What happened
On 24 April 2010, a Bombardier DHC-8-102, registration SX-BIO, was operating a private flight from Exeter to East Midlands. During the flight, the crew observed a significant oil leak from the right engine, which caused oil pressure to fluctuate and eventually drop. Following established procedures, the crew shut down the right engine. Shortly after, the left engine also began exhibiting fluctuating oil pressure, and the crew identified an oil leak from the left engine as well. The commander initiated a diversion to Bristol International Airport, landing the aircraft safely.
The investigation
An investigation by the AAIB examined the maintenance history of the aircraft following a base maintenance check at Exeter. It was discovered that the engine oil coolers had been removed and reinstalled to facilitate repairs to landing gear door pivot brackets. The investigation found that the installation process was difficult due to the confined space, and the technician used a torch held in his mouth to provide light.
Upon inspection of the aircraft at Bristol, oil was found coating the landing gear legs and engine nacelles. Further examination of the oil coolers revealed that the O-ring seals had been compromised. Specifically, on the left engine, a small O-ring had been split and an incorrect size O-ring had been used, while on the right engine, the larger O-ring seal had been cut. Additionally, evidence suggested that the knurled nuts on the pipe fittings had been over-tightened using pliers, rather than being tightened by hand as required by the maintenance manual.
Findings
- The oil leaks on both engines were caused by damaged O-ring seals at the oil cooler fittings.
- The damage likely occurred during the re-installation of the oil coolers during base maintenance.
- The maintenance process involved the use of incorrect O-ring sizes and the use of tools (pliers) that caused over-tightening of the fittings.
- A lack of specific instructions for oil leak checks following the re-installation of the coolers contributed to the failure to detect the leak during ground runs at East Midlands.
- Maintenance personnel were working under time pressure to meet a scheduled painting slot.