ALLIEDSIGNAL Aerospace is flight-testing the latest software for the traffic-alert and collision-avoidance system (TCAS 2). The "Change 7" software is the final iteration of the TCAS 2, incorporating operational feedback from users of the system, and is the basis for the international airborne collision-avoidance system (ACAS2) standard, says AlliedSignal TCAS programme manager Tom Mullinix.

Change 7 introduces 225 system-logic changes covering surveillance, collision-avoidance logic and aural/visual annunciations. Mullinix says that the changes address system-performance issues such as phantom resolution-advisories (RAs) and unclear RA annunciations. AlliedSignal has flight-tested the surveillance-software changes and "everybody's happy", claims Mullinix.

Verbal commands issued by the TCAS 2 have been changed, to improve clarity. An example is the use of "adjust vertical speed", rather than the "reduce climb" or "reduce descent" command now issued after the pilot has responded to an RA. Mullinix says that pilots were missing the word "reduce". The new aural annunciation is coupled with a change to the TCAS vertical-speed indicator to display a "fly-to" green arc instructing the pilot to level off after a climb or descend RA.

Mullinix says that the change is intended to eliminate large vertical excursions caused by pilots over-reacting to RAs. Flight International observed the display in operation in AlliedSignal's Convair 580 testbed and the manoeuvre following the initial 1,500ft/min (7.6m/s) RA, and subsequent levelling off, was almost unnoticably gentle.

Change 7 also refines the collision-avoidance logic to avoid RAs which require pilots to reverse vertical velocity or to descend through the intruder's flightpath. The change also reduces the number of "bump-ups" which occur when flying over busy airports, when fast-climbing aircraft trigger RAs only to level off before becoming a threat, Mullinix explains.

The USFederal Aviation Administration is expected to complete Change 7 development in May. AlliedSignal plans to have a software upgrade available for its TCAS 2 unit by the second quarter of 1998. Mullinix says that the upgrade will cost "$0 to $5,000" per aircraft, depending on the customer, plus up to $1,000 to modify the associated Mode S transponder.

The FAA is expected to issue a notice of proposed rulemaking by mid-year making Change 7 mandatory by 1 January, 2000. Europe is working to make ACAS2 mandatory by the same date, while Australia and Japan are aiming for compliance by 2001. Mullinix says that Europe does not recognise the TCAS 1, fitted to USregional airliners, and that it will require all aircraft with 19 or more seats to carry ACAS2 by 2005.

Source: Flight International