What happened
A Boeing 747SP was being moved from a terminal gate to a maintenance facility via taxi. The aircraft was operated by two mechanics, one of whom lacked the specific qualification required to taxi this type of aircraft. During the movement, the number four engine experienced a hung start and remained non-operational throughout the taxi phase.
Because the number four engine was not running, its associated hydraulic system was switched off. This system normally provides primary power for the aircraft's brake system. Additionally, the inertial navigation system (INS), which is required to provide speed reference during ground operations, was also turned off. As a result, the 747SP accelerated to excessive speeds on the parallel taxiway. When the mechanic attempted to stop the aircraft, there was no hydraulic power available to apply the brakes. The right wing of the 747SP struck the tail section of a Boeing 737, which was lined up on the adjacent taxiway awaiting departure.
The investigation
The airline maintenance supervisor overseeing the 747SP was aware that one of the mechanics was not qualified to taxi the aircraft and knew that the number four engine was not operating. Despite these known issues, the decision was made to proceed with the taxi contrary to established procedures.
Findings
The collision was caused by excessive ground speed due to a lack of braking capability. Contributing factors included the operation of the aircraft by an unqualified mechanic, the disablement of the primary brake hydraulic system, and the deactivation of the speed reference system. The maintenance supervisor's decision to allow the taxi despite known procedural violations played a critical role in the accident.