N798PSikorsky S-76C-2 · 2007

On December 24, 2009, approximately 1231 central standard time, N798P, a Sikorsky S-76C++ multi-engine helicopter, sustained substantial damage when it rolled over shortly after it attempted to land at Vioska Knoll 956-A (VK956-also known as Ram Powell), an offshore tension-leg platform located 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 the 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.

The purpose of the flight was to drop off the five passengers. There were no planned passengers for the return flight.

The Second-in-Command (SIC) stated that when they arrived at the platform, they made a 360 degree high reconnaissance turn around the platform to make sure the deck was clear. The winds were "holding steady" from 174 degrees at 30 knots. Initially, the SIC was the pilot-flying, but due to the direction of the wind relative to the approach and the visibility of the approach from the right seat, he turned the flying duties over to the PIC in the left seat. After a positive transfer of the controls, the PIC flew the visual approach directly into the wind. The SIC reported that the landing was normal and the helicopter was in the center of the aiming circle. However, the position of the tail rotor was blocking the exit stairwell. He said,"Aircraft was hovered approximately 1-foot aft of aiming circle [main gear one foot aft of circle]. Aircraft landing made, all gear on deck." The Pilot-in-Command (PIC) then began to reposition (ground taxi) the helicopter to the right so the tail would be clear of the stairwell. The SIC stated that he was looking out of his window to the right and told the PIC when the tail was clear of the stairwell (about a 20 degree turn). He said, "At that moment, I felt the aircraft rock towards the right. I looked forward again and realized the aircraft had pitched to the right and was continuing to roll rapidly. The initial pitch down-right was abrupt as if the right main gear had compressed. There had been no adverse indications prior to rolling."

The PIC, who was seated in the left seat, stated that at the time they arrived at the platform, the wind was out of the southeast at 30 knots. After landing on the platform, the tail of the helicopter was blocking the stairwell, so he ground-taxied the helicopter to the right about 20-30 degrees. The second-in-command (SIC), who was in the right seat and looking out of the right front window, told the PIC when the tail was clear. The PIC then moved the helicopter about 5 more degrees to the right when he felt the helicopter "squat down." He responded by moving the cyclic to the left, but the helicopter rolled quickly to the right and on to its side. The PIC said he did not make an attempt to pick the helicopter up into a hover.

A review of PHI's critical wind azimuths for ground operations of medium/heavy aircraft on heli-decks and elevated airports revealed that operations with a 0 to 30 degree crosswind of 50 knots or less was permitted. Operations in crosswinds between 30 and 60 degrees in 40 knots or less were restricted and ground operations with a crosswind greater than 60 degrees with winds greater than 30 knots were prohibited.

The PIC, age 53, held an air transport pilot (ATP) rating for air airplane single-engine land and rotorcraft-helicopter. His last Federal Aviation Administration (FAA) First class medical was issued on August 1, 2009. The pilot reported a total of 14,459 flight hours; of which, 2,826 hours were in the same make/model as the accident helicopter.

The SIC, age 57, held a commercial pilot certificate for airplane single-engine land and rotorcraft-helicopter with an instrument rating for both airplanes and helicopters. His last FAA Second class medical was issued on June 1, 2009. He reported a total of 18,103 flight hours; of which, 332 hours were in the same make/model as the accident helicopter.

The Ram Powell platform was equipped with an aviation weather observation system (AWOS); however, at the time of the accident, the wind sensor was out of service and had been inoperative for several months. The reported weather at 1215 was visibility 7 miles, scattered clouds at 1,200 and 1,600 feet, broken clouds at 2,900 feet, temperature 22 degrees Celsius, dewpoint 19 Celsius, a barometric pressure setting of 29.76 inches of Mercury.

A weather forecast issued by PHI at 1000 on the morning of the accident reported a strong cold front pushing east across their entire service region bringing thunderstorms and gusty winds. The winds were forecasted to be from the east between 15 and 25 knots, with gusts of 25-35 knots and possibly 40 knots along the line of storms. As a result of this forecast, PHI issued a weather warning regarding the passage of the cold front. The warning reported a line of thunderstorms along the edge of the front that were moving east across the Gulf of Mexico at 35-50 knots. There was also a tornado watch in effect until 1300 over the eastern half of Louisiana that extended approximately 25 miles beyond the coastline.

The helicopter was equipped with a Penney & Giles dual cockpit voice recorder (CVR) and flight data recorder (FDR), in addition to a Honeywell Aerospace enhanced ground proximity warning system (EGPWS).

Cockpit Voice Recorder

A CVR group was formed on January 26, 2010, and the last five minutes of the recording were transcribed. It began with the PIC calling the platform to tell them that they were 5 minutes out. When the helicopter was approximately 2.5 miles from the platform, the automated passenger briefing was heard. It was also at this time that the SIC transferred controls to the PIC, and he began the approach-to-landing with a 360 degree high reconnaissance turn around the platform.

At 1228:06, the SIC announced that the emergency floats were armed and the Before-Landing checklist was completed. Shortly after, the PIC announced the "decision point." The EGPWS was heard giving the 100-foot and 10- foot-high call-outs, and the SIC confirmed when the helicopter was over the deck of the platform.

At 1230:51, the PIC stated that he was maneuvering the tail to the left and the nose to the right, so the helicopter was not blocking the exit stairwell. The SIC then told the PIC when the tail was clear.

At 1231:10, the sound of the helicopter was heard as it rolled over on the deck, followed by numerous sounds of the passengers exiting the helicopter.

Flight Data Recorder

An FDR Group was not formed, but data was downloaded and provided to PHI, Sikorsky, and the FAA. Due to limitations of the FDR data, an accurate animation of the flight could not be created.

Examination of the EGPWS was conducted at the facility of Honeywell Aerospace, Redmond, Washington, on February 3, 2010, under the supervision of the Safety Board. The EGPWS was received undamaged and was disassembled. The printed circuit boards were also found intact and undamaged. The EGPWS was reassembled and placed on a test bench. Once powered was applied to the test bench, the data retained within the EGPWS was successfully downloaded from the flash memory chips. Using company software, the downloaded data was converted into a readable format. Review of the downloaded data revealed that a takeoff from a PHI facility was recorded in flight leg 2744 at an EGPWS operating time of 2988:11:22. A landing was logged for flight leg 2744 at an EGPWS operating time of 2988:51:05. Three separate bank angle warnings were recorded at an EGPWS operating time of 2988:51:31, 2988:51:33, and 2988:52:22 within flight leg 2744. A second takeoff was recorded at an EGPWS operating time of 2988:52:23 within flight leg 2745. A landing for flight leg 2745 was recorded at an EGPWS operating time of 2988:52:23. Three separate bank angle warnings were recorded within flight leg 2745 at EGPWS operating times of 2988:52:24, 2988:52:32, and 2988:52:40. These bank angle warnings coincided with the time the helicopter touched down on the platform and then subsequently rolled.

A review of the EGPWS data also revealed that at the time the PIC began to move the tail away from the exit stairwell, the collective was not in the full down position, the nose of the helicopter was pitched up, and main rotor RPM (Nr) was 110 percent (the normal operating main rotor RPM of the S-76C++ is 107 percent). The pilot also turned the helicopter approximately 70 degrees prior to it rolling over. These findings were consistent with data downloaded from the FDR.

The Ram Powell platform, also known as Vioska Knoll (VK956), is a tension leg platform with an attached Nabors P-17 platform rig. The heli-deck is 52-feet-wide and 92-feet-long,and approximately 170-feet above mean sea level. The main landing area is 52-feet by 52-feet, with the remaining space utilized for parking. This platform easily accommodated the S-76C++ helicopter, which has a main rotor diameter of 44 feet.

The helicopter was moved to PHI's main facility in Lafayette, Louisiana, and was examined by representatives of Sikorsky and PHI under the supervision of the FAA. The airframe remained relatively intact, but was damaged along the right side of the fuselage. The windshields were intact, but the right side pilot's second chin window was heavily scraped from contact with the deck of the platform.

The right cabin door was damaged, and the upper center and forward lower corner of the pop-out window were displaced outward from the weather-stripping channel. The rain gutter above the right cabin door was damaged. The top of the right forward opera window was partially dislodged from the weather-stripping channel.

The right main landing gear door was cracked at the outboard edge of the non-skid surface; slightly damaged and abraded at the forward outboard corner.

The tail cone was slightly buckled on the left side. The horizontal stabilizer was substantially damaged and had shifted in its mount approximately four inches to the left.

All three landing gear were in the down and locked position. The helicopter was placed on jacks and the landing gear was functionally tested. All three gears operated normally, despite the damage to the right main landing gear door. The nose landing gear doors could not be tested due the position of the jack.

The tail cone and vertical pylon remained intact. Continuity for the tail drive shafts, intermediate and the tail gearbox were verified. The tail gear box fairing and forward section of the tail gear box outer housing exhibited some impact damage.

The main rotor blades (MRB) were not recovered, other than an approximately 3 foot section of spar. Two blades fractured at the spindle and two blades fractured at the blade cuff.

The tail rotor blades (TRB) were not damaged, except for a cuff counterweight on the blue-blade that had fractured and separated.

The engines were examined externally only. Both engines had fractured at least two mounting bolts from their semi-circular forward engine mounts. Remnants of the broken bolts were observed on the deck of each engine's bay.

Impact damage was consistent with a rollover while operating at full operating rotor speed and both engines producing power.

The throttle quadrant was intact. Both engine speed selectors were OFF (fully aft). Both fuel selectors were in OFF. Both T-handles were in the AFT position. The crew reported placing the controls in these positions after the event.

Examination of the flight control system revealed no damage to the flight controls. Hydraulic ground power was applied to the aircraft, and the controls were exercised. Mechanical continuity and smooth operation was observed from the cockpit to the swash-plate. Three pitch change rods were fractured, and damage to the main rotor head prevented testing beyond the swash-plate, or measurement of blade angles.

A slight interference was noted at a cotter pin on the nut retaining the #2 Roll SAS actuator to the roll ratio crank in the mixing unit. The cotter pin had rotated upwards and the free end was rubbing against the 76400-03211-041 walking beam. The scoring on the walking beam was light and appeared to be new. The interference was easily overcome, both with and without hydraulic power applied. Rigging pins were installed in the neutral rig position, and all lined up properly, indicating the basic flight control relationships had not changed. A full rig was not possible, due to the main rotor head and blade damage.

The aircraft was equipped with a Honeywell SPZ-7600 Digital Automatic Flight Control System (DAFCS). This system was a fully coupled, four axis (pitch, roll, yaw and collective) flight control system combining autopilot and flight director functions. The system also incorporated several additional features to reduce pilot workload: auto trim, heading hold, automatic turn coordination, and auto level.

The autopilot provided two basic modes of operation: Stability Augmentation System (SAS) and Attitude Retention (ATT). The SAS mode provided short-term rate damping during hands-on flying while the ATT mode was used for hands-on or coupling to the flight director. The flight director provided steering commands which could have been manually flown by the pilot or coupled to the autopilots for fully automatic flight path control. The system consisted of two FZ-706 flight control computers, a PC-700 autopilot controller, two MS-700 flight director mode selectors, two Penney & Giles air data computers, and an AL-300 command display. It was integrated to a variety of avionics equipment such as NAV receivers (short range and long range), gyros/AHRS, radar altimeters and cockpit indicators including Electronic Flight Instrument System (EFIS) to provide pilot workload relief.

Aircraft control inputs are achieved through the use of series actuators in pitch, roll and yaw. There were three series actuators per system, each with a linear stroke of ±8 mm which is equivalent to ±5 percent of total control authority per actuator. Series actuator positions were displayed on an AFCS indicator panel located in the pedestal. After applying power to the aircraft, it was noted that both SAS roll channels were biased to the left direction, indicating the SAS had attempted to correct the right roll during the event.

Auto trim was achieved through parallel rotary actuators in pitch and roll with 100 percent authority [However, trim actuators are rate limited to less than 10% per second]. These actuators also provided magnetic brake and artificial feel functions. A similar rotary actuator also provided collective control and trim. An electro-hydraulic pedal damper/trim actuator provided yaw trim.

Autopilot malfunctions are annunciated on a DAFCS caution panel located on the instrument panel. Either pilot can override the AFCS functions.

The cyclic, collective, and yaw controls were examined for evidence of malfunctions or pre-impact failures. Mechanical continuity was established for all DAFCS trim and SAS servos. No anomalies were noted.

The instrument panel was intact. Only the standby instruments provided any visual indications, since most of the instruments were glass multifunction display units. The standby airspeed indicator showed 0 knots. The altimeter was set to 29.72 inches of Mercury in the Kollsman window. The standby gyro showed an approximate 40 degree right roll and an approximate 8 degree pitch down. The G/S, NAV, and OFF flags were in view.

Due to the severe weather moving across the Gulf of Mexico, the PIC and SIC were stranded on the platform and mandatory drug testing was delayed for several days. Test results were negative for both pilots.

This impact was survivable and no injuries were reported. Egress for the five cabin occupants was through the left cabin door's pop-out window and the copilot's (left hand) door, which was opened normally (not jettisoned).

According to the FAA Rotorcraft Flying Handbook (FAA-H-8083-21), page 11-7 and 11-8, Dynamic Rollover, "A helicopter is susceptible to a lateral rolling tendency, called dynamic rollover, when lifting off the surface. For dynamic rollover to occur some factor has to first cause the helicopter to roll or pivot around a skid, or landing gear wheel, until its critical rollover angle is reached. Then, beyond this point, main rotor thrust continues the roll and recovery is impossible. If the critical rollover angle is exceeded, the helicopter rolls on its side regardless of the cyclic corrections made.'

"Dynamic rollover begins when the helicopter starts to pivot around its skid or wheel. This can occur for a variety of reasons, including the failure to remove a tie down or skid securing device, or if the skid or wheel contacts a fixed object while hovering sideward, or if the gear is stuck in the ice, soft asphalt, or mud. Dynamic rollover may also occur if you do not use the proper landing or take off technique or while performing slope operations. Whatever the cause, if the gear or skid becomes a pivot point, dynamic rollover is possible if you do not use the proper corrective techniques.'

"Once started, dynamic rollover cannot be stopped by application of opposite cyclic control alone. For example, the right skid contacts an object and becomes the pivot point while the helicopter starts rolling to the right. Even with full cyclic applied, the main rotor thrust vector and its moment follows the aircraft as it continues rolling to the right. Quickly applying down collective is the most effective way to stop dynamic rollover from developing."

Dynamic rollover can occur in both skid and wheel equipped helicopters, and all types of rotor systems. The following are most critical for helicopters with counter-clockwise rotation: right side skid/wheel down, since translating tendency adds to the rollover force, right lateral center of gravity, crosswinds from the left, and left yaw inputs.

A review of PHI's training program relative to dynamic rollover focused on slope operations, stuck/skids/wheels, and tie-downs. It did not include specific ground taxi techniques, hazards or aerodynamics specific to wheeled helicopters.