Aftermath: Improvisation

Accident analysis that goes behind and beyond the NTSB report. Flying

The airport at Los Alamos, New Mexico, lies on a mesa with ravines on three sides and rising terrain to the west. The 6,000-foot runway, 7,200 feet above sea level, is oriented east-west, with a slight upslope to the west. Because of the noise sensitivities of a community just beyond the departure end of Runway 27, all takeoffs are required to the east and all landings to the west, regardless of wind. A restricted area, R-5101, abuts the airport’s south edge and is continuously active up to 12,000 feet.

On a Sunday morning in December 2013, there was a thin layer of powdery snow on the runway, but the airport was VFR. At 7:35 a.m., a New Mexico Airlines Caravan arrived to pick up passengers for a flight to Albuquerque. The pilot of an Aviat Husky approached the pilot of the Caravan to say that he would take off after the Caravan had departed in hope that the big turboprop would have blown some of the snow off the runway. The Caravan pilot had no objection.

The Caravan taxied out at 8 a.m. The wind was out of the west at 10 knots, and the Caravan back-taxied to gain additional takeoff distance. When the Caravan was at the west end, the Husky pilot called on the radio to ask whether takeoffs to the east were mandatory. The Caravan pilot replied that they were, but that the airline was authorized to take off to the west if wind conditions made it necessary. They would request authorization from the airport manager on each occasion and were required to turn east before the terminal building, which is located near the west end of the runway.

“Oh, OK,” the Husky pilot replied. “That makes sense. Thank you.”

Shortly after the Caravan took off, the Husky pilot called again to ask how the weather looked. It was good VFR to the west, the Caravan pilot replied.

We don’t know exactly what happened next because there were no immediate witnesses, only a couple of people who briefly glimpsed the Husky in flight. What is reasonably certain is that the Husky took off to the west, made a tight turn to the left, and after reversing course stalled and crashed about 890 feet south of the runway. The two occupants, both pilots from the Midwest, died, and the airplane was entirely consumed by fire.

The National Transportation Safety Board, referring to an 8:15 automated weather observation record of winds at 12 knots and gusting to 23, blamed the accident on “the pilot’s loss of airplane control while maneuvering after takeoff in gusty wind conditions.”

A local flight instructor, Will Fox, wrote a thoughtful analysis of the accident for the January 2014 newsletter of EAA Chapter 691. Fox inferred from the testimony of a witness who glimpsed the airplane before it disappeared behind a rooftop that the Husky was about 150 feet above the runway elevation when the loss of control occurred. Back-calculating from the handbook takeoff and climb performance, he concluded that the Husky pilot likely did not taxi all the way to the east end before starting his takeoff roll. As for the question of why the pilot turned toward the restricted area immediately south of the airport, Fox speculated that, with climb performance less than half of what he was accustomed to, the pilot may have preferred to turn away from the structures and light poles along the north side of the airport, or he may have turned left instinctively because that is the default turn at most airports. The fact that he turned quite tightly suggests, however, that he was aware of the restricted area.

The Husky’s best angle-of-climb speed is 55 kias, but Fox supposed that if the wind was gusty the pilot would have climbed a little faster than that — say 60 kias, or 1.3 V. To accomplish a 180-degree turn within 890 feet at that speed, the Husky would have to have banked about 45 degrees. This would raise its stalling speed from 46 to 55 kias and pare its margin above the stall down to 5 knots.

These speeds are speculative, of course, as is the assumption that the airplane had completely reversed course when the pilot lost control. Still, a witness at a house just beyond the west end of the runway reported seeing the airplane in a steep bank — he estimated 45 to 70 degrees — and the ground scar, although short, was oriented east-west; so there is evidence that the Husky completed a 180-degree turn within 890 feet before crashing.

The question is: Why did it stall?

Turning downwind — that is, 180 degrees from heading into the wind to having it behind you — is not hazardous in itself. As can easily be demonstrated by flying circles at high altitude in a strong wind, it is impossible to tell from the behavior of the airplane which way the wind is blowing.

When making turns at low altitude, issues arise other than mere direction of flight with respect to the wind. A turn downwind close to the surface creates a strong visual impression of both rapidly increasing speed and the nose pointing outside the desired ground track, to which a pilot may respond with back stick and inside rudder. A sudden slackening of a following wind reduces the rate of climb, again tempting the pilot to raise the nose. Finally, going from a headwind to a tailwind reduces the climb gradient — the angle, not the rate, of climb — and that too may spur the pilot to compensate by raising the nose.

The wind velocity gradient can also be a factor. Wind speed increases with distance from the surface, and so a climbing airplane experiences continuous wind shear. It’s impossible to make a general statement about the importance of this shear because it depends on several rates: the airplane’s rate of climb, the rate of change of wind speed with height, and the rate of response of the airplane to airspeed excursions, which is roughly expressed by its wing and power loadings. A lightly loaded, powerful airplane, like the Husky, responds rapidly to excursions from its trimmed speed; denser and less powerful airplanes take longer. I am inclined to think that a Husky is light enough, and, at a 7,000-foot density altitude, it climbs slowly enough that the wind gradient would not be a significant hazard.

Several factors could be seen to lead to the loss of control. One would be the low altitude at which the pilot began the turn. That would have been a consequence of his choosing not to taxi all the way to the east end — if Fox is right in guessing that that is what he did. Everything, however, would have followed from his decision to ignore the airport rule that all takeoffs should be made to the east.

From the Husky pilot’s conversations with the Caravan pilot, it seems that he was making his plans only moments before his takeoff. Perhaps, with a little more reflection, he would have realized that he could safely take off with a 12-knot tailwind. The runway length was ample, after all; the only problem, for a taildragger, might be directional control early in the takeoff roll. But even at gross weight and at a density altitude of 7,000 feet, the 200 hp Husky would accelerate for only two seconds before the tailwind became a headwind. The important difference was that once airborne, the pilot would have nothing to do but fly straight ahead while gaining speed and altitude. Taking off into the wind, in this special case, may have felt like the natural thing to do, but it was not the right thing.

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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