An angle-of-attack indicator is needed in the cockpit of every airplane. "It’s the most useful piece of information you can get," declared Capt. Ron Rogers, an A320 pilot. Rogers, who is also director of aircraft development and evaluation programs for the Air Line Pilots Association (ALPA), was speaking recently at that august organization’s annual safety meeting.
In two days of discussions about runway incursions, simultaneous offset instrument approaches, "hard" and "soft" control limits on Airbus and Boeing aircraft, and other safety issues of the day, Rogers’ remark struck a nerve. Of the thousands of words uttered at ALPA’s two-day safety symposium, Rogers’ call for an angle-of-attack indicator was the only one that generated spontaneous and fervent applause from the large meeting hall full of pilots.
The concept of angle-of-attack (AOA) is fundamental to flight: it is the angle between the relative wind and the chord of the wing. Think of a wing as a large board that can be oriented anywhere from zero to 90ï¿½ relative to the wind, which is to say edge-on to flat-side. The wing generates lift within a fairly narrow range of rotation—a few degrees. At an angle-of-attack greater than 15ï¿½, most wings stall, which is to say the airflow over the top surface becomes too turbulent to generate lift.
The measure is so critical that the only flight instrument on the Wright brother’s first airplane was a device to measure angle-of-attack. The Wright’s crude instrument consisted of a stick protruding forward of the wing’s leading edge—and clear of the airflow around the wing—with a length of yarn attached to the front end. In flight, the angle-of-attack was easily measured by the position of the yarn streaming back relative to the stick.
Most modern aircraft measure angle-of-attack with more sophisticated sensors, feeding the data to their flight management systems—and to the "stickshaker" to warn of the approach to stall. While the computers may know the angle-of-attack, the pilots sit in ignorance.
To be sure, pilots can infer the angle-of-attack from the airspeed, since a given angle-of-attack corresponds with a particular airspeed at a given weight for the airplane. Slow the speed and the angle-of-attack must increase to generate more lift. At increasing angle-of-attack, up to the stall limit, the coefficient of lift increases. Indeed, a plot of coefficient of lift against increasing angle-of-attack follows a rising line that starts to flatten out near the top. The apex of this curve is the "critical angle-of-attack," or the point at which maximum lift is being generated. Increase the angle beyond that critical point and the wing starts to stall.
The flattening of this curve illustrates the major limitation of presenting pilots with airspeed, but not angle-of-attack information. There is a range of angles-of-attack near the top of the flattening curve that produce about the same high coefficient of lift, but the lift corresponds to a very narrow range of airspeeds—which happen to cluster near the stall speed.
It has been said that stall is the most critical flight regime. If so, it is the regime where critical angle-of-attack information is most needed, and where the airspeed indicator lacks precision.
To ensure safe flight at a speed that correlates to a desired angle-of-attack, pilots often are provided a set of speed cards. A card for each aircraft gross weight outlines the indicated airspeed to be flown, which is sort of a back-door way of getting to the desired angle-of-attack. The shortcoming of this approach is that it does not account for the variance between estimated and actual aircraft gross weight. For example, passengers and their checked baggage may be assumed to weigh 180 pounds.
But underestimating the average weight of, say, carry-on baggage by 5 pounds (2.27 kg) per passenger, can mean the weight estimate with a planeload of 200 passengers can be too low by 1,000 pounds (434 kg). Thus, if the pilot on approach is holding a particular airspeed that correlates to 8.5ï¿½ angle-of-attack, he may find the airplane descending faster than desired (the sudden application of power often seen just before touchdown is a last-minute pilot response to holding a constant airspeed in an airplane that is heavier than estimated). However, if a pilot finds that he’s actually flying 10ï¿½ AOA, not the 8.5ï¿½ AOA implied for that landing weight on the speed card, then he knows the airplane weighs more than reported.
For this reason, raw angle-of-attack information in the cockpit can help ensure that the speed is correct for the actual weight of the airplane during takeoff and for approach and landing. In fact, Navy carrier pilots are taught to make approaches using angle-of-attack instrumentation exclusively, mostly ignoring the airspeed indicator. According to Jim Frantz, who has designed a low-cost angle-of-attack indicator for general aviation, carrier pilots routinely come within 3 feet (0.9 m) vertically and 5 feet (1.5 m) horizontally of the optimum touchdown point on the carrier deck, and within a knot or two of the optimum airspeed.
Angle-of-attack information also adds a margin of safety when flying in a holding pattern. An increasing angle-of-attack can warn of icing. Even with a "clean" wing, the additional wing loading in a turn at low speed can put an airplane into a stall in the blink of an eye. That is, if the crew doesn’t have an angle-of-attack readout to watch with the vigilance of the proverbial hawk.
An AOA indicator also can help prevent emergencies from devolving into crashes. In 1996, a Birgenair B757 crashed 2.5 minutes after takeoff from the Dominican Republic. Insect debris in the pitot tube feeding the captain’s airspeed indicator was suspected. The airplane was flown right into a stall and plummeted into the water. With conflicting airspeed indicators and an autopilot/autothrottle reaction that increased pitch and lowered thrust, the crew disconnected the autopilot and applied full power, but too late to skirt catastrophe.
A few months later an AeroPeru 757 crashed into the waters off Lima under circumstances that represent the flip side of the Birgenair crash. Investigators found the static ports had been taped over as ground crews waxed the plane’s aluminum exterior (the static port provides the reference, if you will, to the dynamic pressure measured by the pitot). Left undetected, the taped-over ports rendered the airspeed indicators useless.
To be sure, in both cases the aircrews could have resorted to a constant pitch/known power setting to fly out of an incipient stall, but hindsight presumes a presence of mind not always present when crews are caught by surprise. In both of these tragedies a direct angle-of-attack readout could have been a real lifesaver.
In truth, an angle-of-attack readout complements the stickshaker. The latter tells the pilot that he’s gone and done it, while the former helps him to avoid doing it. And if the whole approach to safety is to break the earliest causal link in the chain of events leading to an accident, maybe it’s time pilots are provided angle-of-attack information the Wright brothers thought was indispensable.
David Evans is managing editor of Air Safety Week newsletter. For subscription information, call 888-707-5812.