What happened
On August 5, 2017, a Lockheed P2V-5, registration N410NA, was performing a mission to disperse fire retardant over a wildfire near Pocatello, Idaho. During the aircraft's climb, the pilot-in-command attempted to reduce nose-down pressure by adjusting the trim. Following this input, the pilot observed an uncommand and aft movement of the control yoke accompanied by an increase in the airplane's pitch attitude.
The flight crew attempted to regain control by using the trim wheels and the emergency varicam, but the aircraft maintained its pitch-up attitude. The crew eventually deployed 5 degrees of flaps, which helped reduce elevator backpressure. The pilot then jettisoned the fire retardant load over vacant farmland and declared an emergency. To land the aircraft, the crew utilized a wide traffic pattern and used a combination of engine power and wing flaps to manage the pitch attitude. The aircraft landed without further incident, though it sustained substantial damage.
The investigation
Post-accident examination of the aircraft's variable camber (varicam) system revealed damage to the component. The investigation determined that one of the outboard drive stop bolts on the varicam actuator had backed out of the drive coupling, and the two bolts had not been safety wired. This hardware failure caused the left side of the varicam to deform, which in turn forced the left elevator into an upward deflected position.
Maintenance records showed that the universal joints on the varicam had been replaced in July 2016. The investigation found that the mechanic responsible for the work had failed to secure the drive stop coupling bolts with lockwire. Furthermore, the company's quality assurance (QA) process failed to identify the error. The investigation revealed that the lead mechanic and the QA inspector lacked sufficient oversight; the lead mechanic often did not perform spot inspections, and the QA inspector failed to properly annotate critical flight control tasks as Required Inspection Items (RII). An audit of the aircraft's annual inspection package later uncovered seven additional instances where mandatory RII procedures were not recorded.
Findings
- The failure of the varicam hardware resulted in an uncommanded upward deflection of the left elevator.
- The absence of safety wire on the drive stop coupling bolts allowed a bolt to back out of its hole.
- The lack of maintenance oversight by both the lead mechanic and the quality assurance inspector allowed the unsecured hardware to remain undetected.
- The mechanic responsible for the installation was under significant personal stress at the time of the service.
- The company's RII process was ineffective, as the QA inspector failed to verify if critical tasks were properly designated for inspection.