Not just an attitude

By David Jack Kenny

The point is made early on in ground school—it shows up, for example, on page 3-6 of the Jeppesen Private Pilot Manual (1997 edition). We all have to learn it to pass the written test, but those of us who spend our aviation careers smack in the middle of the flight envelope may never come to understand what it really means. Even pilots with the gumption to seek out aerobatic training may be a long time coming to grips with the implications of that deceptively simple sentence: A wing always stalls at the same angle of attack, and angle of attack doesn’t depend on either attitude or airspeed.

This fact continues to take pilots by surprise, experienced and less experienced alike. Those exploring unfamiliar flight regimes or flying aircraft with performance characteristics very different from what they’re used to may encounter it at particularly inconvenient times.

Related Links

Essential Aerodynamics: Stalls, Spins, and Safety online course

"Landmark Accidents: Jet Transition Troubles" Safety Pilot article

"Maneuvering Flight: Hazardous to Your Health?" Safety Advisor

On Feb. 26, 2011, an instrument-rated private pilot flew a BAC 167 Strikemaster from Smyrna, Tenn., to Johnstown, Pa. This was the first leg of a ferry flight to the Columbia County Airport near Hudson, New York, itself the first stage in repositioning the vintage military jet for an air show. The 1,200-hour pilot telephoned some friends from Johnstown, promising to do a low pass over his home field of Kingston-Ulster before flying the last 22 nautical miles to Columbia County. They arrived in plenty of time to hear him call a four-mile final, overfly the runway, and turn crosswind.

They and two other witnesses agreed that about halfway through the downwind leg, the Strikemaster performed a roll. It then turned base, corrected after slightly overshooting final, and descended within 50 feet of the runway. Following a high-speed pass with the gear retracted, it pitched up and began to climb. Estimates of its pitch angle ranged from “20 to 30 degrees” to “near vertical,” but all agreed that when it began to turn left, the nose suddenly dropped. Before the pilot could regain control it crashed through the ice on the Hudson River, killing him.

He had been awarded his type rating in the BAC 167 about ten weeks earlier with 20.6 hours of time in type. This was the only jet experience recorded in his logbook. The NTSB did not report on any prior aerobatic instruction he might have received, but the records of his training in the Strikemaster indicate that he was shown how to do loops, aileron and barrel rolls, and split-Ss. He had eventually been rated “proficient” in loops and rolls, the highest of the three grades awarded by that training program. However, the airplane’s owner told investigators that he had not authorized him to do any aerobatic maneuvers on the ferry flight, adding that he “would never have approved of [the accident pilot] doing solo aerobatics in his airplane,” according to an NTSB investigator.

Whether he’d already planned to do them when he phoned from Johnstown or decided on the spur of the moment, it appears that he had gotten very comfortable with the airplane. Rolling it at pattern altitude probably did nothing to hurt his confidence. Demonstrating the climb performance of a 400-knot attack jet would be a fine way to end a fine day of flying, and much easier than the roll he had just completed.

The low-altitude, high-speed blast down the runway ending in a sudden steep pull-up is sometimes called the airshow pass. Like any other maneuver involving abrupt control inputs, it is best learned at a safe altitude. Its apparent simplicity masks real risks. With lots of airspeed and plenty of power, there wouldn’t seem to be any reason to worry about stalls … unless you think it through. The sharp pitch-up rapidly changes angle of attack, making it easy to reach the critical value at airspeeds that would normally seem safe. The conventional reference to “stall speeds” obscures the fact that where stalls are concerned, airspeed is only a surrogate for angle of attack, and sometimes a pretty poor one.

In a sidebar to his June 2011 story on angle-of-attack indicators, AOPA Pilot Senior Editor Dave Hirschman recalled the time that he managed to stall a biplane flying at full throttle with the nose pitched 90 degrees straight down. That’s convincing evidence that it really is possible to stall in any attitude other than sitting on the ramp—while this accident is convincing evidence of the danger of improvising much beyond your training, especially while close to the ground.