A Boeing 737-400 was operating a scheduled flight between Tenerife (Canary Islands) and Belfast (Northern Ireland), onboard 175 passengers, 4 cabin crew and 2 pilots. This was the 27th flight, accumulating 48 flight hours after both engines on the aircraft had been changed due to a loss of oil on both engines.
Boeing 737-400 (source; Public Domain)
After a smooth take-off, the aircraft was climbing to its initial cruise flight level when the crew noticed that the oil quantity indication for the right-hand (#2) engine was dropping at a rate of approximately 1% per minute. ATC was informed and a return to Tenerife was initiated. During the subsequent descent, the #2 engine Low Oil Pressure warning came on momentarily. In compliance with current procedures, the engine was retarded to idle for the approach. Whit the engine at idle the approach was continued and an uneventful landing was made at Tenerife. After a short investigation into the cause, an oil leak was found at the scavenge pipe between the Transfer Gearbox (TGB) and the Accessory Gearbox (AGB), at the connection of the scavenge pipe to the AGB. Upon disassembly of the connection, a ring of molten yellow plastic was found. When the chip detectors of the engine were checked the chip detector housing of the AGB was found to contain a single piece of deformed and overheated yellow plastic. Both piecses of plastic together made a planking plug, normally used to blank off the scavenge pipe when not installed.
Schematic representation of the blanking plug (yellow) installed in the scavenge pipe. (source aaib.go.uk)
As the engine only operate for less than 30 seconds with the Low Oil Pressure indication activated the engine could remain on wing. The scavenge line was refitted with a new seal and the oil system was refilled. An engine test run was performed, no leaks were present and the engine operated as per Aircraft Maintenance Manual requirements. During assembly of the engine the blanking plug was not removed, the picture below shows the scavenge pipe with the plug installed, and it is obvious this type of plug can not be seen.
After the incident it was determined that the yellow plastic blanking plug lost its structural strength at temperatures around 80ºC - 90ºC. During normal operation the oil temperature of the engine is around 100ºC - 110ºC. Due to the oil system design a temporary blockage of the scavenge pipe would not cause operational issues. As the blanking plug deteriorated over time and got softer over time, it deteriorated so much that oil started leaking during the climbout after take-off from Tenerife. The full report with more details on the incident can be read by clicking here.