You may have noticed, if you follow the adverts in American aviation magazines, a sudden multiplication of the number and variety of angle of attack (AoA) indicators on offer for smaller general aviation aircraft.
THIS BURST OF ACTIVITY was triggered by a new FAA policy allowing AoA instrumentation to be installed in certificated aircraft 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 aimed at the homebuilt market cost as little as $350 – and include offerings from major names like Garmin, Safe Flight and Bendix/King, so they stand a pretty good chance of being adopted.
Speed is a very accurate and sensitive indicator of AoA, provided that the aeroplane is at a typical weight and not pulling any G. The late Jon Karkow of Icon once 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 added, 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 one-G flight at a particular weight. Under any excess weight or G loading, the AoA is greater than the indicated speed would suggest. With two-G acceleration in a 60 degree-banked level turn, an aeroplane 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 one-G, however, indicated airspeed is deceiving, because the heavier the aeroplane 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.
‘People will get impaled on it’
In flight testing, AoA is measured by a pivoting vane on the end of a long pole that sticks out ahead of the aeroplane into undisturbed air. In everyday use, that arrangement won’t do: People will either get impaled on it or knock it off the aeroplane. A vane placed on the aeroplane 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 differences 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 – a Safe Flight product that was on the market decades before the others, disappeared, and is now being resurrected – 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 standardised. Several of the new systems present the pilot with an illuminated pattern of coloured lights or of stripes, chevrons and dots. The leading edge tab system has an analogue 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, which I acquired in 1974, in two homebuilts and flown more than 2,800 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 manoeuvring in the pattern. It is particularly valuable for short-field approaches and for the occasional situations where low-speed manoeuvring 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 glare shield, 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 like autothrottles 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’
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. The Icon instrument, which replaces the dial pointer with an aerofoil, reverses the picture. And 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 aeroplane’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 analogue display – at a level of detail that a conventional stall warning lacks, and it is valid regardless of weight, speed, angle of bank and G loading.
Like all safety improvements, this one will get a few pilots into trouble. They will think they can flirt more boldly with the stall now that they 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.