UK investigators recommended the tightening of maintenance procedures at Flybe Aviation Services after determining that several weaknesses led to a Bombardier Dash 8-100 suffering a serious in-flight oil leak in both engines last year.
Having been dormant in Greece for eight months the Olympic Air aircraft had been flown to Exeter for a C-check, during which both oil coolers were removed - unnecessarily, as it turned out - and refitted incorrectly, resulting in O-ring seals being damaged.
This operation had not been identified as safety critical, says the UK Air Accidents Investigation Branch.
"If a person makes an error while disturbing the oil system on one engine, and then repeats the error on the other engine, the safety of flight of a twin-engined aircraft can be compromised," it adds.
The Dash 8, registered SX-BIO, was then flown to East Midlands for repainting. But 10min into the return flight to Exeter on 24 April 2010 the crew observed a "major" oil leak from the right engine. As the oil pressure fell the pilots shut the engine down.
The left engine's oil pressure then began to fluctuate a few minutes later, and it also showed evidence of an oil leak. After declaring an emergency the crew diverted to Bristol.
Examination of the aircraft showed that its right engine had lost 5.5 litres of oil and the left had lost 3.5 litres. The engines each had a 19-litre capacity, and had contained 17 litres on departure.
AAIB investigators found that requirements for a leak check had been omitted from the maintenance job card at Exeter, and while there was evidence of leaking oil during subsequent tests, the problem went undetected.
"Incorrect diagnosis that the slow oil seepage from both engine nacelles was residual oil from a previous leak led to the source of the leaks not being fully investigated at East Midlands, says the AAIB.
"It is important that the source of any oil leak, even if seemingly very minor, is correctly identified and rectified."
In its report into the incident the AAIB also hints that fatigue within the maintenance organisation may have contributed.