The Swiss Accident Investigation Board released the report of their investigation into a serious incident in which a Boeing 737-800 dangerously converged with an Airbus A319 in Swiss airspace because the 737 initiated a climb without clearance.
On 12 April 2013, TAP flight TP706 and Ryanair flight FR3595 were cruising in Swiss airspace under the control of the Zurich Area Control Centre (ACC). At 16:00:53 UTC, while at FL 370, the crew of TP706, with the radio callsign “Air Portugal seven zero six”, an A319 on a scheduled flight from Lisbon (LPPT) to Prague (LKPR), reported to the Zurich ACC Upper Sector M4 air traffic controller. The crew of FR3595, with the radio callsign “Ryanair three five niner five”, (a B737 on a scheduled flight from Pisa (LIRP) to Lübeck (EDHL) also reported to the controller just a short time later at 16:01:11 UTC, while at FL 360.
At 16:10:43 UTC the crew of FR3595 requested clearance to climb to FL 380 due to expected turbulence; though without mention of their radio callsign. The controller replied as follows: “Six Delta Whiskey, climb three eight zero”. This was the radio callsign for flight Ryanair flight 6DW, an aircraft reporting to the sector shortly before. The crew of flight FR3595 responded to the clearance for flight 6DW as follows: “Flight level three eight zero, Ryanair three five niner five” and initiated a climb. Neither the controller nor the crew of RYR 6DW did respond to this readback of FR3595.
At 16:11:37 UTC the ground-based short-term conflict alert for Sector M4 reported an impending conflict between TP706 and FR3595. After the crew of FR3595 answered in the negative to the controllers immediate query as to whether they were at FL 360, he instructed them to descend immediately.
The traffic alert and collision avoidance system on both aircraft generated resolution advisories (RAs) shortly afterwards; these were immediately followed by both crews.
At 16:11:49 UTC, the closest point of approach between the two aircraft was reached: 0.8 NM (1,5 km) horizontally and 650 ft vertically (200 m).
The serious incident is attributable to the fact that the crew of a commercial aircraft initiated a climb without clearance, which lead to a dangerous convergence with another commercial aircraft.
The following factors were identified as the cause of the serious incident:
– The crew initiated the climb on the basis of a clearance which had been issued to another commercial aircraft belonging to the same aviation operator.
– The air traffic controller did not realise that the clearance issued was not read back by the crew for which it had been intended.
The following was identified as a contributing factor to the serious incident:
– A request by a flight crew for clearance to a higher flight level without specification of their radio callsign;
– The issue of altitude clearance by air traffic control without verification of the crew which had made the request;
– Absent reaction of another crew to whom the clearance was addressed to;
– Insufficient attention was given to the prevailing weather conditions when the decision to combine sectors was made.
- Final Report no. 2211 (PDF)