Overview
On January 24, 2013, during a post-overhaul test flight of a Piaggio P180 Avanti II at Genoa Cristoforo Colombo Airport (LIMJ), the left engine (serial number PCE-RK0104) was shut down at FL200 and then restarted. Upon restart, the oil pressure warning light illuminated and pressure dropped below the acceptable limit. The engine was shut down again, and the aircraft landed on the right engine.
Sequence of Events
- January 24, 2013: During the test flight, the left engine was intentionally shut down at FL200. After restart, oil pressure was low. Landing was performed on the right engine.
- January 29, 2013: A second test flight conducted all planned tests except engine shutdown and restart in flight; all parameters were normal.
- January 30, 2013: Oil leaks were observed on the left engine after the previous day's flight.
- January 31, 2013: Ground engine run with cowlings removed showed no further leaks.
- February 1, 2013: During another flight, restarting the left engine in air again caused oil pressure to rise then drop to 30 psi. Landing was performed on the right engine.
- February 2-3, 2013: Additional checks of engine components found no anomalies.
- February 12-26, 2013: The engine was removed and tested on a dynamometer for about 2.5 hours; no faults were found. Additional inspections of the oil system and other components revealed no issues.
- March 10, 2013: After reinstalling the engine, a test flight with video recording showed the left engine's oil pressure low light illuminated. Video analysis later indicated an uncommanded engine shutdown due to fuel flow drop, followed by drops in oil pressure, torque, generator speed, and inter-turbine temperature.
- March 14, 2013: After replacing the Fuel Control Unit (FCU), a test flight at FL150 (instead of FL200) allowed successful engine restart.
- March 18, 2013: Replacement of Fuel Heater, Flow Divider, and Starter Generator did not resolve the issue.
- March 21, 2013: Compressor Bleed Valve replacement also unsuccessful.
- May 16-29, 2013: The engine was removed again and sent to the maintenance organization. Corrective actions focused on increasing the compression ratio and reducing turbine speed.
- July 3, 2013: After installation on aircraft SP-MXH (the engine had previously been on SP-MXI), a test flight showed normal operation in all phases, including at FL200.
Investigation Findings
The maintenance organization's final tests showed the engine met all criteria in the Overhaul Manual. However, compressor efficiency was lower than that of a new engine, which is normal due to wear. In this specific case, this degradation may have influenced the difficulties encountered when restarting the engine at FL200. The final corrective actions (increasing compression ratio to about 7.4 and reducing turbine speed to 35150 RPM, including sealing the compressor turbine stator and adjusting thermocouple trimming) resolved the issue.
Probable Cause
Lower efficiency of the engine compressor resulting from wear during operation.
Remarks
Two subsequent incidents involving the same engine (serial number PCE-RK0104) occurred on July 21, 2014, and August 20, 2014, with similar inability to restart at FL200. Their probable cause was an overly lean fuel-air mixture provided by the FCU. The investigation noted that successful restart at FL150 (rather than FL200) could be a non-procedural alternative to landing on one engine.