What happened
On the night of 12 December 2002, a Sikorsky S7/6C, registration G-SSSE, was performing a public transport flight from the Trent Oil Platform in the North Sea. After completing a landing on the platform in darkness and overcast conditions, the crew loaded passengers and prepared for a 'towering' takeoff.
During the departure, the commander acted as the handling pilot. After lifting into a hover and initiating a nose-down attitude to accelerate, the aircraft experienced a sudden reduction in airspeed. The co-pilot observed the Vertical Speed Indicator (VSI) indicating a descent and noted that the airspeed was dropping. Despite the commander's attempts to arrest the descent by applying rearward cyclic, the aircraft's airspeed continued to decrease toward zero.
Recognizing the developing danger, the co-pilot requested control and executed a positive recovery by increasing power and establishing a nose-down attitude to regain speed. Once the aircraft climbed through 500 feet, the commander reassumed control. Upon reviewing the on-board Integrated Health and Usage Monitoring System (IHUMS), the crew discovered that several engine parameters had exceeded their limits. Consequently, they decided to return to Humberside for a technical inspection.
The investigation
The AAIB investigation examined the flight recorder data and the crew's recollections. Digital Flight Data Recorder (DFDR) analysis revealed that while the pilots believed they had achieved an airspeed of 50 kt, the actual airspeed remained below 10 kt during the critical phase of the maneuver. The data also showed that the nose-down pitch attitude during the rotation was significantly less than the 10 to 15 degrees required by company procedure.
Engine inspections following the event confirmed that while torque levels were within limits, the T4 temperatures for both engines had exceeded the maximum allowable threshold. The investigation also noted that the aircraft was turning right at a constant rate during the initial lift-off, a movement that was not noted by the co-pilot and may have contributed to spatial disorientation for the handling pilot.
Findings
- The helicopter was potentially close to entering a vortex ring state during the period when the recovery was performed.
- There was a discrepancy between the pilots' perception of airspeed and the actual recorded airspeed, which remained very low during the departure.
- The handling pilot's pitch attitude during the transition to forward flight was significantly less than the prescribed target.
- The lack of a visible horizon due to overcast conditions and the absence of external visual references contributed to the difficulty of the maneuver.
- Engine temperature exceedances occurred during the event, necessitating maintenance and subsequent inspections.