Executive Summary
On 23 July 2017, an ATR72-212A aircraft, registration JU-8802, operated by Hunnu Air LLC, experienced an in-flight shutdown of the left engine while cruising at 22,600 feet (6,900 m) on a flight from Ulaangom to Ulaanbaatar. The flight crew shut down the left engine and feathered the propeller. Due to icing conditions and proximity, the captain decided to divert to Mörön airport, where the aircraft landed normally at 12:10 UTC. No injuries were reported among the 5 crew and 70 passengers, and the aircraft sustained no damage.
History of the Flight
On the morning of 23 July 2017, the aircraft operated flights from Ulaanbaatar to Oyu Tolgoi, Dalanzadgad, and back, followed by a flight from Ulaanbaatar to Khovd and Ulaangom. At 10:31 UTC, it departed Ulaangom (Degli Tsagaan airport) for Ulaanbaatar. At 10:56, it reached 6,300 m and entered cruise. Due to clouds, the crew climbed to 6,900 m around 11:10. At 11:26, they activated propeller, horns, and side window anti-icing systems, and the icing AOA system activated. At 11:44 UTC, while both engines were at power lever angle 74°, the left engine parameters dropped sharply: fuel flow from 288 to 54 lb/h, torque from 58% to 3%, ITT from 658°C to 362°C, Nh from 95% to 71%, Nl from 93% to 51%, while Np remained at 82% and propeller blade angle decreased from 42° to 31°. The crew reduced the left throttle lever and set the condition lever to fuel shut off. Ice detection activated at 11:52, and the airframe de-ice system was engaged. The crew disabled anti-icing systems at 11:59. The master warning did not activate. The aircraft landed at Mörön at 12:10 UTC.
Investigation Findings
The investigation, conducted by the Aviation Accident Investigation Bureau of Mongolia, with participation from BEA (France) and TSB (Canada), examined the left engine's hydromechanical unit (HMU). Post-incident maintenance identified that the HMU, part number 3244871-7 and serial number C388826V, was defective. The manufacturer Honeywell performed a detailed examination and provided a report. The investigation also reviewed the flight operations after the incident.
Probable Cause
The probable cause of the incident was a defective hydromechanical unit (HMU) on the left engine, which led to the in-flight shutdown.