Finnair B752 and Thomas Cook B752 near Tenerife on Nov 20th 2011, near collision
Last Update: April 19, 2013 / 13:56:58 GMT/Zulu time
A Thomas Cook Boeing 757-200, registration G-TCBA performing flight MT-2456 from Manchester,EN (UK) to Tenerife Sur Sofia Reina,CI (Spain) with 217 passengers and 6 crew, was enroute at FL370 nearing Tenerife and the top of descent also set to track ORTIS3G arrival route slightly in trail of OH-LBR, both aircraft were in contact with Canaries Center NNW Sector.
When OH-LBR requested descent, the controller cleared the aircraft direct to waypoint ODULA and to descend to FL250. At the same time the radar label of G-TCBA split now showing targets at FL405 and FL370, both with the same squawk. About a minute later the controller instructed G-TCBA to descend to FL370, to which the crew replied they were already at FL370 prompting the controller to instruct the aircraft turn right 30 degrees. Immediately afterwards OH-LBR inquired to verify their clearance to FL250 reporting they had received a TCAS advisory followed by a report the conflict had been cleared a couple of seconds later.
Both aircraft continued for safe landings on Tenerife.
Spain's CIAIAC released their final report into the serious incident reporting the minimum separation between the aircraft had reduced to 0 feet vertical and between 0.7 and 0.9nm horizontal concluding the probable cause of the serious incident was:
This incident is regarded as having been caused by the clearance given by the controller to aircraft FIN1601 to descend from flight level 390 to 250, crossing through the flight level being occupied by aircraft TCX13CS, FL370, resulting in a violation of the minimum radar separation distance prescribed.
The following are considered to have contributed to the incident:
- A possible garbling problem that made it possible for the label on the radar display for aircraft TCX13CS to show that it was flying at flight level 405.
- The controllerÂ’s failure to detect the fault that existed with the labels.
- The improper response by the crew of aircraft FIN1601 to the climb advisory issued by its TCAS.
The CIAIAC reported that OH-LBR was descending at 3300 fpm after being cleared down when TCAS activated at a separation of 700 feet vertical and 1.3nm horizontal with a resolution advisory "Adjust vertical speed" requiring the aircraft to maintain a vertical speed of 0 fpm, G-TCBA received a traffic advisory. A second later the Finnair TCAS instructed the crew to "Climb! Climb!" (requiring 1500 fpm climb), the Thomas Cook TCAS issued a resolution advisory "Descend! Descend!" (requiring a 1500 fpm descent). Six seconds later G-TCBA began to descend, while OH-LBR was 259 feet above descending at 4200fpm. Another six seconds later OH-LBR had descended through the altitude of G-TCBA reaching a descent rate of 4400 fpm, OH-LBR's TCAS now reverted to "Maintain vertical speed" and G-TCBA's TCAS reverted to "Climb! Climb now!". Another 6 seconds later both TCAS systems announced "Clear of conflict".
CIAIAC analysed that the controller believed OH-LBR was below G-TCBA shown at FL405 probably due to the split/garbled target when he cleared OH-LBR to descend to FL250. However, the controller did not only not detect the split target display, he also missed the opportunity to detect the conflict and his false assessment of the situation when G-TCBA requested descent. In addition he also missed the discrepancy between the flight strip indicating FL370 and the radar display showing FL405.
As result the CIAIAC stated: "The facts exposed in the above paragraphs indicate that the controller did not detect the faults in the system, probably due to his insufficient knowledge of the system and of the means available for detecting them. As a result, the investigation has concluded that the level of knowledge controllers have of this matter should be assessed and, if so warranted by the findings, specific training should be added as part of their continuous training program. A safety recommendation is issued in this regard."
The CIAIAC analysed that the crew of OH-LBR/Y-1601 reacted improperly to the TCAS resolution advisories when they continued to descend instead of arresting the descent and start climbing as instructed by TCAS. The CIAIAC continued: "Studies of close calls involving aircraft receiving TCAS advisories have revealed that the most unfavorable situation occurs when one crew responds as indicated by TCAS and the other in the opposite direction to that instructed."
The CIAIAC reported with respect to the split target, that a technical incident was filed by AENA that seemed to match the characteristics of non-Mode-S radars when the oblique distances between aircraft and radar are very similiar: "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 aircraft TCX13CS splitting from the corresponding aircraft symbol."
This article is published under license from Avherald.com. © of text by Avherald.com.
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