What happened
On July 21, 2001, a Scandinavian Airlines (SAS) flight, operating as SAS1700, encountered severe weather while en route from Stockholm Arlanda to Helsinki-Vantaa. The aircraft, a Douglas DC-9-81 (registration LN-RMT), flew into a heavy hailstorm characterized by large hail stones measuring over 5 cm in diameter.
As the aircraft entered the cell, it experienced intense turbulence. The impact of the hail caused the aircraft's radome to break, the cockpit windshields to crack, and significant denting to the engine inlets, wing slats, and the leading edges of the stabilizers. During the encounter, the aircraft's autopilot and autothrottle disconnected, and the crew received terrain warnings from the GPWS. Despite the intense noise from the hail and the crew's observation of a vibrating sound in the engines, the aircraft was able to continue the flight and land normally at Helsinki-Vantaa.
The investigation
The Finnish Safety Investigation Authority (OTKES) examined the flight preparations, the weather conditions, and the crew's actions during the encounter. The investigation reviewed meteorological data, including the Nordic SW-chart and the aircraft's Quick Access Recorder (QAR) data, which showed airspeed fluctuations between 287 and 327 knots IAS during the turbulence. The investigation also looked into the functionality of the aircraft's radar and the timing of weather warnings issued by the Finnish Meteorological Institute.
Findings
- The primary cause of the incident was incomplete flight planning regarding weather, which resulted in the crew not having an accurate picture of the developing weather conditions.
- The crew failed to review the SMHI Nordic SW-chart and the European area forecast, which contained information regarding embedded convective clouds and thunderstorms in the flight path.
- The aircraft's weather radar was not in use during the cruise and descent phases prior to entering the hail.
- While the crew attempted to activate the radar upon entering the storm, the radar image failed to appear due to an antenna malfunction (the antenna had stopped moving).
- A contributing factor was the delay in the issuance of a SIGMET (Significant Weather Warning) relative to the rapid development of the weather phenomena.
- The crew did not utilize the information provided in the Helsinki-Vantaa ATIS regarding towering cumulus clouds to proactively use the onboard radar.