On 20 November 2011, approximately 100 nm north of Tenerife, Canary Islands, separation was lost in day Visual Meteorological Conditions (VMC) between a Boeing 757-200 being operated by Finnair on a non-scheduled passenger flight from Helsinki to Tenerife South and another Boeing 757-200 being operated by Thomas Cook AL on a non-scheduled passenger flight from Manchester to Tenerife South. None of the 460 occupants of the two aeroplanes were injured during the TCAS RA manoeuvres that followed.
An Investigation was carried out by the Spanish investigation agency CIAIAC. It was established that the Finnair aircraft was at FL390 in the vicinity of the Thomas Cook aircraft at FL370 near to the waypoint ‘BRICK’. Immediately after the Thomas Cook aircraft had requested descent and been told to standby, the same controller gave the Finnair aircraft a direct routing and following a request for immediate descent, clearance to FL250. The controller reported that at this point, the radar display label for Thomas Cook was showing FL400 and the one for Finnair was showing FL390. The Thomas Cook aircraft noted from its TCAS display that the aircraft above was beginning a descent and TCAS activation in respect of it followed, first a TA and then a ‘Descend’ RA and then soon afterwards a reversal to a ‘Climb’ RA. The two RAs were followed and clear of conflict reported to ATC when received. It was found that the Finnair aircraft, descending at 3300 fpm, would have received an RA to ‘Adjust Vertical Speed’ to 0 fpm at the same time as the Thomas Cook aircraft received their TA. One second later co-ordinated RAs were annunciated requiring a climb by Finnair and a descent by Thomas Cook, both at a rate of 1500 fpm. The Thomas Cook aircraft began descent but the Finnair aircraft, still above the other aircraft, ignored the RA and continued descent at an increased rate of 4200fpm with the other aircraft in sight. The effect of this had been to trigger the reversal RA ‘Climb, climb NOW’ for the Thomas Cook aircraft simultaneously with a maintain vertical speed RA for the Finnair aircraft which had by now passed through the level of the Thomas Cook aircraft. ‘Clear of Conflict’ was annunciated six seconds later.
It was found that a fault in the ATC radar system has led to the label for the Thomas Cook aircraft temporarily disappearing from the radar screen and being replaced by two labels, one showing the correct level of FL370 and another showing FL405, with the Finnair aircraft still showing FL390. On the basis of this information, the controller had instructed the Thomas Cook aircraft to descend to FL390. On being advised by the aircraft crew that they were already at FL370, a new instruction was given to turn 30° right. Immediately after this transmission, the Finnair aircraft stated “please confirm we were cleared to descend to level 250. We had a TCAS RA”, in response to which the controller responded in the affirmative.
Following a simulation of the encounter, (see the diagram below) the Investigation concluded that “the aircraft were on slightly divergent flight paths. The TCAS was activated when the aircraft flew within the protective airspace of the TCAS, which at that altitude was 1.3 NM and 700 ft.” The required minimum separation in the airspace involved was 5nm and 1000feet with the achieved separation recorded as 300 feet vertically (Finnair above Thomas Cook) at 0.7nm followed 5 seconds later by 100 ft vertically (Finnair below Thomas Cook) at 0.9nm. The respective radar tracks are shown on the diagram below.
The tracks of the two aircraft whilst in proximity (reproduced from the Official Report)
Investigation of the radar display fault found that it “seems to fit the characteristics of garbling sometimes seen in non-Mode S radars when the oblique distances between each aircraft and the respective radars are very similar”. It was further noted that “given this similarity in the distance, an overlap occurred in the reply received by the radars from the aircraft, resulting in new tracks appearing on the screen and in the label for (the Thomas Cook) aircraft splitting from the corresponding aircraft symbol.” However, it was noted that the flight progress strips for the two aircraft showed the correct flight levels for both aircraft prior to the issue of a descent clearance to the Finnair aircraft.
The Investigation noted that the Finnair crew should have immediately respond as indicated to any TCAS RA even if contrary to ATC instructions and should never execute manoeuvres in the direction opposite to that indicated by a TCAS RA.
The formal statement of the Investigation in respect of Cause of the Incident concluded that it arose because of “the clearance given by the controller to (the Finnair) aircraft) to descend from flight level 390 to 250, crossing through the flight level being occupied by (the Thomas Cook) aircraft, FL370, resulting in a violation of the minimum radar separation distance prescribed.”
It was also considered that the following contributed to the incident:
- a possible garbling problem that made it possible for the label on the radar display for (the Thomas Cook) aircraft to show that it was flying at flight level 405
- The controller’s failure to detect the fault that existed with the labels
- The improper response by the crew of (the Finnair) aircraft to the climb advisory issued by its TCAS
Two Safety Recommendations were issued as a result of the Investigation:
- that AENA ensure that its controllers are aware of the faults that can occur affecting the auxiliary systems used over the course of their jobs and of the way to detect and address said faults. [REC 100/12]
- that FINNAIR as part of the refresher training given to its crews, enhance the training on the procedures to be followed in the event of aircraft encounters resulting in a TCAS activation. [REC 101/12]
The English translation of the Final Report was published shortly after the original Spanish report in October 2012.