An Example of the Behavior of an Aircraft with Accumulated Ice: Latent Instability

Abstract

Many directly measured aircraft performance details related to the unstable behavior of the Desert Research Institute's (DRI) research aircraft after ice accumulation, which led directly to its crash were recorded on its final flight. The data system, with the fully gimballed inertial platform, remained fully operational during the flight, including the final spiraling dive, with negative (upside down) accelerators. The observations show a reduced lift effect involving transition to what seems to be partial stall on the inboard wing. This effect induced, at onset, a reduction of the lift coefficient at a higher angle of attack and at a greater airspeed than was consistent with flight measurements before and after. When normal conditions were temporarily reestablished, lift returned. This anomalous behavior appears to have produced an equivalent to control reversal in pitch, in which forward pressure on the control column could have induced increased lift and a nose up response. This seems to have led to an extreme nose up climb, followed by stall and a deep negative angle of incidence spiral dive, from which recovery could not be effected. The evolution of such instability does not appear to be widely understood and seems to be an apt topic for further investigation. The possibility of its occurrence seems to be a point requiring a cautionary note.