Occurrence
A Bell 206 helicopter, serial number 845, registration C-GPOS, with one pilot and two forestry workers, was hovering approximately 150 feet above a forestry cut block at about 0600 Pacific daylight time. The sky was clear, winds light and variable, and temperature about 14°C. The crew were scanning for hot spots when black smoke engulfed the cabin. The pilot turned right, lowered the collective, and descended toward a road. As he applied collective to reduce descent, rotor rpm decayed. He bottomed then raised the collective during the final flare. The helicopter landed hard, nose down, short of the road; the skid landing gear collapsed, the helicopter turned 180 degrees and rolled onto its left side. The engine continued running but was shut down by the pilot. Smoke was not apparent after landing; there was no post-impact fire. The pilot and front-seat passenger were trapped briefly until the back-seat passenger assisted. The pilot suffered a back injury; both passengers suffered minor injuries. The helicopter was substantially damaged.
Examination
The wreckage was examined at the TSB facility. The engine showed no external anomalies; compressor and power turbine wheels rotated freely. The engine-to-main-rotor transmission coupling was separated, its grease flung out, and one coupling boot burnt. The coupling had been compressed and pulled apart. Main rotor transmission damage indicated it rocked fore and aft, deforming its mounts and tilting forward about 20 degrees. The fuel cell was intact with about 500 pounds of fuel; fuel valve off, throttle closed. Landing gear skid tubes and forward cross tube broke; cross tubes rotated aft and up. After-market step attached to cross tubes jammed against the pilot's door, preventing normal opening. The helicopter had no emergency release systems for doors or windows. The pilot's seat bottom was slightly deformed but not contacting the honeycomb plate.
Analysis
Investigation focused on the input drive coupling failure. Close examination showed the coupling had been compressed and pulled apart; such disconnection was not possible under flight conditions. Damage from the hard landing caused the main rotor transmission to rock and tilt forward, pulling the coupling apart. All damage except to the engine was from the hard landing and rollover.
The engine was tested and showed extreme vibrations; these would have been noticeable if present before the event. Since the engine was not damaged by the hard landing, the imbalance occurred around the time of cabin smoke. The imbalance was caused by separation of GP turbine blade parts. There were no periodic power checks required or performed to detect power loss, nor required inspections for sulfidation corrosion. It was suggested such checks would have detected the corroded components. Sulfidation corrosion weakened the GP turbine blades, fatigue cracks occurred on many blades, and one blade separated. Parts of other blades broke off, causing engine power loss.
The blade loss caused imbalance, likely damaging turbine seals, releasing oil onto hot areas, producing soot and cabin smoke. Alternatively, smoke from the burnt coupling boot was considered but not supported; the coupling failed during the hard landing.
The pilot responded immediately by lowering collective. The helicopter was hovering at about 150 feet at high density altitude and near maximum weight. Regardless of the smoke cause or power loss timing, a successful landing with reduced power was unlikely. Rotor rpm decayed, insufficient thrust to cushion the landing.
When landing gear broke, the cross tubes rotated, jamming the after-market step against the pilot door. Without emergency release, and with the helicopter on its side, the pilot and front passenger were trapped.
Findings as to Causes and Contributing Factors
- Periodic power checks or inspections for sulfidation corrosion, which likely would have detected power loss and corroded turbine components, were not undertaken nor required.
- A blade in the number 1 GP turbine failed due to fatigue resulting from sulfidation corrosion; the engine lost power due to the blade failure.
- The helicopter was operating in a high hover when the engine lost power. The ensuing forced landing was hard, the landing gear broke, and the helicopter rolled over.
Findings as to Risk
- The after-market step attached to the landing gear cross tubes blocked the pilot's door, trapping pilot and passenger.
- Bell 206 series helicopters are not normally equipped with doors or windows with emergency release systems.
Safety Action Taken
On 18 July 2003, TSB sent a Safety Information Letter to Transport Canada (TC) describing the door blockage issue. On 3 October 2003, TSB sent a Safety Advisory to TC highlighting sulfidation corrosion and suggesting periodic inspections be considered. TC initially responded that without substantiating data, scheduled washings were deemed unnecessary and no direct link between sulfidation and blade failure was established. After further TSB Engineering examination (report LP 037/04), TC acknowledged sulfidation as the deterioration process leading to fatigue failure and agreed to recommend the engine manufacturer implement inspections for sulfidation.