Sequence of Events
Finnair Flight AY217 was operating a scheduled domestic service from Helsinki to Mariehamn with an intermediate stop at Turku. The flight to Turku proceeded without incident. The aircraft departed Turku at 1620 GMT with a crew of three, 21 passengers, and an additional individual—a former Finnair pilot—who was present in the cockpit without a ticket or permit.
The leg from Turku to Mariehamn was flown at an altitude of 2 000 ft, and the crew reported no abnormalities. The approach to Mariehamn was conducted using the Mariehamn NDB (MAR), from an east-northeast direction. At 1657 hours, the aircraft reported over the NDB on the inbound track for runway 20.
During the final approach, while maintaining the inbound track, the aircraft struck trees in a nearly horizontal attitude approximately 1,470 meters before the runway threshold. Upon impact, the aircraft flipped onto its back around its longitudinal axis and immediately caught fire. The accident occurred at 1659 hours. One crew member and two passengers sustained serious injuries; the other 22 occupants were killed.
Investigation and Probable Cause
The investigation board determined that the probable cause of the accident was the pilot's misconception of his altitude. It could not be established whether this misconception stemmed from an erroneous altimeter indication or from human error. A defect was found in the pilot-in-command's altimeter, which was deemed sufficient to have produced an incorrect altitude reading that the pilot could not have anticipated. Consequently, the formal cause was stated as: during an instrument approach conducted in weather conditions worse than the approved minima for runway 20 at Mariehamn, a wrong indication from the pilot's altimeter caused the aircraft to descend below the safe altitude and collide with trees on the approach path.