What happened
On February 9, 2008, a Hughes OH-6A helicopter, registration N66372, was performing a mission for the U.S. Border Patrol near Valentine, Texas. The pilot had departed Marfa Municipal Airport earlier that morning to provide aerial support for ground agents pursuing suspects.
While maintaining an out-of-ground-effect hover approximately 30 to 50 feet above the ground, the pilot heard an engine-out warning horn and felt the aircraft begin to yaw to the right and descend. The pilot transmitted a distress signal, maintained directional control, and used the collective to cushion the impact during a forced landing in a wash-out. The aircraft sustained substantial damage, including structural damage to the fuselage, spread skids, and damaged tail rotor blades. The pilot sustained one injury.
The investigation
Investigators from the FAA and CBP examined the engine and discovered that a castellated nut used to secure the governor lever control $\text{rod}$ to the idler bell crank had become loose. The cotter pin intended to secure this nut was missing.
Records showed the engine had been replaced approximately 5.1 hours before the accident. During the reassembly of the engine linkages following an oil leak repair, the contract mechanic reported being distracted by a phone call. The mechanic stated that this distraction led to "tunnel vision," causing him to forget the installation of the cotter pin. Additionally, the mechanic had disassembled the control idler support bracket for the N2RPM Governor and reinstalled it without verifying the attachment bolt.
Findings
- The engine had undergone a 300-hour inspection and engine replacement by a civilian contractor shortly before the accident.
- There were no established written maintenance inspection procedures for the operator to properly accept an aircraft following major repairs.
- While the OH-6A technical manuals only required a standard castellated nut and cotter pin, more recent Hughes 500 repair manuals and FAA Airworthiness Directives required a specific castellated nut with fiber inserts to prevent the nut from backing off.