Introduction
On December 24, 2009, at approximately 1231 central standard time, a Sikorsky S-76C++ multi-engine helicopter, registration N798P, sustained substantial damage when it rolled over shortly after attempting to land on Vioska Knoll 956-A (also known as Ram Powell), an offshore tension-leg platform in the Gulf of Mexico. The helicopter remained on the platform. The airline transport pilot rated captain and commercial rated second-in-command, along with five passengers, were not injured. The helicopter was registered to and operated by PHI, Incorporated, Lafayette, Louisiana. A visual flight rules (VFR) flight plan was filed for the flight that originated at the operator's base in Boothville, Louisiana, and was destined for VK956. Visual meteorological conditions prevailed for the flight conducted under 14 Code of Federal Regulations Part 135.
History of Flight
The purpose of the flight was to drop off the five passengers; no return passengers were planned. The second-in-command (SIC) stated that upon arrival, they performed a 360-degree high reconnaissance turn around the platform to confirm the deck was clear. Winds were reported from 174 degrees at 30 knots. Initially, the SIC was the pilot-flying, but due to wind direction and visibility, control was transferred to the pilot-in-command (PIC) in the left seat. The PIC flew a visual approach directly into the wind. The landing was normal, and the helicopter was in the center of the aiming circle, but the tail rotor blocked the exit stairwell. The PIC then began a ground taxi to the right to clear the tail. After the SIC confirmed the tail was clear (about a 20-degree turn), the PIC moved the helicopter about five more degrees. At that moment, the helicopter "squatted down" and rolled quickly to the right onto its side. The PIC applied left cyclic but could not prevent the roll. The crew reported no adverse indications before the event.
Meteorological Information
The Ram Powell platform was equipped with an aviation weather observation system, but the wind sensor had been inoperative for several months. Reported weather at 1215 included visibility 7 miles, scattered clouds at 1,200 and 1,600 feet, broken clouds at 2,900 feet, temperature 22°C, dewpoint 19°C, and barometric pressure 29.76 inches of mercury. A weather forecast issued by PHI at 1000 indicated a strong cold front moving east, bringing thunderstorms and gusty winds, with winds from the east at 15-25 knots and gusts to 25-35 knots, possibly 40 knots along the storm line. A weather warning was issued, and a tornado watch was in effect until 1300 over eastern Louisiana.
Flight Recorders
The helicopter was equipped with a cockpit voice recorder (CVR) and flight data recorder (FDR). The CVR captured the final five minutes, including the approach and the rollover. The FDR data, while downloaded, could not produce an accurate animation. The enhanced ground proximity warning system (EGPWS) recorded three bank angle warnings on the flight leg coinciding with the landing and subsequent roll. The EGPWS data also indicated that during the ground taxi, the collective was not fully down, the nose was pitched up, main rotor RPM was 110% (normal 107%), and the helicopter turned approximately 70 degrees before rolling.
Wreckage and Impact
The helicopter was examined at PHI's facility. The airframe remained relatively intact but was damaged along the right side. All three landing gear were down and locked, and functional testing showed normal operation despite damage to the right main gear door. The main rotor blades were not recovered except for a three-foot spar section; two blades fractured at the spindle and two at the blade cuff. Tail rotor blades were undamaged except for a fractured cuff counterweight. Both engines had fractured mounting bolts. Impact damage was consistent with a rollover at full operating rotor speed with both engines producing power. Flight control continuity was verified, and no anomalies were found in the automatic flight control system. The standby instruments indicated a 40-degree right roll and 8-degree pitch down.
Additional Information
The FAA Rotorcraft Flying Handbook describes dynamic rollover, noting that it can occur when a helicopter pivots around a skid or wheel, and if the critical rollover angle is exceeded, recovery is impossible. The handbook states that dynamic rollover cannot be stopped by opposite cyclic alone; lowering the collective quickly is most effective. PHI's training program addressed dynamic rollover for slope operations and stuck skids but did not include specific ground taxi techniques for wheeled helicopters.