Piaggio P180 N401WS accident near Boyne City, Michigan

Probable cause

The select/bypass valve's failure to seal correctly because of improper shimming, which resulted in an uncommanded and uncontrollable left turn during landing. Contributing to the accident was the overhaul facility's improper overhaul of the steering manifold.

NTSB analysis

The pilot stated that, during landing, the nosewheel steering was not engaged and that, after the airplane touched down, it made an uncommanded left turn. The pilot used landing gear braking and reverse thrust to control the airplane, but it still continued to veer left and then off the runway. The airplane struck the airport perimeter fence and a tree and came to rest.

Postaccident examinations and tests of the nosewheel steering system and its associated components revealed that the steering actuator did move uncommanded at varying rates and directions when electrical power was off and hydraulics were engaged to the nosewheel steering system. Further, this occurred as a result of the select/bypass valve's failure to seal properly when in an unpowered state due to incorrect shimming within the select/bypass valve, which allowed hydraulic fluid to enter the servo valve when the system was off. The available hydraulic pressure was then ported to move the steering actuator relative to the position of the servo valve spool. Examinations also confirmed that, when the servo valve was electrically powered off, the spool could slightly free-float, and the unpowered spool position could be biased in the direction of the last commanded travel from when the system was last engaged. Examination of the rest of the airplane revealed no other mechanical malfunctions or failures that would have precluded normal operation.

The nose landing gear assembly was overhauled 7 days before the accident. However, the postaccident examinations revealed that the overhaul was not conducted in accordance with the manufacturer's component maintenance manual and did not meet the requirements of Service Bulletin 80-0249, which was required by Airworthiness Directive (AD ) 2009-21-08 rev 1, even though the maintenance records indicated that the AD was completed and signed off by the operator for the accident airplane.

Text from the NTSB. Open the NTSB docket for CEN13FA038 for the full record.