TCAP AN ALTITUDE CAPTURE ENHANCEMENT

Wikipedia

Agrowing sense of alarm over the rising incidents of mid-air collisions and near misses, brought about the introduction of TCAS and today the sky is far safer to fly  in.  However, this also brought about an unwelcome and recurrent side effect that of nuisance or operationally undesired RA commands.

An operationally undesired RA is one that is triggered during 1000 ft level off manoeuvres, while the crew is operating within clearances granted. This normally occurs when the vertical closure rates are high. This accounts for almost 50 % of RAs encountered in today’s environment.

Several recommendations have been made to address this issue.  PANS- OPS Doc 8168, ICAO recommends adopting rate less than 1500 fpm throughout the last 1000 ft of climb/ descent to the assigned altitude, when aware of traffic nearby. The Airbus FCOM recommends that the Vertical Speed be reduced to 1500 ft during the last 2000 ft to level off when known traffic is present in the area. While different regulatory bodies evolved similar solutions, the core aim was a need to alter the altitude capture laws without compromising the climb performance capability. 

In response to the above requirements, Airbus has developed a new system called the TCAS Alert Prevention or TCAP. The principle was to introduce a new altitude capture law that will soften the capture in the presence of other aircraft. Its activation logic is based on the traffic Advisory (TA), triggered by TCAS system, which clearly confirms the presence of other aircraft in the vicinity.  A concept of TCAS availability threshold DZ _avail was introduced to define the upper distance from the selected altitude where a TA can occur with an intruder capturing the same altitude in the opposite sense. The DZ _avail value hence depends on both the vertical speed at the time of TA and its altitude. The triggering conditions require that

• AP,FD must remain engaged
• Aircraft is converging towards its selected altitude
• The distance to the selected altitude at the time of the TA is lower than DZ_avail.

In the  new Altitude capture software enhancement, called the ALT^*_{TCAP control} law,  the vertical speed targets, are computed in decreasing sequence and efficiently to prevent triggering of an RA while ensuring that there is no undue increase in the altitude capture duration.  The law comes into effect independently during early occurrence of TA as well as during ALT* phase. The computed vertical speed is refreshed as long as TA is active so as to boost pilot’s confidence in the system.  The pilot is not required to intervene but only monitor the level off manoeuvre. Simulation studies also indicate that the benefit is available even if only one of the two aircraft involved is fitted with the TCAP system.