What happened
Between January and July 2013, a Piaggio P180 Avanti II underwent a series of flight tests following a scheduled maintenance overhaul. During a flight test on January 24, 2013, the pilot attempted to shut down and restart the left engine at FL200. Upon restart, the oil pressure dropped below permissible limits, forcing the crew to land on the remaining engine.
Subsequent flight tests on February 1, 2013, repeated the issue, with the oil pressure dropping to 30 psi during the restart attempt. Following several unsuccessful attempts to resolve the issue through component replacements—including the Fuel Control Unit (FCU), Fuel Heater, Flow Divider, Starter Generator, and Compressor Bleed Valve—the engine (serial number PCE-RK0104) was removed and sent to a maintenance facility for specialized work. After modifications were performed to increase the compression ratio and reduce turbine speed, a flight test on July 3, 2013, confirmed the engine functioned correctly even during the previously critical flight phases.
The investigation
The investigation was based on reports from the manufacturer and the aircraft operator, as well as materials provided by the engine manufacturer and the maintenance organization. The investigators examined the engine's performance during ground tests and analyzed flight data and video recordings of the cockpit instruments. The investigation also considered subsequent related incidents involving the same engine serial number, which involved difficulties restarting the engine at FL200 due to lean fuel-air mixtures.
Findings
- The primary cause of the engine restart difficulties was reduced compressor efficiency resulting from operational wear and tear.
- While the engine met all requirements of the Overhaul Manual, its efficiency was lower than that of a brand-new engine, which impacted performance at higher altitudes (FL200).
- Corrective actions, including sealing the compressor turbine stator and adjusting thermocouple trimming, successfully restored the necessary compression ratio.
- The investigation noted that while the engine could be successfully restarted at FL150, this does not serve as a procedural alternative to landing on a single engine when failures occur at higher altitudes.