What happened
On 18 November 2011, an Airbus A300-605R, registration TC-MNV, was cleared to depart from RAF Brize Norton on the Malby Standard Instrument Departure. During the ground-to-tower frequency transition, the crew received a departure clearance containing a 'Climb-out Restriction' (COR) of 2,200 ft. However, the crew interpreted this figure as a reduction in available runway length rather than an altitude limit.
As the aircraft took off, the crew maintained a target altitude of 8,000 ft in the Flight Management System. This climb took the aircraft through the altitude of a Boeing Vertol CH-47D Chinook HC2, registration ZA720, which was performing instrument flying training in the vicinity. The close proximity of the two aircraft resulted in a TCAS Resolution Advisory (RA) for the A300 crew. The commander followed the RA by disconnecting the autopilot and adjusting thrust to avoid the helicopter, eventually clearing the conflict and resuming the departure.
The investigation
The AAIB examined radio transcripts and flight data from both aircraft. The investigation established that the A300 crew did not include their passing altitude or cleared level during their initial contact with the approach controller. Furthermore, the investigation found that the term 'Climb-out Restriction' was not standard radiotelephony phraseology and was not included in the CAP 413 Radiotelephony Manual.
Radar data from the Clee Hill and Heathrow sources confirmed that the two aircraft tracks merged on the controller's display. The investigation also noted that the Chinook's transponder was operating in Mode C rather than Mode S, which reduced the altitude resolution from 25 ft to 100 ft increments, though this did not significantly impact the TCAS performance in this specific encounter.