What happened
On 4 May 2020, an air ambulance mission was underway to transport a patient from the Isle of Arran to a mainland hospital. The aircraft, an MBB-BK 117 D-2 with registration G-SASS, arrived at a landing site located at Sandbraes, near Brodick. This site, a grass sports field, was frequently utilized for coastguard, military, and medical flights.
After the patient was loaded, the pilot initiated the departure. Due to the specific constraints of the landing area, the takeoff profile required the helicopter to climb while moving slowly in a rearward direction until reaching the takeoff decision point (TDP). The pilot estimated that the TDP, set at a height of 210 ft, was reached as the aircraft passed over the western edge of the field. As the helicopter transitioned into forward flight and increased power, the resulting downwash struck a nearby static caravan. This force caused a section of the caravan's roof to lift, which subsequently allowed insulation to be blown out from underneath.
The investigation
The investigation examined the takeoff procedures and the physical environment of the Sandbraes landing site. The field was surrounded by various obstructions and several static caravans positioned near the perimeter. While the site was secured by the coastguard during operations and met regulatory standards, the investigation looked into the impact of the aircraft's power increase on nearby structures.
Investigators noted that the operator had previously surveyed the site and had not identified the caravans as a hazard. Furthermore, the landing site had a history of hosting larger helicopters without prior incident. The investigation also considered the possibility that the pre-existing condition of the caravan roof might have contributed to its failure under the pressure of the downwash.
Findings
- The transition to forward flight necessitated an increase in engine power, which generated sufficient downwash to affect the nearby caravan.
- The increase in applied power during the transition to forward flight was the primary cause of the roof lifting.
- The potential condition of the caravan roof prior to the incident could not be determined and may have been a contributing factor.
- The operator has since adjusted the takeoff decision point to a lower altitude to minimize the downwash footprint in future operations.