CREW
AWARENESS
AIRPLANE
OPERATIONS
MANUAL
2-04-40
Page
2
Code
01
REVISION 36
AOM-145/1114
SYSTEM DESCRIPTION
The TCAS was developed to provide crew awareness regarding
possible conflicting air traffic situations. Besides providing awareness,
TCAS also displays to the flight crew the recommended vertical
maneuver to avoid conflicting traffic. TCAS does not provide
recommendations for horizontal maneuvers.
CAUTION:
PRIMARY RESPONSIBILITY FOR EVASIVE ACTION
LIES WITH THE FLIGHT CREW AND ANY ACTION
MUST ALWAYS BE PRECEDED BY A VERY CAREFUL
EVALUATION OF THE SITUATION.
The TCAS computer receives data from the installed transponders,
radio altimeters and air-ground sensor. The signals transmitted by
surrounding airplanes inform their altitude, bearing and identification,
thus making it possible to track any traffic that could enter the
airplane’s protection zone. Based on such data, the TCAS calculates
the predicted path of each intruder airplane, determining whether or
not it may become a target. To determine that, an alert zone is
established, based on separation and speeds of both airplanes. The
size of the alert zone is not distance-based but, rather, is based on
time. Therefore, the caution area corresponds to the volume in space
where a conflict is expected to occur in 35 to 45 seconds, if no action
is taken. A warning area corresponds to an imminent conflict in the
following 20 to 30 seconds. Such time is calculated by dividing
distance between airplanes by their closure rate.
To inhibit the issuing of undesired warnings that constitute a nuisance
effect, the system incorporates a series of protections. These apply
during approaches to crowded airports, to increase protection against
slow closure rates, and to prevent airplanes below 180 ft (380 ft for
TCAS software version 7.0), which are about to land or have just
taken-off, from creating a nuisance.
When an airplane is tracked by the TCAS, the system periodically
interrogates the intruder’s transponder. The exchange of data between
two subsequent transmissions makes it possible to obtain the distance
to the intruder and its altitude, and to predict its path.
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