1、Lessons Learned Entry: 0017Lesson Info:a71 Lesson Number: 0017a71 Lesson Date: 1991-07-29a71 Submitting Organization: JSCa71 Submitted by: R. S. SeiwellSubject: Shorted Wires (From Pinching by an Uninsulated Harness Clamp) due to Reassembly Errors and a Poorly Located Cut Off Switch Resulting in a H
2、eater “ON“ Condition of an Orbiter APU Fuel Line 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 cl
3、amp. 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
4、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 rem
5、ained 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 dam
6、age 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) tyi
7、ng 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.Lesson(s) Learned: A reassembly error (a dislocated wire pinched by the uninsulated area of a harness cl
8、amp) causing a short in a fuel line heater and a poorly located cut off switch can combine to cause a control failure, and lead to potentially catastrophic damage.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Recommendation(s): Changes recommended
9、to systems or procedures included tieing of wire bundles before clamping, using totally insulated clamps, and relocating automatic cut-off switches.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 Space Operationsa71
10、Aeronautics ResearchAdditional Key Phrase(s): a71 Energetic Materials - Explosive/Propellant/Pyrotechnica71 Flight Operationsa71 Flight Equipmenta71 Hardwarea71 Launch Vehiclea71 Parts Materials & Processesa71 SpacecraftAdditional Info: Approval Info: a71 Approval Date: 1993-09-10a71 Approval Name: Ron Montaguea71 Approval Organization: JSC/NS3a71 Approval Phone Number: 281-483-8576Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
