly numerical limits for most permanent resident visa categories. Citizen or permanent resident family members 21 years of age and older can apply for you, but unless you're petitioned for by a spouse, the waiting times are measured in years.

We can review the directory of Visa categories, immigrant, with an eye for transport crew employment. I will elaborate only on the categories that apply to a pilot who isn't eligible for another category (say you're not an Iraqi who has worked for U.S. Govt, etc.).

DV category - Diversity Visa - if you seriously think of working in the U.S., you should be applying every year. It's a lottery. Make sure you apply at the official .gov website; the entry is free.
E Category - Employer-Sponsored. The first and most obvious requirement is that the potential employer must be willing to spend the money (on the order of $10k) to petition on your behalf. If they can find employees already authorized to work, there's very little incentive to petition.

Assuming that your employer is willing to sponsor an E immigrant visa on your behalf, the next step is to obtain a labor certification from the Dept. of Labor. A Pilot would most likely fall under the third preference, assuming that a labor certification can be obtained. And here's the real clincher: given the never-ending supply of commercial transport pilots in the U.S., and a lack of any particular special requirement for the job (we aren't talking about test pilots!), there's a nil chance of the DOL certifying the position. The DOL certification process requires the employer to prove that there is an insufficient supply of qualified workers who are already authorized and qualified for the job. So that is, essentially, where it ends.

Non-Resident Workers

The third category is the most complex one. There are various kinds of work authorization available, under various programs. All of them have special requirements, thus a non-resident is by default ineligible for any work authorization, unless he/she fulfills the requirements. For example, students on F-1 status are authorized to perform on-campus work, and for certain off-campus jobs iff they amount to practical training.

The non-immigrant category (usually Letter-Number, with an optional Letter suffix, e.g. F-1) is indicated on the Visa, but remember that the Visa is merely a document that is necessary (but not sufficient) to enter the U.S. After you're in the U.S., you or your employer can adjust your immigration status, as long as you fulfill the requirements to do so.

Thus, we can review the directory of Visa categories, non-immigrant, with an eye for transport crew employment. I will elaborate only on the categories that apply to a pilot who isn't eligible for another category (say you don't work for NATO or UN, you're not a NAFTA signatory citizen, etc.).

D category - allows you to work as a pilot, but you must be employed by an international airline, and you can't stay over 29 days. Your employment must be bona-fide; it can't be a sham set up just so that you can mostly work as a domestic pilot.

And that is, unfortunately, "it". You may be eligible for work if you're a citizen of one of the countries that U.S. has special agreements with, usually based on humanitarian and similar reasons, but those are few.

As you can see, FAA regulations don't even enter anywhere in the picture yet. The DHS regulations are a primary concern here.


Which compressor is more like to surge and stall - axial or centrifugal? I am studying about stall and surge in rotodynamic compressors and want to understand which one is more prone to these phenomena.

Axial for sure.  Its blades are wings subject to aerodynamic stall within the blade array.

Centrifugal compressors just throw air outward; they don't really "stall" if in-flow is restricted, but will surge due to the pressure drop and resulting load reduction on the compressor.  Centrifugal compressor surge is what is happening when you block a vacuum cleaner and it goes vwwwwwweeeeeeeeeeeeeeeeeeeeee.

Axial compressors, being an array of little spinning wings, depend on a controlled angle of attack on the blades and are much more sensitive to flow disruptions.  And once they do stall, the surge and flow disruptions throughout the engine are more severe (those whomp whomp sounds and flames coming out the front of the engine as reverse pressure waves are induced, that sort of thing).

So if you want efficiency and high compression ratios, you go with axial and live with the sensitivity, the high cost and complexity.  If you want insensitivity to inflow disturbances and can live with a larger diameter engine and less efficiency, and want to make the engine cheaper, you go with centrifugal.


Does the retirement age for commercial airline pilots vary across the globe? Do the retirement age of commercial airlines pilot varies from country to country?

Yes, they do vary between different countries. The international standard for regulations is created by ICAO, but member nations are free to deviate from ICAO rules and several do. ICAO currently limits retirement age to 60 for single-person crew, 65 for multi-person crew, and an exception to 70 for balloon or sailplane.
EASA did a study in 2019 in response to requests from countries to increase the age for single-person helicopter crews to 65. The link to the actual report is broken, but there is a summary posted on the ICAO website. The rationale from these countries is that standard retirement age in those countries is 65. The report lists a few large agencies that deviate from the ICAO standards. Japan raised their limit to 68 for multi-crew ops. Canada, Australia and New Zealand have no age limit, the latter two indicating that it would be considered age discrimination.


Why does a higher load factor increase stall speed? I think that’s self explanatory, why does load factor (G) affect stall speed? Why does load factor result reaching the critical angle of attack at higher airspeed?

What does 'having' a load factor1 mean? It means, by definition, that you are experiencing a force G times your weight. It doesn't matter how it happens: in a turn or loop or turbulence. But in all cases practically all that force (which usually counteracts the weight) comes from lift.

In normal unaccelerated flight, lift = weight. In a more general case, lift = weight $\times$ G. If G > 1, you need more lift than weight.2

How can you get more lift (from the same wing)? Either you go faster, or you increase angle of attack. So inevitably, for a given speed, you'll have to fly at a higher angle of attack, closer to stall. Or in other words, you'll reach stall at a higher speed than normal.

Note that exactly the same thing happens if you 'just' increase your weight, by other means than transient loading with G - say, by having more cargo or fuel. Again, you'll need more lift - with exactly the same consequences.

1 Load factor is applicable in all three axes, but we are implicitly talking about the normal factor in relation to stall speed. When you are accelerating forward or skidding, you'll also have a load factor, but it'll usually be well below 1.0.

2 On the other hand, if G = 0, you don't need wings at all and you'll go ballistic.


How do the compressor stages keep spinning after start in a free turbine? In a ‘free’ (or uncoupled) turbine engine (specifically the Rolls Royce C20-250), what is spinning the compressor blades (N1) after start? It can’t be N2, because it’s a free turbine. I don’t think they’re freewheeling, because otherwise how would I get bleed air? And I’m pretty sure the combustion chamber is ahead of the N2 stack, so N2 wouldn't have its own compressor wheel and if it did, I wouldn't have clean bleed air.

In a C20-250 engine the compressor section is driven by the high pressure turbine (Ng spool, not N1) independent of the power turbine.  The power turbine drives the gearbox and subsequently the output shaft via a concentric driveshaft to the gas core spool.

I believe you’re confusing the configuration of this engine by assuming that all free turbine turboshaft engines are laid out similar to a PWAC PT-6 engine.   While the PT-6 design certainly excels in terms of simplicity as well as modularity for ease of maintenance, it is not necessarily the only configuration for free turbine engine. Plenty of other free turbine turboshafts have other unorthodox layouts but all operate according to the same principles.

A cutaway of the C20-250 engine appears below.  Note the non-axisymmetric air channel carrying air from the last stage of the compressor section to the aftmost part of the engine for introduction into the single can-type combustor.


What is the correct phraseology for changing the runway of a landing clearance? In the liveATC clip on this YouTube video, an American Airlines 767 arriving at SFO (San Francisco International) had been cleared to land on 28R, but, apparently while already on final, was told to switch to 28L.

The initial clearance given as

  American 2293 Heavy, San Francisco Tower, Wind 210 at 18, Caution Wake Turbulence, Heavy 767 mile-and-a-half final, Runway 28 Right, Cleared to Land.

which was read back, as per the usual. However, the Tower later sent the following transmission:

  American 2293 Heavy, Change to Cleared to Land Runway 28 Left

which the pilot read back as

  28L, Cleared to Land. American 2293 Heavy.

However, the pilot was seemingly too focused on flying the final to notice that the runway assignment had changed between the clearances and continued flying final to 28R, nearly resulting in landing on top of a United jet that was subsequently cleared to line-up-and-wait on 28R. Fortunately, the United pilot noticed the big 767 flying towards him and exited the runway on the other side instead of lining up. Tower then instructed American 2293 to go around.

It seems to me that the phraseology used to change the clearance here is too close to sounding like the controller is just repeating the initial landing clearance, rather than directing a busy pilot's attention to the fact that he was just told to change runways while apparently already flying final.

My question is: Was the phraseology correct for this change of the clearance? If not, what is the proper phraseology?

Given that this was in the U.S., I'm looking for FAA phraseology.

The phraseology was slightly incorrect, this is from the FAA's ATC Orders (section 3-10-5):

  3−10−5. LANDING CLEARANCE
  a. When issuing a clearance to land, first state the runway number followed by the landing clearance. If
  the landing runway is changed, controllers must preface the landing
  clearance with “Change to runway.”
  PHRASEOLOGY−
  RUNWAY (number) CLEARED TO LAND.
  Or
  CHANGE TO RUNWAY (number) CLEARED TO LAND

That means the ATC instruction should have been:

  American 2293 Heavy, change to runway 28L, cleared to land

All the correct information was in the transmission you quoted, it was just given slightly out of order. But that could easily be an issue for a pilot who's in a high-workload phase of flight and it's probably exactly why the ATC orders say that the words "change to runway" must come first.

I've had ATC change my landing clearance a couple of times but the controller always asked me first if I could do it:

  Tower: N12345, cleared to land 21L
  N12345: Cleared to land 21L, N12345
  [A few seconds later...]
  Tower: N12345, are you able to switch to 21R?
  N12345: Affirm, we can do that, N12345
  Tower: N12345, change to runway 21R, cleared to
  land

However, landing a C182 at a class D is a bit different from landing a heavy at SFO; I assume the controllers there have less time to talk so they may simply have changed the clearance and expected the pilot to say "unable" if necessary.


Why photographing airports is banned if everything is perfectly pictured in Google Maps? While staying at Premier Inn, next to Abu Dhabi International Airport (AUH/OMAA), I was more than surprised to find out warning signs that I'm strictly prohibited from taking photos of the nearby airport form the hotel's roof pool.

Can someone explain me, why such regulations are still in place or how do they aid to airport security, if everything (what I'm banned from photographing from this distance) is perfectly pictured, in a far better details, in Google Maps?

I believe the sign is to prevent plane watchers from gathering at the spot and taking pictures of every aircraft taking off or landing, which, judging by the existence of the sign, is not how the location is intended to be used.


In the US, are there any operational control towers located in surface-level Class E airspace or in Class G airspace? (This question pertains to the airspace of the United States.)
FAR 91.126 gives some general rules for operating at or near airports in Class G airspace, and FAR 91.127 gives some general rules for operating at or near airports in Class E airspace.
Section (d) of 91.126 and section (c) of 91.127 pertain specifically to operations at airports with operational control towers.
a) This seems to beg the question, are there any operational control towers located in surface-level Class E airspace or in Class G airspace?
b) Were there ever?  (Counting from the September 16, 1993 "alphabet" airspace re-organization onward.)
c) If "no" to a and b, then what is a practical example of how a pilot's behavior might be altered by the need to comply with 91.126(d) or 91.127(c)?  After all, it appears that generally speaking, it is not possible to get within 4 nautical miles of an airport with an operational control tower, at 2500' AGL or below, without entering Class D (or higher) airspace.

There are a handful of towers still in Class E/G airspace. They're rare, but they do exist and are something to watch out for during preflight planning.
As of April 2021, here's the airports I know of:

KJYO - Leesburg, VA
KPCA - Picacho ARNG, AZ
KTNX - Tonopah Test Range, NV (inside restricted airspace)

I wouldn't be surprised if there's others I missed, these aren't really cataloged anywhere.
Many of these do eventually become proper Class D airports once they have proper weather reporting capability. For example, KTRK (Truckee, CA) was an infamous one on the west coast that just got its Class D ring in 2017.
You'll also see temporary towers pop up via NOTAM for special events. These NOTAMs don't change the airspace designation, since changes to airspace need to go through the formal rulemaking process.


Are there any non-bureaucratic hurdles preventing more widespread adoption of synthetic vision? Occasionally, a flight will encounter bad enough weather that the crew will become disoriented. While tools like a terrain radar, ILS, and GPWS/TAWS do help the pilots regain orientation, they're not universal; some aircraft aren't equipped with terrain radar, ILS only works on approach to runways supporting ILS, and GPWS is pretty coarse and primarily aural, not visual (beyond a flashing warning sign) on most aircraft. This means that there's still a risk of overshooting or undershooting a runway, or even controlled flight into terrain.

Some higher-end private aircraft have started incorporating decked-out glass cockpits which, among others, can include synthetic vision systems -- these can show terrain, glide slopes, and other traditionally VFR elements which can otherwise be invisible during night flying or in poor weather.

I haven't seen high adoption of these kinds of systems even in brand-new airlines, though; is there a technical reason why these aren't being included (or even mandated) above a certain type rating? Is it purely pragmatism while the system becomes more ironed out? Or are the hurdles before adoption purely bureaucratic at this point?

It's not bureaucracy in the sense of pointless red tape, it's the demands of certification for brand new systems on Transport Category airplanes.  The hoops that have to be jumped through are epic. Engineering standards, testing this, testing that, covering every conceivable possibility (or trying to).

You always see new innovations in GA, then Corporate, then Transport, because that's the certification difficulty ladder imposed by the demands of having to achieve near perfect safety with a better than one in a billion risk of a catastrophic crash due to a component failure.  (Airliners are, by a far, far margin, the safest way to get form A to B aside from an ox cart - there hasn't been a crash of a heavy in the US in, what, almost 20 years now?)


Why are lavatories located near the exits of an airplane? Is there some scientific or technical reason to locate the lavatories near the plane exits? Wouldn't it create problems during evacuation, if the swing open doors gets unlatched etc on forced landing or some minor crash etc.

There could be numerous reasons,

Putting them near the galley centralizes non seat related things. It also allows you to run less piping as you can pump the same water to the sink in the lav as you would to the galley.

They may be located in a place where a tank can easily be fitted beneath them so as to minimize the amount of piping needed from unit to holding tank.

They are generally a convenient way to segment cabins as they provide an inherent wall (think between first and coach).

As for an emergency situation, Lav doors can generally be locked from the outside (I have seen the little metal flap above the slide often opens and allows you operate the slide). Its common place to do this prior to takeoff and landing. I would think its part of the emergency decent checklist as well so as to keep them shut. There is a discussion on it here.


Why do aircraft sometimes bounce while landing? I've seen a few youtube videos where an aircraft bounces once/twice while landing on the runway. Why does this happen?

The FAA covers this in the airplane flying manual quite nicely:

Bouncing During Touchdown
When the airplane contacts the ground with a sharp impact as the result of an improper attitude or an excessive rate of sink, it tends to bounce back into the air. Though the airplane’s tires and shock struts provide some springing action, the airplane does not bounce like a rubber ball. Instead, it rebounds into the air because the wing’s AOA was abruptly increased, producing a sudden addition of lift...

There is some more info in the doc worth reading as well.
In the simplest terms this can be one of the outcomes of an unstable approach; oftentimes, when the approach speed is too high, bouncing will result. Touching down on the nosewheel first can also cause a very dangerous bounce case known as a porpoise.
In some cases wind shear can cause bouncing. In this case the shear may cause an unexpected downdraft, forcing the plane down onto the runway vertically and inducing a bounce, or it may cause an updraft leading to an overcorrection by the pilot to descend; when the updraft stops, the plane falls rapidly.


Why do jet engines sometimes spit out flames after ingesting birds? After seeing this video of a 757's engine catching fire because of a bird,
I wonder:

What happens to the engine?
Why does it periodically exhaust fire?
Is this some safety feature?

As RedGrittyBrick has commented, what happened in that video is a compressor surge. This is not really an engine fire, but a temporary backfire. To understand what is happening, look at this diagram of a turbine engine:

The air is compressed in the front of the engine, fuel is injected and ignited, and then everything flows out the turbine section and the exhaust nozzle. The only thing supporting the compression at the front of the engine is the rotation of the compressor blades. There are many different things that could cause the compression to fail. In the event of a bird strike, these blades can be damaged.

When that happens, there is highly compressed air in the combustion section, and there is no longer enough compression to prevent it from exiting the front of the engine. This causes the air to exit both the front and rear of the engine. The disruption in air flow happens too quickly for the engine control systems to react. This means the engine is still pumping enough fuel to run the engine at normal thrust, but there is no longer enough air flow to burn all that fuel in the combustion section. The result is that the fuel burns through the turbine and exhaust section of the engine, and behind the engine.

Once this compressed air has exited the engine, it will generally recover. The engine is not on fire, so once the excess fuel burns away, the flames are no longer visible. However, if the problem is still present (and damage from a bird strike would not go away), then the engine surges over and over until it is shut down or set to a lower, more stable power setting.

So this is not so much a safety feature as physics taking over, and there is no real fire of immediate danger to the airplane. The engine system will automatically shut down the engine if it senses there is anything potentially very dangerous or that completely prevents the engine from running. While compressor surges can eventually damage the engine, it's not really a catastrophic event, and it is up to the pilots to assess the situation and shut the engine down if necessary. Reducing power to idle may sometimes be sufficient to stop the surges.

During a compressor surge, the flames are confined within the engine. There should be nothing to burn here besides fuel, so cutting off fuel will stop the fire. Actual engine fires are generally caused by flammable fluids leaking into hot areas around the engine, or the hot engine gases escaping from the air flow sections into surrounding systems. There are detection systems to alert the pilots of this situation, and the engine area is designed to contain a fire until the pilots can use the fire extinguishing systems to put such a fire out or at least get the plane back on the ground.


Why are piston engines for aircraft rarely diesel powered? Why hasn't diesel caught on much for aircraft engines?  The list of diesel aircraft engines on Wikipedia is very short.

The reason I see mentioned is their poorer power-to-weight ratios. (e.g. What are the advantages/disadvantages of diesel/Jet A-1 piston engine?)

I knew this was true in the past but is it true even today? Does anyone have values comparing the power-to-weight rations of typical diesel vs gasoline engines, say for automotive use?

Also, doesn't the better efficiency, especially at higher altitudes compensate for the poorer power-to-weight ratio by allowing lesser fuel and more payload?

50 years ago diesel engines for automobiles were uncommon too but in the last decade they have been steadily making headway in the car market. Do the same developments translate to the aircraft engine  selection too?

Other than weight, are there any other disadvantages of diesel?

You are right, there are few modern aerodiesels around - so few in fact that we can go through the brief list one by one:

Thielert: This is a development based on Mercedes-Benz car diesels, of which more than 1500 1.7 liter engines were built. Currently, a 2.0 liter engine is offered. The original Thielert company went bankrupt in 2008 and has been taken over by Continental.
Austro-Engine, another conversion of a car diesel for aircraft use. It is the result of the disappointment of Thielert's main customer, Diamond Aircraft, with the record of Thielert's initial Centurion 1.7 engine.
SMA:, a French turbocharged diesel with 5 liters displacement. Currently, around 100 of them have been delivered after almost 20 years of development work.
Zoche: The product of a father-son team with many clever details, but a quarter century of development work has not yet resulted in an airworthiness certificate for their two-stroke radial diesels. While Zoche claims to be the victim of a conspiracy by the authorities, the accused complain that the engines could not keep their cylinder heads on for long enough.

What has held the diesels back for so long?

Cost: Development in aviation piston engines has effectively stopped with the jet age. The existing designs were simply good enough until fuel prices went through the roof. This made diesels only attractive in niches, and as a consequence, production runs were far too small to justify the major development effort. Aviation engines run constantly at around 70% of their full power, so regular car engines could not be used without heavy modifications. As a consequence, a conversion kit can cost you $89,000!
Weight and complexity: The higher compression ratio of 1:30 requires a heavier structure, and fuel injection, a gearbox and turbocharging push the part count well above that of a gasoline engine. Once you compare system mass (fuel plus engine), the diesel wins only at ranges which are now mostly flown by jets. Yes, fuel flow is less (10 gallons per hour for a diesel against 13.5 gallons per hour for a comparable gasoline engine), but will only help at ranges above a couple of hundred miles.
Vibration! With that compression ratio, every ignition causes a three or four times heavier jolt to the engine and the aircraft structure. Not only the engine, but the whole aircraft structure needs to be designed with that in mind.

To be fair, a modern gasoline engine will also benefit from fuel injection, a gearbox and turbocharging, and Zoche initially did not use an electric system, relying instead on pressurized air for starting the engine (a pressure bottle loses much less of its stored energy when cooled to -20° than a battery, after all), but the practical aerodiesels are a lot more complex than the engines they are meant to replace, creating an obvious reliability disadvantage.

Thielert Centurion 1.7 in a Diamond Twin Star (picture source)


What is the distance between 2 adjacent waypoints? I am working on an en route air traffic control system. For the application I plan on allowing the user to plot the position of a few waypoints on a square region which is 100 miles in length and width. The aircraft will then follow a route which passes through the relevant waypoints. Therefore, I would like to know whether there is a general estimate for the distance between 2 adjacent waypoints so that I could have rules for where the waypoints can be plotted and how many of them can be plotted in the whole region.

Thank you very much for the help!

The distance between waypoints on a procedure or airway can vary tremendously. For waypoins on a high altitude airway, over 100 NM is common. For low altitude Victor airways, less. For an instrument departure or arrival, 10 to 20 miles is common, especially near the terminal area. On an instrument approach, legs are almost always under 10 miles. So the limit for your application depends on what sort of airspace you're modeling.  At 100NM square, that's more nearly the size of a terminal area than an enroute sector, so 10 to 40 miles is probably a reasonable ballpark.

Might be worthwhile to order an actual IFR enroute chart - they're only a few dollars, and then you have a real example of what airspace actually looks like. Or, any of the free flight planning websites can show you essentially the same thing on-line.


Clearance limit of VFR practice IFR approach? When a VFR aircraft has been approved by atc to perform a practice approach, is the aircraft also cleared for a published missed approach procedure?

I'm assuming you're asking about the US, in which case the answer is no. The AIM 4-3-1(e) says:

  VFR aircraft practicing instrument approaches are not automatically
  authorized to execute the missed approach procedure. This
  authorization must be specifically requested by the pilot and approved
  by the controller. Separation will not be provided unless the missed
  approach has been approved by ATC.


Where can I find full wing polar data to validate an aerodynamic code? I am looking for polar data of a full wing to validate an aerodynamics code I an working on.
I am looking for the Cl, Cd polars (and Cmy) and the planform and airfoil geometry, I am also looking for the polars computed at various Mach numbers.
I looks through the Internet and read many AIAA papers but I never found all the elements I am looking for in one of them.
I heard about the ONERA M6, DLR-F4 and DPW reference wings but I never found a full study of one of those (with numerical data). If someone can provide me with such data I would be very happy.

To go one step further I am also looking for such kind of data for a straight wing.

"Theory of Wing Sections" by Ira H Abbott and A. Von Doenhoff has extensive graphed and tabulated data for Cl and Cd vs alpha of a wide variety of wing sections.

Airfoiltools.com has also got a lot of the data you're looking for, but from a quick scan it doesnt appear to cite their sources. The data is readily available in tabulated format, however.

In either case, its not explicitly mentioned but I'd assume that the data provided is for wings of straight planform. I'd guess that you can find some kind of formula to account for the difference either in the Abbott book mentioned or elsewhere, possibly applying a variation of the blade element method.


What do pilots landing on aircraft carriers do when the ship is not visible due to weather? Say an aircraft takes off of a carrier to do a mission somewhere, and upon returning to the carrier the pilot comes to find that a thick layer of fog has formed just above the surface of the water in addition to low-level clouds. The pilot may have the ship's exact location, but is it still possible to land?
In the civilian world, usually IFR minimums are 200 ft. AGL, but I am assuming this doesn't apply to military aircraft out at sea, mostly because the ocean is usually a flat surface at a known altitude, (0 ft. MSL), and therefore the risks of terrain other than the surface of the ocean would not be a factor. I also assume they don't apply to carrier-aircraft because, well, they're the military.
However, if the ship is completely shrouded by weather, what would the pilot do? Are there instruments/procedures used to execute a super precise spot-landing with virtually no visibility? What if the plane is bingo fuel?

My experience is circa 1990s, but I can offer some perspective on US fixed wing operations.
Besides TACAN and ASR for non-precision approaches, there are (were) 3 precision instrument approach options available:  Precision Approach Radar (PAR), Instrument Landing System (ILS, or “Bullseye”) Automatic Carrier Landing System (ACLS).
PAR: This consists of both azimuth and glide slope radar that allows the controller to issue verbal advisories to “talk down” the pilot.  This is generally a back up for when the other systems might not be working.
ILS: Although it works on different frequencies than civilian ILS, functionally it is identical.  A passive signal is received by the approaching aircraft which powers azimuth and glide slope indicators in the cockpit.
ACLS: This is an active system that locks onto the aircraft and sends a discrete signal to drive the indicators.  For a pilot flying a Mode 2 approach the indications are identical to the ILS, but there is greater precision as well as the option to upgrade to a Mode 1.  In a Mode 1 ACLS approach the pilot can couple the autopilot to the signal to allow it to fly the approach hands-off.
All these options are similar to what civil airfields have available, with one notable exception:  The aircraft carrier has Landing Signal Officers (LSOs) on station.  These hardy souls man the flight deck during all recovery operations, in all kinds of weather, and are the final piece in helping a pilot get aboard when the weather is bad.  In very poor visibility LSOs can see the landing light before the pilot can see the carrier and can help talk them down on short final. (There are lots videos on YouTube of this...)
Because of the unique functionality of the Pilot/LSO team, the concept of “minimums” and “missed approach” don’t really apply at the ship.  At 3/4 mile from touchdown the pilot will be asked by the controller to “call the ball” in reference to seeing the glide slope indicator of the ship’s Optical Landing System.  The pilot will either call the ball in sight, or declare “Clara” which means he/she does not have the ball (ship) in sight.  On the call of Clara the LSO will take over issuing power and line up calls over the radio until touchdown, or instruct the pilot to wave off if a safe landing is not possible.
And finally, if the airplane is bingo fuel they would be expected to divert unless the ship is operating blue water, in which case they would go to the tanker.


What flaps position should be used for taking off from a wet runway? If I have to take off in a C152 or C172 from a runway that's 800-900 meters (2600-3000ft) long and wet but not contaminated, what flap setting should I use to get the shortest ground roll?

The POH for a 152 lists the flap settings for takeoff as between 0° and 10°. It makes no specific mention of take off from a wet runway, however you could follow the same procedures for a soft/rough runway if you are concerned about the surface conditions.

  Soft or rough field take off are performed with 10° wing flaps by lifting the aircraft off the ground as soon as practical in a slightly tail low attitude. If no obstacles are ahead, the airplane should be leveled off immediately to accelerate to a higher climb speed

Source: Cessna 152 POH Section4 (Normal procedures)

I should note though that in roughly 180 hours on C150/152's and a short (650m) hard runway, I've never felt the need to even use flaps, let alone execute a soft/rough field takeoff as described above. It's not a particularly comfortable take off, and a wet hard-surface runway doesn't really present that much of a problem.


What can I do as a bystander on the ground if I see an aircraft above me that appears to be failing? If I'm watching an aircraft fly overhead and it becomes apparent to me that it is failing, what action can I take in response to this incident?

For now I will leave this question generic as to location, but if it becomes necessary to narrow it down I am interested in the US specifically.

Honestly, very little.

First, be clear as to whether you are actually see something failing. If you see flames, or a wing has broken off, then yes, it's a problem. However, if you see someone doing a slow spiral towards an empty field with the power idling, they're more likely to be practicing engine out procedures (something that's required for all US pilots to practice and demonstrate during their checkride for their certificate).

In the event that you actually see an impact, fire, off-airport landing and so on, call 911 and give them your location. They'll send the appropriate crews.

Otherwise, if the aircraft is still flying, there's a strong chance the pilot(s) are already talking to ATC about their problem, if it actually exists. There's nothing you really can or need to do in this case.

Quick Anecdote: I've been told the story of a local police department who gets phone calls virtually every time a citizen lands his small airplane in his private field. Fire/police/medics/etc are summoned, when absolutely nothing bad has happened. This is why you should make an attempt to verify that a problem actually exists before reporting it.


What are the disadvantages for using short-field take-off technique? It is obvious that using the short runway takeoff method(hold the brake till engine get full power) could get the aircraft in air within a shorter runway distance.Then why it is not used for every takeoff for all different runways? My understanding is that occupying less runway should always be a better option, isn't it? What are the disadvantages for using short runway takeoff method?

Three main things:

There is a little bit of increased brake wear because you apply them to stop, they usually creep a bit at full power, and when releasing them, there is also that short period where they are slipping while under pressure.  More wear than not using them at all anyway.  Of the three this is pretty trivial however.
Sitting stationary at full throttle has the engine at maximum heating rate and minimum cooling flow.  The area of the exhaust valve on the head heats up very quickly.  During maintenance, full power engine runs except for very short bursts are to be avoided without a special cooling box shroud, to catch large amounts of air and force it down between the cylinders, installed in place of the cowling.
The full power condition while sitting stationary turns the prop into a pretty powerful vacuum cleaner and it will suck up just about every pebble and bit of grit on the pavement (and even clean looking pavement is full of that sort of FOD).  Or water if you are over a puddle (and water erodes aluminum).  It will leave you wondering why you have to constantly have your propeller dressed out to clean up all the little stone nicks (this is why I always do run-ups over a patch of grass whenever possible).  Of the three, this is probably the biggest item in my experience.


What is this second smaller runway next to London City Airport? I was looking at London City Airport (EGLC) on Google Maps today when I noticed the remains of a very small runway marked 10/28 just across the water from City Airport's main runway. It can be seen here.

To my surprise I couldn't find any mention of this small runway after searching online and looking into the history of the airport. Using the Google Earth historical imagery feature the earliest image I could find that included the runway was 2008, so I assume it was built some time between 2006 and 2008.

Can anybody tell me what this short runway is for?

Good Spot! That was a temporary runway for the Red Bull Air Race:

https://www.forgottenairfields.com/airfield-london-redbull-airstrip-1429.html

You can see "RB" at the eastern end on your satellite imagery


Why isn't tungsten used in supersonic aircraft? Why isn't Tungsten used in supersonic jets or flights even though it has a High Melting and Boiling point?

While tungsten does have a high melting point (I'm not sure what's the relevance of boiling point here), it has little else to offer for aircraft construction. The main issue of course is weight. One of the main goals of aircraft design is to reduce weight while achieving all the required performance parameters and tungsten is among the heaviest elements around, even beating Uranium.

When you factor in its density, it appears less than ordinary- its specific strength is less than 100 KNm/kg, less than half of Aluminum. Another thing is that tungsten is not exactly an easy material when it comes manufacturability. Even its saving grace, the high melting point (over 3000$^{\circ}$ C) is not very relevant as far as supersonic aircraft are concerned. The temperatures experienced in aircraft anywhere other than engines is well within that limit. For example, the surface temperature of Concorde wouldn't even get to a tenth of it.

Concorde surface temperature; image from erau.edu

Surprisingly, tungsten is indeed used in aerospace industry- at exactly the other end of the speed spectrum. it is used as a counterweight for controls in small aircraft and also as balancing masses in rotorcraft blades.


What are good apps to help with VFR navigation on my first solo? Currently I am a student pilot in Spain (Barcelona) and I am 2 days away from my solo. I wanted to ask if there is any good application that will help me on VFR navigation since I'm allowed to go to all Barcelona in my solo, but I am afraid if I get lost I see my self weak in VFR flights and specially to locate my position so any good app in the iPhone device that will help me in navigation because I want to visit the whole Barcelona and not just staying at places that I know because I will get bored for sure.

No, there is no "app" you should be seeking at this stage of your training to do what you're asking about.

First, if you have questions about what you'll be doing while you're flying solo and how you'll be navigating, you should be discussing this with your instructor, not on an internet message board.

Second, whatever tools you'll use in flight need to be ones that you've used under supervision of an instructor, so that you understand how to use them, and what their limitations are.  Student solo flying is NOT the time to go exploring technology and apps you've not used before, in addition to the full-time tasks of maintaining aircraft control, aviating, navigating, and communicating.  Too many dead pilots got focused on some technological toy and, without realizing it, lost situational awareness and/or control of their aircraft.

Third, before you are navigating anywhere solo, you need to be able to (and demonstrate to your instructor that you can) navigate successfully with the tools you have -- a map, compass, stopwatch, and the pre-flight planning you've done.  If you don't feel up to that task (which has been all of us early on in our training -- nobody is born with a built-in INS), work on those skills with an instructor.  Navigation apps are fine, but they supplement, rather than substitute for, your own skills when it comes to VFR navigation.

Fourth, the fact that you would come here to pose such a question, rather than to your instructor, is troublesome: you want an app that can enable you to go off on adventures sightseeing all of Barcelona, but you aren't discussing these plans with your instructor... perhaps he might not approve of all that you're planning?  Perhaps, maybe, you should limit your expectations of how much adventuring & exploring you'll undertake initially, and have a more complete discussion with your instructor of what you are, and are not, expected to do when, or if, you are signed off to solo.  (As stated in comments, the initial solo is typically pattern work only, rather than navigation.)

Flying is about far more than aircraft control; it is a matter of judgement as well, and the difference between sound judgement and the lack of same can make the difference between a successful or a catastrophic outcome of a flight, at least as much as the difference between great and mediocre "stick & rudder" skills.


Does a subsequent crossing restriction supersede earlier crossing restrictions? Does a subsequent crossing restriction supersede earlier crossing restrictions?

Here's an example.

While flying the MIP4 arrival into KLGA, you receive the expected instruction to cross MARRC at FL180. Before reaching MARRC, you receive the instruction to cross BILEY at 13,000. These fixes being expected is not material to the example. The question is, when given the subsequent instruction to cross BILEY at 13,000, does this supersede the instruction to cross MARRC at FL180.

If possible, please cite answers in 14 CFR, AIM, or another FAA source.

AIM 4-4-10 (g) gives us guidance on this situation.

  The guiding principle is that the last ATC
  clearance has precedence over the previous ATC
  clearance. When the route or altitude in a previously
  issued clearance is amended, the controller will
  restate applicable altitude restrictions. If altitude to
  maintain is changed or restated, whether prior to
  departure or while airborne, and previously issued
  altitude restrictions are omitted, those altitude
  restrictions are canceled, including departure procedures
  and STAR altitude restrictions.

For the example in the question, unless the controller restated the requirement to cross MARCC at FL180, that clearance is cancelled by the issuance of the clearance to cross BILEY at 13,000.


Is it bad practice to say my callsign first in a transmission to ATC? Usually ATC is saying the callsign of the plane he speaks with first and then starts with the actual transmission. Pilots usually say their transmission first and end it with their callsign.

However, I somehow started to say my callsign first before every transmission. For example:

Intended:

  Me: Cleared for take-off runway 33, OE-CNK

How I do it:

  Me: OE-CNK, cleared for take-off runway 33

Is this bad practice? Will it annoy the air traffic controller because he has to keep in mind my callsign for the whole transmission? Or is this not very relevant?

It depends on the country where you are flying, and also what the message is.

For example in the UK according to CAP 413 Chapter 2, section 2.48:

  2.48

  Ground to Air: Aircraft callsign – message or reply.

  Air to Ground:

  Initiation of new information/request etc. – Aircraft callsign then message;
  Reply – Repeat of pertinent information/readback/acknowledgement then aircraft callsign.

In Germany the normal procedure is to start a message with the call sign, a reply may have the call sign at the end.  From the AIP, GEN section 3.4-9:

  6.1 Procedures for Radio Communications

  ...

  (4) The call sign shall be transmitted at the beginning of a message.
  A direct answer to a message may be terminated by the call sign
  instead.


How would a tricycle-geared aircraft be recovered from a tip-up? If improper loading leads to a tip up on a tricycle-geared aircraft:

Image source
what methods are used to right level the aircraft?
I can imagine inflating airbags under the tail, or lifting it with a crane, but what prevents it from slamming down on the nose gear once it passes the tipping point?

While I do not have a specific source to cite, which is always important to reference in regard to any aircraft work, I'm basing this answer on my training as an FAA certified aircraft mechanic.  (There are many more experienced mechanics than myself out there, but alas, none who have done this particular process have arrived yet.)

In general, if you have to work on something that does not already have specified methods from the aircraft manufacturer, the go-to reference is:

  AC43.13 1b/2b - Acceptable Methods, Techniques, & Practices of Aircraft Inspection and Repair

But I don't know if that volume covers this mishap.

I can tell you with some certainty that several inspections will be called for on that aircraft.  The skin and structure of the empennage, of course, but also the main gear will almost certainly require a "hard landing check", since the airframe was adversely loaded so much that the nose lifted.

There's every chance that the event will call for a "tail strike" check, which can be extensive.

If the aircraft were tipped forward in an uncontrolled fashion, the nose gear may come down quite hard, but the fact that the forces involved would be unknown (compared, say, to a landing touchdown at a known sink rate and aircraft weight,) would also necessitate a hard-landing inspection of the nose gear.

In a hard-landing event, the fuselage structure may also be compromised, and relevant areas are also checked, as prescribed by the aircraft's particular inspection and maintenance documents.

When an airframe ever does something in an uncontrolled fashion, many inspections will be needed to conservatively account for every possible imagined consequence, and that can cost a great deal of time and money.

All this is preamble to the kind of approach a good mechanic will take to a problem that offers little previous experience.  Every A&P can change a tire, balance a control surface, and check for corrosion, but getting an aircraft back on all wheels safely is not often addressed.

We know a few things:
• The nose gear can support the unloaded nose of the plane (and we know from records what weight range that is).
• Dramatic movements are frowned upon.
• The aircraft was fine before that last load was introduced.
• Personnel safety is a primary concern. (No dangerous heroics!)

Things we don't know:
• How hard you can pull down on the nose gear before damaging something (they were not meant for such loads!)
• Whether the main gear are at or over their load limit already.  (If people were watching the aircraft's weight and balance figures, we wouldn't be in this mess.)
• Whether the fuselage/empennage has been structurally compromised.
• How fast/hard the nose might come down with the weight distribution corrected - too fast?

From these things, we can determine what NOT to do:
• Pile a bunch of baggage handlers in the nose until it tips back down.  Yay, concussions!
• Adding weight (ballast) in general. (Will the airframe be damaged further?)
• Haul downward on the nose with a winch. (Don't tear the nose gear off!)
• Jack up the tail from the rear. (We don't know if it will hold.  Probably, but we don't take chances.)

Variables that could be leveraged:
• Can the cargo be winched back towards the front without endangering anyone?
• Does this model ("type") have fore and aft fuel tanks in the main fuselage, and if so, can fuel be moved between them (or pumped out) for weight shift, in safety?

We're left with two primary goals:
• Decrease the weight moment behind the mains in a controlled fashion, hopefully without adding any weight to the airframe.
• Do not let the nose fall uncontrolled.

My approach would resemble the following, depending on equipment available (or that could be found in the local region):

Provide safe support under the nose of the aircraft that can support the normal full weight of the nose of the aircraft.  We want to ensure the nose cannot come down without our say-so.  A sling is ideal, and the airframe manufacturer will have specifications on how to lift and support the airframe.

In a comment, @TomMcW provided a nice linked visual in this YouTube video:

As seen in the video, the best option would be an overhead crane, with a sling approved for lifting that airframe wrapped under the forward fuselage.

Here's a more extreme example of using a sling to lift an aircraft:

Once the aircraft is in a stable support with minimized risk of unexpected movement, the next phase is to UNweight the rear, in the safest way we can.  That might be to pump out fuel from an aft tank, or winch a cargo container slowly forward, or (the least favorite) manually unload items, using as few personnel as possible.

As weight is removed from the rear, the airframe will weigh down the sling.  You can lower the sling in a slow and controlled way, with the main gear "chocked".

Once the nose is down, sorting out the rest of the cargo and fuel will be trivial, as flight crew can enter and operate internal systems.

But one thing I know for sure: that bird ain't flying today!


Are flexible pavements not recommended in hot climates? I've received a feedback from an airport manager that flexible pavement is not recommended for runways within the regions of an extreme hot climates; this is because of the particles of asphalt that generate through the aircraft traffic along the time over runways, in which a potential risk on aircraft engines may occur by vacuuming these particles, and that some airports, accordingly, have replaced the runways' flexible pavement with rigid one. Is that true? Is there any new recent recommendations from ICAO or FAA in this regard??

Answering for ICAO, no there aren't such regulations/recommendations.

All what matters to the regulatory bodies is the ACN-PCN. If seasonal changes affect the PCN, then different PCN's are to be reported.

  The bearing strength of a pavement intended for
  aircraft of apron (ramp) mass greater than 5 700 kg shall be
  made available using the aircraft classification number —
  pavement classification number (ACN-PCN) method.

  [...]

  Different PCNs may be reported if the strength of the
  pavement is subject to significant seasonal variation.

  — ICAO Annex 14 Chapter 2

Flexible pavements (asphalt) are actually very common in hot climates, see here: Why are most runways made of asphalt and not concrete?


What are Altitude, Attitude and Heading Hold? Can someone give a good and simple explanation about these three dynamics?

Altitude Hold
Attitude Hold
Heading Hold

These are generally 3 different auto pilot modes. The first two are related to pitch and the third to heading/roll a nice overview can be found here

Altitude Hold: Generally speaking setting an autopilot to altitude hold will cause the autopilot to maintain that altitude by varying the pitch of the aircraft. Depending on the system it may attempt to maintain the altitude even its not possible which can lead to a dangerous situation.

Attitude Hold: Is the autopilot setting that will hold the pitch of the aircraft constant when set to this mode. For example if you depart and climb out at an 8 degree nose up attitude, engage attitude hold, the aircraft will hold 8 degrees nose up until set otherwise or shut off. As noted in the comments this can create a very dangerous situation as the aircraft may continue to maintain pitch (generally for a climb) even if power does not allow this can cause a stall at high altitude. More sophisticated auto pilots allow for constant airspeed climbs which are far safer.

In my experience you are generally in either altitude or attitude mode at any given time.

Heading Hold: This is a roll control mode that holds the heading of the aircraft (keeps it on track). It will typically be slaved to a bug on the HSI where you can set the heading the autopilot flies.


How does a flux gate compass system work? I have heard of gyroscopes and AHRS being used for heading indication, but I haven't heard of a flux gate compass system until recently.

What are the principles of its operation and what types of aircraft or systems are currently using it?

Fluxgates are a way to determine a magnetic field, in this case for navigational purposes. The basic principal is based on the lines of flux that the magnetic field of the planet gives off, a nice overview can be found here:

  The flux gate compass that drives slaved gyros uses the characteristic
  of current induction. The flux valve is a small, segmented ring, made
  of soft iron that readily accepts lines of magnetic flux. An
  electrical coil is wound around each of the three legs to accept the
  current induced in this ring by the Earth’s magnetic field. A coil
  wound around the iron spacer in the center of the frame has 400 Hz
  alternating current (AC) flowing through it. During the times when
  this current reaches its peak, twice during each cycle, there is so
  much magnetism produced by this coil that the frame cannot accept the
  lines of flux from the Earth’s field.

There is a good video here as well.

Older, pre-glass HSI's are often slaved directly to them, like this one. On newer systems the flux-gate is a component of the AHARS.

Lots of aircraft use them, they can be installed on everything from a small GA plane all the up though an A380. These days they are fairly small units.


Is there a maximum lift to drag ratio? If I recall correctly, the best competition gliders have a L/D ratio as high as 60:1. What imposes this limit? Is there a maximum theoretical L/D ratio, or could sufficiently advanced materials allow a glider with a L/D ratio of, say, 200:1?

With current technology the L/D might go up to 70 or 75, and going higher would require an almost impractically large wing span. Gliders need to fly in tight circles to use updrafts, and the larger the wingspan becomes, the bigger the speed difference between inner and outer wing will be. Also, landing such a wide wing without dropping a wingtip will be very hard. Smaller wings with a high aspect ratio will have a low chord length, leading to smaller Reynolds numbers, which translates into a steep increase of friction drag if the aspect ratio is increased without increasing wing area. Therefore, only adding wing span will help, and this runs into a soft wall beyond the 30 m of designs like the Eta. In addition, the current mass limit of 850 kg will make bigger aircraft unattractive for competition pilots.

The Concordia (pdf!) is claimed to have an L/D approaching 75, but I have learned over the years to see theoretical predictions as invariably optimistic, and realistic performance, with bugs on the wing and all, will never quite measure up to the hoped-for ideal.

Werner Pfenniger proposed to use boundary layer laminarisation by suction (pdf!) to reduce friction drag and proposed glider designs with L/D ratios in excess of 100. Turbines at the wingtips would drive the suction pumps, so these would still be unpowered aircraft. But so far nobody has dared to turn his visions into reality.

While laminarisation avoids the increase in friction drag of a turbulent boundary layer, a moving wall would eliminate viscous losses altogether. Now the friction generation is between the moving wall (this could be a taut foil running between two cylinders at the forward and aft end of the wing) and the fixed structure. With current materials there is no hope of reducing drag this way, but who knows what tricks will be possible in the future.

A glide ratio of 100 or more looks rather unlikely in the next 50 years.


Are there areas to avoid while flying formation with a larger aircraft? Consider a fighter jet flying formation with an airliner-size aircraft, such as during an interception or mid-air refueling.

Are there areas which should be avoided by the fighter jet? I.e. are there certain positions where the downdraft or turbulence caused by the larger aircraft would send the small aircraft out of control?

In higher altitude the wake turbulence moves downward behind the aircraft. So as the pilot of the lighter aircraft behind the bigger you should keep the same level at least.

photo by: Steve Morris, image source

  I suppose the wake turbulence only becomes dangerous at some distance, when the downwash forms vortices. – Sanchises

No.
The wake turbulence starts forming at about 2/3 of the wing span (measured from body). It starts as a real narrow but powerful vortex. Then it raises its diameter losing power (slowly).

photo by: Josef  Willems, image source

As you can see near the aircraft the  wake turbulence has a sharp profile and is quite predictable. Therefore there is a safe area directly behind the tanker.

related post: How does an aircraft form wake turbulence?


How is close formation flying possible with things like thermals and turbulence? This weekend the Blue Angels came to town. The weather was less than perfect. A quick look at SkyVector during their show indicated there was an active AIRMET for moderate turbulence.

This made me think about the effects of weather (namely phenomena that can change affect the physical location of an aircraft in flight like turbulence and thermals) on planes flying in extremely close formation.

How is close formation flying possible with conditions like this?
Is the net effect of this type of weather minimized for planes of this type?

Disclaimer: I'm not a Blue Angels pilot so don't know their exact procedures/limitations, but have done a fair share of military formation flying.

Answer:

It is possible, it's just a more "bumpy" ride. Not a huge issue if the turbulence is not severe enough (or of a "rotor" type) to cause uncontrollable rolls in three dimensions.

When flying formation you are separated from other aircraft in three dimensions. In turbulent weather conditions the aircraft position usually results in vertical displacement, not so much in horizontal displacement.

This means the aircraft will not roll into the adjacent aircraft (the roll effect happens when you fly into wake turbulence or through rotors for example), but do more of an "up and down" jump.

The same air disturbance will effect several (if not all) aircraft in the formation, sometimes with just a tiny delay.

Also, the heavier and faster the aircraft is, the less effect the same amount of turbulence will have. If weather doesn't permit a small single-engine piston to fly formation, it may still be workable for jets.

Ultimately, it is up to the flight lead to:
- determine if conditions permit close formation
- fly the formation to avoid known hazards (for example if the wind is from the N, don't fly on the S side of hills/mountains)


What is this airplane seen over Delhi? It was flying over Delhi, India 6:41PM IST May 22 2021

That is a Silk Way Airlines Ilyushin Il-76. Here is a snapshot from flightradar24 from 13:03 UTC (6:33 PM IST) where the aircraft is approaching Delhi Airport:


What do flight attendants do if a passenger asks what is going on in case of a potential emergency? This is related to, but not the same as, another question I asked on Interpersonal Skills.

I was on a flight last year, on a route I fly on a fairly frequent basis. Shortly after takeoff, I heard the three double chime signal, which is commonly used by airlines to indicate to the crew of a potential problem. The crew quickly marched up to the front of the passenger cabin and was briefed by the pilots. I also noticed that the plane was going off course. Some time later, I used the onboard Wi-Fi service to query an online flight tracker about the flight I was on, and it said we were diverting back to our origin airport.

Throughout the whole incident, all the crew kept quiet about the whole thing, and didn't let passengers know about the diversion until just a few minutes before landing back at the origin airport.

Before the announcement, while the seat belt sign was turned off and while the crew weren't making any emergency preparations of any sort (just casually chatting with other passengers), I got up and asked the crew what was going on, and they said "nothing". I then showed them the tracker page which indicated the diversion, and they just asked me to return to my seat.

What's the general protocol for cabin crew, when asked by passengers who notice potential issues like this and get curious? Is it common for them to simply say that "nothing" is going on until the flight deck makes an announcement? What if a passenger really insists on knowing what's going on? (As an aviation enthusiast, I feel more comfortable if told about the issue; not knowing about it can make me feel tensed.)

What's the general protocol for cabin crew, when asked by passengers
  who notice potential issues like this and get curious?

The flight crew and pilots are under no obligation to inform the passengers about non critical issues and broadly speaking it's often better to provide only needed information so as not cause some kind of panic. In the vast majority of situations on an aircraft there is nothing the passenger can do other than evacuate when told to do so.

Turning an aircraft around and heading back to the point of origin is not necessarily an emergency although it can be there is nothing to say it is or is not from a passenger point of view.

Even if a passenger insists the cabin crew will likely ask them to take their seats, buckle up and await any further instructions. It's possible that the cabin crew is not even informed of anything as the pilots try and work the problem.

The later part of your question is answered here and the general answer is, unless you are a trained pilot it's hard to actually know if something is going wrong. There is a very interesting example of this from the Air Florida Flight 90 incident which is the only incident I know of where a non flying pilot spotted an issue and took action, this would be a fairly rare case;

  Survivors of the crash indicated the trip over the runway was extremely rough, with
  survivor Joe Stiley – a businessman and private pilot – saying that he
  believed that they would not get airborne and would "fall off the end
  of the runway". When the plane became airborne, Stiley told his
  co-worker (and survivor) Nikki Felch to assume the crash position,
  with some nearby passengers following their example.


Why don't large commercial airplanes have electric motors in the wheels? The cost of jet fuel during taxiing can be a pain. Concorde in particular had very large fuel expenditure for taxiing.

So why not put electric motors in at least 2 wheels to drive the plane that way? They would not have to be the most powerful motors in the world---only strong enough to get the plane up to a few miles per hour, or normal taxi speed.

EDIT: I don't think this is a duplicate of this other question. If anything, the savings cited in the answer of that question seems to imply that my question, asking why it isn't implemented, is actually a pretty good question.

In fact, this is currently on test.

This test is conducted by Airbus, Safran and Honeywell for the Airbus 320 familly on this aircraft.

An engine is installed on the wheels of the main landing gear

As you might know it take long time and lot of money to certified a system in aeronautics, this is why it's not in use yet.

I other hand, you need to run engine in idle for about 5 minutes in order to warm up the engine and stabilise systems (hydraulics, gyroscopes, etc)

You can find more here : http://www.greentaxiing.com/

Edit 1 : edit to add @Ben elements


Why is the landing distance increased on grass runways? Checking the Cessna 152 POH it says to increase takeoff distance by 15% of ground roll (makes sense due to increased friction). Also says to increase landing distance by 45% (doesn't seem to make sense). Would like to understand why.

The long and short of it is that wheels are designed for smooth tarmac. While grass increases friction on the wheels on takeoff (making the aircraft work harder to get up to speed), it also reduces the efficiency of the brakes, thus taking longer to slow down.

And to add to @GdD's point, grass retains moisture much better than tarmac, so in wet conditions it is considerably slippier (and will remain so for longer afterwards).


Why is "proverse" yaw better than no yaw? Why is the tendency to yaw in the direction of the roll more desirable (since it's the "proverse" yaw) than the lack of yaw in response to roll? Is it even possible to have no yawing tendency at all during rolls in a plane?

I would add that it depends on what you mean by "proverse yaw", and that vertical fin size is key.

Proverse implies "too much" yaw in excess of the appropriate yaw rate for the angle of bank, which would be a skidding turn. That's bad.   If proverse just means an into-turn yaw rate that is appropriate for the bank, that's good.

Airplanes with spoilers-only for roll control suffer can suffer from "bad" proverse yaw in that sense, because the drag of the down-wing spoiler, with nothing sticking up on the opposite wing to offset it, pulls the nose into the turn too much, requiring out of turn rudder to keep coordinated flight (I've heard that the spoilers-only Mitsubishi MU-2 is like that).  But almost all airplanes have the opposite, adverse yaw, to one degree or another.

Anyway, what we want is a natural yaw rate that is appropriate to the bank angle so the tail naturally wants to stay lined up behind the nose in the air stream during the turn. Asymmetric drag effects of flight controls aside, the yaw rate you get in concert with bank to get a turn is a function of the vertical fin (and the rudder, to the extent that it's providing a passive fin-like effect, like with a hydraulic rudder, or a mechanical rudder with centering springs).

It's basically a big weathervane.  If you had no fin/rudder, no weathervaning effect at all, you'd just skew sideways when you bank (even ignoring the dynamic yawing effects of the rolling action or asymmetric aileron drag, which make the slipping worse).

You would think it's desirable to have a fin that can make the tail follow the nose immediately and precisely, when side slip occurs as a result of banking, so the bigger the vertical fin, the better.  But there's a problem with that.  To exploit the self-righting effect of dihedral, to give wings-level stability in roll, you need to have a little bit of side slip occur when the wings are banked, to create the self-righting lift differential.

So you need a weathervane, but not too big a weathervane.  The sizing of the fin itself is a balancing act over the need for strong into-turn yaw, and for roll stability due to dihedral effect.  If the fin is too big, into-turn yaw is too strong and immediate; the airplane will immediately yaw (weathervane) into the turn with little or no initial side slip, and it will want to enter a spiral dive any time it's banked.  Vertical fin too small, and the weathervaning effect is too weak and the airplane hunts and slithers around in yaw too much.

There is a sweet spot for fin sizing that gives good "proverse" yaw to give a naturally coordinated turn that only needs bits of rudder to counteract adverse yaw while the ailerons are displaced, while still allowing just enough initial side slip to allow dihedral effect to apply its self-righting tendency when the airplane is banked slightly by a bump.


Is rudder deflection sufficient to maintain directional control on the runway at high speed, or is nose wheel steering needed? If a fighter jet veers off the runaway, is nosewheel steering needed to counter, or can directional control be maintained with the rudder deflection alone?  Is there a chance that the rudder deflection alone won't be enough to change the aircraft direction?  Is there a speed zone or gap were the rudder deflection is not aerodynamically available, while at the same time steering of the front landing gear is ineffective due to loss of friction?  Or does the max speed for nosewheel steering and minimum speed for rudder effectiveness overlap each other?

There is some overlap in effectiveness, and therefore no real magic number, but most aircraft that don't have full time nosewheel steering will have some published airspeed at which one will be favored over the other.  For example, in the EA-6B it was 80 KIAS.  On take off roll you would hold the nosewheel steering button on the control stick down to maintain directional control with the nose wheel.  Accellerating through 80 KIAS it was a call-out to release the NWS button and rely on the rudder effectiveness to keep control up to rotation at around 145 KIAS.  The reason for this was not because the nosewheel was less effective due to less friction, (traction) but rather there was a risk of overcontrolling steering at higher speeds.


What aerodynamic data exists for the Yak-130? This is one of 3 question in "series" - one question split up in 3 to better reward those responding about each aircraft. Other questions: A-10 L-159

Yak-130 is one 3 planes that are used in the ArmA 3 combat simulator, but its aerodynamics modelling currently lacks significantly. I'm planning on writing a short "essay" to developers regarding what and how can be specifically fixed in the flight model, but I'd like to back my research with some hard data. Specifically, I'm interested in any of the following:

L/AoA
D/AoA
Side lift/SA (Slide angle)
Max G/velocity
Flight envelopes

Torques can well be left out - it's not a full fledged simulator, but the above data can give quite a good estimation of high level characteristics already

Are there any good sources on any of the above data? Low-mach data is good enough for me.

I understand this is a tall order, but, well, SE is for expert answers - maybe a wonder happens and someone here has it? :)

The Yak-130 has a maximum g-loading of +8g to -3g and is capable of executing the flight manoeuvres specific to current operational and developmental combat aircraft, including Su-30, MiG-29, Mirage, F-15, F-16, Eurofighter, F-22 and F-35. This aircraft is of classical swept-wing and empennage monoplane design and light alloy construction with carbon-fibre control surfaces. Kevlar armour protection is fitted to the engines, cockpit and avionics compartment.
Flight angles of attack < 40°.
Speeds:
– level flight, maximum   1 050 km/h
– takeoff (full fuel load)    200 km/h
– landing 195 km/h

Side lift and flight envelope is kind of hard to come by, but I got this information from https://thaimilitaryandasianregion.wordpress.com/2015/10/25/yakovlev-yak-130/.
I would also recommend http://www.airforce-technology.com/projects/yak_130/ as another source to look through. Hopefully I was of any help to you in writing your essay.


How does the extension of inboard flaps raise the effective critical angle of attack of the outer wing? I raised a question about this on this post in the comment section and was suggested by Peter Kämpf to create a new question. So here it goes: How does the extension of inboard flaps raise the effective critical angle of attack of the outer wing? And when that happens, why does the outer wing stall first without the help of slats when extension of the inboard flaps induces more lift on the whole wing, not just on the outer wing?

First it is beneficial to have an idea how the pressure field around an airfoil influences the flow around it. With powerful flaps, the suction peak on the upper side is already well developed at smaller angles of attack, so the surrounding air will be accelerated towards that low pressure area.

Now you need to extend that mental picture to 3D. The inner wing has those flaps deployed and, consequently, a suction peak near the leading edge and low pressure on the whole upper side. This will also suck in air from the side. As a consequence, the flow moving towards the outer wing will see a similar upward acceleration as does the flow streaming towards the inner wing, plus a sideways acceleration. This acceleration increases the local angle of attack at the leading edge of both wing sections. If the outer wing was already close to its stall angle of attack before the flaps were lowered, now the increased angle of attack will push the flow beyond the stall condition.

The geometrical angle of attack (measured as the angle between the flow direction at infinity and the reference length of the airfoil) did not change. The "real" angle of attack as experienced by the outer wing, however, will produce a flow similar to what would normally only be experienced at a higher geometrical angle of attack. This moves the outer wing into the stall region when the inner wing still works in the linear realm. This can lead to asymmetric stalls which produce a strong and sudden rolling moment, and at low altitude that is the last thing you need while lining up with the runway.

Adding slats will move the stall angle of attack up a few degrees, and now it will again be the inner wing which stalls first, as it should.


What does ICAO database "engine count c" mean? In the official ICAO database 5 aircraft are marked with an engine count C. What is that supposed to mean? Normally engine count is a number (0,1,2,3...).

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

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

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

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


Can small planes see lights on the ground? I live about 5 miles from a small airport. At night, during rain or snow storms when I imagine there would be poor visibility, I always turn our outside house lights on, in case they could possibly help a little plane in distress. Am I crazy?

Yes we can see lights on the ground from small planes, but your extra lights do not help approaching aircraft. If anything they may serve to disorient pilots. In low visibility situations, provided the pilot is properly rated and the aircraft is properly equipped they may be flying IFR. Under these types of operations when popping out of the clouds we may be looking for runway lighting. Other lights on the ground can (and have been) confused for runway lights in the past.


How do pilots adjust trim on the B777, where the control forces are simulated? I know that pilots of small airplanes (e.g. Cessna 172) use force to trim, i.e. they hold the required attitude using the yoke, then trim the aircraft until no force is required to hold the yoke in that position. In that way, they trim using 'feel' to gauge how much trim is needed.

Airliners and some other planes use trimmable stabilizers for trimming, thus the yoke always returns to center if the pilot leaves it.

So I was reading the B777 FCOM and it's written in there that since the plane is fly-by-wire, it simulates control column forces to let the pilot know it is out of trim.

My question is how does the pilot know how much trim to use? I know one way would be to let go off the yoke and see if the plane keeps its attitude or not, but is there another way?

(sorry for the long question, I am a newbie here and did my best to ensure you understand my question).

The basic idea, as the FCOM says, is to simulate control column forces (as in 'conventional' aircraft) so that the pilot knows that he/she is out of trim. So, once the aircraft has been trimmed, there is no force on the control column- the pilot stops trimming at that point.

Boeing 777 uses a feedback actuator to give an artificial feel to the pilot emulating conventional aircraft- this is no accident, as Boeing B-777: Fly-By-Wire Flight Controls notes:

  The philosophy employed during the design of the 777 Primary Flight Control System  maintains a system operation that is consistent with a pilot’s past training and experience. What is  meant by this is that however different the actual system architecture is from previous Boeing airplanes, the presentation to the pilot is that of a conventionally controlled mechanical system.

The control columns (yokes) have feel units, which provides tactile feedback to the pilots just like conventional aircraft:

  A gradient control actuator is attached to the two control column feel units. These units provide the tactile feel of the control column ... (This) is consistent with a pilot’s experience in conventional commercial jet transports.

The image below shows the Boeing 777 yoke with the trim switches.

... The buttons on the right side of the yoke control the trim surfaces for the elevator.; image from wired.com

This trimming system works just like the conventional aircraft- as you noted "they hold the required attitude using the yoke, then trim the aircraft until no force is required to hold the yoke in that position". Note that, though what happens internally is different from the conventional aircraft, what is presented to the pilot is same. From An Analysis of the Boeing 777 Fly-By-Wire System:

  ... the pitch trim system ... is used in the exact same way, that is, should the pilot find himself pushing the control column forward to keep his desired flight condition, pushing the trim "up" ... will gradually reduce the required force on the control. The opposite goes if the pilot is pulling back on the control. Just like in any aircraft, the ship will be "in trim" once the required force on the control column is zero.


What is this device protruding from the rear of this 787? While looking at the photos from this question
I noticed a small protrusion from the side of the plane. You can see it in the photo:

and in the diagram from l@aeroalias answer:

In the diagram it appears to have something to do with the apu or is fire protection system. There's a shaded line running to it.

What is this object?

Edit: It does look like an antenna. I found a diagram of 777 antenna placement here, but it doesn't show anything in that area.

As you have already imagined, it's the APU fuel line shroud drain, as visible here:

(Source)

Detail:

The fin gives an aerodynamic profile to the drain. The reason this drain exists seems to be:

  Natural fuel leaks from the auxiliary power unit (APU) fueling line
  were supposed to drain harmlessly out the bottom of the structure as
  designed, though rather than do that, the fuel was pooling into the
  APU exhaust cone.

(Source)


Which aircraft is a single-prop, low-wing aircraft with prominent rivets and three canopy windows? We would appreciate help identifying an aircraft that my daughter saw. It is a single-prop, low-wing design with a round fuselage and very prominent rivets.

It had a prominent canopy that stood above the fuselage with visible windows, and the rudder was visibly slated as well.

When I saw your daughter’s drawing I thought of the Vultee P-66 “Vanguard”.

But the Republic P-47 “Thunderbolt” might also be a good candidate.


Why generation of lift is more at mid span of the wing? For the same condition like same density, wing area, free-stream velocity, From the above figure one can see more pressure differential and hence more lift at mid span of the wing?
What causes this phenomenon? Is it due to influence of tip vortex?

The diagram might be a bit more clear if a fuselage were attached to one of the sides.
If that were the case, lift would be less on one side because of the fuselage is interfering with the air flow.  Lift would be less at the other end due to washout (lowering of wing tip angle of incidence), and effects of higher pressure spilling from the bottom around the wing tip towards the lower pressure on the upper wing (vortex formation).
The airstream in the middle of the wing experiences neither of these two interferences, generating lift most efficiently.
This is why gliders favor very long (high aspect) wings, where most of the lift is like the "middle" of the diagram.


What are the most common reasons an aircraft to depressurize while flying? What are the most common reasons that cause an aircraft to depressurize while flying? For example, a broken window, a failure on the Outflow Valve, etc.

The most common failure leading to depressurization I've personally seen is a pack/air cycle machine failure. The equipment that takes pressurized bleed air from the engine, cools and expands it. I've seen it happen two ways. 1) The aircraft is dispatched with one pack on MEL and the second pack fails in flight or 2) The aircraft is flying at high altitude, 1 pack fails, the other pack switches to high flow mode and fails soon after.

Pack failure is by far the most common I've seen. But have also seen a couple failed fuselage skins and a few burst PAX windows. Typically door leaks are enough to be noisy as heck, but not depressurize the aircraft.


Has the wrong engine ever been mounted after maintenance? Keeping in mind that a plane can fly with different engine models (e.g. for test purposes), has an error ever happened during normal operations that led to the wrong engine being mounted? Is some kind of calibration or check necessary before flight to ensure that the engines are the correct ones?

January 2016: a "wrong" or non-functioning engine was mounted onto a Qantas 747. QF63 departed from Sydney to Johannesburg with a 5th engine.

The 747 is capable of carrying these kind of loads through anchor points on each of their wings.

Disclaimer: this wasn't by mistake, it was intended to get the "5th" engine to Johannesburg ASAP, to be fitted onto another aircraft.

As for the possibility of mounting an incorrect engine on an airplane by accident, it's very unlikely (impossible really). Jet engines are designed exclusively with the aircraft in mind. Further, jet engines are expensive you would make certain you order the correct engine for the corresponding aircraft. Aviation mechanics also receive sufficient training and the an engine replacement process isn't a one man job. It circulates through a group of individuals prior to actually making a purchase or deciding to fit a new engine.

Besides, even in the case where some unqualified individuals were to attempt mounting an incorrect engine, it wouldn't work. First, they would easily be able to distinguish the existing engines from the replacement. Second, the mounting the engine in the place wouldn't work, the mount points would be of a different shape and would consist of bolts and screws in different locations.


What is the stall speed for the Dassault Rafale? Looking at this video of a Rafale escorting a high-speed train (TGV)

I was wondering what the stall speed for the Rafale is at close to sea level altitude.

Is this information generally public for fighter aircraft? I could conceive this would be classified.

80 knots for an aggressive pilot, as said here.

It is 148 km/h, a speed of fast but rather typical long range passenger train. No new technology is required for the train, this plane could easily follow a Pacific steam locomotive (top speed 203 km/h).


Does "cleared into the Bravo at 3500" mean I must wait until I hit 3500 before entering it? Let's say I'm two miles away from entering the class B, heading straight for it at 1500 feet and climbing. Approach control gives me my clearance as "...cleared into the class Bravo at 3500 feet...". On one hand, they know my heading and altitude, so I assume that I should continue straight on, entering the Class B in my climb and level out at 3500 feet, which I have done often with no consequences. However, a very literal interpretation of the clearance would have me turn away and remain clear of the Class B until I had leveled at 3500, at which point I could turn back and enter it. If push ever came to shove, which interpretation is correct?

On one hand, they know my heading and altitude, so I assume that I should continue straight on, entering the Class B in my climb and level out at 3500 feet, which I have done often with no consequences.

This is what the controller wants you to do.  It's slightly sloppy phraseology - "Cleared into Bravo, climb and maintain 3500" would be better.  But what she said is not really ambiguous in any practical way.  She would definitely not be expecting you to turn away, then climb, then turn back toward the B.