This was meant to be a routine, unexceptional flight. So how did it end with an ejected pilot and an aircraft in pieces?
Let’s walk through the events as presented in the documentary:
In January 1995, X-31 aircraft number one was scheduled to complete its final flight, with an experienced pilot and control room team. There was one non-routine aspect of the flight: the airspeed sensor had been replaced with a new pitot probe that provided more accurate airspeed readings at high angles of attack, but was also more vulnerable to ice buildup.
The X-31 was prohibited from flying in conditions where ice could form, so the new probe also did not have its heating elements operable. Also of note is that the aircraft’s flight computer used measured airspeed as part of its control logic.
The day of the flight had high humidity at altitude, and the X-31 was operating near clouds. Upon noticing the clouds, the pilot turned pitot heat ON.
A few minutes later, he informed the control room of his action. Control room personnel recognized that pitot heat was not hooked up on the aircraft, but did not immediately alert the pilot. The pilot was then told it “may not be hooked up.” (His reaction is priceless.)
The pilot observed that the aircraft was reporting an airspeed significantly faster than should be possible at its current angle of attack—it was indicating 207 knots, instead of the 130 knots he would expect.
He then attempted to initiate a planned test via a button push. When nothing happened, the control room told him to return to base.
As the aircraft descended, the reported airspeed started to decrease from 200 knots, to 100, to 48. At this point the aircraft became uncontrollable and pitched up aggressively. The pilot ejected, leaving the X-31 to tumble the rest of the way to the ground.
What happened here? The cold, humid air allowed the air data probe to freeze over. This resulted in the system reporting an airspeed faster than truth. And as the aircraft descended, that erroneous measurement decreased.
Because the flight computer used reported airspeed to augment the controls, it began to control the aircraft as if it were flying much slower than reality. And in the end, that’s what led to the loss of the airplane.
Now we can reconstruct the links in the chain that led to this mishap.