(October 2011) Air France 447 went down in the mid-Atlantic in 2009 because all three of its pitot tubes iced up.
Well, not exactly. It wasn’t the loss of functioning pitot tubes that doomed the airplane; it was what the A330’s autopilot, and human pilots, did next. The autopilot — but it would be more proper to say computer, since fly-by-wire airplanes are controlled by decision-making systems far more complicated than the word autopilot suggests — the computer, seeing that it had lost reliable airspeed information, immediately turned over control of the airplane to its human minders.
This was an interesting systems-design choice. What were the human pilots supposed to do that the computer couldn’t? The proper reaction is to change nothing: Maintain power and pitch attitude, don’t touch the trim, make sure the pitot heat is turned on and wait for the ice to melt. You learn that much in training. Why, some people asked, did the computer not do that, rather than throw up its hands and say “your airplane?” Why can’t an autopilot have a coast mode, like the one that allows the GPS in your car to continue tracking even while you’re in a tunnel? After all, many general aviation autopilots know nothing about airspeed, and yet they manage to fly airplanes quite well. Just holding speed and attitude wouldn’t give you RVSM-quality altitude control, but if you’ve lost airspeed data, RVSM is the least of your problems.
But, since faith in technology cannot be shaken for long, the discussion turned to the topic of alternative ways of measuring airspeed.
Some of the more naive suggestions were based on GPS (or inertial, which amounts to the same thing). Such methods might work for a short time, but there is a fundamental problem with them: They measure groundspeed, not airspeed, and cannot separate the local wind component from the motion of the airplane.
GPS, inertial groundspeed and pitch-and-power are no better than second-order surrogates for indicated airspeed — circumstantial evidence, so to speak, leaving room for reasonable doubt. A first-order surrogate would be true airspeed, from which, knowing the density altitude, indicated airspeed could be accurately calculated. Since the airplane’s relationship to the air, not the ground, is what you really care about, you need to find out, somehow or other, how fast that invisible stuff is going by.
One scheme for measuring true airspeed involves pointing a laser beam in the direction of flight and measuring the frequency shift in its reflection from bits of airborne dust. (Similarly, the speed of a train can be deduced from the perceived pitch of its whistle if the whistle’s true pitch is known.) I found several papers about this, going back to the 1990s, and they acknowledge the difficulties — for one thing, airborne particulates are scarce at high altitude — but conclude (as the authors of scientific papers are liable to do, since they hope to obtain funding to continue their research) that the problems are not insurmountable. The technology seems rather delicate and complex, however, and no one discusses its possible cost. I didn’t see any mention of other schemes, such as having the airplane eject its own aerosols and watch them recede into the distance, or eject particles laterally and time them as they go by. There seem to be lots of optical possibilities, at least for getting research grants and writing papers.
Another, perhaps less elegant, idea for directly measuring airspeed might come from the world of meteorology. Wind speed can be measured with what is called a sonic anemometer. The speed of transmission of a sound is relative not to the emitter but to the surrounding air, and so the transit time over a fixed distance changes if the air is moving. Sonic anemometers are quite expensive compared with the spinning-cup kind, but the basic components — ultrasonic emitters and receivers and microcircuits to translate delay time into a meaningful number — are cheap enough.
If, for example, you mount the emitter and the receiver 10 feet apart, the transit time of the audio pulse, which at 40 kHz or so is inaudible to humans but probably very annoying to bats, is on the order of a hundredth of a second. If the system moves through the air at 200 knots, that changes by 30 percent. So there would be no difficulty resolving speed differences of a fraction of a knot.
One paper described a scheme for ultrasonic airspeed measurement, but it involved a rather bulky tubular object attached to the outside of the airplane. I was imagining embedded speakers and microphones that would operate in the open air, say between the vertical fin and the cabin roof of a twin — the beam of sound would have to stay, as much as possible, outside the slipstream and the boundary layer — or between the tip of the horizontal stabilizer and the wing. But maybe the vicinity of an airplane is too noisy, even at ultrasonic frequencies. I trust that some informed reader — or perhaps a whole raft of them — will tell me that this idea, like most patentable-sounding fantasies of mine, has no merit whatsoever.
Cost-benefit analyses are the final resting place of many a technically sweet invention. To give the devil his due, pitot tubes seldom fail, and so we are talking about installing thousands of backup systems that might never be used. Competing with the airborne laser interferometer and the ultrasonic range finder is a small steel tube containing a heating element powerful enough to fry an egg. Obviously, the heated tube wins; it’s simple, cheap, easy to understand and service, and hard to break. Designing it is just a matter of getting the right MTBF — mean time between failures, the criterion of reliability that is of such small comfort to those present when a failure’s time has come. Evidently Thales, the manufacturer of the A330’s heated pitots, hadn’t gotten that part quite right.
The just-sit-there-and-don’t-change-anything solution to loss of airspeed indication is a pretty good one, at least at climbing or cruising speed. For approach and landing, we need a way not just to maintain speed, but also to set it at a desired value. Ideally, we would fall back on our old and much-ignored friend the angle-of-attack indicator. Angle of attack is not of much use for managing speed in cruise, because small changes correspond to large speed differences. At approach speed and below, however, an instrument displaying angle of attack is really superior to an airspeed indicator, because it automatically compensates for weight and bank angle. In fact, airspeed is a surrogate for angle of attack, and not the reverse. I have used an angle of attack indicator — a Safe Flight SC-150 — for 18 years and 2,500 hours; for approach and landing I consider it primary and the airspeed indicator a backup. One of the greatest wonderments of my 50 years of flying is the failure of a display of angle of attack to become a part of every airplane’s instrument panel.
Angle of attack indicators usually consist of some sort of green-yellow-red color scheme rather than absolute numbers. The SC-150’s edgewise dial has four reference points: climb, approach, slow approach and stall warning. But in a pinch it would need only one: approach. And that suggests that in case of loss of airspeed indication, you could use the attitude indicator instead, provided that it has pitch angle markings and that you know the angle of the flight path.
The angle of attack is, to a pretty close approximation, the difference between the pitch attitude of the airplane and the flight path through the air. The angle of attack of a clean wing at approach speed is around six degrees. If the airplane is descending on a three-degree glidepath, such as a VASI or ILS, and its pitch attitude is three degrees nose up, then its angle of attack is somewhere around six degrees — most likely a degree or two more, because the wing is probably set on the fuselage at a small positive incidence.
One way to make an approach without an airspeed indicator is to use the knob on the attitude indicator to adjust the miniature airplane to a position two or three degrees below the horizon while in level cruising flight. Slow down by holding the miniature airplane on the horizon while maintaining altitude. Upon intersecting the glidepath, leave the flaps up, trim to keep the miniature airplane on the horizon bar and maintain the glidepath with power.
It would not be a bad idea to try this once or twice when the airspeed indicator is working, just to see what the result looks and feels like.
The important thing to remember is that the attitude indicator alone is not a substitute for an angle-of-attack indicator. It is only in combination with a known flight path angle, provided by the VASI or ILS, that it can tell you — very roughly — what you need to know.
Read our Flying Tip on flying without airpseed.
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