What happened
On 24 November 1998, a Robinson R4 and registration G-WMCN was engaged in a private flight from Milton Keynes to Denham. The flight, which included the pilot and two passengers, had initially proceeded without incident during daylight hours. However, during the return leg, weather conditions began to deteriorate significantly due to an approaching occluded front.
At approximately 1700 hrs, the helicopter departed Milton Keynes. While the pilot initially believed visibility remained good, the aircraft soon encountered rain and darkening skies. During the cruise, the pilot requested that a passenger hold a map and a handheld GPS unit to assist with navigation, necessitating the use of interior cockpit lighting. Shortly after, the pilot lost visual contact with ground references and believed the aircraft had entered a patch of low cloud. In an attempt to navigate by instruments, the pilot began a descent and a turn to reverse course. During this maneuver, the helicopter struck a tree and crashed into a field, resulting in extensive damage to the aircraft. The pilot sustained minor injuries and escaped the wreckage, but both passengers suffered serious injuries.
The investigation
The investigation examined the meteorological data, the pilot's flight experience, and the aircraft's flight path. Analysis of the recovered GPS receiver revealed that the helicopter had been maintaining a steady airspeed of 100 kt before reducing to 80 kt in the 90 seconds preceding the impact. The data also showed a gentle 30-degree left turn during this period. Investigators also reviewed the various weather forecasts available to the pilot, noting discrepancies between the pilot's interpreted forecast and the actual deteriorating conditions, which included mist, fog, and a low cloudbase.
Findings
- The primary cause of the accident was the loss of visual reference caused by the aircraft entering low cloud.
- The pilot's attempt to study a map and GPS using interior lighting significantly degraded his ability to maintain visual contact with the outside environment.
- The weather conditions, characterized by low visibility and a low cloudbase, were forecast and could have been identified by checking conditions at the destination.
- The aircraft struck a tree during a turn and descent initiated while the pilot was attempting to navigate through the cloud.