Human Systems Integration Division @ NASA Ames

The X-15 was a critical research vehicle in the early days of space flight. On November 15, 1967, the X-15-3 suffered an in-flight breakup. It was the 191st flight of the X-15 and the 65th flight of this third configuration (X-15-3). It was the only fatal accident of the X-15 program. This paper presents an analysis, from a human factors perspective, of the events that led up to the accident. The analysis is based on the information contained in the report of the Air Force-NASA Accident Investigation Board (AIB) dated January 1968.

The X-15-3 was the only configuration of the X-15 equipped with an early experimental adaptive flight control system (the Minneapolis Honeywell MH-96) that provided automated variation of the gains of the vehicle's controls as a function of the dynamic pressure. During a critical phase of the flight, the MH-96 had been malfunctioning for 3 minutes before the pilot became aware of his inability to control the vehicle. The AIB's analysis addressed, primarily, the events that occurred subsequent to the pilot's switching from controlling the vehicle through the MH-96 Adaptive Flight Control System (by using the right-hand control stick) to direct control of the reaction control system (by using the left-hand control stick). The analysis described here suggests that all of the events that caused the accident occurred well before the moment when the pilot switched to direct control. Under the given conditions, by the time the pilot recognized the need to switch to direct control, the destruction of the vehicle was inevitable. Consequently, the analyses and conclusions regarding the causal factors of, and the contributing factors to, the loss of Flight 3-65 presented in this paper differ from those of the AIB based on the same evidence.

Although the X-15 accident occurred in 1967, the results of the presented analysis are as relevant today as they were 47 years ago. We present the main points of our analysis and discuss their implications for the safety of space operations.