Safety Matters and Safety Briefs are based on the AAIB Bulletin and UK Airprox Board reports, with additional material from the US National Transportation Safety Board
Aircraft Type: 1 Cessna 152 2 Guimbal Cabri G2 Date & Time: 17 November 2017 at 1201 Commander's Flying Experience: 1 CPL, 419 hours, around 400 on type Last 90 days: unknown Last 28 days: 19 hours 2 ATPL(H), 25,000+ hours, unknown on type Last 90 days: unknown Last 28 days: unknown
This was the C152 instructor’s second instructional flight of the day, to carry out best rate of climb and glide descent exercises with a student. After takeoff from Wycombe Air Park they climbed steadily to 2,000ft before turning on course to the local training area northwest of the airfield and at 1150 confirmed with Wycombe Tower that they had left the circuit area. There were no further radio communications from the aircraft, which reached 4,000ft then turned left onto a steady north-westerly course and started a sustained descent.
The Cabri G2’s instructor had also completed one training detail that morning, a navigation exercise in the local area to the north-west of Wycombe, routeing via Silverstone and return. The second sortie was to repeat this with a different student. They departed at 1147, climbing initially to the south-west, before turning north then north-west on track to Silverstone. The instructor advised Wycombe Tower they were leaving the circuit to the north, and then climbed to and maintained an altitude of around 1,500 ft amsl. There were no further radio communications from the helicopter.
At 1201 the two aircraft collided. Although no-one saw the actual collision, immediately before they came together a witness on the ground about 0.5nm to the south-west saw them in close proximity, at around 20m apart and “flying fairly low [with] the plane gliding down slightly and the helicopter directly underneath the plane and seemed to be rising underneath it.” A 300m long wreckage trail from both aircraft fell into a wooded area in the grounds of Waddesdon Estate. All four of their occupants were killed.
The C152 had been fitted with a transponder which transmitted aircraft pressure altitude to the nearest 100ft. This showed that it had reached a maximum altitude of 4,130ft which was maintained for thirteen seconds before it began descending, turned left onto approximately 340°T and remained on this track throughout its descent, which continued for the next two minutes and seven seconds, at which point the recorded altitude was 1,530ft.
The Cabri G2 was fitted with a Garmin GTX335 Mode S transponder which responded to radar interrogation and periodically transmitted ADS-B out, allowing its position to be received and recorded. After takeoff it had turned right, climbing to approximately 1,500ft, initially heading north before turning onto a track of 340°T. Throughout the remainder of the flight, its altitude varied between 1,380-1,555ft.
At 1159:10 the C152, with higher groundspeed, was approximately 0.5nm behind the Cabri and 1,950ft above. A minute later it was 750ft above and 1,000ft behind. At 1200:53 the C152 was at 1,500ft, the helicopter at 1,455 ft. Vertical and horizontal separation reduced progressively to the point of the collision.
The AAIB report notes: ‘Air Traffic Services are available in the area of the accident, but the location is on the boundary of several different providers, thus, an aircraft manoeuvring on a typical training flight within the accident area would probably need to keep changing frequencies. External communications can create an additional distraction and increase the workload during an instructional flight, which may be why neither instructor attempted to contact an ATS other than Wycombe Tower. The collision was outside of the promulgated area for Farnborough North LARS and was below their 1,500ft amsl altitude restriction for provision of a Traffic Service. The busy airspace and restricted radar coverage in the area was such that any of the possible service providers would probably only have been able to offer, at best, a Basic Service.
‘As neither aircraft was electronically conspicuous to the other, the only available method of collision avoidance between the two aircraft was “see and avoid”. There are considerable and well understood limitations to “see and avoid” and there was no evidence to suggest that the occupants of either aircraft had seen each other in time to avoid the collision. The C152 was descending from above the Cabri on a similar course and gaining ground. The (C152’s) planned exercise was 8.1 Glide Descent. However, it was descending at a higher average rate and airspeed than would be expected for a best angle glide, so it is possible that a higher airspeed was used, with a correspondingly increased rate of descent. The angle at which the aircraft were closing was such that neither was in the field of view of the other until perhaps a few moments before the collision.’
The damage sustained by the Cessna indicated that initial contact had been between its right wing and the Cabri’s main rotor blades, with the wing being struck from below, and the relative geometry of the two aircraft and the 110° angle of the cuts suggest that the C152 was slightly ahead of the helicopter immediately before the collision and that one or both aircraft may have been manoeuvring immediately prior to the collision. ‘[Thus] the possibility of sudden evasive action cannot be discounted,’ says the AAIB.
Its report continues: ‘Fixedwing aircraft with forward-mounted engines have a restricted view ahead and below the flightpath. As a consequence, to see into the blind spots and to determine that the area into which it is descending is clear, an aircraft would have to be manoeuvred, generally by conducting a series of shallow turns. Radar data indicated that, when the Cessna was 1,950ft above the Cabri, it was only 0.5nm behind it. A simple assessment indicates that, if the C152 was straight and level with zero pitch angle at 2,000ft above the helicopter, it would have to have been at least 1.9nm behind for the pilot to have had any opportunity of detecting a possible conflict without additional manoeuvring. In the absence of a turn, the pilot would need to have pitched the aircraft at least 24° nose-down to have had any chance of observing the helicopter. The situation whereby additional manoeuvring would have been required (by the Cessna) to bring the helicopter