Accident Overview
On August 11, 2008, approximately 1530 central daylight time, a Bell 206L-1 single-engine helicopter, registered as N832AH, sustained substantial damage during a forced landing after a partial loss of engine power while maneuvering near Heber Springs, Arkansas. The helicopter was conducting an external load flight under Title 14 Code of Federal Regulations Part 133. Visual meteorological conditions prevailed, and no flight plan was filed.
Flight Details
The helicopter, operated by Hillsboro Aviation Inc. (Hillsboro, Oregon), had been performing external load operations on the day of the accident. It departed a remote heli-spot about 20 minutes prior to the event. After refueling, the pilot was transporting an empty container on a long line at approximately 300 feet above ground level when the engine "spooled down." The pilot released the external load, executed a 180-degree turn, and initiated an autorotation.
Forced Landing and Damage
During the forced landing, the helicopter landed hard in a tree farm, struck trees, rolled over, and came to rest on its right side. The pilot shut off the fuel and exited the helicopter. Examination revealed the tail boom separated, main rotor blades bent, and the right side of the fuselage damaged.
Post-Accident Examination
A Federal Aviation Administration inspector spoke with a company mechanic who reported that a PC line B-nut was found disconnected on the Rolls Royce 250-C30P engine. The last engine maintenance involving removal and reinstallation of that PC line was 67.1 hours prior, during a scheduled turbine change. No slippage marks (torque paint) were noted on the fitting.
Maintenance Manual Guidance
According to the Rolls Royce 250-C30 Series Operation and Maintenance manual (section 72-00-00), proper tightening of engine tubing connections is critical to flight safety. It states that tubing B-nuts exposed to high vibration and pressure surges are subject to torque relaxation when improperly tightened. Old slippage marks should be removed and reapplied each time the B-nut is tightened.
Service Bulletin
In 1990, Rolls Royce issued an Alert Commercial Service Letter (Revision 1, dated February 5, 2007) titled "Maintenance Warning - External Lines." The letter notes that improper alignment, clamping, and torquing of engine tubing during installation have been factors in aircraft accidents and incidents. It documents examples and causes of tube assembly failures.