IAD05MA078 · Sikorsky S-76C

While facing west on a crowded urban heliport on the leeward side of tall buildings that blocked westerly winds of 15 knots gusting to 23 knots, the pilot executed a rearward takeoff in calm winds slightly above the maximum gross weight of the helicopter. Once clear of the heliport and while out-of ground-effect, the pilot executed a right pedal turn for a north departure over the water, and lowered the nose to initiate an acceleration. Simultaneously the helicopter encountered a left quartering tailwind that was originally blocked by the tall buildings. The helicopter began to settle, and contacted the water. While executing the right pedal turn to transition to forward flight, the pilot noticed an audible "degrading" of the rotor rpm, and the N1 in the "yellow." During interviews with the flight crew, neither pilot could articulate what the maximum allowable gross weight was for the environmental conditions. Each pilot could only say the helicopter was "good" for the takeoff. According to performance data, the maximum gross weight for takeoff was 11,700 pounds, but the data didn't provide a means for calculating power margins. Prior to flying into the heliport on the day of the accident, the pilots had the helicopter was fueled to capacity. This decision was based on picking up five passengers instead of the six that actually boarded. Aware of the additional passenger and without conducting any additional performance planning, the pilot attempted the departure. Prior to contacting the water, the crew experienced uncommanded pitch and roll oscillations, and high levels of vibration consistent with settling with power. Examination of the wreckage, post accident engine runs, and testing of the digital engine control units revealed no preimpact anomalies.

Verbatim NTSB analysis and probable cause from the NTSB dataset