On 27 February 2001, a Shorts SD3-60 (G-BNMT) being operated by Loganair on a scheduled domestic cargo flight from Edinburgh to Belfast Aldergrove with just the flight crew on board suffered a simultaneous and complete loss of power on both engines in day VMC shortly after take-off from Edinburgh. A MAYDAY was declared and a ditching into a rough sea state was attempted near the shoreline of the Firth of Forth but was not successful and the aircraft became submerged and neither pilot survived.
An Investigation was carried out by the UK Aircraft Accident Investigation Branch (UK) (AAIB). The FDR and CVR were recovered and successfully downloaded and the wreckage was recovered and transported the AAIB for further examination. It was found that the 58 year-old Line Training Captain who had been in command and acting as PF had accumulated 13,569 total flying hours on both rotary and fixed wing types including 972 hours on type since joining Loganair almost two years prior to the accident. The 29 year-old First Officer had joined Loganair 11 weeks prior to the accident and had been cleared for line flying 4 weeks prior to the accident. He had accumulated 438 total flying hours of which 72 hours were on type.
It was established that prior to the accident flight, the aircraft had been parked outside for 17 hours facing into strong surface winds in near freezing conditions. During this time, light to moderate snowfall had prevailed for between nine and ten hours. It was concluded that this had made it likely that a significant amount of snow had entered the engine air intakes.
The investigation found that these conditions would have been conducive to "a large build up of ice, snow or slush to occur in both plenum chambers, where it would not have been readily visible to the crew during a normal pre-flight inspection". It was noted that the severe overnight weather had abated by the time the crew had arrived at the airport in late afternoon.
It was established that the flight had proceeded normally with the aircraft commander as PF until the engine anti ice had been selected in quick succession for both engines in accordance with standard operating procedures as the aircraft passed an altitude of 2200 feet. Within 4 seconds of this selection, both engines had almost immediately begun to run down rapidly. It was noted that the aircraft had not been fitted with a continuous ignition system and there was no QRH drill for a rapid engine relight.
The aircraft had just transitioned to overwater after its take off from runway 06 and a MAYDAY had been transmitted as a turn towards the shoreline was commenced. Because of the rapid descent and the relatively high tide status, it appears that it was judged, probably correctly, that there was no likelihood of a successful forced landing and so a ditching in relatively shallow water not far from the shore was made. This had had to be made without flaps because of a lack of electrical power and without any procedural guidance for a ditching with both engines inoperative.
Although it was concluded that, in the circumstances which the crew had faced, the commander had achieved a combination of speed and aircraft attitude, a 6.8° nose up attitude and a speed of 86 knots, “that were probably the optimum under these adverse circumstances”, the ditching was not successful and the impact forces were such that “there was considerable disruption of the aircraft structure”. The submerged aircraft was found nose down some 65 metres from the shore where the water depth was around 6 metres and with the only feasible means of escape, the ‘ditching hatch’ on the roof of the flight deck, closed. It was noted that “the sea state and water temperature were such that, had the crew been able to escape from the aircraft, survival in the water for more than a few minutes would have been unlikely.”
It was noted that the ice protection system for the engine air intake comprised electrically heated mats on each intake lip together with a two-position, two-vane inertia separator driven by an electrically powered actuator which, in the anti ice ‘on’ position, allows solids and liquids to be ejected from the airflow before they are able to enter the plenum chambers and block the air intake meshes.
In respect of the sudden release of frozen contaminant into the core of the engine when engine anti ice was selected on and inertia separator vanes deployed, it was concluded that “At some stage, probably after engine ground running began, the deposits of snow, ice or slush almost certainly migrated from the plenum chambers down to the region of the anti ice vanes. Conditions in the intakes prior to takeoff are considered to have caused re-freezing of the contaminant, allowing a significant proportion to remain in a state which precluded its ingestion into the engines during taxi, takeoff and initial climb.” When the engine anti ice had been selected on, the suddenly altered air flow conditions in the air intake in the presence of snow, ice or slush in the intake system had resulted in the near simultaneous flameout of both engines.
The six Causal Factors formally identified were in relation to:
- the absence of an operator procedure for the fitting of engine blanks in adverse weather conditions and the absence of any such blanks at Edinburgh
- the accumulation of snow in the engines whilst parked prior to flight
- the air intake flow characteristics of the engines which allowed the accumulation of large quantities of snow, ice or slush in a position where it would not be readily visible during a crew pre flight inspection.
- the likely migration of accumulated deposits from the plenum chambers to the vicinity of the anti ice vanes followed by refreezing
- movement of the anti ice vanes altered the airflow and led to the released of the accumulated deposits into the engine.
- the SOPs of selecting both engine anti ice systems on together eliminated a means of defence against a simultaneous double engine failure.
Four Safety Recommendations were made as a result of the Investigation. On 6 March 2001, it was recommended that:
- that the CAA requires the manufacturer to advise all operators of the possibility of snow accumulation in the engine air intakes, when parked, subsequently resulting in engine failures. Further to advise that such a failure may be precipitated by a change of intake conditions resulting from the activation of the anti ice vanes. [2001-39]
Three further recommendations were made at the conclusion of the investigation as follows:
- that the CAA publish information to educate flight crews as to the potential hazards associated with ice, snow or slush accretion in areas of the engine intakes which are not externally visible and highlight the necessity to conduct appropriate detailed inspections when such conditions are suspected. Such information should then be promulgated widely through the industry. [2002-39]
- that Bombardier Aerospace (Short Brothers Ltd) review the following, with regard to the potential for a double engine failure:
- The Emergency Checklist, with a view to establishing a procedure for a rapid engine relight
- The provision of an Auto-ignition system, or suitable crew procedures to ensure that the Ignition systems are activated prior to the operation of intake anti-icing systems. [2002-40]
- that the CAA ensures that its safety oversight programme of AOC holders includes processes to check that operators have made suitable arrangements to provide flight crews with all necessary equipment to carry out all procedures specified in the relevant Operations Manuals. [2002-41]
Safety Action by the CAA corresponding to the recommendation made in 2001 was noted to have been taken prior to completion of the Investigation.
The Final Report of the Investigation was published in March 2003.