Region of reversed commands
THEY DO THINGS DIFFERENTLY THERE
In July 2016, a Cessna 150F taking off from a grass strip in Indiana failed to climb, struck trees and crashed, killing both occupants. A single witness, himself a pilot, who was operating a mower, observed parts of the takeoff. He reported that the Cessna had become airborne at midfield, which was normal, and that when it crossed the departure end it was flying at a high angle of attack. He did not expect the airplane to have any trouble clearing the tall trees 1,100 feet from the end of the runway, however, and turned away. When he later saw smoke rising from the woods, he assumed the property owner was burning something, and it did not occur to him there had been an accident until he heard sirens.
The pilot, 44, was a novice. On his application for a third-class medical, four months before the accident, he reported 60 hours of total time, 25 in the previous six months. According to press reports, he had bought the 150 late in 2015.
The 150F, which has “omni-vision” rear windows, a swept vertical tail, electric flaps and a 100 hp Continental engine, is approved for auto fuel, and before the flight the witness observed the pilot adding fuel to the airplane from plastic jugs brought in the trunk of his car. They spoke; the pilot said he and his companion were flying to another airport for lunch. The witness also reported that when the pilot first acquired the 150, he had come to the grass strip with an instructor to practice short- and soft-field takeoffs.
The wreckage was largely consumed by fire, but the National Transportation Safety Board’s investigator was able to perform the usual control system continuity checks. He found nothing amiss. The flaps had been up at the time of impact. The engine, with 960 hours since overhaul, was examined and borescoped, the spark plugs and magnetos checked. There were no contaminants in the carburetor bowl, and nothing else was found that would preclude normal operation; nor had the witness reported anything unusual about the engine’s sound. The pilot’s toxicology report was clean.
The investigator concluded, therefore, that the probable cause of the accident was improper soft-fieldtakeoff technique.
The strip is 2,081 feet long, 65 feet wide and, to judge from a photograph in the accident docket that must have been taken the day after the event, smooth and well-mowed. Usually, soft-field technique (or rough-field, which is the same) is used for tall grass, snow, slush, mud, bumps and ruts, and so on; it’s not clear why the pilot felt it was called for here.
The aim of the technique is to get weight off the wheels as quickly as possible, either to reduce rolling resistance, in the case of grass, mud or snow, or to mitigate rough-surface impacts on the landing gear — particularly the nose strut, which is the most vulnerable and prone to dig in. The procedure is to use partial flap
THE NTSB INVESTIGATOR CONCLUDED THAT THE PROBABLE CAUSE OF THE ACCIDENT WAS IMPROPER SOFT-FIELD-TAKEOFF TECHNIQUE.
and to hold the nose up from the start of the roll, gradually reducing the angle of attack as the airplane lightens. This is natural in a taildragger; in a tricycle-gear airplane, it requires holding the yoke all the way back early in the roll.
While the wing is close to the ground, the induced drag due to lift, which is normally very high at low speed, is alleviated. As the wing gets farther from the ground, the drag increases. For a low-power airplane like the 150, this can be a problem: The airplane may become airborne, mushing, but be unable to climb unless it accelerates first.
The name of this paradoxical situation is “the region of reversed commands.” It means that an airplane without much excess power, flying at minimum speed, may have to lose altitude first, in order to gain enough speed to be able to go up. Pulling back on the yoke increases drag and makes the airplane sink. Ground effect removes the problem, permitting the airplane to accelerate without losing altitude. The second phase of a softfield takeoff, therefore, is to fly level, just off the ground, to pick up speed.
The 150 may have been slightly overweight — this could not be confirmed because the amount of fuel in the tanks was unknown — and may have had a slight tailwind. Nevertheless, 3,000 feet is ample distance for a 150 to become airborne and clear any normal tree.
If the pilot brought an instructor to this field to instruct him in soft-fieldtakeoff technique, it’s likely that he used that technique for what proved to be his final takeoff. It’s probable, therefore, that he began the roll with partial flap, probably 20 degrees. Flap allows the airplane to become airborne sooner but increases its parasite drag. Once airborne, therefore, he would need to raise the flaps to accelerate more rapidly, but he should not begin to raise the flaps until the airplane has at least a small safety margin above its clean stalling speed.
When the witness glimpsed the airplane over the departure end of the runway, he guessed it was at 50 feet — well out of ground effect. The “high angle of attack” he observed suggests that the pilot may already have retracted the flaps, because flap causes any airplane to assume a more nose-down attitude at a given speed.
According to the POH, the 150’s stalling speed with flaps 20 is 49 mph (the POH was written before the FAA’S adoption of the knot as the aeronautical unit of speed). Clean, it’s 55. Assume that it became airborne at 49 mph in ground effect at the midpoint of the runway. If it did not accelerate at all, it would take about 15 seconds to reach the end, but it presumably did accelerate, and got there a little sooner. The trees at the end of a runway always appear closer than they are, and the pilot probably felt pressure to get cleaned up so he could see at least 60 mph on the airspeed indicator.
Before the switch to electric actuation in the 150, you controlled the flaps with a lever between the seats. That method, however unsophisticated, gave you a direct feel of the effect of flap angle on performance. Although it is possible to bleed the flaps up a bit at a time with the electric switch, a novice is more likely to simply select flaps up and let the motor take care of the rest.
It seems likely to me that the pilot, after becoming airborne, inadvertently allowed the airplane to climb out of ground effect too early. It was still at or near its minimum speed. In that condition, it could neither accelerate nor climb. As the end of the runway approached, he must have remembered to retract the flaps. But the airplane barely had flying speed.
The 150 maintained altitude, but the trees got taller. The pilot probably succumbed to the irresistible urge to pull back on the yoke. In the region of reversed commands, however, things are backward: Pulling makes you go down, not up. And so, the 150 went down.
IN THE “REGION OF REVERSED COMMANDS,” AN AIRPLANE WITHOUT MUCH EXCESS POWER, FLYING AT MINIMUM SPEED, MAY HAVE TO LOSE ALTITUDE FIRST, IN ORDER TO GAIN ENOUGH SPEED TO GO UP.