You may have noticed, if you follow the press releases, a sudden multiplication of angle of attack indicators for smaller general aviation aircraft. This burst of activity is due to a new FAA policy allowing AOA instrumentation to be installed in certified airplanes without a burdensome STC process, as long as it is independent of the existing pitot-static plumbing. The systems on offer are relatively cheap — $1,500 or so, though some systems aimed at the homebuilt market cost as little as $350 — and include offerings from major names like Garmin and BendixKing, so they stand a pretty good chance of being widely adopted into GA aircraft.
Speed is a very accurate and sensitive indicator of AOA, provided that the airplane is not pulling any G’s. Jon Karkow, the lead aero engineer at Icon, has pointed out to me that the design of the round airspeed indicator, with zero at the top of the instrument rather than at the lower left as on most dial indicators, may have been intended to make the needle act like an angle of attack indicator in the low-speed regime. (By the same token, Karkow adds, a test pilot’s trick for helping to maintain a steady altitude is to fly at one of the thousands plus 750 feet — 7,750, 8,750, etc. — so that the altimeter needle points to the left and behaves, again, like an angle of attack indicator, moving up when the nose is high and down when it’s low.)
Airspeed indicators and altimeters aren’t intended to be AOA indicators, of course. They are merely analogous to AOA indicators under certain conditions, namely 1 G flight at a particular weight. Under any excess G loading, the AOA is greater than the indicated speed would suggest. With 2 G acceleration in a 60-degree banked level turn, an airplane stalls at an indicated airspeed 40 percent higher than in straight-and-level flight — 84 kias, for example, rather than 60. To make matters more confusing, it’s the G, not the bank angle, that affects the stall; in a wingover you may have 90 degrees of bank, no G, and no danger of stalling. Even at 1 G, however, indicated airspeed is deceiving, because the heavier the airplane the greater the angle of attack must be at a given speed. A direct reading of angle of attack, unrefracted by the prism of speed, is therefore really the only way to know accurately how hard the wing is working and how close it is to the stall.
In flight testing, AOA is measured by a pivoting vane on the end of a long pole that sticks out ahead of the airplane into undisturbed air. In everyday use, that arrangement won’t do. A vane placed on the airplane itself — on the side of the fuselage, for instance, as in most jets — requires a computer to convert the local airflow angle, typically about twice the actual AOA, to a real value and to make corrections for sideslip and flap position. Such a precise and sophisticated system is complex and costly to calibrate and certify.
Unlike test pilots, most of us don’t need exact numerical values. For stall avoidance, which is the big selling point of AOA systems, what we need to know is how far we are from the stall, whether we’re getting closer to or farther from it, and how rapidly our AOA is changing.
Discarding precise angles makes building an AOA system much easier. Various types of sensors can be used. One kind, found on the Icon amphibian, uses pressure Âdifferences at small holes in the upper and lower surfaces of the wing itself. Others locate the pressure ports in the nose of a special pitot tube or in a separate protruding probe mounted on the wing outside the propeller slipstream. A third type looks just like a stall warning tab, but rather than trigger a simple on-off switch and an audio stall warning, the spring-loaded tab sends a progressive signal reflecting the condition of flow near the leading edge. Yet another style uses a pivoting vane on the wingtip. As far as I can tell none of the inexpensive systems provides flap compensation, but at least the resulting inaccuracies are on the safe side.
Cockpit displays are far from standardized. Several of the new systems present the pilot with an illuminated pattern of colored lights or of stripes, chevrons and dots. The leading edge tab system has an analog display with a moving pointer, as do the Icon and one or two others.
I have had one of the old Safe Flight SC-150 leading-edge tab systems — the company stopped selling that product decades ago but is now resurrecting it in a new version — in two homebuilts and have flown 2,500 hours with it. Like some other systems, it provides, in addition to stall warning, targets for three flight conditions: slow approach, approach and climb. I routinely monitor it, rather than airspeed, while maneuvering in the pattern. It is particularly valuable for short-field approaches and for the occasional situations where low-speed maneuvering is required, for instance when you are asked to make S-turns for spacing on final, have overshot the base-to-final turn because of a crosswind, or need to make a best-angle climb.
The SC-150 uses a rectangular meter that is intended to sit, like most AOA instruments, on top of the glareshield, so that it is near the pilot’s line of sight. The factory-recommended presentation — this was back in the 1970s — had the stall zone at the bottom because the company, whose business was mostly in high-end equipment for turbine aircraft, conceived the instrument as a flight director. If the needle moved downward, toward the stall, it was telling you to get the nose down.
I mounted my instrument upside down, because as a little-plane guy I saw it as an attitude indicator: If the needle went up, it meant the nose was going up. That seems to be the interpretation implied by most of the new vertical displays, which have the red part on the top; but some of them incorporate a bit of the flight-director-style cuing by making the red part into several downward-pointing chevrons. Most of the round dial displays are set up so that the needle points to 3 o’clock in the approach regime, like an airspeed indicator, but even this arrangement is not universally agreed upon. One product uses a quarter-scale round dial with the red zone on the left, a scheme that seems to have no relation at all to other instruments or to the airplane’s attitude.
Pilots who have flown their entire careers without AOA instrumentation and never inadvertently stalled will naturally ask why an AOA indicator should suddenly be considered necessary. They’re right. It isn’t necessary; it’s just useful. What it provides that a stall warning horn does not is trend information. You see, well before the horn goes off, that you are nearing the stall; you see how rapidly you are approaching it; and you see how effective your correction has been. The information is presented — particularly on an analog display — at a level of detail that a conventional stall warning lacks.
Like all safety improvements, this one will get a few pilots into trouble. They will think they can flirt more boldly with the stall because they now know so much more about it, and they will end up stalling anyway. But for the great majority, and certainly for student pilots, an AOA indicator will supplement airspeed in a genuinely helpful and instructive way.
And remember — you may not have experienced an inadvertent stall yet, but it only takes one.
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