What happened
On 6 July 2004, a Robinson R22 Beta helicopter, registration G-LIDS, was conducting a training flight near Elstree Aerodrome. The crew, consisting of an experienced instructor and a student, was performing flight exercises involving changes in airspeed and rotor RPM. Simultaneously, a Hybred 44XLR microlight, registration G-MTJP, was engaged in a private flight between Hunsdon and a site near St Albans.
At approximately 1,200 feet above Welham Green, the two aircraft collided. The helicopter instructor attempted an emergency left turn after detecting the microlight in his peripheral vision, but the impact occurred. The collision caused significant damage to the helicopter's cabin, including the loss of the left door and windshield disintegration. The helicopter was able to perform an emergency landing in a nearby field, though the instructor sustained a serious injury to his left foot. The microlight suffered severe structural damage and crashed into a wooded area, resulting in two fatalities.
The investigation
The AAIB investigation examined the wreckage of both aircraft and analyzed radar data from Stansted. The investigation established that the helicopter'1s left skid and nose made contact with the microlight's wing and upper surface, with the helicopter's main rotor blades striking the microlight's right wing tip.
Investigators also reviewed meteorological conditions, noting bright sunshine and a landscape of shadows and woodland. The study of the aircraft's equipment revealed that the two pilots were operating on different radio frequencies, meaning neither could alert the other to their presence. Furthermore, the investigation looked into the visual limitations of the human eye, specifically the difficulty of detecting small, slow-moving objects against complex backgrounds.
Findings
- The primary cause of the collision was that the occupants of both aircraft failed to see each other and take timely avoiding action.
- Low conspicuity of both aircraft due to their small size, profiles, and lack of relative movement against the background was a major contributing factor.
- The dark color of the microlight made it difficult to detect against the shadows of the woodland and fields.
- The white helicopter was difficult to see against the bright sky, and its anti-collision beacon was likely obscured from the microlight's view by the helicopter's own cabin.
- The use of different radio frequencies prevented any communication or warning between the two crews.