scope:

Description of Driving Event:

A shuttle orbiter Auxiliary Power Unit (APU) heater system failed in the "ON" condition in flight. A post-flight investigation determined that a heater ground-side wire shorted to the uninsulated part of a wire harness clamp. The automatic shut-off switches were located such that they did not turn off the heater due to the short eliminating the switch from the circuit for the fuel and water lines heaters. The heaters were manually switched "OFF" by the crew. The short was caused by an earlier repair that opened the wire harness. When reassembled, the heater wire separated from the bundle, dislocated into the uninsulated part of the clamp, and was pinched by the clamp when it was tightened. Subsequent wear and tear resulted in premature failure of the wire insulation resulting in the short. Had this anomaly remained undetected or manual shut-off not been achieved within five minutes of the occurrence, the hydrazine fuel could have heated to the explosive auto-decomposition temperature within the line. The APU would then have failed and effects of the explosion could potentially have resulted in severe damage to the orbiter. It was further determined that under some failure modes, a shorted condition could remain undetected by both the automatic cut-off sensors and the temperature indicators that alert the crew or ground controllers of over-temperature conditions. Corrective actions included: (1) tying the wire bundles together before tightening the clamp, (2) using a clamp that has all surfaces insulated, and (3) a review of the electrical system design for relocation of temperature sensors.