777 AIRCRAFT MAINTENANCE MANUAL

TCAS -TCAS DIRECTIONAL ANTENNA

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TCAS -OPERATION -SURVEILLANCE AREA

General
The TCAS tracks and evaluates the possible threat of up to 30 airplanes that are within the surveillance area.
These four types of traffic airplanes show on the NDs:
*
Other traffic are airplanes that have an altitude separation of +/-2700 feet and the time until the closest point of approach (CPA) is more than 40 seconds

*
Proximate traffic are airplanes that have an altitude separation of +/-1200 feet and the time until the CPA is more than 40 seconds

*
Intruder traffic which causes a traffic advisory (TA), are airplanes that have an altitude separation of +/-1200 feet and the time until the CPA is between 25 and 40 seconds

*
Threat traffic which causes a resolution advisory (RA), are airplanes that have an altitude separation of +/-900 feet and the time until the CPA is less than 25 seconds.

Surveillance Area
TCAS has a surveillance area that is 10,000 feet above and below the airplane and 30 nm range in front of the airplane. The surveillance range behind and on the sides of the airplane is not as large as in the front of the airplane.
Within the surveillance area, there are two other areas called traffic advisory (TA) and resolution advisory (RA). To calculate RA and TA, the TCAS computer uses the time until the CPA and the relative altitude of the traffic airplane.
EFFECTIVITY
BEJ ALL

To calculate the CPA and the relative altitude, the TCAS computer uses:
*
The TCAS airplane airspeed and altitude

*
Range, altitude, and airspeed of the traffic airplane.

The RA and TA times change between 20 and 45 seconds with the TCAS airplane altitude. The RA and TA times also change with the airspeed and altitude of the TCAS airplane, and the closure rate of each traffic airplane.
Traffic Advisory
When a traffic airplane is an intruder, the TCAS computer makes a traffic advisory (TA) that shows on the NDs.
Resolution Advisory
When a traffic airplane is a threat, the TCAS computer makes these resolution advisories (RAs):
*
Indications show on the NDs

*
Maneuver indications on the PFDs guide the flight crew to avoid a possible collision

*
Aurals come from the aural warning speakers.

System Inhibits
TAs or RAs may not occur if the airplane radio altitude is too low or if a higher priority alert exists.
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777 AIRCRAFT MAINTENANCE MANUAL

TCAS -OPERATION -SURVEILLANCE AREA
This table shows the inhibits and the parameters that cause the inhibits.
BEJ 001-005
CONDITION  PARAMETERS 
Increased descent RA  Inhibited below 1450 ft AGL 
Descend RA  Inhibited below 1000 ft AGL in descent and 1200 ft AGL in climb 
Resolution Advisories  Inhibited below 900 ft AGL in descent and below 1100 ft AGL in climb. (TCAS automatically goes into TA ONLY) 
TA voice message  Inhibited below 900 ft AGL in descent and below 1100 ft AGL in climb 
Advisory priority  TA/RA goes to TA ONLY when there are higher priority advisories from GPWS 

BEJ 006-999

CONDITION  PARAMETERS 
Increased descent RA  Inhibited below 1450 ft AGL 
Descend RA  Inhibited below 1000 ft AGL in descent and 1200 ft AGL in climb 
Resolution Advisories  Inhibited below 900 ft AGL in descent and below 1100 ft AGL in climb. (TCAS automatically goes into TA ONLY) 
TA voice message  Inhibited below 400 ft AGL in descent and below 600 ft AGL in climb 
Advisory priority  TA/RA goes to TA ONLY when there are higher priority advisories from GPWS 

BEJ ALL

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TCAS -OPERATION -SURVEILLANCE AREA

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TCAS -BASIC OPERATION

General
TCAS transmits these two types of interrogation signals:
*
Whisper-shout for ATCRBS (air traffic control radar beacon system) transponders
　
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