Piaggio P180-AVANTI II N700FE

The landing gear did not extend when the copilot selected gear-down during the initial instrument approach to the destination airport. The flight crew was able to extend the landing gear using the emergency extension procedure; however, following the emergency extension, the pilot decided to reengage the hydraulic system to have power-assisted braking and nosewheel steering during landing. The pilot stated that the wheel brakes were less effective than normal and the nosewheel steering was inoperative during the landing roll, which he concluded was because the emergency landing gear selector valve handle was still extended. The pilot reported that in an attempt to resolve the braking and steering issues he reached for the emergency landing gear selector valve, but the landing gear collapsed before his hand touched the valve handle. Postaccident testing determined the directional control valve that controls the flow of hydraulic fluid to the landing gear actuators was stuck in the gear-up position. Examination of the directional control valve revealed a metallic particle trapped between one of the spool lands and the valve housing, which prevented the spool from moving into the gear-down position. The source of the trapped particle could not be conclusively determined. A teardown examination did not reveal any mechanical anomalies that would have resulted in a malfunction of the hydraulic pump package. The airplane flight manual specified that the hydraulic system be turned off before the landing gear was manually extended using the emergency procedures. The flight manual did not include a provision for the hydraulic system to be reengaged following the emergency landing gear extension. As such, the hydraulic system should have remained off during landing. Postaccident testing concluded that the landing gear would immediately retract if the emergency landing gear selector valve was in the stowed position with the hydraulic system turned on. Consequently, because the airplane landed with its gear extended, the emergency landing gear selector valve handle had to be in the extended position upon touchdown. One scenario for the unintended landing gear retraction was if the pilot repositioned the emergency landing gear selector valve to its normal (retracted) position during the landing roll. This would have allowed the unintended pressure in the normal hydraulic system lines to retract the landing gear. Although this scenario is consistent with the reported sequence-of-events, the pilot stated that he did not touch the emergency landing gear selector valve handle during the landing roll. Additionally, the emergency valve handle was found extended after the accident. Another possible scenario was if the pressure in the emergency extension line decreased below that necessary to hold the gear down locks in position. Postaccident testing completed on an exemplar airplane with a simulated failure of the directional control valve and the hydraulic system turned on, concluded that the down locks could release and allow the landing gear to retract if the line pressure in the emergency system decreased sufficiently. After considering the test data and the variables involved, the investigation was unable to conclusively determine which scenario had triggered the down locks to release during landing roll. However, the landing gear would not have retracted had the pilot not reengaged the hydraulic system following the successful extension of the landing gear using the emergency extension procedures. The hydraulic system being turned on, in combination with the failure of the directional control valve in the gear-up position, resulted in unintended pressure within the hydraulic lines associated with gear retraction and created the means for the gear to retract when the gear locks released for undetermined reasons.

Quoted verbatim from the NTSB record.