Incident Summary
A twinjet experienced a runway excursion during the landing roll at its home base after an uneventful positioning flight following maintenance. As the aircraft touched down, the thrust reversers, nosewheel steering, and braking systems did not respond. The pilot activated the emergency braking system, but the airplane overran the end of the runway and came to rest in a ditch. None of the three occupants were injured, but the airplane sustained substantial damage to both wings and the fuselage.
Systems and Failure
The aircraft's electrical system incorporated two squat switches designed for redundancy. These switches were configured to prevent inadvertent activation of the thrust reversers and nosewheel steering during flight and to ensure that the brakes were not already applied upon landing. Postaccident examination revealed that the squat switch assemblies on the left and right landing gear struts were partially detached from their mounting pads, rendering both switches deactivated. The switch assemblies were undamaged and showed no evidence of prolonged detachment. During the taxi before departure, the brakes and steering functioned normally, most likely because one or both switches were making partial contact at that time.
Maintenance and Inspections
It was most likely that the squat switch assemblies were intentionally manipulated during maintenance in an attempt to set the aircraft's systems to "air mode." However, a review of maintenance records found no recent procedures requiring such a configuration, and all involved mechanics denied disabling the switches. The anomaly was missed during two subsequent opportunities: the return-to-service check and the predelivery aircraft and equipment status check. Additionally, the airplane operator's mechanic and the flight crew did not identify the issue during the preflight inspection.
Emergency Braking and Cockpit Voice Recorder
The emergency braking system was independent of the squat switches and operated normally during a postaccident test. Before testing, the emergency brake gauge indicated a full charge. Although evidence suggested that the pilot used the emergency brake handle, it was not activated with sufficient force. The pilot later acknowledged this and stated that the emergency braking system should have been applied earlier in the landing roll. The cockpit voice recorder (CVR) captured the entire accident sequence. The recording indicated that the airplane took just over 60 seconds to reach the runway end after touchdown, during which the pilot made two attempts to engage the thrust reversers. As the aircraft neared the runway end, the copilot made a third attempt, which increased engine thrust and caused the airplane to accelerate. The CVR audio corroborated the copilot's action.
Probable Cause
The official probable cause is the failure of maintenance personnel to reattach the landing gear squat switches following maintenance, which rendered the airplane's steering, braking, and thrust reverser systems inoperative during landing. Contributing to the accident were the failure of both the maintenance facility mechanics and the airplane operator's mechanic and flight crew to identify the error during postmaintenance checks, a failure of the airplane's pilot to apply the emergency brakes in a timely manner, and the copilot's decision to attempt to engage the thrust reversers as the airplane approached the runway end despite multiple indications that they were inoperative and producing partial forward, rather than reverse, thrust.