The module closes with the system that watches the one hazard no ground database holds: the other aircraft. This lesson covers the airborne collision avoidance system from its standard and its deliberately modest promise, through how it sees and sorts the traffic, to the discipline of flying a resolution advisory correctly, including the advisory that asks for nothing at all.
The standard and its promise
ACAS II is the ICAO standard for anti-collision purposes:
ACAS II is the airborne collision avoidance system, the ICAO standard itself;
TCAS II, the traffic alert and collision avoidance system, is the equipment that implements it, and the two names are used almost interchangeably.
Its promise is precisely bounded: it is an anti-collision system, and it does not guarantee any specific separation. Separation is the controller’s job, delivered in miles and thousands of feet; the purpose of ACAS II is the last line behind all of that: when separation has already failed, prevent the collision itself.
How it sees
The system’s eyes are other aircraft’s transponders: TCAS II interrogates the transponders around it, exactly as a ground radar would, and reads each reply for range (from the round-trip time), bearing (from its directional antennas), and altitude (from the reply’s altitude report). It follows that an intruder is detected only if it carries an operating transponder: a mode S (the selectively addressed, data-capable kind) or mode A/C (identity and altitude) transponder to be seen at all, with altitude reporting for the full protection. The installation around the computer:
antennas — on top and bottom of the fuselage;
the TCAS computer;
links to the mode-S transponder (which also carries the air-to-air coordination), the air-data computer for own altitude, and the radio altimeter, the previous lesson’s instrument once more load-bearing: the low-height inhibitions below are timed by it.
The standard detection volume: 35 to 40 NM horizontally, and about 2,000 ft above and below in any setting, extendable to about 10,000 ft above with ABV selected or 10,000 ft below with BLW. Two economies manage the watching. Reduced surveillance: distant, non-threatening traffic is interrogated less often, saving the radio channel for the traffic that matters. And in high-density areas the range may automatically be decreased, because the system can process only a limited number of intruders: better to watch the close threats sharply than the whole sky poorly.
Sorting the traffic
Everything detected is sorted into statuses, each with its own symbol on the horizontal display:
other traffic (an open diamond);
proximate traffic (a filled diamond), within 6 NM and 1,200 ft of the aircraft but not yet threatening;
the intruder, the aircraft the alerts are about: a traffic advisory (TA) (an amber circle, with the call TRAFFIC, TRAFFIC: eyes out, prepare) and a resolution advisory (RA) (a red square, with a command such as CLIMB, CLIMB).
The symbols sit at the traffic’s relative position, with its relative altitude and trend beside it.
The trigger criteria are time and geometry: the time to the closest point of approach (CPA), called TAU, together with the altitude difference. The time limit depends on altitude through a sensitivity level (SL): an RA fires at a TAU of roughly 15 to 35 seconds, tighter low down where traffic is dense, looser high up where closures are fast. The RA aims for an intended vertical separation of 300 to 600 ft (700 ft above FL420), modest numbers, because this is collision avoidance, not separation. And the system knows its limits near the ground and the ceiling:
below 1,000 ft above ground — no RA is generated at all;
below 1,450 ft on the radio altimeter — the increase-descent RA is inhibited;
at high altitude — the aircraft’s own performance inhibits climb and increase-climb RAs it could not fly.
Flying the RA
The commands are deliberately simple: ACAS II commands in the vertical plane only, climb, descend or maintain, and the commands are flown as a manual manoeuvre: the pilot disconnects nothing by reflex but flies the aircraft to the commanded vertical speed by hand. The display makes the command graphic: the RA paints a red area on the VSI, the vertical speeds that must not be flown, and on many variants a green area, the band to fly: avoid the red, fly the green; on variants without the green band, the red alone defines the command, and any vertical speed clear of it complies.