What happened
On December 10, 2019, at 10:46 local time, a Mi-8AMT helicopter, registration JU-6600, operated by Mongol Alt LLC, departed from Chinggis Khaan International Airport in Ulaanbaatar, bound for Ulaan Ude. While cruising at an altitude of approximately 3,570 meters, the flight crew observed abnormal parameters in the right engine. Specifically, the turbine gas temperature (TGT) rose sharply from 799°C to 913° and the compressor speed dropped from 90.2% to 88.4%.
In response to these fluctuating parameters, the crew performed an in-flight shutdown of the right engine. The aircraft subsequently returned to Chinggis Khaan International Airport, landing safely at 11:08. There were no injuries to the four crew members or the four technical personnel on board, and no damage was sustained by the airframe during the emergency landing.
The investigation
The investigation examined the engine's maintenance history, flight data, and physical condition. The right engine, a TV3-117VM, had recently been installed on the aircraft in November 2019. Investigators reviewed the flight data recorder (BUR1-2), which confirmed the rapid temperature increase and subsequent drop in compressor speed.
Physical inspections of the engine's exterior and components, including the exhaust duct and free turbine blades, revealed no external damage. Laboratory analysis of the TS-1 fuel and the Turbonayl 98 oil used in the engine showed both were within normal specifications. The investigation also reviewed the maintenance records of the operator's maintenance organization, noting that while certain inspections had been performed, the full scope of periodic maintenance (Form F4-500+20) could not be fully verified.
Findings
- The right engine experienced a sudden increase in temperature and a decrease in compressor speed during cruise flight.
- Compressor vane wear likely caused an instability in the airflow, leading to the engine shutdown.
- The engine's operational life had been extended via a technical evaluation, but the reliability of the engine during this extended period could not be fully established.
- The flight crew identified the abnormal parameters but did not fully assess the actual severity of the situation before initiating the shutdown.
- Maintenance records indicated that while compressor vane wear was noted in some technical documents, it was not explicitly recorded in the primary maintenance logs.
- The operator's maintenance organization was unable to fully implement the required F4-500+20 periodic maintenance tasks.