What happened
On 6 November 2013, a Eurocopter EC155B1, registration OY-HJJ, was performing a commercial passenger transfer from the Clipper gas platform in the North Sea. During the departure, the flight crew experienced a series of severe pitch oscillations. Following takeoff, the commander initiated a significant nose-down attitude to ensure clearance from the helideck, driven by concerns regarding the aircraft's performance. As the helicopter transitioned to a climb, the crew engaged the autopilot's Go-Around (GA) mode.
Shortly after engagement, the crew perceived the aircraft was descending rather than climbing. This led to manual control inputs that caused the aircraft to pitch up to 18 degrees nose-up, with airspeed dropping to near zero. The aircraft subsequently entered a steep descent, reaching a 36-degree nose-down attitude and descending to approximately 50 feet above the sea surface. A final pitch excursion reached 20 degrees nose-up before the crew successfully recovered the aircraft to normal flight and landed safely at Norwich Airport.
The investigation
The AAIB examined flight data, cockpit voice recordings, and the aircraft's Automatic Flight Control System (AFCS). The investigation revealed that while the crew believed the aircraft was underperforming, the flight data showed the AFCS was functioning as expected. Investigators discovered that the aircraft was heavier than the crew believed due to a combination of an outdated weighing record and a fuel gauge calibration error.
Technical examination of the helicopter identified a loose potentiometer within the collective lever position detector, which had caused intermittent AFCS warnings on the helideck. Additionally, investigators found that a missing component on the stationary swashplate had caused chafing, though this was not a direct cause of the flight path deviations.
Findings
- The crew's perception of the aircraft's flight path diverged significantly from reality, leading to inappropriate manual control inputs.
- The commander's decision to use an exaggerated nose-down attitude, combined with power reduction, triggered the initial descent.
- Pre-existing technical warnings regarding the AFCS predisposed the crew to believe the aircraft was malfunctioning.
- Undetected errors in the aircraft's recorded mass and fuel calibration led the crew to believe the helicopter was underperforming.
Safety action
Following the investigation, the operator implemented several changes, including a prohibition on using the GA mode during takeoff, revised procedures for autopilot engagement, and enhanced simulator training focusing on unusual attitude recovery. The manufacturer also updated repair instructions for the swashplate assembly.