Aftermath: Too Little, Too Soon
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.