What is the purpose of multiple autopilots? I have read that some airplanes have multiple autopilots, specifically airliners.

What is the reason for this, and how does that system work?

Redundancy, particularly during autoland operations.

How it works depends on the specific airplane type. In the most advanced systems with triple (or more) flight guidance computers (FGCs), a "voting" concept is used during critical operations like autoland. All 3 computers calculate their commands independently, and vote on what to do. The middle value, or sometimes the average of the 2 "most similar" values, is then passed to the flight control actuators. In this way, if any one of the computers fails and starts producing garbage commands, those erroneous commands will not be selected (they are automatically rejected by the voting scheme). A monitoring system can also detect a failed computer in this way and take it offline, allowing "fail-active" functionality — an autoland can continue on the 2 remaining functional computers.

Less complex aircraft, and aircraft without autoland, still sometimes have 2 FGCs, but do not support fail-active operation. In these systems only one of the two computers is active or selected at any given time. A transfer switch in the cockpit allows the pilot to transfer control from one computer to the other if one fails.

And it's not just airliners. Business jets commonly also have dual FGCs, and in recent times even small general aviation airplanes like the Diamond DA-42 have dual systems (Garmin G1000 in dual-GFC700 configurations).

This photo (click for high res version) shows a Cessna Citation CJ business jet with dual Garmin autopilots. At the top of the PFD, notice there is a small green arrow pointing left; this indicates the left FGC is currently selected. The photo also shows the XFR (transfer) button on the mode select panel, top center, just underneath the master caution/warning annunciator panel. Pushing this button would switch to the right (#2) FGC and change the arrow on the PFD to point to the right. These indications are fairly representative of other dual-FGC systems.

Here's another photo of a Hawker 800 with Rockwell Collins Pro Line 21. You can see the same arrow on the PFDs. In this aircraft the transfer switch is labeled "AP XFR" and is again on the autopilot panel just underneath the glareshield at the top of the panel toward the left hand side.

And finally a photo of a 767 with 3 FGCs and full autoland capability. It's a bit hard to read in this photo, but just above the autopilot disengage bar (the "spacebar" looking thing on the right side of the mode select panel just under the glareshield) the 3 buttons are labeled "A/P ENGAGE", "L" "C" and "R" for the left, center and right FGCs. During an autoland all 3 would be selected and illuminated.


How is the cargo hold volume for passenger aircraft designed? An A380 has up to 2 decks for passengers and only 1 deck for cargo (for passenger's luggage), whereas a B777 has 1 deck for passengers and 1 deck for luggages. It looks like the volume of luggage per passenger is reduced in a A380 compared to a B777. Is that true? Is the volume of luggage per passenger the same? Is there a method to calculate the volume of the cargo and then to design its shape?

The cargo compartments are not only for luggage. Commercial cargo also travel on passenger aircraft, besides cargo aircraft. With luggage alone, there are plenty of spaces in the cargo deck.

In fact, if you fill all the seats, all the cargo space and sufficient fuel for a long trip in a B777, the plane won't takeoff - its GW (Gross Weight) exceeds the MTOW (Max Take-Off Weight).

Airlines are smart people - if a flight is not full, dispatch will put more cargo on the aircraft to utilize it. A lot of passenger airliners also offer air delivery service.

Some passengers complain, "I can't book the flight. The staff says it's full, yet there are many empty seats!" Commercial cargo yields more revenue than an economy seat. Thus, given the weight limitation, an airline may prefer cargo vs passenger.

I don't have info about A380, but I'd guess that the A380 has less extra cargo space than the B777. But as long as it's enough, it isn't a problem.


What is the safe speed to use highspeed turnoff? I understand that high speed turnoff is for the plane to exit the runway asap. What is its design specification and how fast can a plane use highspeed turnoff? Assuming only A320,737 and larger.

Please be careful applying a blanket figure during high speed turnoff. There are many variables which you should consider. These are:

•   Exact angle of the taxiway(you're turning onto):  If it's a very shallow angle off the runway and therefore does not require much turning, high-speed turnoffs are easier and manageable, and vice versa.

•   Rate of deceleration

•   Side load limitations on the CRJ

•   Tolerance of the nose gear

•   Traffic situation on taxiway

Generally speaking you need both sets of eyes outside, especially if you are joining or crossing another Taxi Way.


How dangerous is flying in a single-engine plane? Some people consider flying small planes very dangerous. Just how dangerous is it to fly in a light single-engine plane? For example, how does the fatality rate compare to driving a car, riding a motorcycle, etc.?

I get this question a lot from people who are apprehensive about flying with a private pilot.  I'm afraid I won't be reducing these fears in any way.  Let's review some general statistics during 2008.  Note - these stats aren't specific to light or single engine aircraft:

NTSB reported there were 1.21 fatalities per 100,000 flight hours for private aircraft (Part 91 operators).
NHTSA reported there were 1.26 fatalities per 100 million miles travelled by automobile

We can equate that to about 2 million hours (estimating an average speed of 50mph). This gives us 0.063 fatalities per 100,000 driving hours.

Private aircraft have a fatality rate about 19 times greater than driving.  It is also true that a majority of the accidents that occur are pilot error (71%) and could have been prevented.

There are risks involved when taking to the sky as a private pilot and understanding these risks is part of the continual learning process.  The key to safety is performing careful planning, keeping current and proficient, knowing when to cancel flights or turn around and not to exceed your capabilities or the capabilities of your aircraft.


Are there any airliners not made by Airbus or Boeing? Are there any commonly used passenger aircraft larger than regional jets (100+ seats) that aren't Boeing or Airbus?

If so, what are they and where are they used?

Yes. Ignoring companies that used to exist but are now bankrupt/merged into Airbus or Boeing, you have companies from countries that aren't historically too friendly with the US and Western Europe. For instance, Tupolev makes the Tu-204, which is currently in production and seats over 175 passengers. The Ilyushin-designed Il-96 is a widebody in current use some places (Russia, Cuba). Before the Cold War ended, Russia had more indigenous airliners; Aeroflot now largely uses Boeings and Airbuses, but there are still Russian aircraft that see use.

In addition, you have newer manufacturers starting to enter the market: Bombardier with their CSeries, Comac (in China) with their C919 (in development), and Irkut with their MC-21 (in development). Embraer makes planes of more than 100 seats (although they're shorter-range, and you might consider them regional jets).


Can a normally poorly handling airframe with high moments of inertia actually have superior flight dynamics under autonomous control? I’m not a pilot but know a thing or two about flight dynamics. As you know there are currently quite a few aircraft under development designed for low-and-slow and (eventually) autonomous flight. These are often referred to as air taxis, eVTOL planes et cetera. Their engineering may require a lot of new thinking but perhaps not. This relies in part on the answer to the question whether they’re designed to be optionally autonomous or exclusively so. My question pertains to aircraft that are NOT to be controlled by humans or only in the case of an emergency, but even then only indirectly through an advanced fly-by-wire system, one probably more advanced than those in modern fighter jets which would be impossible to fly without FBW.
The autonomous air taxis will be more affected by atmospheric turbulence than commercial high-and-fast jetliners and turboprops, and they’ll have to dynamically compensate for this to ensure a smooth and safe ride. To achieve this requires continuous adaptation relying mostly on gyroscope measurements coupled to small to moderate steering actions under autonomous control. Responsiveness is key here and that’s what computers are good at.
So let’s say we’re designing an airplane that’s to be autonomously controlled at all time (obviously with sufficient redundancy). Might for example a plane with a high moment of inertia provide a smoother and at least as safe ride as a “properly” designed airframe designed for manual control? Or do you think the old rules still apply: low(-ish) rotational inertia provides superior handling and passenger experience.
My thinking is that a flight computer that’s programmed to stay within rationally determined limits (to prevent spins et cetera) can fly a plane not just safely but smoother as well. This is akin to how very large ocean vessels are hardly affected by waves and winds compared to small ships. The small vessel is likely to have superior handling characteristics under optimal conditions and yet you'd be much rather off sailing on the large one even if both ships are able to stay afloat in extreme weather.

The underlying assumption of your question seems to be that a higher control bandwidth always leads to better disturbance suppression. Since a higher control bandwidth is easier to achieve with low-inertia systems, aircraft with low inertia would have better disturbance suppression.
This assumption can work in theory, especially for SISO (single-input, single-output) systems. However, for aircraft, we are essentially dealing with MIMO systems. While we can achieve near-perfect disturbance suppression for one degree of freedom (e.g., zero variation in altitude), this will inevitably lead to large excursions of other degrees of freedom. For example, disturbance suppression in altitude will lead to large variations in pitch as we try and keep AoA constant with every gust. So, for a MIMO system like an aircraft, we have no choice but to formulate a cost function of what we consider a 'smooth' ride, and balance the control scheme across various degrees of freedom.
Since we are considering a balancing act, it then makes sense to tune the internal dynamics of the aircraft such that a smooth ride is naturally achieved. So that means: high inertia for degrees of freedom experienced by the passenger, and low inertia for degrees of freedom not felt by the passenger. In practice: larger fuselage inertia and smaller control surface inertia.
Unsurprisingly, this is how it works in practice: larger aircraft are generally more comfortable, whereas at the same time, degrees of freedom not felt by the passengers are exploited in modern aircraft for gust suppression. For example, wings are more and more dynamically decoupled from the fuselage by highly flexible composite wings. Ailerons are split in smaller sections for gust suppression, you may find videos of "dance of the ailerons" for modern aircraft.


What is the relationship between fuel mixture and RPM in a reciprocating engine? I'm doing a project on a safety occurrence but I don't understand how the RPM of an engine is affected by the fuel mixture.
How does a richer than normal mixture result in lower RPMs?

I assume you are referring to a fixed pitch propeller because a constant speed propeller will always hold the same speed as the governor automatically changes the blade angle.

Just like any piston motor there is an optimal mixture. Richer or leaner results in lower power. Planes are set up to run rich at full rich mixture so leaning during cruise or descent will produce more optimal power which will affect rpm slightly in a fixed pitch application. Excess fuel displaces oxygen which decreases power flighty.


How does airfoil affect the coefficient of lift vs. AOA slope? I'm trying to understand whether the airfoil shape can affect the slope of Cl vs. AOA (coefficient of lift vs. angle of attack) in the "linear" portion, e.g., from a few degrees negative to about 10 degrees. As an example, I picked 9 different airfoils from the NACA 4 and 6 series on airfoiltool.com and they all have pretty much the same slope in their linear regions (Rn 1'000'000):

The question I have is, if I would like a steeper slope (more effect on lift with less change in AOA) are there specific airfoils I should look at? Or any specific airfoil characteristics? Or is this slope basically given by physics for "reasonable overall well-performing" airfoils?

The airfoil affects the lift curve slope only very little; theoretically the slope should be a bit steeper for thicker airfoils.
But that pales in comparison to the effects of wing planform. Here the biggest influence is from the wing's aspect ratio and sweep. Another factor is the flight Mach number and the Reynolds number.
But there is one trick to artificially increase the lift curve slope for a given planform and flight speed: Add a camber flap and move it in sync with angle of attack. This will shift the zero-lift angle inversely to the change in angle of attack. A purely mechanical solution would use a flap with a large control horn, so that $c_{r\alpha}$ becomes positive while $c_{r\eta}$ stays negative. However, this is prone to flutter at higher speeds, so depending on the planned speed range a hydraulic or electric actuator, driven by a control unit which is hooked up to an AoA sensor, might be the better solution.


What made Concorde so heavy? The Concorde burnt five times more fuel per passenger mile than the 747. A huge contributor must have been the immense empty weight per seat, three times that of the B747.

B747-300
Concorde

Empty Weight
174,000 kg
78,700 kg

Passengers
400–660
92–128

EW/Passenger
263–435 kg
615–855 kg

This weight excess is distributed among the slender fuselage, thin wings, long landing gear, big engines, and (possibly) material constraints.
But how much of the weight increase comes from each of these components, and why? I am looking for a comparison with the weight breakdown of a conventional jetliner

The Concorde mainly transported fuel across the Atlantic so that enough was available to land safely. Passengers were just an extra on top. Also, being first-class only, it held fewer passengers than what low budget airlines have taken to cramming into their planes lately.
To look at the structure relative to passengers is a bit misleading. Let's look at the fuel load and the mass fraction of empty weight relative to MTOW instead:

747-100
Concorde

MTOW
333.4 tons
186.88 tons

Empty mass
162.5 tons
78.9 tons

Fuel
183,380 liters
119,800 liters

Fuel to empty mass
1.1285 L/kg
1.5146 L/kg

Mass fraction of empty to MTOW
48.74%
42.22%

If you compare passengers plus fuel to the empty mass of each airplane, the Concorde actually comes out well ahead of the 747 and looks like a prize of lightweight design. Having structurally efficient delta wings helped and should explain most of the difference.


What is 20NM reference point in "CTC SOCAL WITHIN 20NM"? In Los Angeles TAC you'll see magenta boxes with SOCAL frequency and other information.

I was just wondering what is 20NM reference point is? in other word, 20NM from where or what?

Short answer:

The primary Class C airport

Longer Answer:

Your question refers to the magenta boxes on sectional and TAC charts that read CTC SOCAL APP WITHIN 20 NM on 134.2 338.2, or equivalent:

These boxes are used in connection with the nearest Class C airspace on the chart. Arriving pilots are requested to contact approach within 20 NM of the primary Class C airport. This is the answer to your question, the primary airport of that Class C airspace. Which airspace and airport exactly will depend on which magenta boxes you look at on the Los Angeles TAC. In the chart examples above and below, that airport would be Bob Hope (KBUR).

The deeper significance behind this instruction is the procedural Outer Area of the Class C airspace which is not depicted on the charts. Though not mandatory, this outer area of the Class C airspace is still considered part of the controlled airspace, and pilots are asked to contact approach before entering this area. See section 3-2-4 of the Aeronautical Information Manual (AIM)

  3-2-4 c. 2. (b)
  NOTE-
  4. Though not requiring regulatory action, Class C airspace areas have a
  procedural Outer Area. Normally this area is 20 NM from the primary
  Class C airspace airport. Its vertical limit extends from the lower
  limits of radio/radar coverage up to the ceiling of the approach
  control's delegated airspace, excluding the Class C airspace itself,
  and other airspace as appropriate. (This outer area is not charted.)

Note that the locations of the boxes on the chart (see image below) roughly correspond to the various approach sectors, with the frequencies contained in the boxes varying accordingly.

Also note that the blue boxes similarly correspond to Class B airspace, though the 20 NM note is absent, since Class B does not have the unmarked Procedural Area.


Is a magnetic compass required for VFR (day) flight in powered aircraft? As the title suggests, Is a magnetic compass required equipment for VFR (day) flight in powered civil aircraft? I have always been under the impression it was.

My question arises from reading CFR section 91.205, article b:

  b) Visual-flight-rules (day). For VFR Flight during the day, the
  following instruments
         and equipment are required:

  ...
     (3) Magnetic direction indicator

This sort of implies I can use anything, including my GPS that indicates magnetic direction. Is my interpretation correct? Has this changed recently?

The phrase "magnetic direction indictor" is interpreted as "magnetic direction-indicator", not "magnetic-direction indicator".
You need a device based on magnetism that indicates your direction. That could be a floating-card type compass, or a fluxgate compass, for example.
Some interpretation of this phrase is given in AC 23.1311-1C section 8.8:

Under VFR operation, part 91, § 91.205, requires a magnetic direction indicator (that is normally intended to be a compass) for heading information.
[...]
Section 23.1303(c), Amendment 23-62, amended the requirement from “A direction indicator (non-stabilized magnetic compass)” to “A magnetic direction indicator.” As new technology becomes more affordable for part 23 airplanes, many electronic flight instrument systems will use magnetically stabilized direction indicators (or electric compass systems) to measure and indicate the airplane heading to provide better performance.
[...]
Part 23 does not prescribe specific accuracy requirements for magnetic gyroscopically stabilized heading systems. Specific accuracy requirements for avionics may be found in the related TSO and, as acceptable means of compliance to § 23.1301, in ACs, notices, or policy statements/letters.

Further, installation of the magnetic direction indicator is regulated by §23.1327:

Each magnetic direction indicator must be installed so that its accuracy is not excessively affected by the airplane’s vibration or magnetic fields.


What is the minimum takeoff fuel requirement policy for commercial aircraft? I am currently working on a fuel burn analysis of a particular route (ATL to DCA). I really want to predict takeoff weight of the flight; however, I realized that it's not easy to get the information. Instead, I may want to gather information about minimum fuel requirement for the flight.

For example, I heard from the pilot of the flight that they should have more fuel (20,000lbs) than the minimum fuel (15,000lbs) before takeoff. Could anyone please let me know where I can get this information such as minimum fuel requirement for A320 from ATL to DCA?

14 CFR 121.639 Requires an airliner to have enough fuel to reach the destination, then fly to the (most distant) alternate, then fly after that for 45 minutes. If you want to know exactly how much fuel that is for a particular airplane on a particular route, you have to refer to that airplane's POH and plan the flight. It will also depend on the winds aloft and what airport is chosen as the alternate.


Is the airspace at KGPT class E or G when the tower is closed? I'm a bit new to airspace and I'm trying to figure out if the airspace around Gulfport-Biloxi International (KGPT) changes to class G or E when the tower closes.  Looking at KGPT's airspace on a sectional, it is Class D with a note to check NOTAMs/Supplement for Class D/E (sfc) effective hrs.

I've checked the chart supplements and it says Class G when the tower is closed, but the AIM leads me to think it would be Class E at 700' MSL (with Class E extensions changing to Class G) when the tower is closed since it has ASOS.  Does this sound right, or am I missing something?

To expand a little more on what @rbp said, the VFR Sectional Note is referring to the Class D and Class E (sfc) designations.  So, when referring the the Chart Supplement, which says AIRSPACE: CLASS D svc (1200-0500Z) other times CLASS G they are referring to the Class D and Class E (sfc) specifically—just as @rbp said.

Now consider what the next level of controlled airspace is when the tower is closed and the airspace over KGPT becomes Class G. It would be 700’ AGL Class E airspace. The AIM goes on to detail this (emphasis mine):

  Where a Class D surface area is part-time, the airspace may revert to either a Class E surface area (see paragraph 3−2−6e1) or Class G airspace. When a part–time Class D surface area changes to Class G, the surface area becomes Class G airspace up to, but not including, the overlying controlled airspace.

Source: Aeronautical Information Manual (AIM), Change 1, May 26 2016, 3-2-5 Class D Airspace (13 MB PDF)

And goes on to explain in Note 2 of paragraph 3-2-5 (with added emphasis):

  Normally, the overlying controlled airspace is the Class E transition area airspace that begins at either 700 feet AGL (charted as magenta vignette) or 1200 feet AGL (charted as blue vignette). This may be determined by consulting the applicable VFR Sectional or Terminal Area Charts.


For light, GA aircraft, is there a "standard" turn, and what does that mean? I have seen instructional sites that seem to indicate a 15 degree bank is a standard turn.  The Turn Coordinator has 15 and 30 degree marks on it.  Is 15 degrees a standard turn and what does this mean?  Is there a general way to figure out what the radius of such a turn is or how long it would take to fly a circle at 15 degrees bank?  Without trying to do the math, it seems that airspeed together with bank angle would determine the turn rate.

The FAA Aeronautical Information Manual defines 'standard rate turn' as,

  A turn of three degrees per second.

This results in the completion of a full circle in 2 minutes. Every airplane must be able to complete a standard rate turn in order to be certified. The standard rate of turn is used as the main reference for bank angle.

GA aircraft are expected to turn at the standard rate of turn during holding, intercepting, tracking, approaches, departures etc.

You're correct that the airspeed determines the turn rate. As the turn rate is standardized, the bank angle becomes dependent on the air speed. From FAA-H-8083-15A Instrument Flying Handbook,

  True airspeed determines the angle of bank necessary to maintain a standard-rate turn. A rule of thumb to determine the approximate angle of bank required for a standard-rate turn is to divide your airspeed by 10 and add one-half the result. For example, at 60 knots, approximately 9° of bank is required (60 ÷ 10 = 6 + 3 = 9); at 80 knots, approximately 12° of bank is needed for a standard-rate turn.


Is any endorsement required to take the Advanced Ground Instructor test? What, if any, endorsement is required to take the Advanced Ground Instructor (AGI) test? 14 CFR 61.35 does make clear what is required for any knowledge test, but not what is specifically required for the AGI test.

I also noted that 14 CFR 61.193 mentioned that flight instructor privileges include providing an endorsement to take the test.

14 CFR 61.213 also lists several eligibility requirements to become and Advanced Ground Instructor including taking a knowledge test while mentioning no endorsement which is required to actually take the test.

No endorsement is needed. The AGI and IGI written tests can be done without anyone's endorsement.

Additions:

You also don't need an endorsement to take the CFI, CFII, or FOI written tests, btw.
Your reference to 61.193 has nothing to do with this. For one, it's 61.193, not 61.139 (but your link itself is correct), and second, these are not endorsements for written tests. In 61.193 it says "to train and issue endorsements that are required for: ..." and that does not include any endorsements for the AGI, IGI, FIA, FII, and FOI written tests. Such endorsements, as they pertain to a ground instructor, would, for example, be a 61.217(d) endorsement (not written test).
"... flight instructor privileges include providing an endorsement to take the test." No, they do not. They include providing an endorsement for a ground instructor certificate/licence (not written test), and endorsements for written tests (in general -- not for the ground instructor).

When an endorsement for the written test is needed, then that prerequisites say that explicitly, such as for the PPL, IR, CPL, ATPL, etc., such as in 61.65.(a)(4), for example.


Does runway 32/33R at Madrid actually have a kink in it? In this satellite image of Madrid airport (MAD/LEMD)...

Source:Google maps satellite view
Alternate link from maps.google.nl still renders incorrectly for me, but appears fine for others...

...it appears that there is a very noticeable left kink in the landing area of runway 32/33R.

According to what I can interpret from Jeppesen's airport data, there doesn't appear to be any indication of this kink:

Page 50 of Jeppesen hosted by uvairlines.com.

Is that kink actually there, or is it an artifact of Google's processing of  different images? There is obviously something going on, as you can see both RWY32 & RWY33 markings in the image, while Jeppesen clearly identifies it as 33.

If that kink is actually there, what impact does that have on a pilot landing? I believe the touchdown target is the two long, solid bars at the top left of the image, which excludes the kink, however, there is enough rubber to indicate that touchdowns have occurred much closer to the threshold than that.

An interesting feature of your question is the apparent double-exposure of runway number in your image. And I suspect it will also account for/explain why you see a kink in an image.

Occasionally, runways change number. The reason for this is basically that runways only have 36 possible designations (1-36) however a runway designated 18 may be orientated anywhere from 175-184 degrees. Usually the actual orientation of the runway is rounded to the nearest 10. hence, a runway pointing at 184 degrees would be designated "Runway 18".

Magnetic variation drifts over the years (eg, 10 years ago Southern UK was +3 degrees, now it is +1). This may make a runway which was once 184 degrees magnetic now 186 degrees and that would require its designation to change from "Runway 18" to "Runway 19".

Now, as for your image, I would guess that Google have taken a grab when it was Runway 32, and (at least partially) again once it had been re-designated (and repainted!) Runway 33. This is backed up with the airport's wikipedia article:

  On 20 September 2012, both runways 15/33 were renamed as 14R/32L (the longest) and 14L/32R (the shortest).

The act of stitching images together, and the image processing algorithms in use, mean you get kind of a partial of one image and partial of the other. It is almost certainly not there in real life, and thus has no effect on the pilot's handling around that runway.


Have there ever been any accidents within holding stacks? At a busy airport, up to a dozen aircraft waiting to land may fly the same holding pattern at the same time, separated vertically by 1,000ft. — a stack. Some ATC towers have dedicated Stack Controllers.

It's all very routine, but have there ever been any instances of incidents or accidents in stacks?

To the layman, it might seem like stacks could be hazardous — e.g. what if a plane in the middle of a stack lost an engine?

The article you linked in your question mentions one incident - loss of separation - that's described in more detail here.

The Aviation Herald is a good source of information on incidents and accidents but it has very few reports related to holding. There are several about low-fuel incidents because aircraft had to remain in the hold for a long time (Avianca 52 is a well-known case), but I could only find one that involved a holding pattern more directly and that was also a loss of separation resolved via a TCAS advisory.

Eastern Airlines 401 crashed while in a hold, but it was an ad hoc holding pattern and not in a stack with other aircraft. American Eagle 4184 crashed due to icing accumulation while holding but again there were no other aircraft involved.

I could only find one example of an actual collision in a holding pattern which was over New York in 1960 and in that case only one of the aircraft was holding (inaccurately).


How does a HUD track a runway? I was looking at the FCOM of the 787, and I came across an interesting thing. There is an HUD takeoff selection on the SID selection page. It got me to wonder, how does the HUD track the runway from so far away?

Is it related to ILS?

Correct, that optional feature on the 787 relies on the ILS or GLS signals.

  On takeoff, HUD provides lateral guidance for takeoff roll and rejected takeoff using ILS or GLS signals.

(FCOM 4.20.7)

PVD (Para-Visual Display) is a similar instrument (in function) that was available on the 747-400 as an option that relied on the ILS (shown below).

(aerowinx.com) PVD.

Further reading:

Peripheral Vision Displays (nasa.gov).
The crash of Singapore Airlines Flight 006 during a takeoff in LVP conditions.


How much oxygen do commercial airliners carry? How much oxygen do commercial airliners carry? Is is enough for every seat to have a 20-30 minute supply, is it a percentage of total flight time (ie 10% of total flight time), or is there a compressor on board?

I would assume the goal of the pilots during depressurization would be to repressurize whether that be by simply lowering altitude or other means. Thus, I would assume (hope) there would be sufficient reserves of O2 to do one or the other.

The masks are connected to a chemical oxygen generator by a lanyard.  Pulling the mask pulls the lanyard, which pulls a pin from the generator and starts the reaction.  They typically last betweeen about 10 and 20 minutes, more than enough time to get down to 10,000 feet where additional oxygen is not required.

There is no need for a % of flight time since the crew will descend to a safe altitude then divert to the nearest suitable field.  Therefore, the only requirement is to last during the (emergency) descent.

You cannot "repressurise" since the cabin altitude will be the same as the external altitude.  The only thing to do is to increase the partial pressure of oxygen by descending.

It's worth mentioning that the flight crew each have their own, isolated supply from a bottle which lasts significantly longer.  This is so that they can continue to fly if there are fumes in the cockpit.  Cabin crew often have their own, smaller, portable bottles so that they can freely move about the cabin to assist the "self loading freight".

Many's a morning when I would arrive to work feeling, ahem, a little "under the weather" and nip into the cockpit for a couple of minutes on 100% oxygen.  Works wonders - and don't worry, they are topped up to full as part of the pre-flight service.


Do Landings in a Simulator Count Towards Landing Currency? (FAA in the USA) If a pilot is trying to complete 3 day or night landings every 90 days in a multi-engine aircraft in order to be multi-engine current per FAA requirements, can those landings be done in a Simulator or a Flight Training Device?

Yes, as long as the simulator is properly certified (to level B,C,or D) you can do your takeoffs and landings to regain currency.

Long haul airline pilots frequently have trouble staying current for 3 takeoffs and landings in 90 days. This is because it takes 4 pilots to complete a long haul flight but only 1 will get the takeoff and landing.

3 trips a month is about all you do, and if you are scheduled to be the augment pilot, you don't get a takeoff and landing.


Why do all fifth generation fighters (J20, F22, SU57) use Trapezoid wings? It seems that trapezoid wings are used by all fifth generation fighters (although the wing of J20 looks quite like delta wing), but why? What made a trapezoid wing so attractive? Why don't they use other forms, like the swept wing on SU27?

Trapezoidal wings have advantages and disadvantages:

Better performance than delta wings at transonic speed
Good stealth performances
Flaps can be easily used.
drag reducing
better Lift/Drag ratio than delta wings

The problem is that they stall at much lower angles of attack than delta wings.

Edit:this problem is overcome using leading edge,LERX,and/or canards


When buying an aircraft which has had multiple owners it is acceptable to get the logbooks from the previous owner only? I know of a case where a small GA aircraft has had three owners since it was manufactured.  However the only airframe, power plant and propeller logbooks available for the airplane are from the most recent owner and no other logbooks are available from the two owners prior to the current owner.  The current owner's logs are good with the required annuals, engine overhauls, etc. documented and incorporates all FAA and manufacturer ADs to date.  Does this decrease the value of the aircraft?  My first thoughts are that yes, it will, as I have no idea as to what happened to the aircraft prior to the date the current owner obtained it, but I'm not totally sure.

It will most definitely decrease the value of the airplane. How much is not easily answered. If the current owner has owned the plane for a long time and all the ADs are complied with then probably not much of a decrease in value. If they have replaced the engine and prop—even better. However, if the current owner has had it for a short while, then maybe he’s selling it for a reason related to the missing logs.

Maybe there was a prop strike or gear-up with a previous owner and they didn’t do the repairs properly. Or maybe there is a lien on the plane from when the last owner didn’t pay the shop. Or it could be that the plane was perfectly maintained but an ex-spouse kept the books in a nasty divorce. That’s why it is difficult to say how much the missing books decrease the value.

You can get the records for the airplane from the FAA in Oklahoma City to see if there was major damage properly repaired. The A&P is supposed to file 337s for major repairs and modifications.

In any case, you’ll need to have an A&P who is familiar with the type do a careful pre-buy.

Whether you should buy a plane with missing logs partly depends on how long you intend to keep it. Just like you are concerned about missing logs, the next owner will be too. If you plan to keep it for a long time, then you probably shouldn’t be too concerned about the difficulty of selling it later. If it’s a transition airplane for you, then know that it will be more difficult to sell.


Why do gliders have bungee cords in the control systems and what do they do? Are they on all control surfaces? What about ultralights? Why do gliders have bungee cords in the control systems and what do they do?

Are they on all control surfaces?

Why don't ultralights have them?

The bungee is just a bidirectional spring that tends to hold the elevator at position x, and if you move the stick you are stretching the spring in one direction or the other.   They are used in the elevator control circuit to provide an adjustable centering force for trim purposes, and on gliders with all flying tails, a measure of stick free static stability (you don't get a lot - many gliders are mildly divergent in pitch if you aren't holding the stick, but this isn't a big deal since you are almost never flying hands off).  The glider's pitch trim control adjusts the "at rest" or relaxed position of the spring so it will tend to fix the elevator at this position or that unless overcome by a control input.

Normally you would have a trim tab on an elevator/stabilizer tail or a trim/anti-servo tab on an all flying tail, as regular airplanes do to provide speed-seeking trim and strong stick-free stability, but gliders operate with much lower loading and forces on the tail and can get away with a bungee spring that acts like a little man under the floor pushing on the stick, this way or that to try to hold it at a given position, so you don't have to.  This does away with all the linkage, tab surface etc, which is a big deal when you are shooting for a 40:1 or more L/D.

You can put bungees in any control circuit if you want to be able to trim it with an adjustable centering force, or want to increase the forces.  Some light aircraft use bungee springs in the rudder control circuit to provide rudder trim.  The spring just replaces your foot by exerting a spring force so you don't have to.  The downside is the spring loads are present when you move the controls out of the trimmed position, adding to the perceived rudder forces when maneuvering.

I'm pretty sure there are ultralights out there that use elevator bungees for trim purposes instead of a tab.  However, driving the surface directly aerodynamically using a tab on the surface provides a more sensitive and robust solution and if I was designing an ultralight, where drag isn't that big a consideration, I'd use a tab for the elevator, and perhaps a bungee in the rudder and aileron circuits if I wanted rudder and roll trim.


Are sudden turns and descents a standard emergency procedure? First, I'm not looking for speculation on an accident, I just want to know if pilots are trained to make certain maneuvers in response to an emergency.

In reading about the jet that just went down in Egypt, the story stated

  "It turned 90 degrees left and then a 360 degree turn toward the right, dropping from 38,000 to 15,000 feet and then it was lost at about 10,000 feet," Kammenos added.

Assuming the report is accurate, are those maneuvers a standard procedure of some kind? Are pilots trained to maneuver like that in an emergency? If so, in what type of emergency would those maneuvers be useful?

There are no emergency procedures I'm aware of that requires a pilot to turn left 90 degrees and then turn right 360 degrees.

There is a procedure for situations requiring rapid descent for things like medical emergencies and fires.  They consist of a 45 degree banked, full-flap, power-idle descent flown at maximum flap extension speed.  So it is a spiraling-turn descent.

Also, the Cessna 172N checklist instructs the pilot to slip in the opposite direction from a wing fire to keep the flames blown in an outboard direction.


Where do military aircraft carry flares? Many sources on the internet say flares are carried in the fuselage, but do not specify the exact location of the compartment.

Does anybody know exactly where they are stored?

The location of the flare/chaff dispenser (FCD) depends on the aircraft type and even in case of the same aircraft, they can be located in different place, depending on the version and customer.

For example, the F-16 carries the FCD in its body fairing, as can be seen in the following image of Hellenic Air Force F-16 Demo Team “Zeus”.

Image from theaviationist.com

The following image shows a close view of FCD.

Image from f-16.net, credited to Capt. Lance

The following image shows an Israeli F-16B 'Nezt' having FCD between its ventral fins.

Image from theaviationist.com

Initially, most of the FCDs were retrofitted to the existing aircraft and were simply attached to them. However, most of the modern aircraft have FCD bays integrated into their fuselage. Still, some aircraft can carry extra flares/chaffs in their pylons (called PIDS (Pylon Integrated Dispenser System)), as shown below in an RDAF aircraft.

Image from f-16.net, credited to Erik Frikke

Also, most of the combat aircraft have the FCD in the bottom, while transport aircraft (and helicopters) have them on their sides.


How are crews managed after a diversion (far from hub)? While reading one of the reports of flight diversion, I started thinking about how airlines manage aftermath of the diversion especially for long haul transatlantic flights.

Many of the reports say airlines provides a replacement plane. Replacement plane flies to the diversion airport, load up the passengers and continue.

If the original flight is full, what happens to original crew? Working with the assumption that new flight has new flight/cabin crew and there is not enough place for all.

If the airport is at significant distance where the replacement comes from or still has to fly significant distance, it's possible that replacement flight crew might run out of flying time.

Do they provide multiple sets of crews - one to fly to diversion and one to fly to actual destination.

I'd say you've answered your question yourself, by listing the various scenario's, any or all of which could be of concern in a particular situation. Handling a diverted flight is a juggle between, safety, time, comfort factor (for crew, for passengers), cost, etc. In turn these are affected by feasibility, location of base (for replacement of crew/airplane), ease of availability of replacements, number of passengers, break up of their final destinations (likely factor when flying to long-haul to a hub), fuel and tech facilities available at the diversionary airfield, whether local 'diversion handling' contracts are in place, or the situation is being handled on-the-fly at an obscure airport. Airlines tend to have their own tailor-made SOPs, or guides.
These issues are ideal ones to be addressed by Airline Alliances such as STAR or 'One World' and traditionally all help is given to such flights. "You today, could be me tomorrow"
Assuming the airplane is ok, The change of crew may not be required if the remainder of the flight fits within the DTL's (Duty Time Limits) and the extra landing for recovery falls within the ambit of the rules. Crew are expected to be rested enough, when reporting for Flight Duty, to operate to their limiting DTL's. There are allowable extensions to these limits too, provided every crew member is upto it, ie not fatigued, and the Captain judges it to be a true reflection of the situation. The Captain would have to document all this in the Regulatory/Company paperwork. Note that Long-haul flights carry more crew than the minimum for shorter flights, so all crew get some time-out and an opportunity for horizontal rest, ideally in the form of a 180deg reclinable seat or bunks, normally in a private crew rest area.
For Ultra Long Haul flights, the hours, and crew numbers are ideally decided on a case by case, 'city-pair' basis, along with the Regulator and related procedures documented by the Operator, acceptable to crew, and approved by the Regulator.
In case a layover has to be declared by the Captain, many Operators/Regulators have a concept of Minimum Rest, which maybe typically 8 or 6 hrs of undisturbed rest, sometimes called (hotel) "key to key" time. This may provide more certainty than organizing a relief airplane that has to fly 12 hours to get there.


Why are Category II ILS approaches prohibited in an E-Jet with inoperative pitch trim? Part of the Pitch Trim Runaway checklist for the E-Jet family states that Category II ILS approaches are prohibited if neither pitch trim channel is available:

If both pitch trim channels are failed:
Cat II Approaches are prohibited.
[E170/175 QRH, page 8-32, quoted in page 2 of NTSB ASR-20-01]

Why are ILS-II approaches (but not ILS-I or ILS-III approaches, or any non-ILS approaches) prohibited in an E-Jet with inoperative pitch trim, even if adequate elevator control is available to do so safely?

Looking at the report, the airplane does not appear to be certified for CatIII in the first place (it would have to have Head Up Guidance and not that many do - the CRJs are the same) so the QRH doesn't need to mention an approach category that the airplane can't do anyway.

As far as CatII goes, the approach with no pitch trim is going to have to be flown by hand, and probably with the existing trim speed different from Vref, unless the trim failure happened while on final while already trimmed to Vref.  So depending on how far away your current, fixed-due-to-failure trim speed is from Vref, you are going to have the hand fly an ILS while holding 5, 10, 15, 20, 30 pounds of force on the column to maintain glide slope at Vref.

This is quite a challenge because the slightest relaxing of concentration will cause you to relax your push/pull a bit and make you drift high or low.  It's enough of an ordeal to do this and stay on glide slope right down to 200 ft.  To hold it down to 100ft, the normal CatII decision height, is pushing it, so the approach is prohibited.


How dangerous is to take-off and fly with snow on the wings? I have seen a video of a commercial aircraft taking-off with snow on the wings. I wonder how dangerous it is to take-off and fly with snow on the wings.

Pilots are allowed to commence a takeoff if the snow is not adhering to the wing and will blow off during the takeoff roll.

It is very difficult to just look at a snow covered wing and know for sure if the snow will blow off. A "hands on" tactile inspection should be conducted and even then you can never be sure about the level of contamination for the entire wing surface.   De-Icing is always the safest option.

To attempt a takeoff with snow and/or ice adhering to the wing is NOT SAFE.  You are gambling with your life, and the lives of your passengers and crew.

That being said, it is possible to do a reasonably safe takeoff with snow and/or ice adhering to the wing, but you will need a much longer takeoff roll, and slow rotation, and climb out.  If you were to have an engine failure you would probably not have sufficient performance to climb out over obstacles, or even remain airborne.

You are gambling that you have enough runway for your "seat of the pants" takeoff and that you will not have an engine failure, because you have no way of knowing the actual aircraft performance.

So again, De-Icing is the only safe option.


Is jet fuel typically injected before or after the flame holder in an afterburner? I don't have the best grasp on the intricacies of afterburners but I'd assume that if the engine has a flame holder then fuel would be injected before it in order to allow it to heat up to autoignition by the time it reaches the flame holder. If not please explain, thanks.

This is what I could find on the matter in my university textbook.

  The fuel spray bar is typically composed of
  one or more rings with distinct fuel injection heads circumferentially distributed around
  the ring. The V-shaped flame holder ring(s) create a fuel–air mixture recirculation region
  in its turbulent wake, which allows for a stable flame front to be established.

Although it does not mention fuel heating explicitly, I would agree with your assumption -- as the mixing of the hot gas and the (colder) fuel of course also evens out temperature differences between the two.

Source: Farokhi (2014), Aircraft Propulsion (p. 199)


How are empennage airfoils chosen? Empennage airfoils are usually symmetric. What is the process behind choosing the airfoil used in the tail of the airplane? Is it related to the stability? And then how would the angle of attack of the tail be determined?

Selection of the profile is only a small portion of the design procedure of the empennage:

Aerodynamic centre of the aircraft minus tail is determined first.
Aft Centre of Gravity is determined.
Static longitudinal stability criteria determine the area of the empennage - upon a disturbance in Angle of Attack, the total aircraft must generate a moment that counteracts the disturbance. Usually, wings and fuselage contribute negative stability, and the tailplane must be sized such that the stabilising moment compensates for this in all circumstances.
Once the area is determined, the aspect ratio, taper ratio, sweep angle and profile thickness are chosen considering a host of factors, including the lift-curve slope, stall behaviour, out-of-trim conditions, structural weight, design diving Mach number.
And then the airfoil shape is chosen. From Torenbeek section 5.5.1.d:

  The basic requirements are that the airfoil section should have a high $C_{L_\alpha}$ and a large range of usable angles of attack. Frequent use is made of approximately symmetrical airfoils with a thickness ratio of 9 to 12 percent and a large nose radius, e.g. NACA 0012. Tailplane stalling - at
  the lower surface - can be postponed by adopting negative camber (e.g. NACA 23012 upside down), upward nose droop, an increase in the nose radius or by means of a fixed slot.

Table 9-2 from the same book contains (dated) horizontal tailplane design data and lists some profiles used:

Cessna 177: NACA 0012/009
Fokker F27: NACA 63A-014 mod.
Lockheed C-130B: NACA 23012 inv.
Cessna Citation 500: NACA 0010/0008
B737/100: 12%/9%
Boeing 707/320: BAC-317


How can this single-prop drone maintain stable (rotating) attitude and position? The short BBC news item and video The drone designed to fly on one propeller shows the flight of something that looks like quadcopter missing three of its propellors and motors. It continuously rotates around a vertical axis through its center of mass tilted at perhaps 30 degrees, and the single propellor can provide different lateral thrust in different directions by (I assume) quickly modulating its power throughout the rotation cycle.

This seems to be only a demonstration of an interesting effect at this point in time, but I'd like to understand how it can retain both stable position and attitude with only one degree of freedom (speed). Yes the attitude is rotating, but that motion appears constant.

So far I haven't found anything to read about how this can actually work. In the video there is a suggestion that this has been calculated and equations are briefly flashed on the screen.

The work is done at ETH Zurich and the Flying Machines Arena.

How can this single-prop drone maintain stable (rotating) attitude and position?

I'm also curious if this is a 'new' effect, or is this a version of something that has been previously demonstrated or at least discussed.

above: GIF made from screenshots of BBC video found here.

above: Screenshots of BBC video found here.

You can read all about it in their paper (which actually reads quite nicely)

http://flyingmachinearena.org/wp-content/publications/2016/zhaIEEE16.pdf

First, the parameter space (so, location of propeller, mass, etc) was searched for a relatively stable solution.

Then, stability is achieve through active control, so indeed by varying propeller thrust rapidly. This is done in a few cascaded control loops; so an 'outer loop' that has a position setpoint, which is then translated to a desired attitude. This attitude is then achieved by varying thrust throughout the rotation.


Why don't black boxes send a signal when above water? Cockpit Voice Recorder and Flight Data Recorder's "Underwater locator beacons" only send their position when underwater.

Why don't they do so when not underwater?

The short answer is that when a plane crashes on land we (generally) have a pretty good idea where to look for the black boxes - there will usually be a relatively clearly defined debris field to search.

When a plane goes down in the water there's a lot more ocean to search, and currents can deposit parts of the airframe over a much wider area, so the underwater locator beacon ("pinger") helps guide people to the debris field.

The longer answer is that sending a signal when you are underwater is relatively easy.  The pinger has a water switch: when it gets wet the water closes an electric circuit and the pinger makes a noise that can be heard on standard passive SONAR gear.
Water is pretty good at conducting sound waves, so the pings can be heard over a large distance and can guide searchers to the black box (or at least the pinger: sometimes they get ripped off, but they're usually still near the debris field).
We get all of that functionality in a fairly small self-contained device that requires effectively no maintenance (you replace the unit when its battery expires).

Sending a signal when you're NOT under water is harder: you can activate the signaling device with a "G Switch" the same way the ELT is activated, but after that the problems get harder:

You can't use sound (effectively)
A "pinger" beeping in the air is much less effective than under water - the range would be extremely limited even at very high volumes. Add to that the fact that crash scenes are pretty noisy with firefighting and rescue crews working and the chances of anyone hearing an acoustic beacon on the surface are pretty slim.
You can't use light (effectively)
A strobe on the black box may work OK in the dark, but it's easily damaged and not terribly effective during the day, or if the recorder is upside-down, in dense foliage, under snow/mud, etc.
Radio locator beacons are problematic
Radio locator beacons require an antenna - if the antenna is damaged or torn off in the crash the effective range of a radio beacon would be drastically reduced, possibly to the point of being completely ineffective.
If the antenna remains attached the signal can still be reduced drastically if the box is buried under natural features (dirt) or aircraft debris (aluminum skin).
(Both of these issues are frequently raised in criticism of ELT technology.)

It's possible a locator system could be devised that works on the surface, but given the challenges and the fact that the location of the black box is generally known (to within the size of the debris field at least) when an aircraft crashes on the surface there hasn't been much effort invested in this.

Finally, remember that while the recorders are a very valuable part of the investigation process remember that from an investigation standpoint it doesn't matter if you find the black boxes on the first day or the last day of recovery operations:  Finding the recorders will not undo the crash or bring the dead back to life, so if it takes a little longer to find them while sorting through a debris field on the surface it's really not a major problem.


What are design or functional differences between sticks and yokes? I've been flying Cessna 172 & 182's my whole flying life up to this point.  I know that Cirruses (Cirri?) have a "side yoke" and Diamonds, Citabrias and others have control sticks coming up through the seat.

Is this a design difference or are there actual functional differences between airplanes that have control yokes (like Cessna 182's) or sticks (like Diamonds) or side yokes (like Cirrus)?

For example, I realize that with the side yoke offers more legroom perhaps, but are there reasons beyond that - like pull/push force required?

There is some link between the type of control and the design maneuverability of the aircraft.

Consider a yoke.  Imagine twisting it to bank.  You are moving it with a twisting motion of one or both arms and in general, aircraft fitted with such yokes are capable of low to moderate roll rates.  Big displacements are uncommon but small, rapid, alternating changes are common.  Watch a wide body PF making control movements during a landing with any kind of xwind or turbulence.  They are less sensitive and give more visual feedback as to the control positions.

Now consider a stick, central to the pilot flying.  This is moved with a movement of the arm, generally only one, and can be rapidly displaced easily.  This is normally associated with aircraft with high rates of roll, especially in an aircraft designed to rapidly null out or reverse the roll.   It is also physically easier to move a stick than a yoke under high-G as the arm movement gives a longer lever.

A great example of this was the Vulcan which, despite its size, enormous wing area and large fin, was so rollable that it could out-maneuver the interceptors of its day.  It has a control column rather than a yoke and the PF would throw it around at altitude as if it were a fighter.  This would not be easy to do with a yoke.

I'm a bit long in the tooth and haven't flown fixed wing for many years.  I am not familiar with the rationale behind side sticks which are fitted to both low and high performance aircraft.  I am guessing they are fitted more for reasons of ergonomics than for performance.


Is there any limit on how narrow aisles can be? Sometimes, when I'm flying on the airlines, I'll board an aircraft where the aisles seem incredibly cramped, where it's almost impossible to move past without bumping every seat. I can't imagine how much of a nuisance it'd be for people larger than me! I once saw an obese person a bit stuck between one of the aisles, blocking the way for others to move into the aircraft.

But then I thought about what would happen in an emergency. What if there were numerous large people on board, and there was a fire?

So, are there any limits to how much the airlines can squeeze their planes' aisles?

For a transport category airplane with 20 or more seats the minimum aisle width is 15 inches from the floor to a height of 25 inches, and 20 inches width above a height of 25 inches.

  §25.815
  Width of aisle.

  The passenger aisle width at any point between seats must equal or exceed the values in the following table:

  1A narrower width not less than 9 inches may be approved when substantiated by tests found necessary by the Administrator.


In aircraft where the thrust reversers are not locked out when airborne, is it permissible to go around after reverser deployment? In general, a go-around, in a jetliner, must either be performed before thrust reverser deployment, or not at all.

This is because attempting a go-around after reverser deployment may result in loss of control of the aircraft.
Loss of control is a possibility because one or more of the reversers could fail to stow during the attempted go-around, potentially resulting in an asymmetric-thrust situation beyond what most jetliners are designed to handle.
This is because the reversers take several seconds to fully stow once deployed, and the aircraft could easily lift off before this happens, in which case any not-yet-stowed reverser buckets or clamdoors will be blown back to their fully-deployed positions by the aerodynamic forces on said piece(s) of reverser hardware.
Lifting off is relevant because, when the aircraft lifts off, the reverser actuators are automatically locked out and depressurised.1
The reversers are locked out when airborne to minimise the risk of an uncommanded in-flight reverser deployment, which, for aircraft not certified for in-flight thrust reversal, is generally bad.

However, some aircraft are capable of, and certified for, the safe use of reverse thrust in flight (typically to steepen descents by using reverse thrust as an airbrake [DC-8, Trident, Concorde, C-5, C-17], or to shorten the landing roll by deploying the reversers while still airborne rather than having to wait until after touchdown [Trident 1C, Il-62, Tu-154]).

To allow this inflight use of reverse thrust, the reversers on these aircraft are not locked out when airborne.
As such, the reverser actuators remain pressurised even after the aircraft lifts off.
Consequently, even if the aircraft lifted off before all reversers had fully stowed, the reversers would simply continue to stow.
As a result, lifting off with one or more reversers still in transit would not produce a significant risk of a severe asymmetric-thrust situation.
Even if, for some reason, a reverser did fail to stow, resulting in thrust asymmetry, aircraft certified for inflight reversing are required to remain controllable even with full forward thrust on one side and full reverse thrust on the other, at least for long enough to shut down the reversed engine.
This would appear, for aircraft certified for inflight reverser use, to essentially eliminate the danger of loss of control (absent other serious failures) in the event of a go-around after thrust-reverser deployment.

This would be especially important for aircraft like the Tu-154, Il-62, and Trident 1C, which routinely deploy their reversers before touchdown; if going around after reverser deployment was prohibited for such aircraft, this would seem to completely preclude very-low-altitude go-arounds with these types.
Are aircraft certified for in-flight thrust reversal (and, thus, not equipped with an in-flight reverser lockout mechanism) allowed to go around even after the thrust reversers have been deployed?

1: There is generally an override switch in the cockpit that can be used to restore pressure to the reverser actuators even when airborne (its main use is to force the reversers to remain stowed if one or more should come unlocked in flight, or to restow then in the event of an uncommanded in-flight deployment), but locating and actuating this switch would take precious seconds which might not be available in the event of an asymmetric reverser deployment at low altitude in an aircraft not certified for the inflight use of reverse thrust (especially as, during the time spent with their actuators depressurised, the deployed reverser(s) would be blowing back to the fully-deployed position, taking them further and further from the stowed position).

An aircraft approaching to land with a reverser deployed or unlocked, particularly one that's deployed unintentionally, will be at idle power, or shut down on the deployed reverser.

A go-around or missed approach is conducted in the same manner as if the engine is shut down for any other reason.


Has there been research into alternatives for the vertical stabilizer? Are there some activities in research for replacing the vertical stabilizer with a turnable horizontal stabilizer? (flight direction stabilization like the birds)

There have been various (and of varying degrees of success) some flying wing designs that have made it into production with no vertical stabilizer. However they had no turnable horizontal stabilizer either. The B-2 Spirit comes to mind,

(source)

The closest thing I can think of that has no proper vertical stabilizer but still does have an epenage is a V tail design, a variety of planes have been made with them over the years including the popular early Beech Bonanza models.

(source)

I am not sure of any planes that have ever been produced with such a tail nor any research projects on such a design but that does not preclude their existence.

A general note on aircraft research and design, the various flying wings and V tail planes built over the years have had issues and been met with hostility by some critics. This is not to imply that they are poor designs however the flying wing does have its share of instability inherent to the design. The V tail gained some negative reputation on the early Bo's because many of them crashed. However many people account this to low time pilots in overpowered (for the time) planes.


Why is the CG envelope wider when the aircraft is heavy? I mean, the same aircraft, but empty or on MTOW. According to MAC, those limits are the same, but the indexes are different. So if I understand that correctly, the CG can be more forward when the aircraft is heavy.

E.g., this Boeing 747 (and all 747s I have seen had an envelope like this), page 20, Report on the serious incident at Oslo airport Gardermoen on 21 September 2004 involving flight KAL520 Boeing 747-400F registered HL7467 operated by Korean Air.

So if I understand that correctly the CG can be more forward when the aircarft is heavy.

Not really, although I guess you could say the index can move "forward" because of the way the form is drawn, but the index is not the CG. Indices are a composite number, namely the moment divided by a reduction factor.

The use of index numbers and a reduction factor greatly simplifies
weight and balance calculations, especially for large aircraft. The
index is a moment divided by a reduction factor and may be found by
this formula:
Index = (Weight x Arm) / Reduction factor
(from http://avstop.com/ac/weightbalance/ch4.html)

While indices make manual calculations easier than using %MAC and weight (the way computerized systems typically do it), they do tend to obscure things in my opinion.
If you want to see a 747's weight and balance envelopes using %MAC and weight, go to 747.terryliittschwager.com and select any 747. The top one on the aircraft list will do, N402YY. When the aircraft comes up, scroll down to OPERATING ENVELOPES.
You can position your pointer over the vertices to see their values. Note that the forward limit for takeoff at 580000 lbs is 11.0 %MAC, staying at that limit up to 778000. Getting heavier from there moves the forward limit aft, not forward, and additional weight from there really sends it aft.
The forward limit for the zero fuel and landing envelopes stays constant throughout the weights of their range.


How do cruise missiles stay aloft? How do cruise missiles fly? Where does the lift come from? On pictures you can see that they have wings what are tiny compared to the body / payload. Do they generate enough lift? Or is the propulsion, other than in most aircrafts, not just used to overcome drag but also to generate lift, i.e. the thrust force is angled down?

Many cruise missiles have wings of some sort, although these can be very small like on the P-700 (barely larger than their control surfaces) or foldable in such a way that makes them not obvious when the missile is in a stowed configuration, like on the Tomahawk or Kh-55.

Additionally, the narrow body of the missile provides lift. Though it is much less effective at it than a wing would be, this is a design trade-off that can be worth it in some scenarios. For supersonic cruise missiles the narrow-body lift is usually enough and the lack of wings is a plus, as they would generate a disproportionate amount of drag at $M_{\infty} \geq 1.0$.

One of the main traits of slender wings (and bodies) is that their effect on the surrounding airflow grows with speed at a much  slower rate than that of a high-aspect-ratio wings, making them suitable for high speed applications.


Can an aircraft be controlled after an ejection? What kind of damage does the ejection process do to the inside of a cockpit or to the aircraft in general?

If ejection were accidentally triggered on the ground (i.e. the ejection wouldn't be followed by a hull damaging/destroying crash), would the damage be minimal enough that it could be flown again by simply installing a new seat and canopy, or would it have to go through a major inspection to ensure no additional damage was done to instruments, wiring, etc?

I'm going to interpret safely as meaning successfully. Ejections mean safety has gone out the window - literally.

There was a case in the Vietnam War where a pilot continued to fly his heavily damaged F-4 after his RIO (back seat radar operator) ejected. He managed to keep it together long enough to get back over friendly territory before ejecting himself. Note, this was an interview I saw some years ago, I haven't been able to find a link to post.

In this case the airplane had been hit badly enough that landing was not going to happen, however if the RIO had ejected accidentally or the airplane less damaged then a safe landing would have been possible. So yes, flying has been done and landing is entirely possible after someone has ejected.


What are the pros and cons of high service ceilings for fighter jets? What are the pros and cons of high service ceilings for fighter jets?
Can they bypass radars and attack effectively at high ceilings?

If your question is about advantages and disadvantages of the high ceiling:

Advantages:

-For military aircraft, reaching a high ceiling gives a tactical advantage in air combat and makes it possible to escape from surface-to-air defense . For civil aircraft, it is above all a question of achieving the best efficiency of the engines or of being able to take advantage of the air currents . For both civilian and military helicopters a relatively high ceiling allows intervention in mountainous regions.

-During the Cold War the race for the altitude record was part of the propaganda war between the two blocks. It is however necessary to distinguish the maximum altitude reached during records, not supported, and the operational ceiling of an aircraft, given for standard mission (military) or operating (civil) conditions, and which is the maximum altitude to which the plane can stay in a sustained manner. For example, the absolute altitude record for a conventional jet plane, held by the Ye-266M (one of the prototypes of the MiG-25 ) is 37,650  m , but this same plane has a ceiling of "only" 24 400  m . In the same vein, the Lockheed NF-104A, a demilitarized version of the F-104A Starfighter used by NASA to train the pilots of X-15 , could climb to altitudes higher than 36 600 meters thanks to its additional rocket engine, but its actual ceiling was only about 15,000  m .

Research continues to push the limits reached during this period, with aircraft such as the X-43 and its supersonic ramjet .
Disadvantages:

but nowadays, missiles have evolved and fly faster, higher .... now it's not a race for altitude.
now you have to be stealthy, have good self-protection systems.

it's time for the hypersonic era!

"The first Russian Avangard hypersonic missile regiment, capable of spinning at more than 33,000 km / h and piercing any existing missile shield, has officially entered service."china have same hypersonic missiles


How does chord length affect wing design? In comments on the answer to a question about transonic speeds and control surfaces I came across this comment and I'm having a hard time understanding it.

  If you increase the chord, lift goes up almost linearly and stability
  decreases, because now the wing's lift force has a longer lever arm in
  flow direction. By increasing chord, you decrease aspect ratio...

There are a few things I am not understanding, but I think they probably relate quite heavily, so I'm going to ask them together.

Firstly, what is the aspect ratio of the wing?  Is that a reference to the incidence of the wing and how a longer wing would mean a larger percentage of the side of the fuselage is covered by the wing (as seen from the front)?  Or am I way off the mark?

Secondly, why does increasing the chord length decrease the stability of the wing? In my mind a longer wing would mean that the craft is more stable because the airflow is being forced into line for a longer period of time.

Lastly, this all relates to how chord length is considered during wing design.  Are there other factors centered on the chord length that should be considered when designing a wing?  And do those factors change depending on the mission of the craft in consideration (eg., trainer v. cargo plane v. jumbo v. Mach 3 spy plane, etc.)?

During design, chord length will be the result of wing span and wing area, because those two are what the designer wants to set. To be more precise, he tries to set a certain wing loading, trying to minimize wing area (and weight) such that the wing creates just enough lift for the estimated aircraft mass in all design cases. Take-off, maximum altitude and minimum turn radius are the classics which drive minimum wing area.

Now I will focus on aspect ratio, because Jan has already answered the rest with his excellent answer. The higher the aspect ratio, the less the wing tips will influence the airflow around the wing. This means that the wing creates more lift for a given angle of attack, but also that the stall angle of attack is lower. The ratio of wing span to aircraft mass (called span loading) is the driving force for induced drag. However, wing span and aspect ratio will also drive wing structural mass, so you need to find a sound compromise between low induced drag and low wing mass.

What is induced drag? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! The wider the wing is, the more air can be used for lift creation, hence less deflection is needed. Consequently, the backwards tilt of the reaction force is smaller, resulting in less induced drag for the same lift.

If you fly fast, there is a lot of air mass streaming past the wing per unit of time, so you need to deflect the air only slightly. Your induced drag is small. That is why induced drag changes inversely with air speed.

Now you know that for high speed at high density, induced drag is not important. If you design an attack aircraft which has to fly at low altitude, a low aspect ratio will help: The lift increase due to gusts is smaller than with a high aspect ratio wing, and the induced drag is manageable.

How the lift curve slope changes with aspect ratio in subsonic flow is shown in the simple plot below. For a slender body (aspect ratio $\approx$ 0), the gradient of the lift coefficient $c_L$ over angle of attack $\alpha$ is $c_{L\alpha} = \frac{\pi \cdot AR}{2}$. Please note that the red line is only valid for AR = 0! Then the lift curve slope increases up to $c_{L\alpha} = 2\cdot\pi$  for $AR = \infty$ (and zero airfoil thickness and no friction effect), as shown by the blue line.

There are more influences, however. Dihedral means that the lift is tilted inwards, and the part which counteracts weight is only growing with the cosine of the dihedral angle $\nu$. Same goes for sweep: Sweeping means that the wing only sees a reduced angle of attack change. If you want to capture all effects, a plot will not suffice. Here is a table with formulas for most cases:

Nomenclature:
    $c_{L\alpha} \:\:$ lift coefficient gradient over angle of attack
    $c_{L\alpha\:ic} \:$ lift coefficient gradient over angle of attack in incompressible flow
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\nu \:\:\:\:\:$  the wing's dihedral angle
    $\varphi_m \:\:$  sweep angle of wing at mid chord
    $\varphi_{LE} \:$  sweep angle of wing at leading edge
    $\lambda \:\:\:\:\:$  taper ratio (ratio of tip chord to root chord)
    $(\frac{x}{l})_{d\:max} \:$  chordwise position of maximum airfoil thickness
    $Ma \:\:$  Mach number

Another consideration is wing volume: In most aircraft, the wing will hold most of the fuel, and a long-range aircraft needs big tanks. Sometimes, only choosing a lower aspect ratio will give enough wing volume for the required range. In order to keep induced drag constant, the span will be kept the same, so wing area increases with chord length. This has the added benefit of more lift, so less complex high-lift devices are needed. Since the structural mass of such a low aspect ratio wing with simple flaps is relatively low, the only disadvantage is the higher friction drag of this larger wing.

If maneuverability is important, wing span needs to be as small as possible. This reduces inertial moments and roll damping, so the aircraft can accelerate faster into a rolling motion and will reach a higher roll rate. This is extremely important in dogfights when the one who points his radar, gun and rockets at the adversary first will win. Here the chord is chosen for sufficient wing area at the minimum practical wing span.

Now we need to talk about viscous effects. Friction between air molecules and between air and wing. The ratio of inertial to viscous forces is expressed by the Reynolds number, and generally a higher Reynolds number means your friction effects go down, translating into less friction drag and a higher stall angle of attack. Especially for model airplanes, but also for gliders, it is sometimes better to reduce aspect ratio in order to gain absolute wing chord. The Reynolds number increases linearly with wing chord, and this can also be a consideration for choosing wing chord.

A normal wing has positive camber, and this means that the center of pressure moves forward with increasing angle of attack. Moment-wise, this means that the wing will create a stronger pitch-up moment when it pitches up. This makes the wing by itself unstable, and you need a tail surface to regain stability. Making wing chord longer will increase the unstable influence of the wing relative to the unchanged tail surface. This is why I said that increasing chord will decrease stability. During design, you watch for a size called tail volume. This is the area of the horizontal tail surface, multiplied with it's lever arm expressed as a multiple of the wing's reference chord. If you keep this constant while changing wing chord, your stability stays the same. But then you change your pitch damping, because this is affected by the square of the lever arm. Digging deeper will uncover more consequences, so I better stop here for today.


How do 3d panel methods model wake-surface interaction? I would like to know more about the general procedure used in modeling wakes for 3d panel methods.

Specifically I am looking at the effect of surfaces(fuselage and horizontal stabilizer) on the wake.

Is it better to merge wake panels with body panels or to allow the wake to clip through the body? Are there any advantages of using one over the other?

This has a couple parts to it, and it illustrates well the flexibility of well-designed panel methods.

First: panel methods can quite easily model multiple, disconnected surfaces simultaneously.  Select a proper implementation of the Kutta condition such that a trailed wake can be created, and you're all set.  The trick is to just make sure that all panels are considered as interacting with each other.  I've read a couple papers with section detailing this, including Sizing Tool for Quadrotor Biplane Tailsitter UAS by Strom.  Whereas, for one wing, you can compute the influence of all singularities on a given panel, for multiple bodies/wakes, you just compute the influence of all singularities -- regardless of what body they belong to -- on a given panel, be it on-body or in the wake.  For lifting panels (i.e., panels associated with a body that should have some nonzero trailed vorticity), the Kutta condition can be computed in the same way (e.g., for an equal-pressure, nonlinear Kutta condition, the strength of a shed wake panel is determined numerically to correspond with the shed wake of any other lifting bodies such that the Kutta condition is satisfied for all shed wake panels simultaneously).

Now that you have a wake and it is being convected in some manner, we get to the question of what to do when multiple bodies are in close proximity and the wake is predicted to convect through a body.  I've seen this treated in a rotorcraft paper (where the rotor wake is impinging on the fuselage) by Govinderajan in Contributions Toward Understanding the Effects of Rotor and Airframe Configurations on Brownout Dust Clouds.

The rotorcraft problem uses a proscribed displacement to keep the impinging rotor wake out of the fuselage.  As shown in the image below (from Govinderajan), the lines represent a straight-line vortex filament connecting various control points along the predicted wake structure.  The dashed blue line/blue dot are the predicted location for the convected wake.  Since the control point is located in the interior of the fuselage, it is moved radially outward to a point 1.1 radii from the center of the fuselage (green dot above fuselage).

A similar procedure is also followed by Lorber and Egolf in An Unsteady Helicopter Rotor Fuselage Interaction Analysis and appears to be also present in Quackenbush et. al's Vortex Methods for the Computational Analysis of Rotor/Body Interaction, all of which present methods that can predict the performance of various aircraft quite well in comparison to experimental data.  Unfortunately, I don't have a comparison to other methods for you, but this seems to be fairly common.


What can I be allowed to do as a engineering/ maintenance intern at a european airline? I am just about to (hopefully) embark on a short internship at the maintenance department of a medium-sized european airline (i.e. under EASA framework).

While the internship has been pushed towards being practical, one thing that bothers me is that I am absolutely clueless what I could be allowed to do (and I'd like to know so I could negotiate to get the most of the time with the carrier).

For instance:

Would I be allowed to handle anything flight-wise with supervision? Could I be allowed to interact with the aircraft or only watch and observe?
Would I be allowed to even loosen a bolt on a wheel, or do you legally need maintenance certification for that?

In europe the MRO are regulated by Part 145, this part precise that you have to be  Certifying staff (owner of part 66) to work on aircraft.
For all other people you can't even loose a bolt except if you are under supervision of qualified Certifying staff.

For engineering work (without work on aircraft) you only need to have a sufficient degree (under company appreciation except for special works like airworthiness where this in precised in concerned regulation (Part M for example)).


How changes in Air masses, Air flow direction and intensity affect lift? I have a question regarding the airflow an aircraft experiences during 3 distinct scenarios:

An aircraft during landing, experiencing a sudden gust of 40 knots from the tail. What would be the immediate consequance- Airspeed, lift?
All this if the aircraft does not correct the situation with pitch/ engine thrust.
An airliner cruising at 270 IAS, suddenly experiencing into an area of tailwind- meaning- the tailwind component of the wind aloft changed and increased by 40 knots. How would the airspeed look- just the following moment, and from there- lift? Regard to the situation without any compensating action by the pilot,
A fighter jet, at 20,000 feet, turning from at 25 degrees per second, meaning doing 90 degrees in under 4 seconds, from a situation of 50 knots headwind to no headwind. If the air mass is stable and blowing at a constant 50 knots, would there be any change in the characteristics of flight to the jet, meaning deteriorating airspeed which he/she would have to compensate for using pitch/thrust adjustment? In this situation, we assume that the pilot doesn't care for maintaining ground speed or track, just keeping the turn at a constant rate of 25 degrees per second.

My question is (and we discussed it in the previous question)- is there any diminishing effect to the fast change of airflow that was just ahead and changed to sideways/behind- when the change is caused by the aircraft itself, just as witnessed (by me recently) in an airliner when suddenly encountering a different air mass (I witnessed increasing Ground speed but decreasing IAS, until the Autothroottles commanded a slight increase in thurst).

For sake of simplicity, lets state that lift is produced by airflow over and under the wings (the rest of the airframe also contributes, but in the scope of this question we can neglect that).

Lift for any given airfoil is basically dependent on airspeed and angle of attack. Up to a certain limit any increase in airspeed or angle of attack will amount to increasing lift and vice versa. So, the question:

Case 1. and 2. The situation is the same. Sudden tailwind of 40kts will lead to a sudden 40kt drop of airspeed and sudden decrease of lift. The groundspeed will start to increase (slowly, because of inertia of the plane). If immediate corrective action is not taken, some altitude will be lost. Near the ground this would be catastrophic.

Case 3. I'm a assuming a level turn here: During or after the turn the wind will not have any effect on the airspeed or lift of the aircraft.

The aircraft is flying embedded in an airmass. If we are considering aerodynamics (lift and stuff), all changes made in the flightpath of the plane are made in relation to the airmass. Not the wind, not the ground. The wind is simply the movement of the airmass in relation to the ground.

If the wind does not change, nothing changes for the plane (overly simplified, of course, there's temp, pressure etc, but that's not on subject here). In cases 1 and 2, the wind changes, as the velocity is momentarily different (gust). In case 3 there is no change in the wind.

Your example of the event you've witnessed, with increasing groundspeed and decreasing airspeed is a simple case of flying into a region of tailwind. Tailwind would simultaneously reduce your airspeed and push you to go faster in relation to the ground, increasing your groundspeed. This event might happen as a result of flying somewhat perpendicularily through top or bottom part of a jetstream.

Since this subject has proven to be somewhat difficult to comprehend (here and in this question too), let us end this answer in a humouresque article on Flying Magazine website, that might help understand the misconception of turning in wind: The Last Word on Downwind Turns, Really.

As a sidenote, in case 3 you have described a rather steep turn without increase in engine power, the aircraft will bleed energy during the turn resulting in loss of airspeed, if airspeed is not maintained with power or glide. I'm also pretty sure that a constant 25°/second rate of turn is just too much to sustain for most fighters, even with full afterburners, and it is definitely too much for any pilot to handle for long. F-16 has a max rate of turn of about 27°/sec at about 380kts and 9g's.


Are aircraft with counter-rotating props or Down Between Engines less likely to develop asymmetric icing? It is suggested by some (and somewhat substantiated) that twin-engine propeller aircraft which do not have counter-rotating props (all modern regional turboprops are this way) can develop asymmetric wing icing due to the airflow pattern generated by the propellers:

 (from the FAA FFFSCR report, via the linked source)

Has it been demonstrated that twins with counter-rotating props (are any such twins rated for FIKI?) are immune to this asymmetric icing phenomenon?  What about quad-turboprop planes such as the A400M, with its Down Between Engines configuration?

Ice accretion is a very complex phenomenon on airplanes that depends on several factors. One of the main factor is the composition of the cloud that the airplane is flying and the exposure time.

Another significant effect are the conditions on the imprigment surface on the airplane, which depends of the flow path and the pressure over the surface.

So, having the properlers rotating in the same direction means that the propellers are providing energy in a different direction. Also that will mean that, for optimizing the airplane, the fuselage will be slightly assymetric (very difficult to notice).

So, to the first statement you are mentioning, I can confirm you that the imprigment areas affected by this assymetric effect will develop different ice shapes.

What I can tell you is that FAA and EASA employ real experts for airplane certification and they are concious of this effect, and I can tell you that any doubt they have about aircraft performances will request the manufacturer to demonstrate in flight.

My answer to your question is that those airplanes are not protected in the sense that they will develop assymetric iced conditions but are strongly protected in the sense that this condition is considered and very likely tested with airtificial iced shapes, so the airplanes are really safe. So globally, yes, the airplane is "protected" by design.

For your questions about 4-engined airplane, the answer is the same but having more complexity due to the number of engines.

I can tell you as well that in iced conditions, due to the complexity of the phenomena and the prediction and simulation difficulties, there is a lot of conservatism in the way manufacturers design the airplane and significant cost in airplane testing. FAA and EASA are really doing a good job.


Is altitude and heading your discretion when you are cleared visual approach? If I'm north of KSAN heading east and the controller clears me for the visual approach for runway 27, can I descend and turn the plane at my discretion so that I  can get ready for landing?

Also, once a controller clears me for the visual approach and I accept it, am I responsible for my own terrain and obstruction clearance?

Visual approach procedures are discussed in AIM 5-4-23. A visual approach is an IFR procedure and you must be on and stay on an IFR flight plan. It authorizes you to proceed visually clear of clouds to the airport. You may make whatever normal maneuvers necessary to safely conduct the approach and landing including descents and turns. Under most conditions, you are responsible for your own terrain clearance and separation. The exception is if you have the airport in sight but not the aircraft you're following. ATC can still clear you for the visual but will retain radar separation responsibility.

Note that a visual approach is different from a "Charted Visual Approach Procedure," which KSAN happens to have for Runway 27. If cleared for the charted visual, then you must fly it as depicted.


What is the minimum turning radius of an SR-71? What is the minimum turning radius of an SR-71 at Mach 3.2 and an altitude of 80,000 feet?

I have heard that if an SR-71 were to cross the Pacific coast over San Francisco and pull a hard turn to the right, it would return over Seattle.

I'm interested in real-world performance, taking into account things like inlet unstart (failure to capture the engine inlet shockwave in the intake) from too sharp an angle of attack.

The Turn Radius of the SR-71 would depend on its speed. The faster it went  the wider its turn radius was.

The SR-71 had a minimum turning radius at altitude of about 80 nautical miles (NM) . It was not an airframe limitation but a matter of wing area. At 80,000ft, the air is too thin and the wings too small to allow for much lift to turn with.

At a turn radius of 80 NM, the SR-71 would cover about 145 miles, taking about 4 minutes in the process of making a 180 degree turn.

Details are provided in the SR-71 flight manual handbook, which is now declassified:


Why does aircraft stability increase when it transitions from subsonic to supersonic flight? I am puzzled why would the aircraft be more stable at high speed flight at high altitudes. Given that the Centre or Pressure moves gradually towards the 50% MAC region, once past Mcrit.

Has this got to do with moments and the neutral point?

Static longitudinal stability (also called static margin) is determined by the distance between the center of gravity and the neutral point¹. The neutral point is at a quarter of the chord at subsonic speed and moves back to half chord once the aircraft flies fully supersonically. This means that the static margin will greatly increase when moving from sub- to supersonic flight.

In order to keep the center of pressure at the center of gravity, in supersonic flight the elevator needs to create lots of downforce. The bigger the difference in lift per area between wing and elevator, the stronger pitch stability will become.

This effect makes the transition between both regimes tricky: When accelerating, aircraft experience the shift in the center of pressure as Mach tuck, and when the trim facilities are not sufficient, the aircraft might enter an uncontrollable dive.

In the reverse direction, an aircraft trimmed for supersonic flight will experience a pitch-up moment from the forward shift in the center of pressure once it slows down, which can stall it or overload the structure.

¹ I could also talk of the center of pressure here, but that would be less precise. The neutral point is the center of pressure for all angle-of-attack-dependent lift forces, and it is those which determine pitch stability.


Do planes have odometers? I'm curious to know if planes have distance counters similar to the ones in cars indicating how many kilometres/miles the plane has flown since being put in service.

A Hobbs meter measures the time spent in a general aviation aircraft with the power on.

Planes differ in that time is easier to record than distance. For a given airspeed, the distance covered by the aircraft depends on the wind, altitude, and other factors.

Modern airliners would record the period each engine has run, flight time, cycles (one cycle is one pressurization/depressurization cycle), etc., digitally.

Those records are then used to track the time between overhauls (TBO). Same as with cars.

For small planes, a tach timer is used to record the revolutions the engine has made, this one is used for TBO.

(Source) Image of tach timer.


If a tail dragger is converted to a tricycle gear airplane, what will be the effect on weight distribution? If a tail dragger airplane is converted to one with conventional tricycle landing gears, how will this affect the balance of the airplane? It is possible to offset the effect by shifting weights around the airframe?

There are some planes out there that exist in both configurations. One of the more common and first that comes to mind is the popular Cessna 150, the Cessna 172 also has an available conversion.

As a taildragger

(source)

And with tricycle gear

(source)

Since the majority of the mass remains unmoved (the engine, fuel tanks etc) the CG will see a bit of a shift but will not be that drastic. The biggest weight shift will be moving the mains up front. However since your gear is located in a different place and thus is loaded differently your CG limits may change.