Timberland Helicopters Aerospatiale AS-355E TWIN STAR accident

The flight was planned to transport a vegetation removal crew to power distribution lines in mountainous, high-elevation terrain. As the helicopter approached the drop-off point, the pilot observed through the engine instruments that the No. 2 engine was operating at reduced power and that the main rotor speed was decaying. He maneuvered away from power lines and made adjustments to the engine trim settings; with power seemingly back to normal, he began the approach again.

However, as the helicopter approached the landing zone the main rotor speed again began to decay such that the pilot knew it would be insufficient for landing. He turned the helicopter away from terrain, and the helicopter began to descend into the surrounding trees, where it landed on its belly and rolled over. The engines were still operating after the accident, and the pilot shut them down and secured the electrical system.

The compressor section of the No. 2 engine had been replaced almost two weeks before the accident. Both the pilot and maintenance personnel stated that following the replacement, they encountered intermittent problems synchronizing the engines. While troubleshooting they performed a series of adjustments to the engine trim system followed by test flights, after which they were eventually able to match the engines. Following the compressor change, the engine was making a loud “howling” sound that neither the pilot nor maintenance crew had heard on this helicopter before. Despite this, they continued to fly the helicopter on other missions leading up to the accident.

Postaccident examination did not reveal any anomalies with the engine trim system or No. 1 engine. Examination of the No. 2 engine revealed that the compressor bleed valve had a hole in the rolling section of its diaphragm, which was causing the valve to intermittently fail. Such a hole would have resulted in the intermittent engine performance observed leading up to the accident flight, and the reduction in power observed before impact.

Examination also revealed that the compressor section incurred a balance and clearance anomaly during its overhaul. These discrepancies would have resulted in a minimal reduction in engine efficiency and would likely have explained the howling sound, but not the abrupt loss of power.

The bleed valve was manufactured under an FAA Parts Manufacture Approval (PMA) process. Although the diaphragm used the same materials specified by the manufacturer, it failed about 77 hours short of its service life.

The pilot adjusted the engine trim setting during the emergency in an attempt to increase power to the failing engine. However, the trim system was differential in design, such that using it in this manner would have caused a reduction in power to the running engine. Regardless, the loss of power occurred at an altitude and flight phase that would have made a safe recovery challenging considering the close proximity to the terrain.

Verbatim NTSB analysis and probable cause from the NTSB dataset