Incident Description
While on short final to land at approximately 200 feet above ground level, the APU fire warning light illuminated. The crew performed the appropriate procedures, which resulted in the fire warning light extinguishing. After landing, the crew requested that the fire department visually inspect the aircraft and follow it to the gate, where passengers were offloaded in the normal fashion.
Examination and Findings
Examination and testing of the APU revealed that a fire had occurred in the APU exhaust. The fire was a result of oil entering the hot exhaust gas stream due to leakage around the aft turbine bearing seal. The seal evidenced scoring around its circumference, and the seal surface had been treated with Metco spray in accordance with the maintenance manual.
Maintenance Background
Due to a previous history of oil leakage around seals treated with Metco Spray, in 1988 Allied Signal changed the treatment to Tungsten carbide, which eliminated the leakage problem. However, because some operators experienced difficulty in obtaining the Tungsten carbide coating, the Metco spray coating procedure was left in the overhaul manual.