What happened
During a flight, the crew experienced an Electronic Control Unit (ECU) failure on the right engine. Following established operator procedures, the crew decided to proceed with the planned flight, noting that the pilot flying was experienced in managing such limitations. The crew was aware that the failure meant reverse thrust would be unavailable on the right engine, but they had developed a plan to manage this.
Upon arrival, the aircraft landed on a wet runway with a tailwind. The landing roll was longer than anticipated, which decreased the remaining runway available for stopping. During the rollout, the outer right tyre locked up, triggering the anti-skid system to release brake pressure on both outboard wheels. This left only the inboard wheels providing braking force, significantly reducing deceleration effectiveness.
As the crew focused on the urgent task of stopping the aircraft before the runway end, they missed a standard cockpit call that would have prompted the captain to use the tiller for directional control. Consequently, the aircraft drifted laterally. Due to the narrowness of the runway, the left landing gear eventually left the sealed surface. The crew did not realize a runway excursion had occurred until a post-landing inspection revealed mud on the tyres.
The investigation
The investigation examined the effectiveness of the operator's procedures regarding ECU failures. It was found that the procedures used to decide whether to continue the flight did not prompt the crew to consider other environmental variables, such as the impact of a wet runway, a tailwind, or the increased risk of asymmetric deceleration on a narrow runway.
Additionally, the investigation reviewed regulatory compliance regarding in-flight performance calculations. While regulations require crews to calculate landing performance based on current reports, the investigation noted that the guidance material regarding the transition from dry to wet runway conditions was not sufficiently clear. This ambiguity meant the requirement to discontinue an approach when a runway surface is unexpectedly wet was not explicitly understood.