Event
Shortly after takeoff, while the aircraft was climbing through approximately 4,000 feet, the auxiliary power unit (APU) fire warning light illuminated in the flight deck. In response, the flight crew discharged the fire extinguishing units and requested an emergency landing.
Investigation
Following the incident, a teardown inspection of the APU was conducted. The examination revealed evidence that a hard object had been ingested into the first-stage compressor impeller, resulting in the bending of two of its blades. The object then continued through the engine and impacted the exducer blade. The exducer blade failed at a pre-existing crack and subsequently penetrated the exhaust pipe assembly. As a consequence, hot gases escaped into the cooling air passage, causing overheating of the tail cone and the aft portion of the APU.
Findings
The teardown did not identify the specific nature or source of the ingested hard object. The examination confirmed that the blade failure and subsequent gas leakage led to the fire warning. No other anomalies were reported in the APU system.