B5/2001L Aircraft Damage in Hailstorm West of Helsinki on 21 July 2001

On 21 July 2001 there was an aircraft incident west of Helsinki, in which an MD81 airliner operated by SAS on a scheduled flight from Stockholm to Helsinki was damaged in severe hail and turbulence while descending in a cumulonimbus cloud, at an altitude of about 4600 meters. The incident occurred about 55 km west of Helsinki-Vantaa airport. On 23 July 2001 the Accident Investigation Board (AIB), Finland, set up a commission to investigate the incident. Troubleshooting coordinator Heikki Tenhovuori was appointed as investigator-in-charge, and the other members of the commission were airline captain Lasse Seppänen, cabin safety officer Sami Sievä and meteorologist Ossi Korhonen. Moreover, the commission consulted air traffic controller Ari Huhtala as an expert, and he also assisted in writing the investigation report.

The slat panels on wing leading edges were dented by hail, as were the leading edges of horizontal and vertical stabilisers and engine inlet cowls. A hole was torn in the radome, as a result of which the radar antenna surface was also dented. The outer surfaces of windshields in front of the captain and co-pilot were cracked. The aircraft structures and engines sustained no other damage. The cabin was not damaged. All overhead stowage compartment doors remained closed and all hand baggage remained in place. There were 63 passengers and six crew members on board. None of the passengers was injured, but two cabin attendants got bruises and mild muscle strains when they fell down on the floor.

The flight crew reviewed the aviation weather information available before the flight. Weather in Helsinki and in Turku, which was selected as an alternate airport, met the applicable requirements. However, the crew failed to study the significant weather chart for the route, nor did they take it with them. This chart showed occasional embedded cumulonimbus clouds with tops at 10 600 meters and thunder for the final part of the route in south-western Finland. The pilots did not form a correct picture of the weather conditions so that they would have used the airborne weather radar. Finally the flight crew tried to switch the weather radar on right after the hail shower began. At that stage, the weather radar could not produce an image, since the hail had damaged the radome so that the radar antenna was not able to move. The total time of flying in the turbulence was about 1.5 minutes. Vertical acceleration varied between -0.23 and +2.34. The final approach and landing were uneventful.

The Aeronautical Weather Services, Southern Finland, prepared a warning message on significant weather (SIGMET) for the area. However, the warning was distributed only after the plane had flown into the hailstorm. The Investigation Commission sees that the warning message was prepared too late considering the development of weather conditions.

The Investigation Commission recommended that SAS should take appropriate actions to ensure that the pilots pay sufficient attention to weather conditions during flight preparation and in flight, and make efficient use of airborne weather radar. The Investigation Commission also recommended that the Finnish Meteorological Institute should consider creating a system to facilitate the monitoring of rapidly developing weather phenomena and preparation of warning messages on significant weather. Both SAS and the Finnish Meteorological Institute have initiated actions as a result of these recommendations.

B5/2001L Report (pdf, 0.84 Mt)