Incident Overview
While in cruise flight, the 'RIGHT REVERSER ACCUMULATOR LOW' caution light illuminated. Following landing, examination disclosed that the auxiliary power unit (APU) turbine wheel had separated in an uncontained failure. The separation caused damage to numerous components in the compartment below the number two engine pylon.
APU Status and Investigation
The APU had been placarded as inoperative two days prior to the incident. The investigation did not determine when an inadvertent attempt to start the APU had occurred. However, it did reveal that no procedures, other than the inoperative placard, were in place to prevent start attempts.