What aircraft was involved and where did it happen?

The accident on 2010-01-03 involved a MD-82,, registration I-SMEB, at Aeroporto di Torino Caselle, IT.

What was the probable cause of the accident?

The APU starter relay contacts welded in the closed position, causing the starter motor to remain energized far beyond its operational limits, which led to overheating and an incipient fire.

APU Starter Relay Failure Causes Fire Risk on MD-82

A malfunctioning starter relay caused an electrical fire in the APU compartment of a McDonnell Douglas MD-82 during ground operations at Turin Caselle.

What happened

On January 3, 2010, a McDonnell Douglas MD-82, registration I-SMEB, was at Turin Caselle Airport awaiting passenger boarding. During ground operations, cabin crew reported a strong, acrid odor emanating from the rear of the aircraft. Upon inspection of the Auxiliary Power Unit (APU) compartment, a maintenance technician discovered smoke and sparks originating from the APU starter.

The technician immediately requested the flight crew to shut down the APU. Working with a second technician, the ground crew used nearby fire extinguishers to suppress the incipient fire. The flight crew followed emergency procedures, instructing the cabin crew to evacuate the aircraft while the pilots deactivated the APU. The incident resulted in damage to the APU starter and adjacent electrical components, but there were no injuries and no fatalities.

The investigation

The ANSV investigation, supported by the NTSB, focused on the electrical components of the APU starter system. Laboratory analysis of the APU starter relay (R2-63) conducted by the NTSB revealed that the internal contacts (A1 and A2) had become deformed and heavily damaged. Specifically, the A1 contact showed significant loss of its original geometry and material degradation, with traces of silver and tungsten found on its surface.

Further examination of the APU starter itself showed evidence of severe overheating, with melted material both inside and outside the unit, and a seized output shaft. The investigation also reviewed Boeing's historical data, noting that similar failures had been reported on DC-9, MD-80, and MD-90 aircraft, particularly those using specific APU models.

Findings

The primary cause of the event was the APU starter relay remaining electrically closed even after the engine reached 35% RPM.
This failure prevented the centrifugal switch from disconnecting the power, causing the APU starter to be powered for an excessive period beyond its designed duty cycle.
The prolonged electrical current caused the contacts to overheat and eventually weld together, leading to the electrical fire.
The operator had not yet implemented the manufacturer's previous suggestion to replace the older relay design with an improved version (P/N AHJS-CXA-40 or P/N A-400G-2) to mitigate such risks.
The crew's ability to monitor the battery voltage for signs of starter disengagement was likely compromised by the need to focus on taxiing and ground collision avoidance.