The AAIB reported that a Saab 340B runway excursion accident was the crew likely failed to use rudder during takeoff in cross-wind conditions.
On January 2, 2015, the Saab 340B was performing flight BE6821 from Stornoway Airport (SYY) to Glasgow (GLA). At 08:32 hrs G-LGNL was cleared to enter runway 18 at Stornoway Airport and take off, and the ATC controller transmitted that the surface wind was from 270° at 27 kt. The commander commented to the co-pilot that the wind was across the runway and that there was no tailwind. As the aircraft taxied onto the runway, the co-pilot applied almost full right aileron input consistent with a cross-wind from the right, and the commander said to the co-pilot “charlie , one hundred, strong wind from the right”. The commander advanced the power levers, the co-pilot said ““autocoarsen high” and the engine torques increased symmetrically. The commander instructed the co-pilot to “set takeoff power” to which the co-pilot replied “apr armed”. Approximately one second after this call, the engine torques began to increase symmetrically, reaching 100% as the aircraft accelerated through 70 kt.
During the early stages of the takeoff, left rudder was applied and the aircraft maintained an approximately constant heading. As the aircraft continued accelerating, the rudder was centralised, after which there was a small heading change to the left, then to the right, then a rapid heading change to the left causing the aircraft to deviate to the left of the runway centreline.
The pilot applied right rudder but although the aircraft changed heading to the right in response, it did not alter the aircraft’s track significantly and the aircraft skidded to the left, departing the runway surface onto the grass at an IAS of 80 kt. The power levers remained at full power as the aircraft crossed a disused runway and back onto grass. During this period the nose landing gear collapsed before the aircraft came to a halt approximately 38 m left of the edge of the runway and 250 m from where it first left the paved surface.
After the aircraft came to a halt, the captain saw that the propellers were still turning and so called into the cabin for the passengers to remain seated. One of the passengers shouted for someone to open the emergency exit but the cabin crew member instructed the passengers not to do so because the propellers were still turning. The co-pilot observed that the right propeller was still turning so operated the engine fire extinguishers to shut down both engines. When the passenger seated in the emergency exit row on the right of the aircraft saw that the right propeller had stopped, he decided to open the exit. He climbed out onto the wing and helped the remaining passengers leave the aircraft through the same exit, instructing them to slide off the rear of the wing onto the ground. The left propeller was still turning at the time the right over-wing exit was opened and the passenger seated in the left-side emergency exit row decided not to open the left exit.
The technique for controlling the direction of the aircraft on the runway is to use rudder assisted by nose wheel steering (NWS) at low speeds because the rudder has reduced effectiveness below 40 kt.
When rudder is used, the requirement for NWS to assist directional control will reduce progressively as speed increases above 40 kt and rudder effectiveness increases. It is likely that no assistance will be required by 60 kt and, therefore, there will be no step-change in NWS directional effect when the pilot releases the steering control. During this attempted takeoff, rudder was approximately neutral from 40 kt, the point at which it would have become effective, and directional control was probably maintained through NWS alone (asymmetric thrust or differential braking having been discounted). If rudder had been applied, there would have been a reduced NWS requirement at any given speed and therefore there would have been a reduced likelihood of a change directional effect when the NWS control was released. The lack of data showing NWS commands meant that these considerations could not be verified.
- AAIB report (PDF)