On 8 June 2009, an Airbus A318-100 being operated by Air France on a scheduled passenger flight from Belgrade, Serbia to Paris CDG in day Visual Meteorological Conditions (VMC) came into conflict with a Boeing 737-800 being operated by Ryanair on a scheduled passenger flight from Nottingham East Midlands UK to Bergamo Italy. The conflict was resolved mainly by TCAS Resolution Advisory (RA) response and there were no injuries to any occupants during the avoidance manoeuvres carried out by both aircraft.
An Investigation was carried out by the Swiss BFU which classified the event as both a Serious Incident, requiring an Investigation under ICAO Annex 13 principles, and a Category ‘A’ AIRPROX under the International Civil Aviation Organisation (ICAO) system (meaning a collision nearly occurred).
It was established that the loss of separation initially arose as a result of an error made by a supervised ATC trainee and that both aircraft had been correctly following their ATC clearances. It was found that the Airbus A318 had been flying east towards the Trasadingen VOR at FL 380 when given a descent to FL 360 by the trainee controller which was not noticed by the OJTI. Short Term Conflict Alert (STCA) activation alerted to the potential conflict with the B737 heading north towards the same position at FL 370 and the OJTI told the trainee to instruct the A318 to descend at a minimum of 3000 fpm. In the absence of a reply from the A318 to the instruction to increase rate of descent even after it had been repeated, the trainee requested that the OJTI take over and the latter instructed the B737 to descend to FL 360 and the A318, which had already reported a TCAS RA descent, to turn 20° to the right.
The PF on the A318 was the First Officer and the PF on the B737 was the aircraft commander. TCAS on both aircraft was confirmed to have generated coordinated RAs which were followed, with the B737 RA (only) reported to have been preceded momentarily by a Traffic Advisory (TA). As a result of the RA descent by the A318, the RA climb by the B737 and the turn given by the ATC OJTI to the A318 and actioned, the two aircraft crossed above Trasadingen at an actual separation of 1.4 nm laterally and 725 ft vertically. It was found that in order to achieve the ATC trainee-instructed rate of descent of 3000 fpm for only a 2000 ft change of cleared altitude, the A318 crew had disconnected the AP prior to the TCAS RA activation. The commander of the A318 (the PM) reported having seen the B737 (only) during the right turn.
The Investigation concluded that the following factors contributed substantially to the occurrence:
- An intervention by the supervising air traffic controller which was too late, because he had not realised that the trainee air traffic controller was overwhelmed by the challenging traffic situation.
- Within the Zurich air traffic control unit there was no function for monitoring clearances with regard to possible conflicts.
The Investigation also concluded that the following factors contributed to the development of the occurrence:
- The fact that air traffic control received no confirmation of a high rate of descent by (the A318)
- The fact that the supervising air traffic controller was working without a headset.
One Safety Recommendation was made as a result of the Investigation:
- That the Federal Office of Civil Aviation (FOCA) should ensure that the Zurich Area Control Centre is equipped in such a way that planned clearances are monitored for possible conflicts. (No 424)
Safety Action taken by ANSP Skyguide following the investigated event - the introduction of a third workstation for all Zurich ATCC sectors which would be available to supervisors of trainee controllers - was also noted.
The Final Report of the Investigation: Final Report No. 2088 was published on 30 November 2010