Visual Angle of Attack Indicators and Systems Engineering Theory : Air Facts Journal

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“What is chiefly needed is skill rather than machinery.” Wilbur Wright

“The best safety device in any aircraft is a well-trained crew.” A.L. Ueltschi, Founder of FlightSafety International

Angle of attack indicators have not been a popular upgrade so far.

By far the best and most appropriate way to consider the utilization of visual angle of attack indicators (VAOAI) in general aviation is using the techniques of systems engineering. This means studying how all of the pieces of a system work together, not just isolated flight tests or personal preference. (The equivalent in biology is called ecology.)

The ”system” for VAOAI is pretty much all of general aviation and not just the hardware on the airplane. Rather, the system, in the largest sense, includes VAOAI advocates and promoters, regulators, pilots, aircraft, regulations, training, airports, air traffic, airspace, weather, publications, public opinion, and more. It’s no surprise that YouTube videos ignore so many of these components.

Evaluating VAOAI is not that simple. We like to think that we consider topics based solely on their merits because our thought processes are rational and logical. However, our society loves to promote the concept that something is wrong and must be remedied by change, not by addressing the underlying problem itself. To come to the best conclusions about VAOAI, we must recognize the biases in our thought processes. Systems engineering techniques can help us do just that.

Systems engineering is, in fact, a well-defined engineering discipline.[1] NASA provides this excellent but unavoidably long definition:

At NASA, “systems engineering” is defined as a methodical, multi-disciplinary approach for the design, realization, technical management, operations, and retirement of a system. A “system” is the combination of elements that function together to produce the capability required to meet a need. The elements include all hardware, software, equipment, facilities, personnel, processes, and procedures needed for this purpose; that is, all things required to produce system-level results. The results include system-level qualities, properties, characteristics, functions, behavior, and performance. The value added by the system as a whole, beyond that contributed independently by the parts, is primarily created by the relationship among the parts; that is, how they are interconnected. It is a way of looking at the “big picture” when making technical decisions. It is a way of achieving stakeholder functional, physical, and operational performance requirements in the intended use environment over the planned life of the system within cost, schedule, and other constraints. It is a methodology that supports the containment of the life cycle cost of a system. In other words, systems engineering is a logical way of thinking.

The Most Controversial Part of the Overall System

“The first principle is that you must not fool yourself and you are the easiest person to fool.” Richard P. Feynman, Nobel Prize winning physicist

The most controversial part of the (larger sense) VAOAI system is not even acknowledged as a part of the system. That part is the advocates and promoters of VAOAI who, as will be seen, have motivations unsupported by complete systems engineering analysis.

An historical antecedent is the concept of the unstable approach. This idea came from the airline industry, perhaps in the 1990s, with the well-intentioned goal of reducing airliner accidents. The “logic” was that since so many accidents come from unstable approaches, if unstable approaches were reduced, that would reduce accidents. Unfortunately, nobody analyzed all of the approach data, only the accident data. This analysis would have shown that unstable approaches were in fact quite common and almost never resulted in an accident. This was not acknowledged until decades later. Stable approaches are, of course, good practice, but are an unusable descriptor of accidents.

The “logic” was that since so many accidents come from unstable approaches, if unstable approaches were reduced, that would reduce accidents.

With this optimism established, almost nobody bothered to examine all of the approach data. It was almost like the emperor’s new clothes. Related and perhaps overly harsh terms to describe this situation include confirmation bias, mob psychology, peer pressure, and virtue signaling. And this criticism is not to be taken as a condemnation of well-intentioned, dedicated individuals.

This same situation applies to VAOAI. Many well-intentioned individuals, not having done a full analysis, enthusiastically support VAOAI or their own related variants. As will be seen in the sections below, such analysis does not support VAOAI.

Then there’s the matter of personal preference. Those with a need for certainty will demand that the entire pilot population adapt their way of looking at things, and those with an engineering bent will demand that all pilots understand the underlying engineering concepts, even when there are more applicable and simpler explanations. After all, who cares about Reynolds numbers, LaGrangians, or Hamiltonians?

Noted sociologist Thomas Sowell said, “It is usually futile to talk facts and analysis to people who are enjoying a sense of moral superiority in their ignorance.” And there is the conflict in discussing VAOAI — optimistic advocacy vs. objective systems engineering. The first is fun, easy, and seems obvious, the second is way too much work.

But they disagree.

VAOAI — Example of Inadequately Justified Support

A simple example makes the point. FAA SPECIAL AIRWORTHINESS INFORMATION BULLETIN SAIB: 2024-07, December 26, 2024, has been quoted in the press as the FAA supporting angle of attack systems for general aviation. However, the text of the SAIB justifies its recommendations by citing transport category aircraft events, such as American Airlines flight 965 and Northwest Airlines 6231. It recommends angle of attack systems (to be incorporated into instrument scans, implying VAOAI for experimental, amateur-built aircraft, but doesn’t even mention such aircraft in its “justification.”

General aviation research results are conspicuously absent. SAIB does have a brief section on angle of attack as a theory, but lacks the considerable relevant detail needed for a full systems engineering justification. In fact, it lacks almost any justification for general aviation angle of attack systems.

This document can be objectively described as FAA virtue signaling.

I emailed the point of contact for this SAIB to learn the history of this SAIB, who initiated it, who approved it, etc., as it apparently originated from FAA Flight Deck Human Factors. That email was not acknowledged. Finding no information that the point of contact had any first-hand flying experience, not even private pilot, I filed a FOIA request. That request promptly generated a phone call from an individual who had great difficulty understanding what kind of information I was requesting – she thought I wanted a copy of the SAIB. I copied the relevant Chief Scientific and Technical Advisor, whom I casually know, and that was also ignored.

In the absence of any background information from the FAA on this SAIB, the description of this SAIB as virtue signaling remains in force.

Two Historic Systems Engineering Fails–Avionics Collision Avoidance Systems

TCAS, Traffic Collision Avoidance System, is required on airliners to reduce the chance of a midair collision. In its earliest implementation, however, it was initially a systems engineering failure — although those problems have long since been resolved.

When the human factors folks were working on TCAS, they worked on the displays and how those displays would interact with the pilot, and they did a great job. However, the systems aspect that was neglected was how pilots would handle situations in which TCAS commanded one thing and ATC another. Eventually, all this got worked out, but not at the start of TCAS implementation – the pilots had to figure out how to use it because the systems engineering did not consider the entire system.

ADS-B traffic uplinks are arguably another systems engineering failure. ADS-B traffic information is uplinked to the pilot with no real guidance other than here it is, you figure out what to do with it. There were and still are no ways to integrate uplinked ADS-B information with ATC communications, because ATC only cares about visual sightings. Also, ADS-B traffic displays do not necessarily aid in visual traffic detection or in traffic avoidance. In other words, pilots have to figure out how to use ADS-B because the systems engineering is still incomplete.

ADS-B traffic information is uplinked to the pilot with no real guidance other than here it is.

Two More Concept Failures

One recent general aviation fad that did not catch on, fortunately, was the continuous turn from downwind to final approach. There are many factors in play, but the short version is that a continuous turn to final is just a conventional traffic pattern with a really short base leg. Looking at things this way, it’s clear that the assumed advantages don’t accrue and that disadvantages may increase. This shortcoming was published as a letter to the editor in EAA Sport Aviation, page 6, December, 2017.

Another unfortunate fad is flying approaches at “optimum” angles of attack. And note the unfortunate ambiguity of the English language, that while “optimum” is implied to mean “the best,” it does not convey why that thing is the best, or what it is the best at, or in what situations it is the best. “Optimum” with no supporting context is as meaningless as “more than.”

More specifically, many VAOAI advocates recommend flying at “optimum” angle of attack, even if the resulting speed is below manufacturer’s recommendation (read my previous article on the topic). However, there has apparently been no consideration given to the consequences of routinely flying that slowly.

VAOAI—Justifying a Transition

It is little discussed these days that if something is even a little better, that obviously justifies implementation. That approach neglects the cost of the transition, handling the transition when both old and new are currently in play, and handling those who just won’t be able to make the transition. Sometimes this last group is referred to in general aviation circles as the unreachables and unteachables.

The second point of justifying the transition is that the new system must be so much better than the old system that the costs of the transition is justified. As will be seen below, general aviation is so complex that a new system, like VAOAI, is unlikely to be appropriate for every situation, and may be a step down in many situations. And what data there is suggests that any performance improvements from AOA are insignificant.[2]

VAOAI—The Scope of the Problem

VAOAI is often proposed as a replacement for airspeed, or a supplement, or something, because stall airspeed changes with weight but stall always occurs at the same angle of attack. But think back to your own flight training, and how often airspeed was referenced in how many maneuvers, how you as a pilot developed a feel for what airspeed errors were tolerable, what different airspeeds felt like in the control wheel and what they sounded like.

Now consider that VAOAI is to be added in to the cockpit. A full description of how VAOAI is best used would have to be determined for all those cases where airspeed was discussed. When VAOAI is suggested as being comparable to airspeed, the technique for flying the airplane with VAOAI replacing airspeed or being used in conjunction with airspeed must be documented. And those cases where VAOAI does not replace airspeed must also be documented, such as gear and flap extension speeds, rotation on takeoff, etc.

Sometimes VAOAI is touted as providing additional information, but those arguments neglect what is to be done with that additional information. One video said that VAOAI could tell the pilot how far the airplane is from stall angle of attack in a steep turn, but so what? And considering the workload in a steep turn, holding altitude and speed, not many pilots would have the ability to process VAOAI information, given that such information would not be used to meet steep turn completion standards.

It’s no wonder that VAOAI advocacy has had such limited scope.

VAOAI—Training Required

There are major, major problems in stating that VAOAI will work fine with proper training.

As will be seen below, there are so many different situations and conditions in general aviation that the best training on VAOAI will include statements as to when VAOAI is of no use and should be ignored.

A second major problem is that VAOAI cannot be properly trained until the system, in the largest sense, is completely defined. In the meantime, it can be no more than an option. There are a number of VAOAI advocates who acknowledge that training is required, but they ignore training details like who, what, when, where, why, and how.

The third major problem is that the state of aviation training is terrible. This bold statement is easily believed after listening to stories from Designated Pilot Examiners at flight instructor conferences. The situation is analogous to secondary teachers preparing students for exams, not teaching the material. Randall Brooks hints at this when he states, “Proficiency within the normal flight regime gives no hint at what lies beyond the boundaries of our everyday operations.” (https://safeblog.org/2016/05/07/addressing-i-loc-more-training/)

Lastly, stating the VAOAI will work with proper training is an enormous cop-out. What it says is that all problems with using VAOAI will be training problems, because VAOAI is intrinsically applicable and useful.

It’s also curious that none of the youtube videos show how to actually fly VAOAI in even the most benign conditions. Whereas there are numerous discussions of pitch/power for airspeed/altitude, there are no such discussions for VAOAI. It seems to be more a case of, if you install it, they will figure it out.

VAOAI—Meteorological Conditions

While VAOAI can provide correct information in calm weather, it can be useless in other circumstances.

One problem is turbulence. None of the YouTube AOA advocates have addressed this, although one did state that AOA would not be useful in all circumstances. Obviously, there is no point in declaring a system primary when another system must be relied on in less-than-ideal circumstances.

Here’s a video that shows that VAOAI can be hard to read on final in only moderate winds (note how calm the airspeed indicator is by comparison):

Some will say that the noise in the VAOAI readout (noise is the correct engineering term) can be mitigated by filtering the AOA data, but filtering means averaging over time according to some algorithm. The net effect is that filtering inherently makes the AOA system sluggish and slow to respond. Newer GA glass cockpits may have the data and compute power to address this, but the cost to implement and product liability may hamper implementation, especially if the augmentation algorithms must be tailored to invidual airplanes or installations.

In light airplanes, thermals and downdrafts displace the airplane significantly from the desired final approach path. This makes the approach “unstable,” but light planes fly successfully through such conditions all the time. Unfortunately, I don’t have a video of flight in such conditions with AVAOAI.

Similarly, many types of planes will have recommended final approach speed increased in gusty conditions and/or for crosswinds. Manufacturers have recommended any number of algorithms for increasing final approach speed, such as increase the final approach speed by half the steady state wind and all of the gust factor. Since gusts are given in unit of speed, and the final approach increment is given in speed, VAOAI cannot provide guidance for such gust compensation.

When such information is lacking, many pilots will increase approach speeds based on wind conditions.

Related to gust compensation is compensating for reported wind shear. It’s easy enough to add speed on final to compensate for reported wind shear but not possible to do that with VAOAI.

VAOAI—Flight Dynamics

There are interesting aerodynamic characteristics of the airplane, independent of whether VAOAI is installed or not. Let’s cut to the chase and avoid the ugly math (which is easy enough for the geeks to derive from the lift equation).

If the airplane is in cruise flight and pitch is slowly increased, airspeed will follow pitch and AOA will also follow. In other words, we can set the AOA we want by setting the correct pitch attitude and AOA will catch up, similar to airspeed. In engineering-speak, pitch leads AOA.

But here is where it gets interesting. If pitch changes rapidly, causing g load to change more rapidly than airspeed, AOA will change faster than pitch. This is backwards from the smooth pitch change case. Using that same engineering speak, now pitch lags AOA instead of leading it. (Similarly, if g load changes due to gusts, AOA will change before pitch has a chance to react.)

So, the pilot can use pitch to set AOA except when she cannot. NASA Tech Note D-6210 from 1971 discusses why pilots were unenthusiastic about flying AOA instead of airspeed in a Piper Twin Comanche. That Tech Note did not explicitly derive the lead-lag characteristics, however.

VAOAI—Failure Modes

The above-mentioned SAIB: 2024-07 states that airspeed and AOA are independent systems, but this is not true for most low-end general aviation systems. For many systems, AOA is sensed from the difference between pitot pressure and another pressure source, but not static pressure.

If VAOAI is primary, then failure mode remedies must be determined, published, and trained. Pretty soon, the solution will come to pitch and power to determine approximate airspeed.

Back when I gave primary instruction, I wouldn’t solo a student till they had an hour of pattern work with the airspeed indicator covered. They were terrified on the first approach, apprehensive on the second, and by the end of the hour, bored stiff. They were ready to handle an airspeed failure.

What would be the appropriate VAOAI failure scenario, and appropriate training? And if the answer to that question is airspeed, then that diminishes the value of VAOAI as a requirement.

VAOAI—Even More Considerations

Suppose VAOAI goes mainstream. What would the implications be for ACS, regulations, and Advisory Circulars? The present level of virtue signaling would not be acceptable.

And the quality of the discussion needs to improve as well. One VAOAI attempts to promote VAOAI by creating a bogeyman called seat of the pants. This kind of either-or discussion is great rhetoric and terrible engineering.

VAOAI—Summary

Those who do the work will conclude that VAOAI for general aviation is a special purpose instrument which may have utility in special circumstances. It is not usable in all conditions nor by all pilots.

I would rather have questions that can’t be answered than answers that can’t be questioned. Richard P. Feynman

Footnotes:

[1] A New Approach To System Safety Engineering by Nancy G. Leveson, MIT Press, is only one of numerous texts on systems engineering theory, many with interesting perspectives.

[2] Shu W. Gee, Harold G. Gaidsick, and Einar K. Enevoldson, “Flight Evaluation of Angle of Attack as a Control Parameter in General-Aviation Aircraft,” NASA TN D-6210, March 1971.

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Ed Wischmeyer

Ed Wischmeyer is an ATP and CFI-I whose computerized logbook tallies nearly 200 makes and models of aircraft flown. He has written hundreds of pilot reports, articles and columns, many for Kitplanes. He has owned four experimental RVs, an AirCam and a Cessna 175. His professional experience includes Apple Computer, SRI International, Honeywell, Boeing, and Gulfstream -all in research and advanced development. He also taught gradudate school courses in aviation safety for two years. He earned his Ph.D. from MIT.

Visual Angle of Attack Indicators and Systems Engineering Theory