WPR13FA227

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.

The crew of the twinjet reported that the positioning flight after maintenance was uneventful. However, during the landing roll at their home base, the thrust reversers, steering, and braking systems did not respond. As the airplane approached the end of the runway, the pilot activated the emergency braking system; however, the airplane overran the end of the runway, coming to rest in a ditch. None of the three occupants were injured, but the airplane sustained substantial damage to both wings and the fuselage. Two squat switches provided redundancy within the airplane's electrical system and were configured to prevent inadvertent activation of the thrust reversers and nosewheel steering during flight and to prevent the airplane from landing with the brakes already applied. Because postaccident examination revealed that the squat switch assemblies on the left and right landing gear struts were partially detached from their mounting pads such that both switches were deactivated, all of these systems were inoperative as the airplane landed. The switch assemblies were undamaged, and did not show evidence of being detached for a long period of time. The brakes and steering were working during taxi before departure, but this was most likely because either one or both of the switches were making partial contact at that time. Therefore, it was most likely that the squat switch assemblies were manipulated on purpose during maintenance in an effort to set the airplane's systems to "air mode." Examination of the maintenance records did not reveal any recent procedures that required setting the airplane to air mode, and all mechanics involved in the maintenance denied disabling the switches. Mechanics did, however, miss two opportunities to identify the anomaly, both during the return-to-service check and the predelivery aircraft and equipment status check. The anomaly was also missed by the airplane operator's mechanic and flight crew who performed the preflight inspection. The airplane's emergency braking system was independent of the squat switches and appeared to operate normally during a postaccident test. Prior to testing, it was noted that the emergency brake gauge indicated a full charge; therefore, although evidence suggests that the emergency brake handle was used, it was not activated with enough force by the pilot. The pilot later conceded this fact and further stated that he should have used the emergency braking system earlier during the landing roll. The airplane was equipped with a cockpit voice recorder (CVR), which captured the entire accident sequence. Analysis revealed that the airplane took just over 60 seconds to reach the runway end following touchdown, and, during that time, two attempts were made by the pilot to activate the thrust reversers. The pilot stated that as the airplane approached the runway end, the copilot made a third attempt to activate the thrust reversers, which increased the engine thrust, and thereby caused the airplane to accelerate. Audio captured on the CVR corroborated this statement.

The airplane was equipped with a Universal CVR-30 cockpit voice recorder. The unit did not sustain any damage, and was sent to the NTSB Vehicle Recorder Division's Audio Laboratory for audio extraction and content summary. The unit had recorded the last 30 minutes of the flight on two separate channels, which included the intercom system and the cockpit area microphone (CAM). The recording essentially supported the pilot and copilot statements, and began with the airplane at flight level 280, with the pilot flying as pilot-in-command. Over the next 27 minutes, the crew communicated with air traffic control, initiated an uneventful descent, and completed checklists in a challenge/response fashion. The pilot asked for flaps 8, and the copilot acknowledged, as air traffic control personnel advised them to switch to the McMinnville advisory frequency. The flight progressed with the landing checks, and the landing gear was heard to extend. The landing checks were completed during the descent. A short time later there were three audible clicks recorded by the CAM, followed by a sound of "bump-bump," similar to touchdown. Four seconds later, the pilot reported that the thrust reversers had not come out. The copilot asked if the control was all the way back, and a click was heard followed by a decrease in engine noise. The engine noise then increased, and the pilot stated, "no. they're sure not workin". A click and snap sound was then heard, and the pilot stated, "I can hear 'em though". The copilot stated that he had taken off his headphones to try and listen for the reversers. The engine noise again decreased, and the pilot stated he would try the thrust reverser's again. Thirty seconds had now passed since touchdown, and the engine sound again began to increase. The pilot then stated that they were not working, and the copilot exclaimed, "you got it?" to which the pilot replied in the affirmative, and the copilot said, "slow'er down". Over the next 10 seconds, they continued to exchange words regarding slowing down and exiting the runway. The pilot then stated, "oh, emergency" and the sound of three snaps were heard followed by an increase in engine noise. A few seconds later, the pilot stated, "emergency brake...okay" and the engine noise began to decrease. Over the next 8 seconds the crew expressed concern that they could not stop before the end of the runway, and then a thump sound was heard similar to impact. For the next 33 seconds until the end of the recording, sounds similar to an engine shutdown and crew-egress were recorded. The total time from the sound of the landing gear making contact with the runway, to the sound of the presumed impact, was 63 seconds. McMinnville airport was located at an elevation of 163 feet. Runway 22 was 5,420 feet long by 150 feet wide, and paved with asphalt. An additional 1,025 feet asphalt blast pad/stopway extended beyond the departure threshold. A complete report of the airplane systems examination is contained within the public docket. Squat Switches The airplane, by design, was equipped with a redundant series of squat switches located on the left and right main landing gear strut torque arms. The switches were positioned to activate when the landing gear was extended, and the airplane was on the ground. The switches were connected within the electrical system, and configured to prevent inadvertent activation of core airplane systems during various modes of flight. The design was such that when the airplane was on the ground, the following occurred: - A circuit was broken, which prevented the airplane from landing with the brakes applied. - A circuit was completed, which energized the squat switch relay panel, in turn disabling the nose wheel steering system, and preventing deployment of the thrust reversers. Post-accident examination of both the left and right landing gear struts revealed that each of their squat switch assemblies were partially detached from their upper scissor arm fork pads. On both sides, all three switch attachment screws were loose, with the switches rising between 1/8 and 1/4 inches from the pads. The switch roller bearing surfaces were not making positive contact with their respective scissor arm cams, thereby deactivating the switches. Under these circumstances, the airplane systems would interpret that the airplane was in air-mode, when it was in fact on the ground. The switches were removed, and their mounting screws and pad surfaces were examined. All the bolt threads were sharp, with no flattening damage noted to the thread crests. The scissor pad and squat assembly mating surfaces were lightly coated in dirt, grime, and paint debris, and no fretting damage was present. Thrust Reversers The airplane was equipped with two Garrett (Honeywell) TFE731 series geared turbofan engines, and an Aeronca electro-pneumatic thrust reverser system. The Thrust Reversers supplement of the airplane's flight manual stated that during landing - once the airplane has touched the ground, the thrust reverser levers must be pulled back to the Reverse Idle/Deploy position, during which time the UNLOCK light will illuminate on the reverser panel. The light will remain illuminated while the reversers are translating, and once they reach the fully deployed position, the corresponding DEPLOY light will illuminate. At that point, the throttle solenoid will unlock allowing further retraction of the thrust lever and an increase in reverse engine thrust. A correlation between the thrust lever positions and the engines power lever angle (PLA) was accomplished at various forward and reverse thrust lever positions. In all cases, the lever positions matched the manufacturer's nominal engine PLA values. According to the Learjet 35A maintenance manual section, "THRUST REVERSER - MAINTENANCE PRACTICES", a throttle control interlock switch, solenoid, and pawl incorporated in the control quadrant assembly prevented application of reverser thrust above approximately 45 percent N1 rpm until both thrust reversers are fully deployed. According to representatives from Honeywell, a 45 percent N1 speed correlates to about 531 pounds of static thrust per engine. Emergency Braking System The airplane was equipped with an emergency air brake system consisting of an emergency brake control valve connected to a high-pressure air bottle. The bottle was connected to the hydraulic brake system via a series of shuttle valves. The control valve was mounted on the pedestal within the cockpit, and manually operated by the pilot. Lowering the valve handle admitted high-pressure air into the brake hydraulic lines, thereby applying pressure to the braking system. An emergency air pressure indicator needle was mounted within the cabin, indicating an operating range of between 1,800 and 3,000 PSI. According to the emergency procedures section of the airplane's flight manual, the emergency handle must be pushed down approximately 2 inches before braking action begins. The manual recommends applying the brakes smoothly, and with small movements to prevent the tires from skidding. Examination of the emergency brake system following recovery revealed that the pressure indicator needle was one needle-width below the fully charged pressure of 3,000 psi. An emergency brake system test was subsequently performed by activating the emergency brake handle and simultaneously observing the movement of the brake caliper pistons, pads, and disks for both wheels. With the emergency lever depressed by 2 inches, the pads began to move and make contact with their associated disks. The lever was cycled to its full down position of 4.5 inches, five times. Resistance was felt in the lever, with corresponding movement of the pads and the sound of hissing air. Upon completion, the emergency air gauge had moved to about 2,500 psi. In a subsequent interview, the pilot stated that he felt minimal resistance in the br