What happened
On 16 December 2010, a Saab-Scania SF340B, registration G-LGNI, was performing a scheduled passenger flight from Inverness to Lerwick. During the approach to Kirkwall Airport, the aircraft encountered weather conditions including a 27-knot crosswind, light showers of hail, and a runway covered in 3 mm to 5 mm of slush.
Upon landing, the pilot flying attempted to move the power levers into the beta range to engage ground pitch control. However, the levers initially remained stuck in the flight idle position. Simultaneously, the nosewheel steering system failed to respond to inputs. To maintain directional control during the landing roll, the commander utilized a combination of differential braking and rudder inputs. As the aircraft decelerated to taxi speeds, the power levers became movable, and the nosewheel steering returned to normal operation. The aircraft subsequently taxied to its parking stand without further incident, and there were no injuries to the 3 crew members or 17 passengers on board.
The investigation
The AAIB examined the aircraft's nosewheel steering and flight idle (FI) stop systems. The investigation focused on the mechanical and electrical logic that governs the movement of the power levers and the availability of hydraulic pressure to the steering actuator. The investigation also looked into the physical effects of the slush-covered runway and the aircraft's landing gear characteristics.
Findings
- The primary cause of the incident was the delayed closure of the main landing gear weight-on-wheels switches combined with the slow acceleration of the main wheels.
- Due to the relatively stiff nature of the main undercarriage oleos, the aircraft's weight settled onto the wheels slowly, which delayed the activation of the weight-on-wheels switches.
- The presence of slush on the runway likely delayed the main wheels from reaching the required speed of 25 kt to trigger the flight idle stop release.
- During the landing roll in a strong crosswind, the nosewheel castored at an angle exceeding 20°, which caused the nosewheel steering system to rely on the main landing gear weight-on-wheels switches rather than the nose landing gear downlock.
- No technical defects were found in the aircraft's systems during subsequent engineering inspections.
Safety action
Following the investigation, the manufacturer issued warnings in the Aircraft Operations Manual. These warnings advise flight crews that landing on slippery surfaces in crosswinds may cause a temporary delay in the opening of the flight idle stop or render the nosewheel steering system temporarily inoperative.