Incident Overview
On 4 August 2016, a Fokker F27 Mk50 (registration JU-8258) operated by Aero Mongolia LLC experienced an in-flight shutdown of its right engine approximately 28 minutes after takeoff from Degli Tsagaan Airport in Ulaangom, Mongolia. The aircraft was on a scheduled passenger flight with 4 crew and 40 passengers on board. No injuries were sustained, and the aircraft returned to Ulaangom without further incident.
Flight History
The flight departed Ulaangom at 09:25 UTC (18:25 local time) en route to Ulaanbaatar. At an altitude of 5700 m (about 09:47 UTC), the right engine oil pressure indicated 53 psid, in the yellow band. The captain instructed the co-pilot to consult the Quick Reference Handbook (QRH) and asked the senior flight attendant to visually check for oil leakage from the right engine. At 09:48 UTC, the senior flight attendant reported no visible oil leak, but oil pressure fluctuated between 45 and 50 psid. The crew followed the QRH procedure for oil pressure not in the green band. Oil pressure then began to decrease slowly from 45 psid. The crew contacted Altai air traffic control to declare a return to Ulaangom. At 09:54 UTC, when oil pressure dropped below 41 psid, the right engine was shut down. The aircraft landed safely at Ulaangom at 10:16 UTC (19:16 local time) and parked at 10:17 UTC.
Investigation and Findings
The Aircraft Accident Investigation Bureau (AAIB) of Mongolia conducted an investigation. A post-incident inspection on 5 August 2016 found no external damage to the airframe or engine. There were no signs of oil leakage from the engine exhaust or breather tube. Traces of oil were found on the inside of the lower engine cowl. The scavenge oil filter and main oil filter indicator were normal. Chip detection indicators on the Maintenance Test Panel were normal.
Analysis of maintenance records revealed that the right engine (serial number 124064) had a history of high oil consumption. The fuel pump gearshaft seal (part number 3022375) had been replaced shortly before the incident. The investigation determined that the seal was not installed correctly according to the manufacturer's instructions.
Probable Cause
The AAIB determined that the probable cause of the incident was that the fuel pump gearshaft seal on the right engine was not properly installed in accordance with the manufacturer's instructions, likely causing oil leakage that led to a drop in oil pressure and subsequent engine shutdown.
Safety Recommendations
The investigation issued safety recommendations to prevent recurrence. These recommendations addressed maintenance procedures and oversight related to fuel pump seal installation.