(3)
The directional antenna lets the TCAS computer transmit interrogations and receive replies on one of four antenna beams. The TCAS computer electronically points the antenna beam in one of four different directions to find the bearing of intruder airplanes. The antenna does not have to move. The TCAS adjusts the drive level and phase of each of the four antenna elements in the directional antenna to point the beam.
(4)
The TCAS finds airplanes that have a Mode S Transponder by listening for Mode S squitter transmissions. Mode S Transponders transmit squitter data once every second. TCAS receives Mode S address and adds it to the interrogation list. TCAS could later interrogate those airplanes discreetly. The TCAS also finds airplanes that have transponders that do not reply to Mode S interrogations but do reply to ATCRBS interrogations. The TCAS must interrogate intruder airplanes that have Mode A and C transponders because they do not transmit squitter data. When an intruder is found, the TCAS monitors the intruder. The TCAS can monitor up to 45 intruders at once.
(5)
The TCAS interrogates intruders continuously to monitor the intruders. When an intruder is interrogated, transponders reply after a fixed delay. The TCAS measures the time between an interrogation and a reply to find the range of the intruder. The TCAS can calculate the relative altitude of the intruder if the intruder has a Mode C or Mode S transponder. The TCAS uses the directional antennas to find the bearing of the intruder.
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747-400
MAINTENANCE MANUAL
(6) The TCAS puts intruders into groups as non-threat, proximity, TA, or RA threat group airplanes. It uses the relative speed and position calculated from the reply data to put the intruder in the correct group. TCAS provides one or more of these data and instructions to the pilots:
(a) TCAS shows an intruder airplane as a symbol on the Navigation display. The symbol position on the display shows the range and bearing of the intruder. The symbol's shape and color tells if the airplane has been grouped as a non-threat, proximity, TA or RA threat. The intruder's altitude is shown above the symbol if the intruder is above you, and below the symbol if the intruder is below you. A directional arrow trend next to the intruder airplane symbol shows if the intruder airplane is ascending (up arrow) or descending (down arrow) faster than 500 fpm.
NOTE: TCAS can only group and show an intruder airplane as
____ an RA threat if the intruder is reporting altitude (Mode C or Mode S).
(b)
The TCAS shows other traffic-related indications on the ND. When TCAS cannot determine the bearing of an intruder airplane, TCAS will display a no bearing message, red RA or yellow TA. There are a maximum of two no bearing messages. The one with the highest priority is displayed first. When a potential threat is out of the display area, the OFFSCALE indication will be displayed.
(c)
The TCAS supplies a TRAFFIC alert on the cockpit audio system if an intruder airplane enters the TAU area. If the intruder airplane is grouped as a TA, the TRAFFIC indication on the ND is colored yellow. If the intruder airplane is grouped as a RA, the TRAFFIC indication on the ND is colored red.
NOTE: The TAU area represents the time to the closest point of
____ approach by an intruder airplane. The TAU varies with the airplane altitude from 20 seconds to 45 seconds.
(d) The TCAS supplies an RA alert on the cockpit audio system and the corresponding vertical movement RA on the Primary Flight display.
(7)
An airplane with TCAS has a Mode S Air Traffic Control (ATC) transponder and an ATC/TCAS control panel. The Mode S Transponder does the Mode S Transponder functions necessary for TCAS and the non-TCAS functions of ATCRBS (Modes A and C) Transponders.
(8)
When two airplanes with TCAS are threats to each other, the TCAS in each airplane supplies data to the other and they automatically make a decision which resolution advisory occurs in each airplane. The TCAS will not allow the same vertical movement RA to occur in both airplanes.
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