TCAS More 'Foolproof' Than Generally Recognized

By Bruce D. Nordwall/Aviation Week & Space Technology

15-Jul-2002 8:17 AM U.S. EDT

The traffic-alert and collision avoidance system (TCAS), somewhat maligned and misunderstood in the wake of the July 1 midair collision in Europe, provides more protection than is commonly understood.

Pilots generally have a high regard for TCAS, and its European counterpart ACAS (airborne collision avoidance system). From the cockpit they are perceived as a safety net when the air traffic control system fails to provide safe separation.

HOWEVER, CONTROLLERS have some reservations, largely because they have no way of knowing what TCAS may be telling flight crews, and they are concerned about the results if there is a conflict between the dictates of a human (controller) and machine (TCAS), as appears to have happened in this accident. Reports of the mishap say that the DHL Boeing 757 pilots followed TCAS commands to descend, but the Russian pilots of the Bashkirian Airlines Tupolev Tu-154 did not respond to ACAS commands to climb, instead following the controller's order to descend.

TCAS will give protective warnings even from the approach of a non-TCAS-equipped aircraft, or one whose TCAS is not working, so long as it has an operating transponder. What is lost if both aircraft are not equipped with TCAS or ACAS is the ability of the two systems to communicate intentions.

A TCAS-equipped aircraft approaching within 20-48 sec. of another aircraft, within 10,000 ft. of its altitude, will receive a traffic advisory (TA) with both an aural and symbolic "Traffic, Traffic" alert, and a visual indication of the vertical separation. If they continue to converge, the pilot of the TCAS aircraft will get a resolution advisory (RA) directing him to descend or climb. If both aircraft have TCAS, they will communicate to avoid mirror-image maneuvers.

TCAS commands avoidance maneuvers only in the vertical plane. When one aircraft's TCAS tells its pilots, "Descend, Descend," it also directs the other TCAS aircraft to remain level or climb. With the currently mandated systems (TCAS-II with version 7.0 software and ACAS-II), if a descending aircraft detects the other also in a descent, it will make a dynamic adjustment and direct the pilots to reverse the maneuver: "Climb, Climb."

THE PRECEDING TCAS-II, in use in the U.S. since 1994 employing version 6.04 software, could make a dynamic reversal when a TCAS-equipped aircraft encountered a non-TCAS aircraft that made a mirror maneuver. But it had a quirk when such a conflict occurred between two TCAS aircraft. If they were both descending to avoid contact, the algorithms could tell one to further increase its descent rate, but could not tell it to reverse and climb. The7.0 software introduced in 1999 and mandated in March 2001 has the capability to adjust, according to Tom Staggs, formerly Honeywell's business manager for TCAS.

Honeywell is one of three manufacturers of TCAS-II, building the system developed by AlliedSignal before it bought Honeywell, which spun off its TCAS-II to be produced by L-3 Comm, now in partnership with Thales. The other producer is Rockwell Collins.

A TCAS or ACAS installation has a radio transmitter and receiver, directional antennas, computer and cockpit display. TCAS, like secondary surveillance radars on the ground, sends out an interrogation to which transponders within range respond. The TCAS computer uses the time between an interrogation and reply to calculate the distance. Directional antennas give the bearing for the cockpit display, but the collision algorithm is based on range rate--a constant change in range (like a steady bearing) means that a collision will occur. The transponder's Mode C or Mode S response gives altitude directly, which is displayed as "+02" if the other aircraft is 200 ft. higher, or "-14" if 1,400 ft. below. Climbs or descents at rates greater than 500 fpm. are indicated by an arrow pointing in the appropriate direction.

STILL, THE ISSUE of controllers not knowing what TCAS directions are being given, and the possibility of conflict, is important. In the U.S., the directions are clear--obey TCAS. Even the "general prudential" rule that instructions from air traffic controllers are always to be obeyed unless an aircraft is in extremis would indicate following TCAS. The logic would be that by the time a TCAS alert is received, the ATC system has somehow failed to provide safe separation, and so the aircraft is in extreme danger. TCAS information on the other aircraft's proximity is updated one and perhaps two times a second, compared with the controllers' information, which is updated at the 12-sec. interval of a secondary surveillance radar antenna's rotation.

Staggs said trials to data-link TCAS resolution advisories to controllers have been held in Boston. Europe is working to mandate delivery of TAs and RAs to controllers over the Mode S transponder, but he points out that Mode S is an inefficient modem by modern standards, with a data exchange rate of 300 baud, so messages would have to be carefully designed.

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Aviation Week & Space Technology:
July 15, 2002
 

TCAS, Human Factors
At Center of Midair Probe

JENS FLOTTAU/MUNICH

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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