Bell 407 N373RL incident near Viosca Knoll 989

Probable cause

Engine power surges during takeoff for reasons that could not be determined during postaccident testing of the engine and the engine control unit.

NTSB analysis

The pilot reported that, about 30 seconds after takeoff from an off-shore platform, about 400 ft mean sea level, the helicopter experienced a series of compressor stalls and engine surges. The pilot adjusted the collective pitch and began a slow descent. After the pilot decreased the engine power, the surges and stalls ceased. However, when the pilot began increasing power while attempting to return to the platform, the surges and stalls recurred. The pilot reported hearing the low rpm horn and observing that the power turbine gauge was near the redline. The pilot subsequently entered an autorotation, inflated the floats, flared the helicopter, and then landed it on the water.

The engine was test run with an exemplar engine control unit (ECU) and T1 sensor installed and found to be operational, and no surges were noted. Although the engine produced 2 percent less power than specified on its power assurance chart, it could not be determined whether this affected the engine during the flight. Although the engine's bleed air valve closed early during the test run, the valve would have been closed per specification during the incident sequence of events.

The ECU had been removed from the engine and was examined separately. An acceptance test procedure and a vibration test were conducted with no discrepancies noted. Although the ECU failed an initial pressure check, after the ambient pressure sensor was recalibrated, it passed the check. A review of the ECU's recorded data indicated that the engine experienced an engine surge condition during the flight. However, the data did not reveal any anomalies that would have caused the engine power surges.

Text from the NTSB. Open the NTSB docket for CEN15IA046 for the full record.