ONGOING EDIT NOTE (By OP) This question is right now being comprehensively reworked to make it less broad over the course of the next number of days. (That's being done mainly offline, so as to offload version management). If you are interested in this question, I welcome you to leave a comment or some in the meantime and check back later.

EDIT NOTE: (By OP) [previous and ongoing] The text of this question was changed comprehensively to correct formal issues while taking care not to break coherence with previously given answers. I explicitly own any issues in that quarter.

Original title (for reference only): Why are aircraft manufacturers even permitted to sell aircraft without an AoA indicator?

Owing to that, answers are definitely not out of scope that cater to whether seeing an AoA indicator as important enough makes sense in the first place.


Given the work done and published by regulation authority in the last two decades, what are the missing elements to forbid airliners without AoA indicator to be sold?

// Soft back edge of second rework pass here //

An AoA indicator is arguably a relevant piece of equipment on a flight deck. Despite recommendations in widely followed accident reports towards mandating its presence in aircraft, little has generally happened in that quarter since, and with observable reluctance in some cases.

This could suggest to suspect that something may be missing from the statutes or reality of the regulations and procedures that one might expect to be sufficient to cause aircraft manufacturers to follow suit.

Whether or not the suspicion holds (which might or might be opinion), this question is interested (not in whatever possible opinion but) in looking at the elements that the relevant procedure consists of and learning whether the relevant procedure has missing elements or elements that are establishably too weak to effect AoA indicators to be mandated.

( I am interested to know what elements ... ) // WIP - EDGE of second rework pass here //

As a side effect, I expect to potentially learn something about regulations in general (as all regulations can't have been made just because of AoA :D)

Evidence to consider

// WIP - Another EDGE of second rework pass here //

[ outline: 1. AoA (science, more or less: (theory), practice, accidents) -> 2. Recommendations (implement) -> 3. Effect (generally none or not from 2.) -> 4. opposing forces (anecdotically visible: press) -> 5. elements of procedure (a. generally capable in terms of process b. capable of overcoming 4.) (or lack thereof, either in elements' existence or capability) ]

[ As far as opinion is concerned, it may be unfathomable and thus left to opinion whether element X in procedure Y is apparently not acting strongly enough in case A to overcome resistance Z and accomplish result R. But it is presumably possible (and I think not opinion) to establish whether a procedure Y: (S1->S2) to move a matter from state S1 to state S2 exists in the first place and whether Y contains elements Xk such that S1--[(all(Xk))]-->S2 is at least theoretically sound and can logically be expected to work. We could then explore (where necessary) which Xk are counteracted by Zj and whether all Xk are (and have been successfully) designed to overcome any respective Zj - or, OTOH, designed (or "somehow" become) such that overcome(Xk,Zj) could provably not work or reasonably be expected not to work ("is too weak to effect", "is incapable of insuring" etc.) Sort of like the logic about moving control surfaces - no hydraulics, no move.

NOTE: Question will then not cover such reasons for 3. as are NOT procedure but are e.g. such as "the procedure was all set but nobody cared" or "Politician P just didn't wish to inconvenience manufacturer M into complying" - unless that's a matter of clear evidence and has been thoroughly established outside this network. Or maybe not even then, as the question will need to be narrowed down to something like procedure. Other stuff might be content matter for another question. ]

old tldr

This question asks about regulations and procedures governing aircraft design and implementation. As it was originally prompted by looking at a prominent example of an aircraft design element that got recommended for consideration to implement by a governing body (which still didn't happen), the question looks at that example only as some sort of anecdotal background and, well, example (which may in itself be open to opinionated discussion). The question, however, is about factual properties of the regulations and processes governing the field that e.g. the example exists in, both as set down and as observed in reality (not strictly restricted to the example).

So a governing body recommended looking at making this feature mandatory, then although that means something, it didn't happen. So what are the properties of how regulations are and work that facilitate deviating from such recommendations, what influences are at play and how do the processes work that result in the application of regulations as well as the changes that regulations and processes are subject to themselves.

Angle of Attack

The Angle of Attack (AoA) of an aircraft in flight is a well-known critical element of a stable flight attitude. Correctly observing AoA corresponds directly to flight safety and arguably to the relative effort and ease of maintaining this aspect of flight safety -

No matter your weight, if you fly at the right AOA you’ll have the right stall margin*

On a side note, it interestingly also corresponds to flight economy in a way recently discussed in more detail here (ibid.)

[this] angle of attack ... represents the most efficient way to fly [at that chosen optimum]

[Source:] "Low-Cost Accurate Angle-of-Attack System", Borja Martos and David F. Rogers, Journal of Aircraft 2018 55:2, 660-665

*) (I won't nitpick whether and how airspeed should have been mentioned along with it)

Angle of Attack indicator

Based on these observations, there is arguably ground for considering whether it makes sense for aircraft to be equipped with instrumental indication of Angle of Attack on the flight deck (AoA indicator [1]), or whether that should perhaps even be mandatory, based on the importance of AoA for flight safety as indicated in investigations.

*On a purely textual note, "AoA" in this question will be used to mean "Angle of Attack" but I seem to recognize that "AoA" can sometimes be used to mean "Angle of Attack indicator" as well, depending on context. As for this question, please feel free to edit in case there's a problem with that.* [1] [rather broad, better illustrative source welcome]

Background in aircraft accidents

While there has been quite considerable argument about the ifs and hows of the actual necessity and usefulness of having an AoA indicator on the flight deck, investigation into a number of aviation incidents turned lethal accidents, such as AF447 stalling into the Atlantic Ocean, and the more recently Boeing 737MAX crashes, has suggested that it might have helped to avoid the fatal outcome of these events if the respective aircraft had been equipped with an AoA indicator.

For quick reference, Flight AF447 avoidably crashed out of a sustained pilot-induced stall that went unnoticed or misinterpreted, aggravated by other factors. The aircraft did not feature an AoA indicator. In the 737MAX crashes, again flight crews had no indication of the aircraft's AoA available to help in mitigating the avionics' acting up following internal disagreement about the aircraft's AoA (which it obviously did measure but was not designed to display, such as on the glass panel).

After each of these accidents, (non-) indication of AoA was discussed as an arguably relevant factor in the sequence of events leading to the respective crash. The more recent events led to the aircraft manufacturer eventually adding a proper AoA indicator into the glass panel PFD on the Boeing 737MAX. The same action was not taken in the type of AF447, the Airbus A330. (Sources as given above.)

In section 4.2.2 of the AF447 investigation final report, the BEA as the investigating body recommended

  • that EASA and the FAA evaluate the relevance of requiring the presence of an angle of attack indicator directly accessible to pilots on board aeroplanes

However, this has apparently not led to broad adoption, much less to a making mandatory of AoA indicators, though some have been lately appearing in some aircraft (as mentioned above) on an arguably less mandatory basis.

After investigation into the 737MAX accidents had started showing that an AoA indicator would have been vital in identifying and changing a sequence of errors leading to at least one of these massively fatal crashes, the manufacturer could be observed in the market as having an AoA indicator for the aircraft type more or less ready for delivery but was apparently fighting to not deliver it to customers unless they paid a presumably significant upgrade price for "options".

While that may be understandable from a (ethically agnostic) purely monetary perspective as designing and actually building an AoA indicator into an aircraft arguably costs money, practically withholding from flight crews a potentially crucial instrument - which was the reported effect in a number of less deep-pocketed airline operators - motivated the original wording of this question: Why are transport aircraft manufacturers, or any aircraft manufacturers and designers, for that matter, even permitted to build and sell aircraft with no AoA indicator built into the flight deck as a standard item?

While that original take drew opinion, the more factual point this arguably seemed to point to was problems about regulations and procedures and what would be happening with respect to them in (the cruel?) reality.

Question (revamped)

This question now asks for insight about the general aspect of regulations and processes, both in place and actually used, to ensure flight safety based on aircraft design and the parameters of operation to go from there.

The issue of whether aircraft generally are, and should be, equipped with an AoA indicator, may serve as an example of issue and application.

I would like to gain insight primarily focused on contemporary commercial transport - and possibly GA - aircraft over the last 40 years or so to present.

Broader insight into issues found in the topic, being of a more general nature or reaching further back in aviation history, will be welcome within the reasonable necessity of not letting the question get to broad.

I am less interested in the matters of opinion already mentioned, rather in the properties of the processes involved with the inception and outcome of aircraft regulations. Insofar as there exists opinion that hasn't already been mentioned, I'll be happy to be pointed to the existence of such opinion or discussion for more insight, but still:

the subject of this question is about hard facts about regulations and processes concerning the aircraft industry in creation, delivery and use of its products, and how these regulations and processes have been (and are being) developed and applied in reality and how this plays out.

To be sure I'm not asking about speculation, albeit such speculation may be mentioned as existing in an answer for illustration. I am interested in the way regulating the aircraft industry for equipment and safety issues works.

Answers might point to relevant aspects of how both set-down and real-life procedures in regulations are put in place and executed and how these procedures observably are followed and play out.

Such as, what would it mean for the regulations and procedures to be worked and developed "right by the book", what influencing factors and deviations would be encountered, and how would the procedure building and executing process go about deviations to mitigate the effects of ill influence or eliminate it/them as far as that is possible.

In the face of an AoA indicator apparently still not being a standard feature of an advanced aircraft at present, but being reported to appear in some aircraft for arguably different reasons (which may in itself be beyond the scope of this question), I would be interested to know

  • what became of the BEA's recommendation,
  • how does that relate to how regulations and procedures work (or how they should be working and how things do play out in reality)
  • what would one expect about regulations and procedures working the way they should (and what would that be in the first place)
  • if this AoA issue is indicative of anything, would this be a typical case of what happens (or can happen) or (observably) rather an edge case

I am not asking about opinion on these points but about observable properties of e.g. regulations typical of governing the procedures used in establishing the design of aircraft and what they are explicitly made out to effect (think of must, shall, should and what is being unambiguously mandated or aimed at, and of compliance observed in reality and how that gets handled, end-to-end.)

and more generally, as far as the scope of the question may go without being too broad, how are regulations and procedures in the aircraft industry developed and applied after all with respect to issues relating to critical equipment.

I particularly welcome insight into how money and other unilateral or colluding interests come into play and how do regulations act and develop to mitigate the effect of such interests.

With respect to the established processes in developing and getting approval from authorities for an aircraft design, I wonder what properties of the overall process and its surroundings would be capable of leading to some safety-critical decisions about aircraft equipment being left to the unilateral considerations of aircraft manufacturers. (capable of meaning obvious capacity not opinion. This should be verifiable by e.g. whether it could possibly happen in context i.e. other properties of reality not precluding it etc., and going from there, if there are cases where that has clearly happened, potentially also cases (fact) where it arguably happened (then, I take that whether it did and whether that was due to it being possible in the first place because, would be opinion - the mere existence of it would be again fact and could be noted - but the fact wouldn't.)

Historically (and currently), if one were to assume that an AoA indicator should be mandatory in an aircraft, just for the sake of making observations, then, have other comparable safety features or measures seen the same predicament (or have been known to are viewed this way by some)?

What properties of the regulations and processes used in the last decades as implemented in reality have allowed the state of affairs to become what it is, and in case changes to that have been discussed, what became of it?

  • 1
    $\begingroup$ I feel this question would bring opinion. Note that many equipment you can find today on airliners (I think your question is restricted to airliners but you should explicit if this is the case) didn't exist when the first jet airliners were certified. Then it is not surprising that compulsory equipment are added little by little to new aircraft types. $\endgroup$ – Manu H Jun 9 '20 at 6:35
  • 2
    $\begingroup$ In case of the AF447 the AOA would have shown 40° or so and the pilots would have thought it was a sensor error. And in the B737 Max crashes the AOA indicator would show a high value that doesn't change, so again not useful for the pilot. $\endgroup$ – Jan Jun 9 '20 at 7:04
  • 1
    $\begingroup$ While I personally agree about AoA usefulness and desirability, I must say that its presence on every Soviet airliner didn't make them enjoy much better safety record, even with respect to stall-related incidents. $\endgroup$ – Zeus Jun 9 '20 at 8:10
  • 1
    $\begingroup$ @Jan is correct, the problem wasn't one of instrumentation or information available to the pilots, it was that they were fitting their problems into the wrong model of their situation $\endgroup$ – Dave Gremlin Jun 9 '20 at 13:01
  • 4
    $\begingroup$ That... is a lot of text! I understand you're trying to refine the question so it can be reopened, and I applaud you for your efforts - sometimes it's not easy. Unfortunately, from what I'm reading now, this is likely too broad: "This question now asks for insight about the general aspect of regulations and processes, both in place and actually used, to ensure flight safety based on aircraft design and the parameters of operation to go from there." could take an entire book to answer... $\endgroup$ – FreeMan Jun 18 '20 at 16:35

While your argument for the inclusion of AoA Indicators is insightful, there are a few things to take into consideration. Namely, that regulations move slower than the technology they regulate. The basic cockpit instrument panel equipment that every powered aircraft (big or small) is required to have is regulated in the FAR/AIM by Part 91.205. Stall warning devices and AoA devices are not required equipment. To make them required would require a change in the regulation. The way stall warning is achieved is regulated in Part 25. Part 25 does not require a device, instrument or indicator though one is possible as supplemental. Warning can be achieved by aerodynamic buffeting.

§25.207 Stall warning.

(a) Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position must be clear and distinctive to the pilot in straight and turning flight.

(b) The warning must be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself. If a warning device is used, it must provide a warning in each of the airplane configurations prescribed in paragraph (a) of this section at the speed prescribed in paragraphs (c) and (d) of this section. Except for the stall warning prescribed in paragraph (h)(3)(ii) of this section, the stall warning for flight in icing conditions must be provided by the same means as the stall warning for flight in non-icing conditions.

(c) When the speed is reduced at rates not exceeding one knot per second, stall warning must begin, in each normal configuration, at a speed, VSW, exceeding the speed at which the stall is identified in accordance with §25.201(d) by not less than five knots or five percent CAS, whichever is greater. Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began.

Another consideration is that as technology advances, aircraft equipment requirements will change. This will be predicated on the perceived value of the equipment to improve safety. The old saying is that regulation are written in blood. The more deaths from a particular cause, the more regulation will be enacted to prevent the cause.

Unfortunately, more technology does not fix many of the root causes of accidents. For instance, stall indicators such as stall warning horns and AoA indicators will not prevent accidents in the case of: sensors supplying the information being misaligned, damaged or inoperative; instruments being faulty; instruments giving incorrect information; the information given being ignored, misinterpreted, excluded from importance, fixated upon, or unnoticed due to over saturation of information. Some of these issues could be considered as unavoidable or acts of God. Others can be considered under the category of manufacturer, maintenance, or pilot error. The accidents that you noted might or might not have happened with or without an AoA indicator. Ultimately, pilots must be able to fly the aircraft with less than even the basics of equipment in 91.205. First and foremost, the pilot has to understand their aircraft and its limitations. No new equipment will fix or change that simple fact. Especially, when things start to go wrong.

If you are a pilot, you may understand the concept of task saturation. It is like juggling. Juggling one ball is not even juggling. Juggling two is easy. Juggling three is a skill. As you continue to add balls, the task of juggling gets harder and harder. If you add one more than your max, the tendency is not to drop that one or a different one ball. All of the balls fall.

At some point, adding another piece of information to the PFD is counterproductive. There is not enough real estate on the screen nor in the pilot’s conscious mind.

If the pilot has visual reference to the horizon an AoA indicator is superfluous. If the pilot does not have a visual reference to the horizon, the Attitude Indicator will substitute for the absent natural horizon. If the Attitude Indicator is faulty, cross referencing the other instrument will substitute for the Attitude Indicator. If the pilot gets too task saturated to take in all of the above information, the stall warning indicators should warn them of an impending exit of the normal flight envelope. If the pilot has to rely on the proper function of every and all equipment plus one more, then we have a bigger problem in aviation than the equipment list.

If you are not a pilot, maybe this example can sum up the last few paragraphs. The AoA Indicator is like the low fuel warning indicator in your car. The low fuel warning works off of the information from your fuel gauge. Your fuel gauge has to work properly for the warning to work properly. But, if you are an attentive driver, it should not take a low fuel warning for you to know that you are low on fuel. For a responsible driver, the low fuel warning should never come on. An exceptional driver should not even need the fuel gauge. They would just fill the car’s fuel tank after a certain amount of use taking into consideration a margin for/of error. Yet, we still have people run out of gas even in cars with low fuel warning indicators.

  • $\begingroup$ "horizon an AoA indicator is superfluous" I presume you mean "attitude indicator" not "AoA indicator"? $\endgroup$ – FreeMan Jun 9 '20 at 16:06
  • $\begingroup$ @FreeMan - I see your point. But, no I meant the AoA is superfluous. The AI is a convenience when VFR. Neither is necessary. I like having the AI at night. Although, both can be a distraction. I’ve had the AI go out in flight a couple of times in VMC day & night. I’ve had to cover the AI with a sticky note to keep from fixating on it even though my conscious brain knew that it was not operable. The few times I’ve flown with AoAs, I’ve actually found them annoying when doing slow flight and intentional stalls. $\endgroup$ – Dean F. Jun 9 '20 at 16:28
  • $\begingroup$ and... that's why I didn't edit. :) However, (remember, IANAP) if I can see the horizon, how does that negate my need for an AoA indicator? Also, since you go from there into talking about the AI, it's confusing... $\endgroup$ – FreeMan Jun 9 '20 at 16:35
  • $\begingroup$ @FreeMan - Don’t get me wrong. I love the idea of an AoA. I love technology in general. I don’t think we should rely solely on it. It’s just a useful tool when used correctly. It’s a danger when not used correctly. If I have allowed complacency to set in, and I am ignoring what the aircraft is telling me, the AoA might save my bacon. Hopefully, I won’t let things get that far. And, help me remember IANAP. I am drawing a blank right now. $\endgroup$ – Dean F. Jun 9 '20 at 16:50
  • 1
    $\begingroup$ You ought to edit the title to actually be a question. $\endgroup$ – Michael Hall Jun 18 '20 at 14:30

Not the answer you're looking for? Browse other questions tagged or ask your own question.