NTSB Identification: CEN13FA038
14 CFR Part 91: General Aviation
Accident occurred Wednesday, October 31, 2012 in Boyne City, MI
Probable Cause Approval Date: 08/28/2014
Aircraft: PIAGGIO P180, registration: N401WS
Injuries: 2 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

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

Postaccident examinations and tests of the nosewheelsteering system and its associated components revealed that the steeringactuator did move uncommanded at varying rates and directions when electricalpower 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 sealproperly when in an unpowered state due to incorrect shimming within theselect/bypass valve, which allowed hydraulic fluid to enter the servo valvewhen the system was off. The available hydraulic pressure was then ported tomove the steering actuator relative to the position of the servo valve spool.Examinations also confirmed that, when the servo valve was electrically poweredoff, the spool could slightly free-float, and the unpowered spool positioncould be biased in the direction of the last commanded travel from when thesystem was last engaged. Examination of the rest of the airplane revealed noother mechanical malfunctions or failures that would have precluded normaloperation.

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.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

Full narrative available

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