Summary
The FAA proposes to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 757 airplanes. This proposed AD was prompted by reports of precoolers that failed due to a wear-out condition, combined with latently failed overheat detection thermal switches. This proposed AD would require an inspection for heat damage on the engine strut structure, repetitive tests of the thermal switch temperature and ground wires, replacement of the precooler on Model 757-300 airplanes, and applicable on-condition actions. The FAA is proposing this AD to address the unsafe condition on these products.