What happened
On April 14, 1999, a Gates Learjet 55, registration OH-IPP, was performing a private flight from Helsinki to Saint Petersburg. After a brief stop in Helsinki for refueling and inspection, the aircraft departed at 21:56 local time. The flight proceeded normally through the initial climb and cruise phases.
While cruising at approximately 23,000 feet over the Gulf of Finland, the aircraft experienced an unexpected, violent explosion and intense vibration originating from the rear of the aircraft. The right engine immediately failed, with Interstage Turbine Temperature (ITT) and rotational speed (N1 and N2) dropping to zero. There were no prior warning lights or fire alarms. The crew followed standard shutdown procedures, using the high-pressure fuel valve to secure the engine, and successfully returned to Helsinki-Vantaa, landing on runway 04 using only one engine. There were no fatalities and no injuries to the two crew members or three passengers on board.
The investigation
An investigation conducted by OTKES, with technical assistance from the NTSB in the United States, focused on the physical destruction of the right engine. Upon inspection at Helsinki-Vandanta, investigators found that the engine cowling had been torn outward by debris.
Detailed teardown inspections revealed that the low-pressure turbine (LPT) had suffered catastrophic damage. While the first-stage LPT disk remained intact, the second and third-stage LPT disks had been ejected from the engine through the turbine casing. The investigation also found that the LPT shaft and connecting bolt had fractured, with the bolt showing signs of significant unilateral bending force. Metallurgical analysis of the fracture surfaces was inconclusive due to the extent of the damage, though evidence of overload was present.
Findings
- The primary cause of the incident was a failure within the low-pressure turbine.
- The specific failure mechanism could not be definitively determined because the critical components—the second and third-stage LPT disks—were ejected from the engine and lost in the Gulf of Finland.
- The destruction of the engine was an uncontained event, where turbine components breached the engine casing.
- While the sixth bearing and its seals were also destroyed, investigators considered the likelihood of this being the initiating factor to be low, as the oil system and filters remained clean.
- The engine had previously undergone maintenance in 1997, during which some LPT vanes had been repaired following domestic object damage (DOD).