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located on airplane centerline forward of the main cabin door.
• System computer
• Cockpit displays
• Radio Tuning Unit (RTU) and Mode S ATC Transponder
The directional antennas interrogate and receive the transponder signals of other
airplanes and also continuously transmit a high speed data code (squitter) to
TCAS / ACAS equipped airplanes. Transponder responses from other airplanes
are processed in the TCAS / ACAS computer to determine relative closure vectors
for traffic within the defined alerting envelope. The alerting envelope has altitude
limit values that may be modified with selections made on the RTU.
The TCAS / ACAS computer communicates traffic information to the Display
Controllers (DCs) and Symbol Generators (SGs) for subsequent display on the
cockpit EFIS units. The TCAS / ACAS display is automatically selected to the NAV
display on airplane power up. The flight crew may deselect the TCAS / ACAS
display by using the Line Select Keys (LSKs) on the MAP menu of the DCs. The
TCAS / ACAS traffic display may also be shown on the CAS screen (DU #4) by
selecting TCAS / ACAS on the SYSTEMS menu on either DC. The TCAS / ACAS
traffic display will automatically “pop up” on DU #4 whenever conflicting traffic
penetrates the TCAS / ACAS alert envelope. Although the system tracks up to fifty
(50) airplanes, only fifteen (15) airplane targets may be displayed at one time due
to the limitations of the SGs.
The TCAS / ACAS computer has an internal synthetic voice component that
annunciates the presence of traffic over cockpit speakers and interphone. If the
OPERATING MANUAL
2A-34-00 PRODUCTION AIRCRAFT SYSTEMS
Page 56
October 11/01
Revision 5
computer determines that an airplane presents an imminent collision hazard, the
synthetic voice directs the flight crew to make changes in the vertical speed of the
airplane to avoid the conflict. Any evasive maneuver directed by the TCAS / ACAS
computer is shown on the Primary Flight Display (PFD) in a cue format that guides
the flight crew to a safe separation distance.
NOTE:
TCAS / ACAS displays and alert messages are tools
to increase flight crew situational awareness. Other
tools include windshear and other EGPWS alerts.
Displays and annunciations are prioritized so that any
Windshear alert will override a EGPWS or TCAS /
ACAS alert, and any EGPWS alert will override a
TCAS / ACAS alert. In instances where TCAS / ACAS
alerts are overridden, only Traffic Advisories are
displayed and aural messages are temporarily
inhibited.
2. Subsystems, Units and Components:
A. TCAS / ACAS Antennas:
Two antennas (upper and lower) antennas are installed. The antennas
(shown in Figure 27) are directional receivers and omni-directional
transmitters operating in the L-Band of the radio spectrum. A signal
strength algorithm in the computer determines which antenna is used for
best system operation. Using the directional features of the antennas and
the elapsed time between interrogation and reply, the system computer
determines bearing and distance to the other airplane transponder. If the
received transponder signal is from an airplane with Mode C or Mode S,
the altitude encoded in the signal is used to locate the airplane in three
dimensions. Since TCAS / ACAS transmits and receives on the same
frequency as the ATC system, a pulse suppression circuit is incorporated to
prevent the TCAS / ACAS antennas from transmitting simultaneously with
the onboard transponder.
B. TCAS / ACAS Computer:
The TCAS / ACAS computer is installed in the right radio rack, and is
powered by φC of the Left Main 115V AC bus. The computer uses
transponder replies received from other airplanes to monitor flight path
tracks and determine potential conflicts. Conflicts are detected by
comparing airplane range versus range closure rate and airplane altitude
versus altitude closure rate. TCAS / ACAS can exchange data with other
airplanes having Mode S transponders at a range of forty nautical miles (40
NM) and can detect (but not communicate with) Mode S airplane targets up
to one hundred twenty miles (120 NM). For airplane targets with Mode A or
Mode C transponders, the range is twenty nautical miles (20 NM).
A TCAS / ACAS alerting envelope is formulated based upon the amount of
time available to the flight crew for evasive maneuvers at computed closure
rates. The alerting envelope is a three-dimensional space surrounding the
airplane that varies in size and sensitivity with altitude. Sensitivity levels
determine the alarm time, size of the protected area and the vertical
threshold for alerts. At higher altitudes the sensitivity level is expanded to
　
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