Accident Overview
A Bell 206B helicopter (serial number 1743) operated by an exploration company experienced an engine failure while in cruise flight. The pilot had just delivered supplies at a camp and was en route to pick up two workers approximately three miles west. At an altitude of about 100 feet above ground level, the Allison 250C20 engine suddenly lost power. The pilot attempted an autorotation but stretched the glide to cross a creek and reach a level landing area. This caused rotor rpm to decrease and loss of lift. At touchdown, the left skid dug into the ground, and the helicopter rolled onto its right side, resulting in substantial damage. The pilot was uninjured.
Pilot and Maintenance
The pilot held the necessary certifications and qualifications, with approximately 8,000 total flight hours, including 6,000 hours on the Bell 206. The helicopter was maintained per regulations; a 300-hour inspection had been completed 29.3 air hours before the event, during which the fuel nozzle was replaced and the tail rotor rebalanced. No unserviceabilities were reported prior to the flight.
Fuel and Engine Examination
At the time of the failure, the helicopter had about 39 U.S. gallons of Jet B fuel on board. The fuel tank ruptured on impact, spilling a considerable quantity. All fuel lines and filters were intact and contained fuel without visible contaminants. Examination of the wreckage revealed no pre-impact structural or system failures, except for the engine-driven fuel pump (EDP). The EDP drive shaft could rotate but did not drive the pumping gears due to excessive wear on the splines of the drive shaft and driver gear.
The EDP (Model MFP263, serial number AKV0216) had accumulated 6,55.5 hours since new, far below the recommended overhaul interval of 3,500 hours. This pump model, introduced in late 1994, had experienced two other similar failures within a week of this occurrence, both on the ground. Disassembly and examination at Chandler Evans Control Systems Division (CECo) confirmed that all components met specifications except for the spline wear. A reddish powder deposit, oxidized spline material, was found, along with small spline fragments in the high-pressure fuel filter. The root cause of the spline wear could not be determined.
Meteorological Conditions
Weather at the time included an overcast ceiling at 300 feet agl, visibility of 8 miles, temperature 15°C, and light winds. Weather was not considered a factor.
Probable Cause and Safety Actions
The investigation concluded that the engine failed due to the engine-driven fuel pump malfunction, specifically the disengagement of the drive shaft splines from the driver gear splines caused by excessive wear. The pilot's low altitude (100 ft agl) limited available landing sites, and the decision to stretch the glide reduced rotor rpm, affecting the landing outcome.
In response, CECo initiated collection of most delivered MFP263 pumps for tear-down examination, finding abnormal spline wear in several units. Allison Engine Company issued a Commercial Engine Bulletin on 21 August 1997 requiring inspection of affected pumps for drive shaft backlash within 25 operating hours, with recurring inspections every 100 hours thereafter.