How does a TCAS system avoid unnecessary (RAs) resolution advisories when aircraft are operating on separate closely spaced parallel runways (e.g. 2500 feet or less separation between rwy centerlines).

Reference the KSFO Quiet Bridge Visual RWY 28L/R approach: (rwys 28L/R are separated by less than 800 feet)

As an example, suppose there is an aircraft on final approach at KSFO for rwy 28R about 15 miles out and another aircraft is converging and inbound from the southeast past "SIDBY" for rwy 28L. Then suppose the aircraft inbound from "SIDBY" (for rwy 28L) reports seeing the other aircraft (on final for rwy 28R) and is instructed by ATC to maintain Visual Separation (from the aircraft on final for rwy 28R), thereby allowing for separation between both aircraft to possibly become less than standard radar or procedural separation. How does the TCAS system deal with this type of situation and not generate an unnecessary RA? so quiet bridge visual


The TCAS logic is adjusted when the aircraft is in the terminal area. It does this through the following inputs:

  • Radio Altitude - Controls TCAS parameters when below 2350 feet AGL
  • Landing Gear, Flaps, Slats Position - TCAS II logic considers aircraft performance capabilities in all configurations

Resolution Advisories are inhibited below 1000 feet AGL to prevent unnecessary TCAS alerts in terminal areas.

[From Collins Avionics TCAS II Reference Notes.]

And I can confirm from flight test that when flying approach in the Sabreliner, there are only TAs for VFR traffic flying the pattern on the parallel runway (about 1500 feet separation).


To add to Gerry's answer, in the parallel approach scenario, the TCAS can be switched to TA ONLY mode instead of TA/RA; one airline's SOP notes:

Use of TA ONLY may be indicated in one or more of the following circumstances: [...] During parallel approaches when the other aircraft has been positively identified visually (VMC) or by the controller (IMC) [...]


  • TA ONLY: traffic advisory only
  • TA/RA: traffic advisory and resolution advisory

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— @AmericanAir on twitter.com [cropped]


To add to previous answers, TAs, and RAs when above 1000ft, are based on predicted collision time.

On general when flying parallel approaches standard 1000 ft separation applies until both aircraft are established on the localizer, ie. tracking accurately inbound the airfield on same direction. When in final approach the RA threshold is only 15 seconds (or 20s above 2350ft AGL) so it is unlikely that that threshold will be crossed as both aircraft are tracking on same course at approximately same speed.

Should one of the aircraft deviate from the localizer towards the adjacent runway an RA will indeed be issued by the system when above 1000ft, which would be appropriate.


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