Although the TCAS RA function is inhibited when an aircraft is on the ground or when the aircraft is airborne at very low altitudes, it is very important for operational safety that the system is selected on before take off, to ensure that the RA function will be active as soon as the built-in system constraints allow this during the initial climb. It would be completely impracticable for flight crew to routinely select TCAS on (or off) whilst airborne at low altitude.
In any case, the TCAS Display can continue to provide flight crew with useful situational awareness information, both below the RA inhibit altitudes and while on the ground when an aircraft is about to line up on the take off runway, or when crossing an active runway whilst taxiing in or out. This is particularly useful at non-radar airports, or in poor visibility, as a means to check the runway approach for aircraft about to land when lining up for departure, or when crossing an active runway whilst taxiing.
However, present-generation TCAS equipment may cause interference to radar surveillance systems, which operate in the same frequency band (1030-1090 MHz), and to other (airborne) TCAS systems. Because of this, it is very important that aircraft only select their TCAS to TA/RA when approaching the holding position for their departure runway or, if so desired, when approaching to cross an active runway during taxiing. In the latter case, the setting must revert to ‘Transponder Only’ once clear of the active runway. It is equally important that TA/RA is deselected as soon as possible after clearing the landing runway. This circumstance is addressed in ICAO PANS-OPS Doc 8168 (and also the ACAS Manual Doc 9863) where it is recommended that TCAS II not be operated (at all) whilst taxiing. Despite this, it is widely recognised that TCAS II selection should be used to enhance the safety of an active runway crossing as described above - see EUROCONTROL ACAS II Bulletin No. 9 issued July 2007.
The most appropriate way to ensure that TCAS equipment is used in this way and to thereby avoid interference with ATC systems, is to ensure that aircraft operator normal checklist content and use are appropriate. These procedures should include:
- Entries for selection of the TCAS from ‘Transponder Only’ to TA/RA included in the Checklist used before take off, and for selection from TA/RA to ‘Transponder Only’ in the Checklist used after landing.
- These entries should be positioned near the beginning of the sequence of Checklist items that are commenced in the vicinity of the holding point immediately before take off, and as soon as safely clear of the runway after landing.
- An SOP for use of these Checklists, which requires that they are carried out at an appropriate time in relation to their function - i.e. only when cleared to enter an active runway for departure and as soon as safely clear of the runway after landing.
- If an operator requires selection of the transponder to TA/RA to activate the TCAS Display whilst crossing an active runway, then the SOP should contain a clearly specified procedure, and applicability, which covers the timing of selection and de-selection of TCAS for active runway crossings.
ATC need to bear in mind that issuing a clearance to enter or cross an active runway can be expected to be followed by TCAS selection to TA/RA. ATC may also expect that the corresponding selection to ‘Transponder Only’ will be promptly made after the aircraft has exited the active runway.
This method of ensuring that, until next-generation TCAS is developed, existing TCAS equipment is not used in a way which may be prejudicial to safety, is compatible with procedures relating to Mode S transponder and TCAS operation on airport movement areas which are (July 2008) being proposed for inclusion in a revision to ICAO PANS-OPS Doc 8168.