What happened
On September 7, 2021, a P-1 fixed-wing patrol aircraft, registration 7033, was conducting a post-manufacturing test flight at Gifu Airfield. During the landing roll on Runway 28, the aircraft began an uncontrolled veer to the right. The captain attempted to correct the deviation using left rudder pedal inputs and by applying asymmetric thrust via the No. 4 engine, but the aircraft continued to drift. The first officer also attempted to intervene using the steering wheel, but the deviation persisted. The aircraft eventually exited the runway and came to a stop in a grassy area, rendering it unable to taxi. There were ten people on board, including the captain, first officer, and flight engineer, and no injuries were reported.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data recorder records, onboard maintenance computer logs, and the physical condition of the steering assembly. Investigators focused on the Steering Control Valve (SCV) and the Steering Actuator (S-ACT). The investigation revealed that while the pilots did not receive any cockpit warnings regarding a steering malfunction, the aircraft's heading deviated significantly from the runway centerline. Physical inspection of the steering system identified oil contamination and specific scratch marks on the SCV spool, suggesting that foreign material had interfered with the valve's internal mechanics.
Findings
- The investigation established that the steering angle was continuously shifting to the right despite active pilot corrections.
- Foreign materials mixed into the SCV during the aircraft assembly process became lodged between the sleeve and the spool.
- This contamination restricted the movement of the spool and caused the neutral position to remain stuck in a state that directed hydraulic oil to steer the aircraft to the right.
- The crew did not recognize a system failure until the aircraft had already left the runway, primarily because the onboard maintenance computer showed no errors and no cockpit alerts were triggered.
- The damage to the aircraft, including dents to the fuselage and deformation of engine fan blades, was caused by the aircraft rolling over soil and pebbles in the grassy area.
Safety action
Following the investigation, the manufacturer implemented several measures:
- Updated work instructions for component suppliers to strictly enforce cleaning protocols to prevent contamination.
- Enhanced training for assembly personnel to prevent foreign object debris (FOD) from entering critical systems.
- Increased the frequency of steering system functional tests with the return port filter removed to ensure no debris remains in the system.
- Clarified emergency procedures, instructing pilots to use the "STEER MODE" switch to transition to Caster Mode if a steering malfunction is detected.