TCAS, Human Factors At Center of Midair Probe




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TCAS told Boeing pilot to descend, Tupolev pilot to climb. But Tupolev pilot followed ATC 'descend' commands

German investigators are looking at work-rule compliance and potential system deficiencies at Swiss air traffic control provider Skyguide, as well as cockpit procedures, to determine what caused the midair collision between two commercial jets over southern Germany on July 1. Traffic in the region is handled by Skyguide.

The aircraft--a DHL Boeing 757 (A9C-DHL) and a Bashkirian Airlines Tupolev Tu-154 (RA-85816)--collided over the northern shore of Lake Constance near the town of Ueberlingen close to Akabi waypoint. All 71 on both aircraft died.

OFFICIALS AT GERMANY'S BFU accident investigation authority released initial cockpit voice recorder data from both aircraft early last week. According to the BFU, both jets received a TCAS "traffic, traffic" alert a little less than 1 min. before the collision. Around 15 sec. later, both systems went to resolution advisory (RA) mode urging the Tu-154 to climb and the 757 to descend. One second after the RA call, the Skyguide controller told the Tu-154 to "descend flight level 350, expedite, I have crossing traffic." Fourteen seconds later he again urged the Tu-154 crew to "descend level 350, expedite descent." Both aircraft were on the same frequency and no air traffic control warning was issued to the 757.

Both aircraft descended from 36,000 ft., the 757 pilot obeying the TCAS command and the Tu-154 pilot obeying ATC instead of the TCAS "climb" command. Both initiated their descent around 30 sec. before impact, according to an early Skyguide statement. The impact of the right-angle flight paths occurred at 35,400 ft. at 11:35:33 p.m. local time.

The latest version of TCAS is supposed to reverse the command if it detects the other airplane is taking the wrong action (see p. 36). The TCAS on the 757 was this latest model, a Honeywell TCAS 2000 version 7, and as of late last week, it was not clear if it reversed the command from "descend" to "climb" after the Tu-154 started descending.

Skyguide claimed in a statement immediately after the collision that it had received an unidentified TCAS descent call from what it believed was the 757. The call's timing has not yet been clarified in the investigation. Flight data recorders are still being analyzed at BFU's Braunschweig facilities.

The 36,000-ft. altitude was correct for both routes. Bashkirian Airlines Flight 2937 was a charter service from Moscow to Barcelona heading almost due west, and had been under Skyguide control for about 5 min. It had been in German airspace where the route altitude was FL360, and should have descended to FL350 for a different airway at the Trasadingen beacon located a few miles beyond the crash point.

DHL's scheduled cargo Flight 611 was from Bahrain via Bergamo to Brussels in a roughly northerly heading. According to unconfirmed reports, the 757 had been handed over from Milan ATC to Zurich (Skyguide) around 7min. earlier.

Skyguide's short-term conflict alert (STCA) system was down for maintenance from around 11 p.m. local time. Zurich control's main telephone line was also inoperable, and a second controller who normally would have assisted the controller in charge had taken a break. During the course of events, the controller was handling 4-5 aircraft on two frequencies and two different screens. The controller tried to make a connection with Friedrichshafen tower to coordinate an arrival between 11:25:43 and 11:33:11 p.m., but failed to get through on the reserve line, Skyguide stated.

MEANWHILE, GERMAN ATC provider Deutsche Flugsicherung (DFS) confirmed that it received an STCA warning for the two relevant flights at its Karlsruhe center. A controller tried to alert his Swiss colleague in Zurich between 11:33:36 and 11:35:34 p.m. (a second after the crash), but the telephone line was busy initially, and from 11:34:45 p.m. the calls were not answered, DFS stated.

Peter Schlegel, head of the BFU, said that with STCA down, minimum lateral separation for aircraft at the same altitude should have been 7 naut. mi. instead of the regular 5 naut. mi. Skyguide should have told the Tu-154 to descend to FL350 around 90 sec. before the crash to comply with the extended lateral separation and with 1,000-ft. vertical separation, he said. The first order to descend came 44 sec. before impact.

Meanwhile, Georg Fongern, a spokesman for German pilot union Vereinigung Cockpit, said the Tu-154 pilots should have followed the TCAS command under any circumstances, neglecting ATC commands. However, a Russian state official said that under Russian standard operating procedures it was the pilot who would have ultimate responsibility for the decision. The transponders on most domestic Russian aircraft don't work with TCAS, and the domestic procedure is to follow ATC commands.

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July 15, 2002 The McGraw-Hill Companies Inc.

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