OR SEARCH
 

PROFILE

Use Mine
Change Mine

BRUCE D. NORDWALL/WASHINGTON

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.

HONEYWELL

The pilot's perspective in the white aircraft is shown in sequence, from left to right in the top row, and then bottom row. A stand-alone TCAS instrument is shown, but the display would be the same on an Electronic Flight Information System, or glass cockpit. In the far left display, (Non-Threat Traffic) RNG 10 is the range displayed from own aircraft (blue cross) to the edge of the white circle. The other non-threat aircraft (white diamond, open center) is 200 ft. higher (+02). Proximity Traffic: The other aircraft is closer (white diamond, white center) and still 200 ft. higher. A Traffic Advisory is given when other aircraft (yellow circle) is within 48 sec. of closest point of approach (CPA), still 200 ft. higher. Resolution Advisory--about 35 sec. before CPA--aircraft is a red square, relative altitude (+200 ft.) is in red, and the vertical speed indicator is green for the commanded descent (between 1,500 and 2,000 fpm.), with red showing that any climb or descent less than 1,500 fpm. is wrong. Descent Established:The pilot has established a descent with white needle in the green area, and vertical separation has increased to 400 ft. Softening Command:With 600-ft. separation showing in red, the pilot is directed to ease the descent rate to between 0 and 500 fpm. down. Level Off: The white arrow shows the aircraft is level, red number shows threat is 700 ft. above, and aircraft symbol is still red. Clear of Conflict: As the aircraft pass, with 700-ft. separation in the example, the target symbol returns to yellow circle indicating proximity traffic. At this time the pilots should notify ATC of the deviation and return to their assigned altitude.

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, CONTROL-
LERS 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 employingversion 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 interrogationto 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.

© July 15, 2002 The McGraw-Hill Companies Inc.