Aftermath: Too Little, Too Soon

A novice pilot runs out of everything he needs.

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It was an old story. A relatively inexperienced pilot practicing landings got too slow on final approach, stalled and crashed.

The pilot, 56, was flying a club Cherokee 180, a type in which he had logged 13 of his 130 hours. He was making a short round-robin cross-country flight, ostensibly for practice, with landings at two airports along the way. He had landed at the second of them, taxied back and was making a second approach to Runway 17 when the accident occurred.

The National Transportation Safety Board attributed it to “the pilot’s failure to maintain airspeed during an approach in gusty crosswind conditions.”

The NTSB report on the accident (ERA12FA163) says the wind was from 230 at 13 knots gusting to 20, but that is somewhat misleading. The numbers come from tabulations of hourly automated observations at the uncontrolled airport. In the first place, the wind speeds on those tabulations are listed in miles per hour, not knots, and so during the course of the hour preceding the accident the wind actually averaged 11 knots; gusts of up to 17 knots occurred with unspecified frequency. There is no official information about what the wind was doing when the accident occurred, but a witness who ran to the scene after seeing the airplane go down reported that the surface winds were only 5 to 7 knots, with some stronger gusts. The mention of “gusty crosswind conditions” probably exaggerates the role of wind in the accident.

Initially approaching the airport from the south, the pilot had overflown it at 2,000 feet, made a sweeping right turn, joined the pattern on the 45 and completed a routine approach and landing. The ground track of his second circuit was similar to that of the first, but he was slightly higher and considerably slower as he turned base to final a little more than a mile from the touchdown point, which is displaced 800 feet from the threshold. As he approached the runway, he got progressively slower and dropped below the glideslope, which was marked by a 4½-degree four-light PAPI. The last groundspeed and height recorded on his GPS, still a quarter mile from the aim point, were 45 knots and 69 feet agl; 16 seconds earlier, he had been at 49 knots and 197 feet. The second final approach was, on average, about 20 knots slower than the first.

The gross weight stalling speeds of the Cherokee are 59 knots clean and 53 knots with 40 degrees of flap. Accident investigators determined from the wreckage that the flaps had been set at about 25 degrees. Between 25 and 40 degrees, flaps add more drag than lift, but let’s say the gross weight stalling speed was around 55 knots. The airplane was well below gross weight, however, having taken off with 36 gallons of fuel and flown only about 45 nm with a couple of landings before the accident. Supposing that it weighed 1,900 pounds, its stalling speed would have been under 50 knots. The target approach speed, 30 percent above the published stalling speed, would have been around 70 kias.

A 10-knot wind 60 degrees off the nose produces a 5-knot headwind component. The groundspeeds in the high 40s, therefore, probably represent airspeeds in the low 50s. The calculation is superfluous, however; whatever the wind, the Cherokee stalled. Whether it stalled out of unaccelerated flight or because of some abrupt action by the pilot, we can’t know. A pilot witness reported that the Cherokee porpoised several times when very low on the approach, and then, at an altitude of 50 feet, pitched up suddenly. Another witness reported the wings rocking “hard” — a behavior that seems consistent with the forecast of gusty crosswinds but was not mentioned by the first witness. Both of these descriptions suggest a pilot who was working to control the attitude of his airplane while failing to deal with the two fundamental requirements of airspeed and height.

If there were gusts, their role is unclear. As far as pilots are concerned, a gust is simply a sudden change in the speed of the wind, either an increase or a decrease, or in its direction. At altitude, wind gusts can come from any direction, but close to the ground they have to be horizontal. When gusts are expected — and really any time a strong wind is blowing because its speed may fluctuate — pilots are advised to add a few knots to the approach speed.

As is customary in discussions of flying slow, everything is expressed in terms of speed. Speed, however, is just a surrogate for angle of attack.

An old hangar riddle takes this form: “A Cub is flying at 50 kias into a 25-knot headwind. The wind suddenly dies. What happens?” Pilots give various answers, ranging from “Nothing, because the airplane is part of the air mass” to “The airplane stalls.”

The “correct” answer actually depends on how long a time frame you consider. Initially, the only thing that happens is that the airspeed indicator registers a 25-knot dip. Nevertheless, the Cub does not instantly stall because its angle of attack has not changed, and so air is flowing over the wing in the same way as it was before the wind died, although at a lower speed. Because of the lower airspeed, however, the wing loses lift — three-quarters of it, given the numbers in the question. The airplane therefore drops out from under the pilot.

Now, suppose the airplane has a bit of altitude, and the pilot does nothing. Consider a paper airplane thrown too hard: It arcs upward, stalls, then drops its nose into a dive and resumes gliding. The Cub is no different. The wing might stall momentarily unless the pilot reacted quickly to the loss of lift by pitching the nose downward. Even if the pilot did nothing, the Cub would drop its nose because of the upward pressure on the horizontal tail produced by its mushing descent. It would dive and regain flying speed. It might overshoot, but after a couple of oscillations it would settle back to its trimmed speed. This is the behavior of all naturally stable airplanes, model and full-scale alike.

But it is not the behavior of all pilots. If we encounter a downdraft while cruising at altitude, we pull back on the stick or increase power or more likely do both because we attach importance to maintaining altitude accurately. On the ILS, we handle brief excursions from the glideslope with elevator and longer-term ones with power. Our habit, in other words, is to manage altitude ourselves, not leave it to the natural, but slower-acting, dynamics of the airplane. Furthermore, our normal reaction to a transient loss of altitude is to pull back, and it works, provided that we have some surplus airspeed. If we have no airspeed margin — and this pilot didn’t — then the appropriate reaction would be just the opposite: Push the nose down. Unfortunately, we use the pull-up reaction so much more often that, despite stall-recovery training, the push-down one does not become instinctive.

Why our Cherokee pilot allowed himself to get so low and so slow — shades of Asiana at SFO — we’ll never know. One noteworthy thing about this approach, however, and also about his previous one, whose ground track was nearly identical, was the length of the final approach. The FAA’s recommendation for the traffic pattern is to turn base when the touchdown point is 45 degrees behind you. This implies that after the power reduction on downwind you fly only as far past the touchdown point as you are laterally from the runway — probably no more than half a mile. If the distances from power reduction to base turn, from base to final, and from final turn to touchdown are the same, you should dispose of about a third of your altitude on each segment. Flying correct heights and positions in the pattern won’t keep you from getting too slow, but it can help keep you from getting too low, and that is half the battle.

This article is based on the NTSB’s report of the accident and is intended to bring the issues raised to our readers’ attention. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.

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