The LOGANAIR flight G-BNMT accident was notified to the Air Accidents Investigation Branch (AAIB) at 1744 hrs on 27 February 2001 by Air Traffic Control, Edinburgh Airport. The investigation was conducted by: Mr D King (Investigator-in-Charge), Miss G M Dean (Operations), Mr P R Coombs (Engineering) and Mr R James (Flight Recorders). An AAIB Special Bulletin (Number S1/2001), containing preliminary information about the accident, was published on 1 May 2001. During the course of the investigation, Mr P D Gilmartin was appointed to replace Mr King as Investigator-in-Charge.
A LOGANAIR flight G-BNMT crew of two was operating the aircraft on a scheduled mail service from Edinburgh Airport to Belfast International Airport, with 1,040 kg of cargo aboard. The aircraft, a twin engined turboprop type, suffered a double engine flameout shortly after takeoff. The flight crew ditched the aircraft in shallow water in the Firth of Forth, close to the shoreline. The aircraft was severely damaged on impact with the water and the forward fuselage section became submerged. Neither crew member survived.
For some 17 hours prior to the accident, the aircraft had been parked on a north-easterly heading, facing into the prevailing strong surface winds, in near freezing conditions. In addition, overnight, there was light to moderate snowfall and drifting.
LOGANAIR flight G-BNMT had been parked with the engine air intakes unprotected from snow ingestion. Thus, there was an opportunity for a significant amount of snow to enter the engine air intake systems.
Tests showed that conditions were ideal for 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.
The investigation established that, following a selection by the LOGANAIR flight G-BNMT of the anti-icing systems on the aircraft, specifically the selection of the intake anti-ice vanes, the subsequent movement of the vanes precipitated the near simultaneous engine flameouts. Interaction between the moving vanes and the residual ice, snow or slush contamination in both intake systems is considered to be the most likely cause of the engine failures.
The investigation identified the following causal factors:
- The operator did not have an established practical procedure for flight crews to fit engine intake blanks (‘bungs’) in adverse weather conditions. This meant that the advice contained in the aircraft manufacturer’s Maintenance Manual ‘Freezing weather – precautions’ was not complied with. Furthermore intake blanks were not provided on the aircraft nor were any readily available at Edinburgh Airport.
- A significant amount of snow almost certainly entered into the engine air intakes as a result of the aircraft being parked heading directly into strong surface winds during conditions of light to moderate snowfall overnight.
- The flow characteristics of the engine intake system most probably allowed large volumes of snow, ice or slush to accumulate in areas where it would not have been readily visible to the crew during a normal pre-flight inspection.
- 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 intake 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.
- Movement of the intake anti-icing vanes, acting in conjunction with the presence of snow, ice or slush in the intake systems, altered the engine intake air flow conditions and resulted in the near simultaneous flameout of both engines.
- The standard operating procedure of selecting both intake anti-ice vane switches simultaneously, rather than sequentially with a time interval, eliminated a valuable means of protection against a simultaneous double engine flameout.