On 24 April 2010, a Bombardier DHC8-100 operated by Olympic Airways which had, some weeks earlier, been flown to the UK for heavy maintenance at Exeter was positioning from East Midlands to Exeter in day Visual Meteorological Conditions (VMC) with just the two flight crew on board when it experienced a significant oil loss from one engine en route and responded by shutting it down and declaring a ‘PAN’ to ATC for radar vectors direct to destination. The remaining engine was then found to be losing oil, and the declared status was upgraded to a MAYDAY and a successful diversion to the nearest suitable airfield, Bristol, was made.
An Investigation was carried out by the UK AAIB. It was established that the aircraft had not flown between July 2009 and March 2010 and early in 2010, two new engines had been fitted to the aircraft and it had been flown from Athens to Exeter on 18 March 2010 for a ‘C’ Check by Flybe Engineering during which both engine oil coolers had been removed and refitted.
The previous (uneventful) flight to the incident one had been a non-revenue positioning flight eight days earlier from Exeter to East Midlands after completion of the ‘C’ Check and was to enable re-painting at East Midlands.
It was concluded that oil had already been weeping slowly from both oil coolers when the aircraft arrived at East Midlands Airport on 16 April, and that it had continued to do so during the ensuing week, manifesting itself in oil drops underneath the nacelle butt joints on either side of the oil coolers. However, these leaks had appeared to have stopped or slowed just before the aircraft departed, only restarting (unknown to the crew) as the aircraft taxied out and then leaking at a higher rate during the incident flight. It was estimated that the engines lost about 5.5 litres and 3.5 litres of oil respectively during flight. It was noted that the oil cooler fittings had not leaked at this rate during the earlier flight from Exeter to East Midlands and yet there was no evidence that the oil coolers had been disturbed while the aircraft was at East Midlands. It was therefore concluded that either engine vibration or loads imposed during the landing at East Midlands had slightly shifted the position of the oil pipes so that the effect of the cuts in the O-ring seals were exacerbated. Shortly after landing there, the engines had been shut down and so, with reduced oil pressure, any oil leak would have slowed. It was considered probable that any oil leak during the low power engine run carried out prior to departure on the incident flight at the request of the supervising engineer would have only been apparent had the lower forward cowlings been opened and the oil coolers inspected. The two minutes for which the engine run was carried out was considered unlikely to have been sufficient time for the oil to appear externally.
The Investigation concluded that the oil leaks from both engines had been caused by damage to O-ring seals at the respective oil cooler fittings during re-installation of the oil coolers after their removal during the ‘C’ Check. It was also considered that a number of factors had led to this damage and to the subsequent failure to carry out oil leak checks prior to release to service.
It was noted that Flybe Engineering had implemented a ‘Critical Task Checklist’ for each aircraft type that it maintained after a CAA Audit in 2009 had found that there was no such list. As introduced, it had required that specified tasks which could present a risk of multiple errors on critical systems when carried out at the same time as a similar task by the same personnel should be “separated by at least one flight, or carried out by different personnel, or if this cannot be achieved, a re-inspection of the work after completion of all similar tasks should be completed and worksheets annotated accordingly”. However, it was also found that engine oil cooler removal / refit was not on the list. It was also noted that awareness of the new list was limited.
The Investigation also examined the long working hours recorded by maintenance personnel responsible for overseeing the ‘C’ Check and noted the considerable opportunities for the effects of fatigue to manifest themselves.
Six Safety Recommendations were therefore made as a result of the Investigation to the effect that:
- Flybe Aviation Services revise their practices and procedures to ensure that their repair instructions are adequately detailed and specify the necessary access and removal requirements. (2011-014)
- Bombardier Inc. amend the Aircraft Maintenance Manual for the DHC-8-100 series aircraft to emphasise the correct procedure for securing the inlet and outlet pipes to the engine oil coolers, including the method for tightening the associated knurled nuts. (2011-015)
- Flybe Aviation Services review their defect rectification processes to ensure that important safety checks, such as oil leak checks, are not omitted. (2011-016)
- Flybe Aviation Services remind all staff of the importance of investigating the source of every engine oil leak. (2011-017)
- EASA expand the advisory or guidance material in Annex II (Part 145) of European Commission Regulation (EC) No. 2042/2003 on how approved maintenance organisations should manage and monitor the risk of maintenance engineer fatigue as part of their requirement to take human performance limitations into account. (2011-018)
- The CAA include the following areas in their Part 145 audits of Flybe Aviation Services:
- practices and procedures for detailing repair instructions,
- identification of safety critical tasks,
- planning of defect rectification and
- management of maintenance engineer fatigue. (2011-019)
The Final Report AAIB Bulletin: 6/2011 EW/C2010/04/03 of the Investigation was published on 9 June 2011