The school had a second A36. The two airplanes were similar in most respects, but one — the accident airplane — had an altitude-compensating fuel pump and the other did not. In principle, the one with the altitude-compensating pump would take off at a full rich mixture setting, regardless of density altitude. The NTSB suggested that the accident airplane might have been leaned anyway, robbing it of power. But instructors who used both airplanes said that they leaned them both in the same way, setting the EGT at 40 degrees F rich of peak (“best power mixture”). This was logical, regardless of the type of fuel pump. An instructor who had flown with the instructor on the accident flight said that her technique was the same, and that she was “very thorough and diligently followed company rules regarding takeoff and performance data on the company form.” Their standard takeoff procedure, incidentally, included a 60-knot call-out and response.
In order to ascertain whether the compensating fuel pump could have played a role in the accident, an NTSB pilot flew a number of takeoffs under conditions similar to those of the accident and in a similarly equipped A36. The effects of leaning were negligible. In fact, the only way the pilot found to seriously compromise the airplane’s takeoff performance was to reduce power to 2,300 rpm and 23 in Hg. At that setting, the airplane would not accelerate to flying speed with full flaps; but with takeoff flaps it used only 500 feet of extra runway.
Notwithstanding its own tests, the NTSB decided that the probable cause was “a partial loss of engine power due to the certified flight instructor’s failure to comply with the pilot operating handbook requirements for the mixture setting during takeoff. Also causal was the instructor’s inadequate supervision of the flight, failure to monitor the airplane’s performance, and failure to initiate an aborted takeoff in a timely way.”
It’s hard to imagine that something did not feel strange to the instructor at the very least, even supposing that the 174-hour student was oblivious. If one were programming a computer to handle the takeoff, it would monitor speed and distance covered and would abort the takeoff if rotation speed were not reached in the expected distance. It would do this more precisely than a human being could, but even a mere human can make a mental note of the location of landmarks along the runway and judge whether the takeoff is progressing as expected.