TCAS SYSTEM

TCAS involves communication between all aircraft equipped with an appropriate transponder (provided the transponder is enabled and set up properly). Each TCAS equipped aircraft interrogates all other aircraft in a determined range about their position (via the 1,030 MHz), and all other aircraft reply to other interrogations (via 1,090 MHz). This interrogation-and-response cycle may occur several times per second.
The TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their range (garnered from the interrogation and response round trip time), altitude (as reported by the interrogated aircraft), and bearing (by the directional antenna from the response). Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
TCAS and its variants are only able to interact with aircraft that have a correctly operating mode C or mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a mode S transponder .
The next step beyond identifying potential collisions is automatically negotiating a mutual avoidance maneuver (currently, maneuvers are restricted to changes in altitude and modification of climb/sink rates) between the two (or more) conflicting aircraft. These avoidance maneuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.
A protected volume of airspace surrounds each TCAS equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter. The illustration below gives an example of a typical TCAS protection volume.

System components

A TCAS installation consists of the following components:

TCAS computer unit

Performs airspace surveillance, intruder tracking, its own aircraft altitude tracking, threat detection, RA maneuver determination and selection, and generation of advisories. The TCAS Processor uses pressure altitude, radar altitude, and discrete aircraft status inputs from its own aircraft to control the collision avoidance logic parameters that determine the protection volume around the TCAS aircraft.

Antennas

The antennas used by TCAS II include a directional antenna that is mounted on the top of the aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the aircraft. Most installations use the optional directional antenna on the bottom of the aircraft. In addition to the two TCAS antennas, two antennas are also required for the Mode S transponder. One antenna is mounted on the top of the aircraft while the other is mounted on the bottom. These antennas enable the Mode S transponder to receive interrogations at 1030 MHz and reply to the received interrogations at 1090 MHz.

Cockpit presentation

The TCAS interface with the pilots is provided by two displays: the traffic display and the RA display. These two displays can be implemented in a number of ways, including displays that incorporate both displays into a single, physical unit. Regardless of the implementation, the information displayed is identical. The standards for both the traffic display and the RA display are defined in DO-185A.

TCAS operation

The following section describes the TCAS operation based on TCAS II, since this is the version that has been adopted as an international standard (ACAS II) by ICAO and aviation authorities worldwide.

TCAS operation modes

TCAS II can be currently operated in the following modes:

Stand-by

Power is applied to the TCAS Processor and the mode S transponder, but TCAS does not issue any interrogations and the transponder will reply to only discrete interrogations.

Transponder

The mode S transponder is fully operational and will reply to all appropriate ground and TCAS interrogations. TCAS remains in stand-by.

Traffic advisories only

The mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform all tracking functions. However, TCAS will only issue traffic advisories (TA), and the resolution advisories (RA) will be inhibited.

Automatic (traffic/resolution advisories)

The mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform all tracking functions. TCAS will issue traffic advisories (TA) and resolution advisories (RA), when appropriate.

TCAS works in a coordinated manner, so when an RA is issued to conflicting aircraft, a required action (i.e., Climb. Climb.) has to be immediately performed by one of the aircraft, while the other one receives a similar RA in the opposite direction (i.e., Descend. Descend.).

TCAS alerts

TCAS II typical envelope

TCAS II issues the following types of aural annunciations:

• Traffic advisory (TA)
• Resolution advisory (RA)
• Clear of conflict

When a TA is issued, pilots are instructed to initiate a visual search for the traffic causing the TA. If the traffic is visually acquired, pilots are instructed to maintain visual separation from the traffic. The pilot training programs also indicate that no horizontal maneuvers are to be made based solely on information shown on the traffic display. Slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable.
When an RA is issued, pilots are expected to respond immediately to the RA unless doing so would jeopardize the safe operation of the flight. This means that aircraft will at times have to manoeuver contrary to ATC instructions or disregard ATC instructions. In these cases, the controller is no longer responsible for separation of the aircraft involved in the RA until the conflict is terminated.
On the other hand, ATC can potentially interfere with the pilot’s response to RAs. If a conflicting ATC instruction coincides with an RA, the pilot may assume that ATC is fully aware of the situation and is providing the better resolution. But in reality ATC is not aware of the RA until the RA is reported by the pilot. Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter. Hence, the pilot is expected to “follow the RA” but in practice this does not yet always happen.
Some States have implemented “RA downlink” which provides air traffic controllers with information about RAs posted in the cockpit obtained via Mode S radars. Currently, there are no ICAO provisions concerning the use of RA downlink by air traffic controllers.
The following points receive emphasis during pilot training:

• Do not manoeuver in a direction opposite to that indicated by the RA because this may result in a collision.
• Inform the controller of the RA as soon as permitted by flight crew workload after responding to the RA. There is no requirement to make this notification prior to initiating the RA response.
• Be alert for the removal of RAs or the weakening of RAs so that deviations from a cleared altitude are minimized.
• If possible, comply with the controller’s clearance, e.g. turn to intercept an airway or localizer, at the same time as responding to an RA.
• When the RA event is completed, promptly return to the previous ATC clearance or instruction or comply with a revised ATC clearance or instruction.

Types of traffic and resolution advisories

Type	Text	Meaning	Required action
TA	Traffic; traffic.	Intruder near both horizontally and vertically.	Attempt visual contact, and be prepared to maneuver if an RA occurs.
RA	Climb; climb.	Intruder will pass below	Begin climbing at 1500–2000 ft/min
RA	Descend. Descend.	Intruder will pass above.	Begin descending at 1500–2000 ft/min
RA	Increase climb.	Intruder will pass just below	Climb at 2500 – 3000 ft/min.
RA	Increase descent.	Intruder will pass just above.	Descend at 2500 – 3000 ft/min.
RA	Reduce climb.	Intruder is probably well below.	Climb at a slower rate.
RA	Reduce descent.	Intruder is probably well above.	Descend at a slower rate.
RA	Climb; climb now.	Intruder that was passing above, will now pass below.	Change from a descent to a climb.
RA	Descend; descend now.	Intruder that was passing below, will now pass above.	Change from a climb to a descent.
RA	Maintain vertical speed; maintain.	Intruder will be avoided if vertical rate is maintained.	Maintain current vertical rate.
RA	Adjust vertical speed; adjust.	Intruder considerably away, or weakening of initial RA.	Begin to level off.
RA	Monitor vertical speed.	Intruder ahead in level flight, above or below.	Remain in level flight.
RA	Crossing.	Passing through the intruder's level. Usually added to any other RA.	Proceed according to the associated RA.
CC	Clear of conflict.	Intruder is no longer a threat.	Return promptly to previous ATC clearance.