What is the constant in this formula for additional lift? Let’s consider the case of a plane flying s/l in a headwind with a positive gradient. A wing produces lift by inducing a bound vortex superimposed on the relative wind., and in the particular case of flying in a positive gradient, one can imagine an extra pseudo-vortex associated with the gradient itself, that re-inforces the bound vortex producing additional lift.

The question is now how to quantify that additional lift. From the variables that enter in the circulation theory of lift, one can guess that the variables involved are the density of the air rho, the airspeed v, the wing area A, the chord c and of course the wind gradient ß.

From dimensional analysis, it results that the extra lift L is:

L = k · rho · v · A · c · ß

where k is a constant to be determined. It can be probably done with a ‘material experiment’, but my question here is whether it can be derived from already known data or from a ‘mental experiment’.

The formula you have given looks strange to me, I have never seen it before. But possibly I can help you with the following brief explanation about kinematic relationships when flying under wind conditions.

Flight path velocity (VK) is the vector sum of aerodynamic velocity (VA) plus wind velocity (VW), see sketch. - When flying in a headwind condition aerodynamic velocity will increase and a pilot will need to decrease Alpha to keep the lift constant. It is the opposite way around in a tail wind condition. Now a pilot will have to increase Alpha in order to keep the lift constant and additionally will have to pay attention not to come too close to Alpha max. Or increase thrust.

If one uses the sketched kinematic relationships properly one does not need an additional formula for analysis.


Lost radio communication after take-off in IMC - what are my options? This might be "yet another lost comms questions", but I've tried to read a lot and there are different answers everywhere. Here's the scenario:

(The flight plan as filed is - KSSI SSI V441 MONIA GNV OCF V581 DADES KTPA)

I'm taking off from an uncontrolled airport - KSSI to fly to KTPA (ICAO IDs). The current weather at KSSI is 003 OVC and 2 SM. I call MACON FSS for my clearance on 121.5, and here's what I get -

MACON FSS: "ATC clears N123CW to KTPA Direct SSI and then as filed, climb and maintain 4000 expect 8000 two-zero minutes after departure, departure control will be 126.75, squawk 1234. Released for departure, clearance void if not off by 20:45, time now 20:30, frequency change to advisory approved."

After read back, I take off exactly at 20:35 and after a couple minutes I contact Jax Center on 126.75 as I'm crossing 1300ft. I hear no other traffic on the frequency, and I try to contact them 2 more times while I'm heading direct to the SSI VOR. I try to contact MACON FSS with whom I'd just spoken a few minutes ago - and I hear nothing back. I try a couple of other frequencies that I can get from charts - no luck anywhere. I'm in complete IMC.

Now, I know that I've lost my radio communications capabilities and hence squawk 7600, and figure out my route and altitude to fly.

I was cleared direct to the SSI VOR, and then as filed. Looking at "AVEF", ATC expects me to fly the route I filed. Looks good - no problems here.

I was asked to climb and maintain 4000ft, and expect 8000ft 20mins after departure.

Looking at "MEA", ATC expects me to be at 8000ft since it's the highest amongst the Minimum En-route Altitude, the expected altitude (which is n/a) and the assigned altitude).

I'm still at 2400ft, climbing to 4000ft.

My questions are:

1) When should I start my climb to 8000ft? Should I level off at 4000ft and wait until 20:55 (which is 20 minutes after my departure time) before initiating my climb, or does ATC expect me to be at 8000ft by 20:55?

2) Along my route (not immediately after departure, but after a while), for e.g. at the GNV VOR, if I briefly enter VMC, should I go ahead and land at the nearest airport as practicable or should I still continue through my route? I do know that the regulations advise to land as practicable when you're in VMC, but if the airspace ahead is being cleared for you when you're under IFR, is it advised to try and land into VFR traffic who have no idea that you're coming-in out of nowhere?

Appreciate all the answers. Thank you!

PS - Here's the Low-Enroute chart for quick reference:

The answers to these questions can be found in the AIM in section 6-4-1. All quotes are from that page.
To the first question, you should expect to receive a clearance to 8,000 feet twenty minutes after departure; therefore, don't climb above 4,000 until that time. If you are still in radio contact after 20:55, the "expect" portion of the clearance expires:

If the pilot received an “expect further clearance” containing a higher altitude to expect at a specified time or fix, maintain the highest of the following altitudes until that time/fix:
(1)   the last assigned altitude; or
(2)  the minimum altitude/flight level for IFR operations.
Upon reaching the time/fix specified, the pilot should commence climbing to the altitude advised to expect. If the radio failure occurs after the time/fix specified, the altitude to be expected is not applicable and the pilot should maintain an altitude consistent with 1 or 2 above.

For the second question, the answer comes down to the definition of "briefly". Is your hole big enough for you to get under the clouds, and is the ceiling high enough that you can remain VMC to land? If so, you should do so. But if you just briefly see the ground, or aren't confident that you could safely fly VFR to a suitable airport, then you should maintain IFR and continue flying your clearance. You may inconvenience a few controllers, but if you leave your cleared route only to discover that your intended airport is socked in, getting back to your route to resume your original destination puts you potentially low to the ground, and not along the route that ATC is expecting you:

Pilots should recognize that operation under these conditions may unnecessarily as well as adversely affect other users of the airspace,
since ATC may be required to reroute or delay other users in order to
protect the failure aircraft. However, it is not intended that the requirement to “land as soon as practicable” be construed to mean “as soon as possible.” Pilots retain the prerogative of exercising their best judgment and are not required to land at an unauthorized airport, at an airport unsuitable for the type of aircraft flown, or to land only minutes short of their intended destination.

That said, VFR traffic should always be looking for other aircraft - with and without radio contact. Radios are not required equipment, at least at non-towered airports, so landing among VFR traffic without a radio should not be a big deal.

Cell phone, Call the tower, that's what the controllers want. Might not work with a jet, but most GA should be low enough. Always try to have the towers number.

Your over complicating things.


Shouldn't the instructor in this scenario try to avoid a deadstick landing? In the Netflix drama "The Crown" (season 1, episode 4) Prince Philip has flight lessons with Group Captain Townsend (below is an image of that scene, I was unable to identify the plane).

After Prince Philip complains about the noise of the engine, Group Captain Townsend deliberately turns it off, and they both enjoy the silence for a while. After some chat, Prince Philip notices the ground getting closer:

Prince Philip: "Shouldn't we get her started now? The ground seems terribly close."
Group Captain Townsend: "No, it's too low to restart, sir. We'll do a dead-stick landing."

And they land safely.
My question is: if that was a real event, shouldn't Group Captain Townsend (as the instructor) interrupt the relaxing gliding with a warning? Something along the lines of "excuse me, sir, we're losing altitude, if you want me to restart the engine we have to do it now", or even "I'm restarting the engine now". I mean, shouldn't he be checking the limit altitude for restarting the engine?
I'll not ask why Group Captain Townsend didn't do that, it would be off topic, opinion-based and, despite the Group Captain being a real person, that scene is fictional anyway (The Crown is not 100% historically accurate). I'm just curious about the most probable behaviour of a real instructor in a situation like that.

if that was a real event, shouldn't Captain Townsend (as the instructor) interrupt the relaxing gliding with a warning? Something along the lines of "excuse me, sir, we're losing altitude, if you want me to restart the engine we have to do it now", or even "I'm restarting the engine now". I mean, shouldn't he be checking the limit altitude for restarting the engine?

This question appears to be predicated on the idea that the instructor had no plans to land the plane with the engine turned off. Having watched the scene, I'm far less convinced that this is the case. In fact, it's not until Philip remarks that the ground is coming up that the Captain tells him they'll be landing deadstick.
In the series, this instructor featured is a renowned flying ace with huge amounts of combat flying experience including a number of forced landings and ditchings.
Bob Hoover (who you could argue with cut from a similar cloth) regularly displayed and demonstrated with the engines turned off, up to and including landing in that configuration.* On the basis that turning the engine off was deliberate and not an emergency, I don't think the scene after that point is particularly unrealistic. They appear to be making a straight in approach and land on an airfield - it's entirely plausible the Captain made a deliberate decision to land with no engine from the very start.
*Example:

First, a real instructor wouldn't have turned the engine off at all. The engine is kind of important, after all.*
Second, you're never "too low" to start the engine. After all, you can start it when you're on the ground, and that's about as low as an airplane can get.
Third, to actually answer your question, yes, if there was some limit to how low they could go, then the instructor should have said something before reaching that point.
The airplane in the screenshot is a de Havilland DH.82 Tiger Moth.
*There are some instances of instructors turning the engine off to test their students on emergency engine-out procedures. But that's for that specific purpose, not just because they don't like the engine sound. Note that I'm not going to argue one way or another as to whether or not it's a good idea, just pointing out it does happen.


How can I find the optimum chord wing length for this wing? This Aviation SE forum looks the perfect place to ask this question: people here usually make maths the root of most solutions and I think this may solve my aerodynamic optimization problem!

I am having a very hard time optimizing the wing size of my rc plane. I already went to the excelent RC GROUPS community but, despite the generous help of many people, this subject got a little more confusing and personal opinions (practical experience) are conflicting.

My question is this: I already have an rc plane (with tail, fuselage...) and I want to optimize the wing size in order to get efficiency and the longer flight time. I already built dozens of rc planes but this time I want to make this one the most efficient as possible.

IMHO to get longer flight time I need to consume the least amount of energy from my lipo battery as possible - so I need to keep my brushless motor running in the slowest RPM possible, consuming less Amps. Having said that, my total plane weight (not considering the wing, of course) is around 300g (summed weight of ESC, servos, 1000kv motor, 11 inch prop, fuselage, tail, receiver and battery). The weight is fixed, 300g. My wingspan is also fixed, 90cm. I cant have a wing longer than that. The wing shape is rectangular (I know elipses would be the better shape but let's fix the wing shape to simplify this discussion).

If everthing is "fixed" so what is the variable? Well, I want to find the optimum chord wing length for this plane which is gonna bring me the highest flight duration.

I am using the CLARK Y airfoil but you can assume any other one you want. I wouldnt like to discuss airfoil, I would like to discuss the size of the chord and how to optimize it.

My experience and lots of study into this subject says that high aspect ratio wing are usually more efficient than low AR. My wingspan is fixed and all other variables are fixed (to make this discussion more specific) except chord.

So my dear friends, I am struggling a lot to find the optimium chord to get the best flight duration. It looks easy but I am almost 2 months trying to get this answer from many different sources and I still cant find a good solution.

PS: I know I could optimize many of my other fixed parameters, but to avoid having this question marked as too broad/global/generic, I really would like to only optimize the wing chord.

PS (2): the plane that I want to optimize the chord is in the image below. The wing that I show on the picture already provides good flight time (around 22 minutes) but I would like to optimize it in order to get better flight time.

You want to fly at the polar point of the lowest energy consumption. If you describe the drag with the usual parabolic equation, you can use the optimum point derived in this answer. It is
$$3\cdot c_{D0} = \frac{c^2_L}{\pi\cdot AR\cdot\epsilon}$$

Induced drag is three times the parasitic or zero-lift drag when you fly at the polar point for minimum energy loss. But the minimum induced drag at a certain speed is also fixed once wingspan and mass are given - you can only mess up by creating a non-elliptic circulation over span, but never get better than this limit.

Now get the zero-lift drag down as much as you can (pick a thin, highly cambered airfoil which produces a high lift coefficient at the given Reynolds number, definitely no Clark Y) and size chord such that this airfoil will work at the optimum lift coefficient.

Note that this will be an iterative process. Picking a chord will influence your aspect ratio and zero-lift drag, so the optimum lift coefficient will be depend on the chosen chord.

Maybe you can get something from the Daedalus:

  The MIT Aeronautics and Astronautics Department's Daedalus was a
  human-powered aircraft2 that, on 23 April 1988, flew a distance of
  72.4 mi (115.11 km) in 3 hours, 54 minutes, from Iraklion on the island of Crete to the island of Santorini. The flight holds official
  FAI world records for total distance, straight-line distance, and
  duration for human-powered aircraft....  Airfoils Mark Drela had
  written the program XFOIL which enables the design of aerofoils and
  which can accurately predict the performance at any Reynolds number
  which is actually superior to a wind-tunnel test of a proposed
  section. The wing sections never existed on paper, either as lists of
  numbers or as drawings. The information came out of the design program
  onto disc, and this guided the cutter. The spar of the MLE (Michelob
  Light Eagle) was three tubes aligned vertically.

Source

  This scheme was abandoned for the final Daedalus design where the more
  usual single tube was used. Construction of Light Eagle took 15,000
  hours work by 18 members of MIT. The airfoils which were used are:
  DAE11, DAE21, DAE31, and DAE51.


Should I log flight time as a passenger? If I only have a private pilot license, and am not the pilot in command, should I log my flight time?

If so, what are the limits of this? i.e. is that as true in the backseat as it is in the front seat? Must I have that type rating?

No, you should not*.

In my opinion, you should limit your logbook to records of your own flight experience. This would include any time spent training, as PIC, or as required crew.

The time you can log as PIC is that which you spend as the sole manipulator of the controls, the sole occupant, or as the pilot in command when the required crew is >1. Here's an article from an FAA FSDO describing the logging of PIC time in more detail. Rod Machado has some good coverage of this as well.

If you're just hanging out in the airplane, it doesn't matter if you're in the front seat or the back seat. If you're receiving instruction, you should get an entry and a logbook endorsement from the instructor you're flying with. Most General Aviation aircraft don't require a type rating, so I'm unsure what the context of your final question is.

* It's worth noting that you can technically put whatever you want in your logbook, though it's not a great idea; when it comes time to fill out an FAA 8710 form for later ratings, it can get confusing. In my opinion, you still shouldn't log any time, but if you're just wanting to record flights that you took with friends, that's fine. As an instructor who's had to slog through some weird logbooks, I'd really, really encourage you to track that elsewhere, though. Your logbook is exactly that - it's your logbook, for the logging of your flight time, and a record of your flight experience and training.

No. You're only supposed to log time when you are "manipulating the controls" i.e., PIC/SIC or as a rated crewmember. Safety pilots and many others can log time without touching the controls, but as a passenger, you have no role in the operation of the aircraft, and therefore cannot log time.


What are "approved weather reporting services" and do they apply to part 91? When I look up Torrance Zamperini (KTOA) alternate minimums, it states, "NA when control tower closed, except for operators with approved weather reporting services."

My main questions are:

Can Part 91 pilots file this airport as an alternate when the control tower is closed?
Do the "operators" restrictions apply to Part 91 pilots?
What is an "approved weather reporting service"?

While we are on the topic, what exactly does the word "appropriate" refer to in the text below? FAR 91.169(c) seems to indicate that almost any source of weather information is acceptable (emphasis mine):

  IFR alternate airport weather minima. Unless otherwise authorized by the Administrator, no person may include an alternate airport in an
  IFR flight plan unless appropriate weather reports or weather
  forecasts, or a combination of them, indicated that, at the estimated
  time of arrival at the alternate airport, the ceiling and visibility
  at the airport will be at or above the following weather minima:

First, within the context of the question, the term "operator" in the Alternate Mins section refers to a person or company that has an operating certificate.  Not just Joe the Pilot who is "operating" an aircraft. So, a "Part 91 pilot" (taken from the context to mean: "pilot flying for pleasure or personal business") cannot file this airport as an alternate when the tower is closed.

Second, the term "approved weather source":  In the FARs, various Parts apply to various operators (121, 125, 135). They each have similar verbiage about approved weather reporting services.  For example, Part 121.101 (for domestic and flag operations) says the following (paraphrased):

  No certificate holder may use any weather report to control flight
  unless it was prepared by the U.S. National Weather Service or a
  source approved by the U.S. National Weather Service. Each certificate
  holder shall use an approved system for obtaining forecasts and
  reports on each route to be flown and at each airport to be used.

You can see this related question: What sources of weather are approved by the U.S. National Weather Service? for more information.

BUT... the important point from that FAR Part 121 quote is the second sentence. ALL operators must have Operational Specification (OpSpecs) approved by the FAA before they can operate. One of the items within the OpSpecs will specifically address how and from where these weather reports and forecasts will be obtained.

So, this is the bottom line answer to the question: only "operators" can use this airport as a filed alternate when the tower is closed, since that operator will have their own "weather source" that is approved by the FAA and listed in that operator's OpSpecs.

Disclaimer: I did not reference the plates.

Operator would include a Part 91 flight.  Approved Wx service is normally ATC, FSS, NWS or a trained observer (which I understand to include pilots).

As for what appropriate means...

I like to interpret things colored by asking myself, "What would the accident review board say?"

In light of that I would consider appropriate to mean that there are METARs and nearby TAFs.  While it appears to be general practice to use a nearby airport with reporting services, it is not clear that is a good practice.  Especially during frontal passage Wx at two nearby airports can be radically different.

So in conclusion, rely on METARs, TAFs, perhaps even PIREPs, but not the Channel 5 news.


What is the correct answer to this Mass and Balance calculation? I was practising some mass and balance questions and I found that my answer is a little bit too far from the listed answers. Can anyone help me to see if I did something wrong?

The question was:

  Given that the Dry Operating Mass (DOM) is 27 372 kg, there are 12 000 kg of
  trip fuel and 4 000 kg of fuel are expected on landing, what is the
  maximum allowable traffic load for the Medium-Range Jet Transport
  (MRJT) aeroplane?

  23,923
  23,528
  19,005
  19,428

My solution was:

  Operating mass = DOM + Usable fuel

  $27 372 - (12 000 - 4 000) = 19 372 kg$

That doesn't match any of the options, so what is the correct answer?

The answer is 19,428, but I could only get there by Googling 'MRJT' to get the missing information:

DOM + all of the fuel is 43,372.

And since MSTOM is 62,800, that leaves you with 19,428 for payload (allowable traffic load).

$$62,800 - (27,372 + 12,000 + 4,000) = 19,428$$

Below you can see the calculations for MTOM, MLM, and MZFM. Since the traffic load for the 'MTOM limited case' is the lowest, the maximum traffic load for the flight is 19,428 kg.

(Table courtesy of DeltaLima.)

It strikes me that your formula is correct, you just didn't follow it -- Operating Mass = DOM + usable fuel = 27 372 + (12 000 - 4 000) = 35 372


Have Eddy current brakes ever been used/tested on an aircraft? Eddy current brakes (wiki) --- a.k.a. magnetic brakes --- get quite some attention from train engineers. They are used on Shinkansens, roller coasters, and even (as far as I know) metro trains on Lane C in Prague. They basically consist of a magnet and a rotating disk, where these two never touch. The kinetic energy is converted to heat, as in conventional brakes. While Shinkansens give them up, from what I view it is in favour of recuperating brakes and whence it is irrelevant with respect to aviation.

I wonder whether anyone considered their use in aviation. To me it seems that it could be a good option.

Eddy current brake is a misnomer: it's a damper, not a brake as in a device that works on friction forces and can completely stop motion. Eddy current dampers do provide deceleration forces, by inducing whirly electromagnetic currents in a rotating wheel - they could provide extra emergency braking power without installing extra brake pads.

However, aircraft wheel brakes are dimensioned for emergency deceleration, can provide this adequately using friction pads, and their main problem is getting rid of the heat that is created. Eddy current dampers create the same amount of heat...

In my opinion , its an amazing idea to down the speed of an ground running aircraft to a big extend using eddy current braking. The main adavantage of this technology is that ,formation of eddycurrent can be done uniformly and hence it wont create irregular braking impact. Rather thinking about stoping the aircraft it is more relavent to think about reducing the speed within a short time period ,so that , shorter runways will be required for massive aircrafts to land.Hence for takeoff purpose only lenghty runways are required. And its will also make the pilots easy to face the impact of overshooting the runway during landing.

  As we know that, there are many accidents taking place now a days in the time of landing (overshooting the runway),which makes the entire flight a bad memmory for the passengers. Like every part of flight , safe landing is also very important to ensure that the entire flight is not life challenging.So it is time to think about an effective braking system that makes the aeronautical industry more safe and less challenging like the "EDDY CURRENT BRAKING SYSTEM".


May an ultralight vehicle pass directly above KEGE at 9200 MSL at 1600Z with no prior authorization without violating FAR 103.17? May an ultralight vehicle pass directly above KEGE at 9200 MSL at 1600Z with no prior authorization without violating FAR 103.17?

http://vfrmap.com/?type=vfrc&lat=39.643&lon=-106.916&zoom=10

The A/FD entry for KEGE says "AIRSPACE: CLASS D svc 1400–0200Z‡; other times CLASS E."

There is more to this question than meets the eye-- KEGE appears to be an oddball case.  More detailed descriptions of the airspace descriptions at KEGE, including the page numbers for these airspace descriptions in the FAA's "Airspace Descriptions and Reporting Points Documents" JO 7400.11C, are given in this related question Can the Class-E-to-surface airspace near Eagle County Regional airport KEGE be ignored when the tower is open?

Please note that this is NOT a question about:

Whether an ultralight needs prior authorization to fly in the airspace above the surface footprint of E4 "extensions" such as at KNYL or KMFR, as per Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?
Whether an ultralight needs prior authorization to fly in the airspace above the surface footprint of the rectangular projection of E2 airspace at KEGE-- this rectangular projection was the subject of another related question Can the Class-E-to-surface airspace near Eagle County Regional airport KEGE be ignored when the tower is open?
Whether an ultralight needs prior authorization to fly over the ceiling of Class D airspace in a case where the E2/E4 airspace designations have been assigned in a more conventional manner, with the footprint of the E2 airspace fully coincident with the footprint of the Class D airspace, such as at KMFR or KNYL http://vfrmap.com/?type=vfrc&lat=42.374&lon=-122.874&zoom=10
Whether an ultralight needs prior authorization to fly in Class E airspace in general

Yes, an ultralight aircraft may fly over KEGE ( http://vfrmap.com/?type=vfrc&lat=39.643&lon=-106.916&zoom=10 ) at 9200' MSL at 1600Z -- i.e. above the ceiling of the Class D airspace when the control tower is open and the Class D airspace is in effect-- with no prior authorization without violating FAR 103.17.  Just as is the case above any Class D airspace with projecting areas of Class-E-to-surface airspace where the airspace designations have been handled in a more conventional manner, such as at KMFR ( http://vfrmap.com/?type=vfrc&lat=42.374&lon=-122.874&zoom=10 ).

The design of the airspace at KEGE as described in the FAA's "Airspace Designations and Reporting Points" document, JO 7400.11C, is unconventional and possibly flawed, as described in more detail in this related question Can the Class-E-to-surface airspace near Eagle County Regional airport KEGE be ignored when the tower is open?.  Here are all of the most plausible interpretations of how the Class-E-to-surface airspace around KEGE is intended to function.  None of these interpretations require an ultralight aircraft to seek prior authorization before overflying KEGE at 9200' MSL when the Class D airspace is in effect.

All of these interpretations address this question -- "During the hours that the Class D airspace is active at KEGE, is there any E2 airspace at KEGE, and if so, where?"   To understand why, generally speaking, E2 airspace, and only E2 airspace, appears to be the best fit for the meaning of "the surface area of Class E airspace designated for an airport" in the context of FAR 103.17, see this related answer Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?

1) The best description of the situation at KEGE appears to be that the airspace has been designated in the same way as the 21 airports described in this answer to a related question What are all the US airports where E4 "extensions" change to E2 and become part of a larger E2 "surface area" when the tower closes for the night? , except that no E4 airspace description has been provided to create Class-E-to-surface airspace in the projection to the east when the tower is open and the E2 airspace is inactive.  according to this interpretation, this omission is accidental.  A careful study of the "Airspace Designations and Reporting Points" document makes it clear that any given block of designated airspace (e.g. E2, E4, or Class D) is intended to become effective or ineffective as a single unit, so when the Class D airspace is in effect at KEGE, none of the E2 airspace is in effect. So technically, as the description has been written, it appears that there is no Class-E-to-surface airspace in the projection to the east when the tower is open. Local ATC treats the airspace as if there is Class-E-to-surface airspace in the projection to the east 24/7, i.e. as if an E4 airspace description had been provided to cover the airspace in the projecting area to the east when the tower is open and the E2 airspace is not in effect.  Naturally, it would be wise for a pilot to treat the airspace in the projection to the east as if it were some form of Class-E-to-surface 24/7, because this is the practice of the local ATC.  Nonetheless, an ultralight overflying KEGE above the Class D ceiling when the tower is open is not within the lateral boundaries of any active E2 airspace, so no prior authorization is required by FAR 103.17.

2) The airspace around KEGE has been designated in the same way as the 21 airports described in this answer to a related question What are all the US airports where E4 "extensions" change to E2 and become part of a larger E2 "surface area" when the tower closes for the night? , except that no E4 airspace description has been provided to create Class-E-to-surface airspace in the projection to the east when the tower is open and the E2 airspace is inactive.  According to this interpretation, this is intentional and is based on the theory that when the Class D airspace is active, only that portion of the E2 airspace which coincides with the Class D is superseded by the Class D, while the rest of the E2 airspace remains in effect.  (KEGE appears to be the only airport in the US where this unusual theory has been applied; normally any given block of designated airspace (e.g. E2, E4, or Class D) is intended to become effective or ineffective as a single unit.)  However, a fundamental characteristic of any given column of E2 airspace is that it contacts the surface.  Therefore when some portion of the E2 airspace is superseded by Class D airspace up to 9100' MSL, all the immediately overlying E2 airspace also detaches from the rest of the E2 airspace and becomes E5 airspace.  When the Class D airspace is in effect, none of the airspace above the ceiling of the Class D can be considered to be E2 airspace.  Therefore an ultralight overflying KEGE above the Class D ceiling when the tower is open is not within the lateral boundaries of any active E2 airspace, so no prior authorization is required by FAR 103.17.

3) The airspace around KEGE has been designated in the same way as the 21 airports described in this answer to a related question What are all the US airports where E4 "extensions" change to E2 and become part of a larger E2 "surface area" when the tower closes for the night? , except that no E4 airspace description has been provided to create Class-E-to-surface airspace in the projection to the east when the tower is open and the E2 airspace is inactive.  According to this interpretation, this is intentional and is based on the theory that when the Class D airspace is active, only that portion of the E2 airspace which coincides with the Class D is superseded by the Class D, while the rest of the E2 airspace remains in effect.  (KEGE appears to be the only airport in the US where this unusual theory has been applied; normally any given block of designated airspace (e.g. E2, E4, or Class D) is intended to become effective or ineffective as a single unit.)  Therefore the airspace above the ceiling of the Class D can be technically be construed to be E2 airspace 24/7.  However, FAR 103.17 does not specifically and explicitly reference E2 airspace.  While in most cases, as noted in the related answer Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?, E2 airspace, and only E2 airspace, is the best fit for the meaning of "the surface area of Class E airspace designated for an airport" in the context of FAR 103.17, any piece of E2 airspace that is not actually part of a column of Class E airspace extending all the way to the ground is clearly an exception to this general principle.  An ultralight may overfly KEGE above the Class D ceiling when the tower is open with no prior authorization without violating FAR 103.17.


What does the balance ball actually indicate? Given answers and comments from a previous question, I realize that I need to clarify what the ball actually measure. I think it does not measure side slip but sideways acceleration, but I'm not quite sure.

Let's say, I put a ball in a roller coaster. Whatever the maneuver performed, there can't be side slip (no wind, the roller coaster is basically a train). Those maneuver include:

boat turns (the ball should be on the external side of the turn)
straight horizontal trajectories (the ball should be centered)
straight trajectory with a bank angle (the ball should be on the lower side)
a simple roll

How does this differ from in flight situations? Given what the ball actually measures, what makes it reliable to be used as a side slip indicator?

bonus question: if I take a long aircraft (e.g. a AN-225) and put a ball in several places in the aircraft, will all those indicator concur in a not coordinated turn?

1. Preamble

The ball reacts to the body force (or acceleration), with the exact same working principle as a bubble level. In airplane frame, the force is expressed as:

$$Y+mg\cos{\theta}\sin{\phi}=m(\dot{v}+ru-pw)$$

where $Y$ is all external side forces minus gravitational component, $m$ is airplane mass, $g$ is gravitational acceleration, $\theta$ is pitch angle, $\phi$ is roll angle, $\begin{bmatrix}u & v & w\end{bmatrix}^T$ is the inertial velocity in vehicle frame, $\begin{bmatrix}p & q & r\end{bmatrix}^T$ is the angular velocity in vehicle frame.

First, as any accelerometer, the ball does not react to gravity. Second, and this is a preference in bookkeeping, you can consider the accelerometer (or the ball) to only react to the external forces (i.e. $Y$), or only react to the inertial forces minus gravity (i.e. the right side of the equation minus the gravitational component).

2. In Flight

Taking thrust out of the question, then in flight, the only thing that can contribute to $Y$ is aerodynamic forces. What's the largest source of lateral force in air? Sideslip. There you have the relation between ball measurement and sideslip angle measurement. If you're unconvinced, read the sub-sections below.

2.1 Symmetric Thrust and Airframe

For symmetric thrust and airframe, the only yaw moment sources come from aerodynamics, and the largest contributors are sideslip and rudder (ailerons and spoilers also contribute due to differential drag, but the effects are even smaller).

Using the B737 model in AVL at Mach 0, flap 0 and AOA 0 for a rough idea, -5 deg rudder generates about 4.4deg of sideslip (nose left), at which point their yaw moments cancel each other out. However, the total $C_y$ is 0.055 to the right.

Comparing the stability/control derivatives (per rad), $C_{y_\beta}$ is -1.2, whereas $C_{y_{\delta_r}}$ is 0.44, which means that at the aforementioned conditions, the sideslip is overpowering the rudder by 140%, in terms of side force.

2.2 Asymmetric Thrust

In the case of one engine inoperative (OEI), we need to overcome the yaw moment from thrust asymmetry via rudder.

For wings level, there is side force from rudder that must be compensated. This can be accomplished by generating more sideslip into the live engine (which means more rudder deflection is needed than to overcome the thrust yaw moment). At steady-state, the net side force is zero, the ball is centered, but there is steady non-zero sideslip.

If we allow the wings to bank toward the live engine, a portion of gravity can be used to compensate for the side force from rudder, which means that less sideslip induced side force is required to maintain steady-state (therefore less rudder required). However, since the total side force minus gravity ($Y$) is non-zero, the ball is skewed to the live engine.

In either case, with thrust asymmetry, the relationship between sideslip and side force are "broken".

3. On Ground

On ground, any point of contact with the ground provides additional sources to $Y$. This is why tilting the bubble level with your hand makes it not measure level: it's not that the level is responding to gravity, but rather that it's responding to the normal forces exacted by your hand.

For an airplane, the source is obviously coming from the landing gear contact. At high speed taxi on a straight runway, with crosswind, the sideslip creates aerodynamic side force. The landing gear resists lateral motion via an opposite side force (however, you'd need to deflect nose wheel or rudder to resist yaw moment). At steady-state, the yaw string would measure sideslip, but the net side force is zero, so the ball would measure nothing.

Things are different when it's a circular runway. Because the airplane needs to track a curvature, the combined lateral forces must provide the centripetal forces necessary to maintain the track curvature. Therefore, the ball would react to the equivalent centrifugal force.

4. Sensor Positioning

Thus far, we've been assuming that the ball is placed exactly at the airplane CG. If the sensor is offset, then it will also react to the angular rates. That's why large airplanes need to calibrate the sensor measurements to the assumed CG position.

To answer your last question, unless the airplane is tracking linear motion, placing the sensor at different locations will give you different readings.

Much of this answer is based on the related answer How does sideslip indicator react during crosswind?.

We should start by understanding what the deflection of the "inclinometer" or "ball" actually indicates.  To a good approximation, it responds to the net sum of all lateral (sideways) accelerations acting on an aircraft, excluding the lateral acceleration component due to gravity.  By "lateral" or "sideways" we mean in the aircraft's own reference frame -- for example, in a turn, the direction meant by "lateral" or "sideways" would be constantly changing as seen by an observer on the ground, or for that matter, an observer in a balloon drifting by with the airmass.  In a turn, by "lateral" we don't mean the "centripetal" direction-- rather we specifically mean "sideways toward the wall of the cockpit".  In other words, the direction that is parallel to the aircraft's wingspan.  In a coordinated turn, there is definitely a lateral acceleration component, but the aerodynamic force in the lateral direction is zero.  In other words, in a coordinated turn, the net lateral acceleration other than that due to gravity is zero.  To a first approximation, the only thing that causes an aircraft to experience a lateral acceleration, other than gravity, is the aerodynamic sideforce generated by a sideslip as the airflow strikes the side of the fuselage.  That is why the ball serves as a slip-skid indicator, i.e. a sideslip indicator.***  There are some reasons why the ball is not a perfect indicator of sideslip, which will become evident later in this answer, but the idea that the ball is basically a sideslip indicator is a good starting point.

To be complete we should also point out that for a given aerodynamic sideforce, the motion of the ball is also influenced by the G-loading, or more precisely, by the upward aerodynamic force generated by the wing.  The higher the "upward" G-loading, the more aerodynamic sideforce it takes to cause a given deflection of the ball.  At zero "upward" G's, the slightest aerodynamic sideforce will put the ball all the way into one of the far corners of the glass tube, and at negative "upward" G's, the ball tends to just stay stuck in one of the corners even when sideforce is zero.  This is why many aerobatic aircraft have a second slip-skid ball that is mounted upside-down.

If the ball is an accelerometer, how can it be deflected in a situation like a steady-state wing-down slip along a straight-line flight path, where net acceleration is clearly zero?  The answer is that net acceleration is zero, but there is still an aerodynamic force component, and therefore an aerodynamically-driven acceleration component, acting in the sideways direction in the aircraft's reference frame, caused by the air impacting the side of the fuselage.  Just as a panel-mounted G-meter reads "1" not "0" in straight-and-level flight-- net acceleration is zero, but there is still an aerodynamic force component acting in the upwards direction in the aircraft's reference frame-- the lift force from the wing.  Just as the slip-skid ball measures the net lateral (sideways) acceleration other than that caused by gravity, so too does the G-meter measure the net vertical acceleration component other than that caused by gravity, which is essentially just the upward acceleration caused by the wing's lift vector.

What reference frame is the slip-skid ball operating in?  We've seen that the slip-skid ball measures lateral acceleration (other than that due to gravity).  We can measure net acceleration in any valid inertial reference frame and we'll get the same answer, except for differences due to the tilt of one frame relative to the other.  An accelerating airmass is not a valid inertial reference frame.  Nor is the aircraft itself, unless net acceleration is zero.  Therefore it is not wrong to assert that in the context of an accelerating airmass, the slip-skid ball is operating in the reference frame of the earth, rather than the reference frame of the aircraft or the airmass, except that the reference frame is tilted to match the orientation of the aircraft in space at any given instant in time, and so is constantly changing its orientation with respect to the orientation of the earth's reference frame.  In other words the direction called "lateral" is fixed with respect to the aircraft, not the earth-- it changes (relative to the earth) as the aircraft changes heading, or as the aircraft banks.  Yet the actual acceleration in that (ever-changing) lateral direction is measured in relation to a valid inertial reference frame, not in relation to the aircraft itself.  It is a valid question to ask how it can be that the slip-skid ball which in a sense is tied to the earth reference frame, can give essentially the same indication as the yaw string.  Perhaps the above paragraphs have shed some light on those questions-- the answer has to do with the fact that the slip-skid ball is only measuring the lateral component of acceleration, and the fact that the lateral component of acceleration is intimately related to sideslip.  Keep in mind that in many instances in this answer, we use phrases like "in the aircraft's own reference frame", but we really only mean to describe the orientation of the reference frame involved.  At any instant time, the actual reference frame that an on-board accelerometer such as the G-meter or the slip-skid ball is operating in would indeed be an actual valid inertial reference frame such as that of the earth itself, just tilted differently.  It is correct to observe that an accelerometer is not actually operating in the reference frame of the aircraft itself in any case where the aircraft is accelerating, or in the reference frame of the airmass in any case where the wind is not steady.  If the slip-skid ball were actually operating in the reference frame of the aircraft itself, it would always read zero, and likewise the G-meter.

In any situation where lateral accelerations can be caused by something other than sideslip, the ball is not perfect as a slip-skid indicator.  One example would be any case where the fuselage is streamlined to the relative wind but the rudder is deflected.  The sideforce from the rudder itself will move the ball slightly off center opposite to the direction that the pilot would normally expect, i.e. in the same direction as the rudder is deflected.  Therefore when using the rudder to streamline the fuselage to the airflow, it is optimum to leave the ball slightly off-center in the direction that it would be deflected if the rudder were not applied at all.  Normally this effect is negligible in actual flight, but not always.  The scenario of a twin-engine airplane with one failed engine is the best-known case where a pilot will typically actually take this effect into account, leaving the ball significantly deflected in the direction of the pilot's rudder input, i.e. toward the working engine  -- see the eighteenth paragraph, and also the footnote, in  this related answer for more on this.  Of course the sideforce from the rudder will cause a turn unless we cancel it by banking toward the good engine.  But fundamentally, the bank is not what is displacing the ball-- for the same rudder input, the ball would show the same displacement regardless of whether the plane was banked as needed to continue in a straight line, or was kept wings-level and allowed to do a flat turn toward the good engine.  The yaw string would be centered in either case.  (Obviously, if we constrain the flight path to be linear, then everything changes and the slip-skid ball doubles as a bank angle gauge.  From this viewpoint, we know we should bank a bit toward the good engine, but the reason why is not obvious.)

The tail rotor of a helicopter also generates an aerodynamic sideforce.  Again, just as with the twin-engine airplane with one failed engine, when the fuselage is streamlined to the airflow in forward flight, the slip-skid ball will be slightly off-center, deflected opposite to the direction of the tail rotor's sideforce-- at least in cases where the body of the helicopter tends to tilt along with the disk of the main rotor.  If the disk of the main rotor can be tilted to counter the tail rotor's sideforce while the fuselage or body stays completely vertical, then the slip-skid ball can stay centered even when the fuselage or body is completely streamlined to the airflow.

A little-known fact is that it is also possible for the yaw string and slip-skid ball to disagree when an aircraft (glider) is being towed, or when an aircraft (airplane) is towing another aircraft, because the towline can impart a lateral force component whenever the two aircraft are not directly behind each other. In a turn on tow, where the heading of each aircraft is tangent to the circular path of turn, the towline is imparting a lateral force toward the center of the turn on each aircraft, so the ball will be displaced toward the outside when the yaw string is centered.

Note that we will see less deflection of the slip-skid ball for a given deflection of the yaw string when a gust strikes a heavily loaded aircraft, than when it strikes an aircraft of identical shape and size but less mass, travelling at the same airspeed.  A much simpler analogy would be two toy cars of the same shape and size but different mass, each with a flag on it. When a wind gust strikes both cars and blows both flags in an identical manner, the wind will accelerate the lighter car faster than the heavier car. The flags are analogous to the yaw string, and the cars are analogous to the aircraft accelerating sideways and tending to "leave the ball behind", so that the ball deflects sideways inside its tube.

Likewise if the pilot uses the rudder to induce a given sideslip angle as measured with the yaw string, the ball will deflect less when an aircraft is heavily loaded than when it is lightly loaded, if the shape and size and speed of travel are all identical.  This means that the heavily-loaded aircraft will require less bank angle to travel in a straight line while the pilot is using the rudder to maintain a given sideslip angle, as measured with the yaw string.  Or in other words, for a given aircraft travelling at a given airspeed, a given deflection of the ball indicates a larger sideslip angle when the aircraft is heavy than when the aircraft is light.  However this is not something that ever needs to be considered in actual for flight for any practical reason-- normally we just keep the ball centered, unless we are using the wing-down (slipping) method of crosswind correction just before touch-down, in which case we bank as needed to allow the desired ground track while keeping the nose aligned with the runway heading.

Obviously on the ground the connection between sideways airflow (sideslip) and lateral acceleration is broken, unless the aircraft is wings-level on a flat icy surface.  The wind can blow (creating sideslip) without accelerating the aircraft.  Also unless the ground is level, it can push up against the tires in a way that exerts a lateral force component on the aircraft.  (Even though the ball doesn't sense the downward acceleration component due to gravity, it does sense the upward accceleration component of the ground pushing up on the tires-- this is why the ball functions as a bubble level when the aircraft is parked and stationary, just as it does in flight whenever net acceleration including gravity is zero.)  The question also asked about a roller coaster or train -- there are many situations where the track will exert a lateral force on the vehicle, so a slip-skid ball would not always be centered.  Clearly sideslip (sideways wind) has no influence on the slip-skid ball at all in this situation, just as in the case of the aircraft on the ground.

The ball is not quite perfect as a sideways accelerometer.  Changes in the rate of yaw rotation about the aircraft's CG can also influence the ball.  Imagine that we mounted the aircraft on a pivot at its CG in a vacuum-filled hangar and twirled the airplane like a pinwheel in the yaw dimension.  Whenever the yaw rotation rate was increasing, the ball would deflect opposite the direction that the nose was moving.  The further the cockpit from the CG, the more pronounced this effect would be.

In other words, a change in yaw rotation rate causes a local lateral acceleration, as measured at some specific point along the length of the fuselage.  Also, changes in the rate of pitch rotation change the local G-loading as felt at given points along the length of the fuselage, which changes the indication of the slip-skid ball-- see below for more.

  bonus question: if I take a long aircraft (e.g. a AN-225) and put a
  ball in several places in the aircraft, will all those indicator
  concur in a not coordinated turn?

In a steady-state turn-- meaning constant rates of rotation in pitch and yaw-- then the only apparent centrifugal force induced by the rotation will act in the longitudinal direction, and all the inclinometers will concur, regardless of whether they are centered or not.  (Extra detail-- strictly speaking, for this to be true, the inclinometers must all be mounted along the aircraft centerline, so that they are exactly tangent to the arc of movement during a yaw rotation.) Whenever the yaw rotation rate is increasing, inclinometers at the front will be biased toward a deflection toward the outside of the turn and inclinometers at the rear will be biased toward a deflection toward the inside of the turn.  Whenever the pitch rotation rate is increasing, the "felt" G-loading will be increased at the front of the aircraft and decreased at the rear of the aircraft, which will tend to bias the inclinometers at the front of the aircraft toward less deflection (in whatever direction they are deflected), and will tend to bias the inclinometers at the rear of the aircraft toward more deflection (in whatever direction they are deflected).  Note that as we progressively increase the bank angle, the yaw rotation rate increases and then starts to decrease again, while the pitch rotation rate continually increases.  A shallow-banked turn involves mostly yaw rotation, while a steep-banked turn involves mostly pitch rotation.  So we can think through how the indications of the inclinometers located at various points along the fuselage will change over time as we progressively increase the bank angle.  In short, all the indicators will not always show exactly the same reading.

Normally these effects are negligible in actual flight.

Note that yaw strings mounted at various positions along the fuselage will not show exactly the same deflection during a turn-- the rotation of the aircraft in the yaw axis causes different parts of the aircraft to be moving through the airmass in different directions at any given instant in time.  In other words the undisturbed free-stream airflow or "relative wind" is actually curved in a turn. To read about how one pilot takes this into account in actual practice, see the Soaring magazine article Circling the Holghaus way by Richard H. Johnson.  Links to some other articles discussing the curvature of the relative wind in turning flight are embedded in this ASE answer.

If you really want to challenge yourself to better understand the behavior of the slip-skid ball, consider the following thought experiments: A) While flying in a moderate to steep turn, the pilot pushes the stick forward hard to unload the wing, or pulls the stick aft hard to load up the wing with extra G's.  Obviously the turn rate will vary.  Does the slip-skid ball move very much?  Does it move at all?  Does the yaw string (if present) deflect?  B) Imagine a wingover-like maneuver where the aircraft is "unloaded" to less than 1 G while steeply banked, in a (initially) near-level or slightly-climbing pitch attitude.  Obviously gravity is accelerating the flight path downwards, and obviously this downward acceleration is largely in the lateral direction in the aircraft's reference frame, at least initially.  If the pilot uses the rudder as needed to "help" the vertical fin keep the nose pointing directly into the airflow so that the yaw string stays exactly centered, is the slip-skid ball deflected?  Now go do the experiments, ideally in a sailplane or some other aircraft with a yaw string and no propwash over the nose.  The latter experiment in particular cannot be done in a useful way with no yaw string-- the aircraft definitely will sideslip if the pilot doesn't use the rudder as needed to prevent it.


What instructions should I give to an untrained passenger for Hand propping Cessna 172N as a pilot? Does anybody know what is the most effective way of hand propping a Cessna 172N, with a passenger who is not mechanically inclined.

So you need to give them the most easy to follow and simple instructions. Like after pulling the hand brake, and priming and opening the throttle 1/4 inch what do you tell them to do?

I would push the throttle gently in after I'd hear the engine coughing to power when hand propped, while keeping my feet on the brakes, till  engine's smooth revving and maintaining power.

But to a person that has no familiarity with the noise of the engine and does not know what to expect, what should I say.

Edit
I thought I should elaborate a bit to shed some light on what happened.

We had come back from a half a day of  hiking around the hills of Santa Ynez in the afternoon with my hiker friend. After trying to crank the ignition a couple of times, it just moaned jerking half way through a hesitant arc.

We looked around for something to use as chocks, found two rocks like 8-10inched and kind of heavy and pushed them to a locking position in front of the tires.

I set the throttle; told him not to touch it before I jump back after cranking and wave him all clear. Then he pushes the throttle by half an inch while pushing the breaks. We brought the engine back to life after a few tries.

After I felt we have a enough of prop rotation inertia I got on to the right seat and brought the RPM down. He got out and removed the rocks. Got back in and we flew back to Van Nuys.

That is crazy.  DON'T just rely on an person who's new to airplanes and only training is 5 minutes of showing them what to do, who may or may not react correctly when it springs to life, as the only thing preventing the plane from heading off somewhere while you try to dive clear.  Don't. Do. It.

Tie the tail. To something.  Anything.  Use the passenger and show them what to do and give them a careful briefing, but tie the tail down anyway.  You don't know how the passenger is going to react if things go off kilter.  And if tying the tail is simply not an option, and you have no choice because you have to leave because zombies are approaching, at least point the airplane toward some obstacle just beyond the normal turning radius, like a wire fence, for it to run into when it takes off with your passenger confused and frightened because they pushed the throttle in too much and let off the brakes when the thing jerked forward on them, with you diving for your life.

Once it's running at idle and everything is kosher, set the parking brake and you can walk back to untie it with your passenger ready to switch the ignition off if it starts to roll away.

And make sure to use the correct terminology with your assistant.  Don't say "switch on/switch off".  It's "Switch off" for ignition off, and "Contact" for ignition on (it's old fashioned sounding, but it works).

I have been personally burned by a guy who did things the usual way, when I said "switch off", then repeated "switch off" to make sure he heard it.  He thought I'd said "Switch off" then "Switch on" when I'd just said switch off twice.  I neglected to brief him on the use of the word "contact" so it was my fault in the end.  Anyway, I got a nice noisy surprise when I turned the prop over thinking ignition was off.

Hand propping is quite a dangerous operation, especially on tri-gear airplanes where you have to lean forward to do it.  I trust you yourself have been properly trained, and know things like not to hook your fingers over the blade so you don't get pulled into it if it kicks back, that sort of thing.

Most important: tell your untrained passenger to keep the brakes applied no matter what. Then try to push the plane to make sure he's really on the brakes.


Why was the Boeing 377 "Stratocruiser" frequently landed nosewheel first on touchdown? Why was the Boeing 377 "Stratocruiser" frequently landed nosewheel first on touchdown? In the mid-1950's I had often observed Northwest Airlines' B377's frequently being landed that way at the old Minneapolis-St. Paul airport.

A family friend and PanAm First Officer who frequently flew the SFO-HNL run in B377's told me that (as best as I can recall now) the reason for landing the B377 in that configuration had something to do with help in "setting or unlocking the nosewheel steering" for directional use when the aircraft was fully down on the ground.

As the B377 was Boeing's prototype for the "Stratocruiser", I've wondered if their wartime B-29 "SuperFortress" also was, or could be, regularly landed nosewheel first as well?

And then, during WWII, were there any other tricycle geared two- and four-engined "heavies" that were also meant to be, or could be, landed nosewheel first?

Apparently, the nose first landing was necessitated by the aircraft's tendency to enter into a stall if the nose was brought up too far up. According to one of its pilots:

No amount of heaving back on the pole would induce the mainwheels to make contact first. Various explanations were offered for this peculiarity... Others pinned the blame on the pernicious lift-spoilers.
Why build a splendid wing and then fit lift spoilers? The story was that the Civil Aeronautics Administration (CAA: the FAA-to-be) would not give the type its Certificate of Airworthiness because a wing was apt to drop when the aircraft stalled. The CAA wanted the nose to drop first, and the only swift remedy was to destroy an area of lift near the fuselage.

A similar discussion can be found here. However, this seems to have changed later. From the same article:

Ironically, after BOAC had operated the type for about nine years it was decided that lift-spoilers were not necessary, and they were removed. I was then flying later types, but those who were still on Strats told me that it was then a much better-behaved aircraft.

Another thing that supports this theory is that the military version, the C-97 Stratofreighter seems to have landed normally, according to its pilot:

The control wheel is eased back at 90 mph to clear the nose wheel from the runway. The airplane will fly itself off at 100 to 120 mph depending on the load.

and

Normally the nose wheel is kept off the ground for the first thousand feet, but it can be dropped immediately for cross-wind landings.

So, this seems unique to the Stratocruiser (and its derivative, the Super Guppy). B-29s as far as I know, landed main wheels first.

Airplanes with nose wheel touching first behavior are dangerous.

If you have to land with gusty crosswind, you may not be able to perform a perfectly smooth & tangential landing. One bounce of the nose wheel will immediately cause a pitch up momentum, increasing angle of attack. The initial bounce becomes a lift-assisted bounce. Nose wheel contact combined with yaw angle due to crosswind, will induce a yawing momentum in the wrong direction, which will increase asymmetric airflow speed between left and right wing.
And that's how to start a spin at 10m above the runway.

Whatever the cause of nose wheel first touch tendency is, it may be wise to decrease average angle of incidence of the wing, by reducing flaps deflection, allowing main wheels to touch first. This immediately pitches down the aircraft and prevents bounces, this also tends to align aircraft to runway. You can start braking, steering, and stop.

One airframe modification would allow full flaps down deflection combined with slight upwards ailerons deflection, which will decrease wing's average angle of incidence.

An other better modification which also leads to a lighter airframe, is to shorten the front gear's leg.

To answer question : Why was the Boeing 377 “Stratocruiser” frequently landed nosewheel first on touchdown?

It's either because B-377 pilots frequently approach the runway way too fast, maybe afraid of stalling since there's a huge speed difference between stall speed when fully loaded, and stall speed when empty, or there is some bad design of both wing and consecutively elevator angle of incidence. First option is way more likely tu be true.


Would a drone strike be more dangerous than a bird strike? Let's say you have a seagull and a drone of a comparable size.  Pretend either one was going to be ingested by large turbo fan engine, like the one found on a 737 (eg., the CFM-56)

Which would cause more damage and why?  I'm particularly interested in how the fan blades and compressor stages will be affected by the difference in materials.

There is a numerical study on the topic at VirginiaTech, where they simulated a 5kg drone being ingested by a turbofan, some pictures related to the study below:

On the same  Aerospace America Article it can be read that:

FAA will begin testing of drone ingestion next year
The metric for danger is the density of the material, so metallic parts are more critical than the bird bones/tissues. Modern drones have more aluminum parts, but the most critical component of a Drone ingestion would be the batteries.

Comparing different sizes, a commercial quad copter is around 1.4kg, while the most frequent bird impact (from FAA database) is around 1.8kg.

This is not the worst case scenario since larger drones are commercially available, and a goose, which can weight up to 6kg.

While a collision with anything during flight would be bad, please keep the fear of drones in perspective.  the OP's scenario has never happened.

There have been more than a million hours of flight of small drones, yet there is not one verifiable report of a collision between a personal drone and a manned aircraft. Not one.

There is also not one verifiable report of a drone crash in the U.S. that resulted in a serious injury as defined by the NTSB to someone not connected to the flight. Not one. It is a safety rate that all other segments of aviation would be jealous to have.

The only significant metal in most consumer drones is the motors, about the size of a large hailstone.  The batteries feel dense, but they are mostly a liquid polymer electrolyte.

Yes, the FAA plans to include a drone in their engine ingesting tests next year.  The problem I see is which drone?  The333.org has examined 5469 Exemptions through 7/12/2016 and the data contains 902 Models of UAS from 256 manufacturers.  So, which one is "typical"?


Are turn rate indicators calibrated to a certain speed? If so, what speed? Are turn rate indicators calibrated to a certain speed? If they are, what speed are they calibrated to, and why aren't all the turn rate indicators calibrated for the same speed?

They are not calibrated for a certain speed.

If by "calibrated", you mean they deflect together / to the same side, then you're using the instruments wrong. The two instruments have are not linked to each other, they operate independently and they operate on different principles. They are located next to each other in the cockpit out of convenience.

The "slip indicator" is a ball in a U-shape tube. It tells you whether gravity (or experienced acceleration) is pointed down, or to the side. Period. It is as simple as that.

The "turn coordinator" (little white plane / white needle) indicates your yaw rate. It operates by using a spinning gyro; the precession effect of yawing causes the gyro assembly to roll.

The roll is restricted by a spring, which is calibrated to indicate standard rate turn when the yaw rate is 3 degrees per second. The turn coordinator works at any airspeed (part of the preflight check is to turn the plane while taxing and verify the turn coordinator works).

As per the clear intent of the original question, this answer deals with the deflection of the needle in a turn rate indicator (or the deflection of the symbolic airplane in a turn "coordinator"), not the deflection of the slip-skid ball.  "Turn and slip indicator" or "turn and bank indicator" are names that are commonly applied to the whole package (at least in the case of the turn rate indicator with the needle rather than the turn "coordinator" with the little symbolic airplane), but it's clear that the original question is not asking about the movement of the slip-skid ball (inclinometer).
Also, this answer only deals with actual gyroscopic instruments, not their more modern counterparts.
Also, we'll assume throughout this answer that the slip-skid ball (inclinometer) is kept centered via appropriate rudder inputs by the pilot, or by the aircraft's yaw damper or other automated systems, if needed.
Also, please don't confuse the ideas explored in this answer, with the fact that various turn rate indicators designed for use in various types of aircraft with varying performance use different values for the turn rate that is achieved when the needle is at the primary index marking, and also use different values for the turn rate achieved when the needle approaches full deflection.  For example, airliners often perform half-standard-rate turns (four minutes per turn), so many turn rate indicators intended for use in such aircraft have the primary index markings set to correspond to a half-standard-rate turn.  Such instruments will be marked with the label "4 minute turn" rather than the more customary label "2 minute turn".  But that's not what this answer is about.

Are turn and slip indicators calibrated to a certain speed?

Essentially, yes.  On any given turn rate indicator, each turn rate index marking will only be completely accurate at one specific True airspeed.  On an instrument with multiple turn rate index markings, they wouldn't necessarily all need to all be calibrated to be accurate at the exact same True airspeed, though it seems logical that they might be.
Most illustrations of turn rate indicators with multiple turn rate indices appear to depict older instruments, often of British or other non-U.S. manufacture.  Many turn rate indicators bear only one index for turn rate in each direction, corresponding to either a standard-rate (two minute) or half-standard-rate (four minute) turn.  In this case the answer to the question would be "yes, on any given turn rate indicator, the turn rate index is only completely accurate at one specific True airspeed."
However, the variation in the accuracy of a turn rate indicator as True airspeed varies is small enough that it is not generally something that a pilot needs to take into account in actual practice.
But it does exist.
For example, in one source we read that:

The indicators are calibrated for rate 1, 2 and 3 turns at specified
angles of bank and TAS, and looping error is taken into account in
this calibration

Original source: "a rather elderly version of RAF publication AP1234"
Quoted in: this post on the PPRuNe aviation forum
In another source we read:

Turn rate indicators are calibrated to Rate 1, 2 and 3 turns at a
specific TAS. A rate 1 turn is 180° a minute or 3° a second. A rate 2
turn is twice that, 360° a minute or 6° a second. A rate 3 turn is
three times as much, 540° a minute or 9° a second. The angle of bank
required to achieve a given rate of turn increases with the TAS.
It is inherent in the design of the instrument that in any yaw
condition the gyro axis will tilt. and the gyro will become sensitive
to pitch rate.
If the aircraft is then rapidly pitched nose up (in a loop or recovery
from a spiral dive) this pitch input can deflect the gyro to read
maximum turn rate. This is called looping error. This pitch rate error
also affects the instrument readings in normal turns.
To compensate for this and for the difference between yaw and turn
rates the indicators are calibrated to show rates of turn correctly in
balanced turns for Rate 1, 2 and 3 turns at specific angles of bank
and TAS.
Although the indicated rate of turn will be incorrect at speeds away
from these datums the errors are not significant in normal operation.
The direction of rotation of the gyro is chosen so that when an
aircraft banks into a balanced turn the gyro precesses in the opposite
roll sense to keep its axis more or less horizontal and therefore keep
it more sensitive to turn rate.  If the gyro rotated in the opposite
direction it would only work satisfactorily at very low yaw rates and
small angles of bank.

Source: Bristol Groundschool ATPL Training Material sample, v6.1.5, page 3.13, published November 13, 2013, accessed via this on-line link, italicization and bolding added.
In other words, the calibration of the indicators to show specific turn rates is carried out at specific bank angles and specific True airspeeds.
Actually, both of these quoted statements are a little less than crystal clear, because if the turn rate is specified, then we only need to specify one additional parameter, either bank angle or True airspeed, in order to fully constrain the other parameter as well. They can't vary independently of each other, assuming that the slip-skid ball is kept centered. Still, the point is made that the calibration of the instrument for turn rate is not independent of True airspeed.
Here's a quote from an on-line discussion forum that actually cites the specific airspeed used to calibrate the turn rate indicator in one specific aircraft:

Also, if you consider that in a balanced turn you have yaw roll and
pitch, the pitch element makes the instrument over-read as the
aircraft will be at more than 1g. For this reason, the turn indicator
is calibrated to read correctly with this increased g by adjusting the
tension on the springs, but at one speed only . The instrument fitted
to the Chipmunk in RAF service was actually calibrated to 180 knots,
so I don't think this correction is too significant.

Source: this post on the PPRuNe aviation forum

and why aren't all the turn rate indicators calibrated for the same
speed?

It would make no sense that turn rate indicators intended for use in aircraft with vastly different performance would all be calibrated to have maximum accuracy at exactly the same airspeed.  Especially given the fact that the critical parameter is actually True airspeed, which varies even more widely between different aircraft types than does Indicated airspeed.
Why should the calibration of a turn rate indicator be airspeed-dependent?
The basic reason that the calibration of a turn rate indicator is airspeed-dependent is that for a given turn rate, the spin axis of the gyro wheel cannot be horizontal relative to the earth at all possible bank angles.
We can start by thinking of a turn rate indicator as a yaw rate sensor.  A turn is comprised of a pitch rotation as well as a yaw rotation.  For a given turn rate, the yaw rate is dependent on the bank angle.  For a given turn rate, the steeper the bank angle, the lower the yaw rate.  And for a given turn rate, the higher the True airspeed, the steeper bank angle.  So for a given turn rate, the yaw rate is dependent on the True airspeed.
But the story is more complicated than that, because a turn rate indicator is actually more than just a yaw rate sensor.
Let's explore this in more detail.  (This will get a bit complicated-- the reader is encouraged to make some sketches, or better yet, to make a simple model of a gyro wheel such as a pencil skewered through a round disk that he or she can hold in different attitudes in space.)
In this You Tube video entitled How Gyroscopic Turn & Slip Indicator Works, we can see what a turn rate indicator looks like inside.  At rest, the gyro's spin axis is oriented laterally (parallel to the aircraft's wingspan).  The direction of spin is clockwise as viewed from the right wingtip, so that the top of the gyro wheel is spinning away from the pilot.  The instrument's pivot axis or "gimbal axis" is oriented longitudinally-- fore and aft.
In the video, we can see the gyro's spin axis roll (bank) to a new orientation in response to yaw.  The degree of movement is rather substantial.  The rotation of the gyro wheel converts the aircraft's yaw rate to a roll torque on the gyro, forcing the gyro's spin axis to roll (bank) against the direction of the yaw, i.e. opposite to direction that the aircraft is banking in a coordinated turn.  Gears convert this movement to a motion of the pointer in the opposite direction, i.e. in the same direction that the aircraft is banking in a coordinated turn.
Note that the gyro is not acting like a "stable platform" like the artificial horizon-- purely rolling the instrument has no effect on the pointer, as it would if the gyro's spin axis were tending to stay fixed in space.  Apparently the centering springs are strong enough to overcome any such tendency.  Instead, as the aircraft yaws, the roll torque created by gyroscopic precession is acting against the centering springs to cause the gyro's spin axis to come to rest in an orientation that is "banked" relative to the instrument case.  Note also that the direction that the gyro's spin axis "banks" relative to the instrument case is also the direction that would tend to keep the gyro's spin axis more or less level (horizontal) with respect to the earth.  However, since the gyro is not acting like a stable platform, there's no reason to think that the gyro's spin axis will stay exactly horizontal with respect to the earth.  In fact, for a steady-state turn at a given turn rate, the gyro's spin axis can only be horizontal with respect to the earth at one particular bank angle, i.e. at one particular True airspeed.
Whenever the gyro's spin axis is banked relative to the instrument case and the aircraft-- i.e. whenever the aircraft is turning-- the gyro responds to pitch rotation as well as yaw rotation.  For simplicity, consider a case where the gyro's spin axis is banked almost ninety degrees relative to the aircraft, which would never actually happen.  Now yaw rotation will barely be sensed at all, but any pitch rotation by the aircraft would be converted by gyroscopic precession to a roll torque on the gyro, tending to roll the gyro's spin axis to a steeper bank angle.
This effect is called "looping error".  One consequence of "looping error" is that any time the flight path is curving upward-- i.e. any time the aircraft is experiencing more than the normal G-loading for the bank angle-- the Turn Rate Indicator will read higher than the actual turn rate.
It appears that whenever the gyro's spin axis happens to be exactly horizontal with respect to the earth and the aircraft's flight path is not curving upward or downward, the deflection of the indicator needle will be exactly proportional to the turn rate.  (Citation needed.)  In essence, "looping error" is exactly compensating for the fact that the turn comprises of both pitch and yaw, rather than pure yaw.  However, as noted above, a turn rate indicator cannot be designed so that this happy situation occurs at all possible bank angles for any given turn rate.  Rather, in a steady-state turn at a given turn rate, this situation can only occur at one possible bank angle.  In many instances the gyro's spin axis will not be exactly level with respect to the earth.  So for any given actual turn rate, the indicated turn rate ends up being somewhat dependent on bank angle, which means that the indicated turn rate cannot be independent of True Airspeed.
It also appears that the stiffer the centering springs of the instrument, the less sensitive the instrument will be to transient "looping error" caused by pulling excess "G" in relation to the bank angle, but the less accurate the instrument can be over a wide range of True airspeeds in a steady-state turn, because the spin axis of the gyro will tend to stay more aligned with lateral axis of the aircraft (i.e. the wingspan) and less horizontal relative to the earth.
In a turn coordinator, with a symbolic moving airplane on the front of the instrument rather than a needle, the situation is even more complicated.  The orientation of the gimbal axis is different.  Rather than being oriented purely fore-and-aft, i.e. parallel to the aircraft's longitudinal axis, it is tilted (with the forward end raised and the aft end lowered) by about 30 degrees.  The purpose of this is to make the instrument sensitive to roll as well as yaw, to better signal the earliest stage of a developing turn or change in bank angle.
Apparently not all turn coordinator gyros spin the same direction.  If the gyro spins clockwise as seen from the right, with the top of the gyro moving away from the pilot, "looping error" is similar to what we see in the turn rate indicator, and if the gyro spins counter-clockwise as seen from the right, with the top of the gyro moving toward the pilot, "looping error" is reversed, so that the instrument under-reads the turn rate whenever the G-loading is excessively high for the bank angle.  Note that in the latter case, with the gyro spinning counter-clockwise as seen from the right, the symbolic airplane must be geared to tilt in the same direction that the gyro is tilted, not the opposite direction.  This means that in a steady-state coordinated turn the spin axis of the gyro actually is tilted away from level with the earth's surface, rather than toward level with the earth's surface.  In a steady-state turn, this would seem to make the displayed turn rate more dependent on bank angle, and therefore more dependent on True airspeed, than would be the case if the gyro spun the opposite direction.  For more on the direction of spin of the gyros in turn coordinators, and the resulting effect on the direction and magnitude of "looping error", see this related answer.
Any slight error in the turn rate indicated by a turn rate indicator or turn coordinator in a stabilized turn due to the calibration being only valid for one True airspeed is insignificant enough that it is rarely taken into account in actual practice.  This error is the clearly the least of a pilot's worries when flying in cloud.  But the answer to the question is that the calibration of any given turn rate index on any given turn rate indicator or turn coordinator can only be exactly accurate at one specific True airspeed.
Some related links--
How Gyroscopic Turn & Slip Indicator Works (You Tube video) -- in the cutaway, note how far the gyro's spin axis tilts to the side (rolls) in response to yaw rotation-- the tilt at full deflection appears to be about 40 degrees.  We can see that the tilt is in the direction that would bring the gyro's spin axis closer to vertical with respect to the earth during a normal coordinated turn.  By the fact that the gyro's spin axis rolls to the left when the instrument is yawed to the right, we can see that the gyro must be spinning clockwise as viewed from the right, so that the top of the gyro is spinning away from the pilot.
"How a Turn Coordinator Works-- Inner Workings"  -- Again we can see that gyro's spin axis rolls in response to yaw, and in this particular turn coordinator, the roll is in the direction that would bring the gyro's axis of rotation closer to vertical with respect to the earth during a normal coordinated turn.  Again, by the fact that the gyro's spin axis rolls to the left when the instrument is yawed to the right, we can see that the gyro on this particular turn coordinator must be spinning clockwise as viewed from the right, so that the top of the gyro is spinning away from the pilot.
Bristol Groundschool ATPL Training Material sample v6.1.5, published November 13, 2013 -- see page 3.13 regarding the dependency of the calibration of the turn rate indicator upon True airspeed.  See figure 3.16 on page 3.12 for an illustration of a turn rate indicator with multiple rate markings-- the typical British practice?
A link to an aviation website giving a rule of thumb for bank angle for a standard rate turn-- the rule of thumb is that the bank angle in degrees should be roughly .15 * (True airspeed in knots).  Reaffirming the idea that if the calibration of a Turn Rate Indicator or Turn Coordinator depends on bank angle, then it cannot be independent of True Airspeed.
https://www.pprune.org/tech-log/82721-turn-gyro-axis.html -- from an airline pilot's discussion forum-- not all the ideas here appear to be correct, especially the comment about the rotation about the gimbal axis being limited to 6 degrees of motion.
https://www.pprune.org/tech-log/528255-turn-rate-indicator-turn-coordinator-looping-error.html#post8164645 -- from the same forum as above--  a comment on the problems that can sometimes be posed by a Turn Rate Indicator's tendency to over-read under excess G-loading, and the greater problems that can be posed by a Turn Coordinator's tendency to under-read under excess G-loading.
Related ASE answers --
May turn rate indicators (and turn coordinators) be calibrated to be accurate at all airspeeds, or only for one? -- a good answer to what is essentially a duplicate question
During "partial panel" flying, in what ways is a turn rate indicator more useful than a turn coordinator? -- see all answers
In some "turn coordinators", does the gyro spin opposite to the direction it spins in a "turn rate indicator"? -- more on how the tilted gimbal axis of a turn coordinator allows roll sensing as well as yaw sensing, and also affects "looping error".  More on why if the gyro spins counterclockwise rather than clockwise as seen from the right, the instrument will under-indicate rather than over-indicate the turn rate whenever the G-load is unusually high for the bank angle.  Includes several links to videos and diagrams showing the internal workings of turn rate indicators and turn coordinators.
What is the formula for the bank angle required for a turn in line-abreast formation? -- calculating the required bank angle as a function of turn rate and True airspeed


Why don't commercial passenger safety videos ever explain policy instead of merely dictating it? British Airways announced and published their new in-flight passenger safety video:

I'd like to draw your attention to two pieces of instruction given in the video, but also stated in every passenger safety video I've ever seen on every airline I've flown:

Never inflate the life-vest inside the aircraft.
Always put on your own air-supply mask before assisting others.

These instructions are always given in a dogmatic or dictatorial, way - the airlines expect passengers to comply with these instructions despite never giving any the reason for these policies - and I'm concerned certain passengers would disregard the instructions because they think they know better (and I included myself in that category until I found out why these policies exist).

I understand the reasons, respectively, are:

While being inflated makes it harder to move around the plane when your torso just doubled in volume - more crucially: if the plane does take-on water it means you'll be trapped inside the plane because you won't be able to submerge yourself to access an underwater door or exit.
Useful consciousness at altitude is measured in seconds, not minutes, and you will likely be unconscious and suffering hypoxia if you get distracted - it will take you a good 10-15 seconds to put your own mask on, let alone someone else's. It's entirely possible you'll die or suffer serious brain damage in your selfless attempt to assist your child.

Before I was aware of these facts I simply assumed the reasons were more mundane: inflating your life-jacket would simply make things slightly harder for yourself, and you should don your own mask first simply because it's easier - and I thought that inflating my own jacket inside the plane would actually save me time, and that I believed I was perfectly capable of assisting family members with their masks before my own if I held my breath. While laughable when informed, I genuinely believed these facts about myself until fairly recently. It would not surprise me if many passengers today feel the same way about themselves.

I understand the the real reasons for the policies make for grim reading and putting those kinds of visceral descriptions in the videos would spook passengers - but I assumed that because the videos never give any dire warnings, and are always said with a cheerful tone of voice, that the instructions weren't all that important, so it would somehow be tolerable if I did my own thing and disregarded the rules because I thought I was acting in my own best interest.

So why don't they at least add some transparency and explanation, for example they could say:

Never inflate your life-jacket inside the aircraft so you can fit through smaller openings and to avoid being trapped if the aircraft takes on water.
Always put on your own air-supply mask first before assisting others because seconds matter in low-oxygen situations.

While the sample explanations I gave don't go into too much detail, they still make it clear to passengers that the consequences of noncompliance are serious, if not fatal - and could still be read with a cheery tone of vocie :)

I compare this to the problem with "Danger: Confined Space" warning signs: the layperson thinks a confined space is dangerous because they might casually bang their head on a low ceiling - no thought is paid towards the common, very real risks and dangers of dangerous gasses pooling in a narrow shaft - yet it isn't too much trouble to add a short message to the sign saying "Because of trapped carbon-monoxide and other gasses", for example. It's odd because we already have other explanatory danger signs like "High voltage" (don't touch it without gloves), "Moving parts" (don't touch it, period) and "Falling objects" (so wear a hard-hat) - so why not airline safety instructions?

There is no time. You need to convey the information in as little time as possible or people (those few that watch at all) lose interest and start doing other things.
Reasoning tends to invite arguments, there's always someone who thinks he knows better. You don't want that.
Simplicity. Make things as easy to understand as possible using simple words. That way less intelligent people and people who have trouble with your language are more likely to understand what's expected of them.

There might be other reasons, but those are the ones that come to mind.

You'll hate this, but IMHO you misunderstand the purpose of the briefing/video.

It's primarily safety theatre to ensure that the "self-loading freight" (sorry,  "valued passengers") feel that they are embarking on a slightly hazardous enterprise but are in good hands due to the skills of the airline staff.

In fact, the chances of anyone being in a life-threatening incident on a civil airliner are vanishingly small. Even for airline employees, the vast majority will go through their entire career without such an incident. When incidents do occur, they are often either non-threatening or a total loss (no amount of seat-belts, oxygen masks or locked away tray tables will save you when the plane slams into a mountain at 600mph).

Commercial flights on small aircraft are a lot more hazardous than jets, but omit the safety video and often any kind of briefing beyond the pilot looking over her shoulder and checking y'all strapped in, eh?

Flying would still be safer than driving if they had no cabin crew, took all the seats out and had the passengers hang on to grab rails, just like the airport bus (which is way more likely to come to a sudden stop than an airliner).


Why was the "Dispatch Reliability" of the A320 "low" during its first years? I found the following Statement on the German Wikipedia page about the A320 family of aircrafts.

  Nach erheblichen Schwierigkeiten in den Anfangsjahren erreicht die A320-Familie mittlerweile eine Abflugzuverlässigkeit von 99,7 Prozent.
  (https://de.wikipedia.org/wiki/Airbus-A320-Familie#Wirtschaftliche_Aspekte)

This roughly translates to:

  After significant difficulties in its early years, the A320-family reaches a Dispatch Reliability of 99.7% nowadays.

I could not find more information about this statement, so my question would be why the Dispatch Reliability of said aircraft family was supposedly significantly lower in its early years? Is it normal for completely new types of aircraft to have such a low DR that it's worth mentioning?

The A320 saw entry into service on 18 April 1988 with Air France.
About a year later Flight International covered the dispatch topic. For British Airways – whose initial acquisition of the A320 was through the British Caledonian takeover, and not an order – the problems weren't with the plane, but with the Boeing culture that had to adapt.
Otherwise there was nothing notable.

(...) The "culture difference", says Bracken, materialised in apparently trifling (but actually problematic) ways, such as line mechanics having trouble with Airbus literature. There was nothing wrong with the literature, it was just different.
November 1 came. "It wasn't a very good start," says Bracken, "but the reasons were outside the aircraft." By January, BA's A320 technical dispatch rate had not climbed out of the 96 per cent range, reveals Barry Gosnold, chief engineer technical services.
That is a revealing figure. BA had expected much more, even though the performance is not unusual for a completely new aircraft.The A320's performance with Air Inter at the same time was between 2 and 3 percentage points better, although Air Inter had received its first A320 months later than BA.

Source: AIRBUS A320 AT WORK, Flight International, 17 June 1989

The A320 was innovative in a number of ways, especially its Fly By Wire control system, and now it is an incredibly popular and mature platform, partly because of its despatch reliability.

All the elements that allow a despatch of the aircraft for a commercial flight contribute to despatch reliability. This can be anything from having spare parts for anything that needs to be replaced, to crew being familiar with inspection and preparation procedures (checklists) for the aircraft, to pilots correctly understanding messages on the flight management system.

When aircraft are first introduced, there are often problems with logistics and the (unintended) interactions between aircraft systems. As the aircraft accumulates more operating hours, these problems get solved. As aircraft reliability improves over time, it can also be safe to operate in "MMEL" conditions (master minimum equipment list).

For example, if you know it is safe to despatch with a certain warning message because you have proven that the 3 redundant backup systems are functioning properly - this further helps despatch reliability, because now you know it is safe to dispatch whereas on day one with the new aircraft, you did not have that option.

When I worked as a supplier to two major airframers, they were always incredibly keen to improve despatch reliability by fractions of a percent.