Incident Overview
On 12 December 2002 at 1737 UTC, a Sikorsky S76C helicopter, registration G-SSSE, was conducting a night departure from the Trent Platform in the North Sea. The commander was the handling pilot. After lifting into a hover and turning into wind, the commander began a 'Towering' takeoff. At about 15 feet radio altimeter height, he applied forward cyclic control. Both pilots saw 50 kt indicated on their airspeed indicators, and the commander saw a momentary indication of 70 kt. The commander then checked back on the cyclic control. The co-pilot observed the airspeed decreasing and the vertical speed indicator (VSI) showing a slight descent. He called "Airspeed Decreasing" and then "Rate of Descent". The commander applied more rearward cyclic to arrest the descent. By then, the airspeed was near zero. The co-pilot called "I have control" twice and the commander handed over control. The co-pilot executed a positive recovery by increasing power, selecting a nose-down attitude, increasing speed, and establishing a climb. At 500 feet amsl, the commander reassumed control and levelled at 1,500 feet amsl. The crew then interrogated the Integrated Health and Usage Monitoring System (IHUMS) and found exceedances on six systems. They decided to return to Humberside for an inspection. An uneventful run-on landing was made at Humberside.
Flight Recorder Data
The Digital Flight Data Recorder (DFDR) data showed that G-SSSE lifted into a low hover for about 6 seconds, with pitch attitude between 3° and 7° nose-up and heading changing from 025°M to 050°M. Over the next six seconds, radio height increased to 31 feet, pitch attitude decreased to about 2° to 3° nose-up, and heading changed to 068°M. In the next second, radio height increased to 160 feet with pitch attitude increasing. Over the subsequent 5 seconds, radio height slowly increased to 175 feet, pitch attitude reached 17° nose-up, and heading changed to 080°M. Pitch attitude then reduced momentarily to 12° nose-up before increasing to 19° over the next six seconds. Heading increased to 110°M and radio height reached 210 feet. Then pitch reduced to 0° after 8 seconds; the aircraft descended to 180 feet and stabilised on heading 123°M. Collective pitch was increased and engine torques increased to approximately 107%. Pitch was then reduced to 15° nose-down. Up to this point, there was no indication of airspeed above the recording cut-off of 10 kt. Then, as the aircraft descended to 160 feet radio height, DFDR airspeed started to register, main rotor speed reached its minimum of 100%, heading remained constant, airspeed increased quickly to 70 kt, and radio height increased to 500 feet. At that height, the rate of climb reduced and airspeed further increased before the aircraft climbed away at approximately 0° pitch attitude and turned left onto heading 085°M.
Engineering Information
After landing, engineering interrogation of the IHUMS card was inconclusive, so DFDR readings were consulted. The recorded values for engine torque were 99% (No 1) and 122.4% (No 2); gas generator speeds were 102.1% (No 1) and 102.3% (No 2), within the 2.5-minute rating limit. However, T4 temperature limits of 885°C were exceeded: 900°C (No 1) and 913°C (No 2). Main gearbox torque and output torque were within limits. Maintenance for an overtemperature event was completed; no faults were found, and the helicopter was returned to service. After three days of uneventful flying, the engine manufacturer requested inspections for an overtorque event, leading to removal of No 2 engine module 5. No faults were found, and a successful post-maintenance air test was completed before G-SSSE returned to service.
Operational Information
The commander was dual qualified on the Sikorsky S76 and Eurocopter AS365N. In the 28 days prior, he flew 7 hours on the AS365N and 38 hours on the S76, including 19 hours of instrument flying on the S76. At the time of the incident, he had been on duty for 6 hours following a 26-hour rest period. The co-pilot was also dual qualified, with 1,600 total hours, 502 on the S76. In the 28 days prior, he flew 31 hours on the AS365N and 14 hours on the S76, including 4 hours of instrument flying on the S76. He had been on duty for 6 hours.
A company Flight Safety Officer flew with another crew in similar visibility to evaluate Trent Platform lighting. He considered the platform brightly lit, with a marked contrast for the incident crew lifting off at night with an overcast cloud base. It was not possible to see the sea surface. No fixed installation or oil rig was located in the direction of the departure, so the crew had no visual reference in that direction.
Discussion
Both pilots were qualified and well rested; no unserviceabilities were detected with G-SSSE prior to the incident. The operation was familiar to the crew. The aircraft entered an unexpected and hazardous situation, causing the co-pilot to take control. One conclusion from the operating company's investigation was that the helicopter was potentially close to a vortex ring state when recovery action was taken. Without CVR evidence, crew recollection was important. The DFDR was vital in quantifying aircraft parameters. Both pilots described the selection of nose-down attitude as positive, but the pitch attitude did not go below 0° nose-down, compared to the normal target of 10° to 15°. With an airspeed of 50 kt noted by both pilots, the commander raised the nose to initiate a climb. DFDR evidence indicated airspeed was unlikely to have exceeded 10 kt; with a surface wind of 100°/25 kt, the helicopter would have been travelling rearward. With the co-pilot confirming a rate of descent, positive correction was required. There was a short delay after the co-pilot's first call to take control. The co-pilot had a clearer appreciation of the dangers and made a correct assumption of control.