Cat. 2 investigation report: simplified-format report, adapted to the circumstances of the occurrence and the investigation stakes.
The pilot of the PA-34, the only person on board, was carrying out an IFR flight between Heubach airport (Germany) and Strasbourg-Entzheim airport (Bas-Rhin). He was in descent and in a layer of cloud, in IMC.
The crew of the A320 took off from Baden-Baden airport (Germany), bound for Hamburg (Germany).
The crews of both aeroplanes were in contact with Strasbourg approach. At 14:44, on first contact with the approach, the pilot of the PA-34 received clearance to descend to FL80 on heading 275, in order to be vectored for an ILS for runway 23. The pilot read back the clearance.
A few minutes later, the crew of the A320, in initial climb, received a clearance to climb to FL70 initially and then turn onto heading 045. The controller then asked the crew of the A320 to reduce the rate of climb, informing them of the position of IFR traffic (the PA-34) “5 NM east of your position, 1000 ft above your clearance.”
Observing that the PA-34 had flown through FL80 in descent, the controller asked the pilot to confirm his flight level and then to climb immediately to FL80: the pilot read back but did not climb.
The controller then asked the crew of the A320 to immediately turn left onto
heading 045 and then a few seconds later, the pilot of the PA-34 to immediately turn left onto heading 210. The two crews read back. The crew of the A320 followed the clearance and increased the left turn while the pilot of the PA-34 turned right towards the north, contrary to the clearance he had received.
The controller asked the pilot of the PA-34 what had happened. The latter replied that he had encountered difficulties with the autopilot and that he was trying to hold the aeroplane. The controller again asked him to climb to FL80 using the emergency phraseology but the pilot continued to descend.
At 14:51:51, the crew of the A320 reported a TCAS resolution advisory and started to descend. The minimum separation between the two aeroplanes was reached with a vertical separation of around 750 ft. The PA-34 reached its lowest point, FL74, and then started climbing. A few seconds later, the crew of the A320 reported that the conflict was over, that they were returning to their heading and joined FL70.
Complying with the controller’s instructions, the pilot of the PA-34 reported that he was established on the Glide and LOC five minutes later. He landed on runway 23 at 15:02.
When the controller identified that the pilot of the PA-34 was descending below the clearance for FL80, he immediately identified a potential conflict with the Airbus A320 taking off. His instructions were given to the crews of both aeroplanes using the emergency phraseology. The controller was not aware of the difficulties encountered by the pilot of the PA-34 and had no means of understanding his manoeuvres.