What happened
On 26 July 1998, an Aerospatiale AS355 F1 Ecureuil II, registration G-MASK, was performing an air ambulance mission near Rochester Airport when it struck an electric power cable and crashed. The aircraft, operated by Kent Air Ambulance, was returning to base after attending a road traffic accident near the M2 motorway.
Following the medical task, the crew departed from a field adjacent to the motorway. During the flight, the helicopter followed a path involving several legs, eventually heading towards Rochester Airport. Witnesses on the ground reported hearing unusual noises, describing the engine sound as spluttering, misfiring, or running rough. Some observers also noted seeing flashes of light near the rotor head during the initial takeoff. As the aircraft approached the airport, it was observed performing a sharp, steep turn and a sudden, near-vertical climb followed by a steep dive, ultimately impacting the ground and bursting into flames.
The investigation
The AAIB investigation examined radar data, radio recordings, and extensive eyewitness testimony. Radar tracking from the Debden radar head showed the helicopter flying at altitudes between 300 and 600 feet above the ground during its final approach. While radio communications from the commander appeared normal, a significant number of witnesses—14 out of 18 who provided specific comments—reported that the aircraft sounded mechanically abnormal.
Investigators also scrutinized the aircraft's mechanical condition. Although the intense post-crash fire destroyed much of the wreckage, making it difficult to confirm specific mechanical failures, there was no definitive evidence of a pre-existing technical defect that would have forced the pilot to deviate from his approach. The investigation also noted that the G-MASK was not equipped with a wire-strike protection system.
Findings
- The primary cause of the accident was the collision of the helicopter with an electric power cable.
- The aircraft was flying at a low altitude during its final approach to the airport.
- Numerous witnesses reported highly unusual engine noises, though no specific mechanical cause for these sounds could be confirmed due to fire damage.
- The helicopter lacked any specialized equipment to detect or mitigate strikes against overhead wires.