What happened
On October 24, 2015, an American Eurocopter AS350B3 helicopter, registration N911WL, was performing simulated engine-off training near Folsom, California. The crew, consisting of a flight instructor, a pilot undergoing instruction (PUI), and a tactical flight officer, had departed from Mc Clellan Airfield to perform patrol duties and practice autorotations.
After completing routine patrols and several uneventful autorotations over flat terrain, the crew transitioned to a peninsula on the northern shore of the Fulosm Lake Reservoir. The instructor intended to perform "enhanced" autorotations, a maneuver where a specific landing point is selected for an emergency landing.
During the power recovery phase of the maneuver, the flight instructor moved the throttle twist grip from idle to flight at approximately 100 feet above ground level. The engine did not respond as expected, and the instructor observed that the engine and rotor RPM needles remained split. As the aircraft approached the ground, it moved forward onto downsloping terrain. To avoid an unusual attitude, the instructor aimed for a flat touchdown on the skids. The helicopter landed hard and tipped forward, causing the tailboom to bend and the aft airframe to wrinkle.
All 3 occupants were not injured, though the aircraft sustained substantial damage.
The investigation
Investigators examined the engine, which was equipped with a Turbomeca Arriel 2B1 engine and controlled by a dual-channel FADEC system. Post-accident testing of the engine and digital engine control unit (DECU) showed no mechanical anomalies or faults attributed to the accident flight; the engine performed to nominal specifications during test runs.
Maintenance records were reviewed regarding the electrical operation of the twist grip. While a previous service bulletin recommended inspections to prevent corrosion from causing unintentional power changes, investigators found no evidence of corrosion or malfunctions in the microswitches during the post-accident examination.
Findings
- The flight instructor's choice of a power recovery initiation altitude of 100 feet agl was lower than the 200 feet agl recommended in a recent manufacturer safety notice.
- The selected landing area on a pinnacle was unsuitable, as the surrounding downsloping terrain provided a limited maneuvering envelope for recovering from a delayed engine response.
- The helicopter was operating near its maximum gross weight, which increased the risk of a hard landing.
- The presence of the tactical flight officer during the training maneuver represented an unnecessary risk.