Background
On 11 October 2015, a formation of two Puma helicopters (designated A21 and A22) was conducting approaches to SOC, a helipad adjacent to a Persistent Threat Detection System (PTDS) aerostat. The PTDS was moored near the helipad and its tether was marked with flags to increase visibility.
Sequence of Events
The formation's departure and transit to SOC were uneventful. After a go-around due to civilians on the helipad, the formation flew downwind. There was a miscommunication between the crews regarding whether another approach would be conducted immediately. The formation leader elected to orbit, but did not communicate this to A22.
During the latter part of a conversation about ground features, both pilots of A22 focused their attention on a building to the left. The lead aircraft moved rapidly to the right, causing A22 to lose visual contact with A21. Approximately 13 seconds before striking the tether, A22 was within 530 m of the PTDS and turning right at about 20° angle of bank. The aerostat was outside the captain's field of view. The captain had stated he was "visual with the wire" 20 seconds prior, but was likely looking for A21 in the moments before impact.
At about 430 ft AGL, A22 initiated a rapid left roll. The main rotor blades contacted the tether on the right side. The tether was drawn onto the tail cone and failed under tension. The area microphone recorded a sharp "thwack" sound, and a passenger reported hearing a "pop" and feeling a jerk.
Contributory Factors
The investigation panel identified the following contributory factors:
- Loss of visual contact with the lead aircraft.
- Lack of situation awareness regarding the PTDS tether.
- The tether markings were considered an "Other Factor" as they did not directly cause the accident but reduced the crew's ability to identify the hazard.
Findings
The panel concluded that the crews were aware of the PTDS but did not routinely mention it due to the frequency of approaches and use of ground features for procedural clearance. The RWAMMWAS (wire alerting system) was not used. The tether and flags were difficult to see, and the proximity of the helipad to the danger area boundary meant aircraft operated unavoidably close to the hazard.