What happened
On the night of 9 October 1998, a Eurocopter AS355N Ecureuil II, registration G-EMAU, was preparing for a police air support mission near Welford, Northamptonshire. After assessing the weather as suitable for visual flight, the crew of three—consisting of the pilot and two observers—commenced a departure from their helipad. The aircraft began a gentle right turn over open farmland, following a standard procedure for the site.
At an altitude of approximately 25 to 30 feet and a low airspeed, the helicopter suddenly encountered a bank of fog. To mitigate glare from the landing lights, the pilot switched them off, but this sudden loss of external visual references coincided with the aircraft entering the fog. During the subsequent attempt to transition to instrument flight, the pilot struggled to maintain control. The helicopter struck a line of trees, causing the aircraft to roll and eventually come to rest inverted. A fire brokeed out shortly after the impact, destroying the wreckage. One passenger sustained fatal injuries, while the pilot and the second observer sustained serious injuries.
The investigation
The AAIB investigation examined the meteorological conditions, the serviceability of the aircraft's instruments, and the pilot's transition to instrument flight. Investigators found that while the weather had recently cleared, the presence of saturated ground and light winds had created localized mist and fog.
Technical examination of the main attitude indicator (AI) revealed that its internal lighting was unserviceable, meaning the instrument was being illuminated by an external pillar lamp. This arrangement created shadows across the display, making it more difficult to assess the aircraft's bank angle. The investigation also looked into the aircraft's structural integrity, noting that the AS355N model featured unlined structural fuel tanks, which are more susceptible to rupture during high-impact accidents compared to models with self-sealing cells.
Findings
- The sudden and unexpected loss of visual cues when entering the fog bank disrupted the pilot's ability to transition to instrument flight.
- The pilot became disoriented due to the lack of external references and the difficulty in interpreting the poorly illuminated attitude indicator.
- The pilot failed to detect the aircraft's yaw acceleration during the turn, leading to an uncertain perception of the helicopter's position.
- The use of an external pillar lamp to illuminate the instruments increased the mental workload required to maintain situational awareness.
- The aircraft's low airspeed during the transition made the flight inherently more unstable and harder to control by instruments alone.