What happened
On February 21, 2021, at approximately 18:07, an Airbus A350 operated by Finnair experienced a loss of directional control while taxiing at Helsinki-Vantaa Airport. The aircraft, which was being prepared for a cargo flight to Tokyo, was operating in freezing conditions with active freezing rain. The crew had previously performed deicing procedures at position 604.
As the aircraft exited the deicing area and entered a sharp turn on a taxiway, the nose wheels lost traction on the icy surface. Despite the pilot's attempts to use braking and asymmetric engine thrust, the aircraft could not be steered effectively. The aircraft slid straight, with the left main gear exiting the taxiway. During the excursion, the left engine struck a taxiway instructional sign, causing damage to the engine's fan blade cowling. The pilot eventually managed to steer the aircraft back onto taxiway W.
Following the incident, the crew requested a technical inspection. The aircraft was taxied to a maintenance hangar, where mechanics confirmed the impact damage to the engine cowling but found no other significant issues. Due to the continued slippery conditions on the apron, the aircraft could not be moved to its intended parking position and had to be relocated via a tug after the surface had been treated with grit.
The investigation
The investigation examined the environmental conditions, the airport's winter maintenance effectiveness, and the aircraft's handling characteristics. Investigators reviewed weather data, airport surface friction measurements, and cockpit voice recordings. The investigation also looked into the airport's management of deicing operations and the physical layout of the taxiway intersections.
Findings
- The primary cause of the excursion was the extremely low friction level of the taxiway surface caused by freezing rain and ice.
- The airport's maintenance classification for the taxiway route was "poor," the lowest possible rating.
- The design of the taxiway curves at the deicing area was found to be difficult for wide-body aircraft like the Airbus A350 to navigate, especially when the nose wheels require significant steering input to keep the main gear on the pavement.
- The decision to centralize all deicing operations at a single location (APN 6) contributed to the risk, as it forced aircraft to navigate complex, narrow turns in hazardous weather.
- The use of both engines during taxiing, while intended to maintain control in slippery conditions, resulted in increased thrust that contributed to the loss of steering when the nose wheels lost grip.