ERA16FA311 · Sikorsky S61 N

The helicopter flight crew, consisting of a pilot, copilot, and maintenance crewmember, was performing 20-knot rearward flight about 200 ft above ground level as part of a post-maintenance functional check flight (FCF). While the pilot was flying and recovering from the first rearward flight maneuver, unusual sounds were heard, which the flight crew identified as a compressor stall. The pilot then told the maintenance crewmember that they were returning to their home airport; however, after discussing compressor stalls and engine exhaust gas temperatures with the copilot, the pilot changed his mind and told the maintenance crewmember that they were going to try the maneuver again in a different direction relative to the wind (with the wind off the nose). While the pilot was recovering from the second rearward flight maneuver, there was a change in background noise, which the maintenance crewmember identified as a compressor stall. About 2 seconds later, there was another change in background noise, consistent with a decay in drivetrain rpm, which was followed by the helicopter descending and impacting the ground.

Although the crew identified the sounds and loss of power as compressor stalls, a sound spectrum study could not characterize the gas generator speed (Ng) behavior, due to overdriven audio on the CVR, to determine the engine anomaly the crew identified as a compressor stall. It is likely that the cause of the overdriven audio is related to an engine anomaly. At the same time the engine anomaly occurred, the sound spectrum revealed Nr quickly decayed due to a dual loss of engine power coupled with a high collective setting. Because main rotor speed (Nr) decayed at the same time the overdriven audio occurred, it is likely both engines lost power nearly simultaneously. No anomalous damage to the engines was found that would have explained the dual loss of engine power and the main rotor drive system did not exhibit evidence of preimpact mechanical malfunction. Fuel exhaustion was unlikely given the estimated fuel load at the time of the accident. After the overdriven audio, the rate of Ng decay for each engine appeared similar to engine Ng decay when at a lower power setting. Due to a lack of a flight data recorder or a cockpit image recorder, the behavior of the engines as well as the position of the cockpit engine control levers at the time of the anomaly could not be determined.

During postaccident examination, the left (No. 1) freewheeling unit (FWU) was found to rotate freely in both directions of rotation instead of rotating freely in only one direction as it was designed to do. Had the left FWU failed in flight resulting in a loss of drive between the No. 1 engine and the main gearbox, the No. 1 engine power turbine would have experienced an overspeed due to the sudden loss of load. A No. 1 engine power turbine overspeed would also have resulted in an overspeed of the AC generators due to a through shaft connecting the left FWU and the AC generators. However, during the accident loss of power event, the frequencies of the main gearbox planetary mesh and the AC generators did not diverge but remained synced, which is not consistent with a left FWU overspeed. Therefore, the anomalous finding of rotation in both directions for the left FWU is likely a postaccident artifact.

Review of the rotorcraft flight manual (RFM) emergency procedures revealed, "2. If an abnormal engine conditions occurs such as engine stall, flame-out, or overtemperature, transition to single engine flight or landing…."

Verbatim NTSB analysis and probable cause from the NTSB dataset