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VSI/TCAS and ATC-transponder.


This is a combination of ATC MODE-S transponder and TCAS control.

TCAS control

1. TCAS vertical scan selector.
ABOVE - Expands the vertical scan to 9900 ft above and 2700 ft below the aircraft.
N - Normal position. TCAS vertical scan is +/- 2700 ft.
BELOW - Expands the vertical scan to 2700 ft above and 9900 ft below the aircraft.
2. Function selector.
STBY - Both ATC interrogators are powered but cannot transmit.
TA/RA - This is the normal operating TCAS mode. In this mode, Traffic and Resolution Advisories are enabled.
TA - Only TA (TRAFFIC ALERT) is displayed; no RA. This mode prevents TCAS from issuing RA when intentionally flying close to other aircraft, i.e closely spaced parallel approaches.
XPDR - In flight: The selected ATC transponder will transmit replies to ground interrogations. The unit not selected remains in standby. On ground both transponders are in standby.
3. TEST switch.
Initiates ATC transponder and TCAS self test.
4. Code selector knobs.
The large (inner) knob is used for selection of the first two digits in the reply code and the small (outer) knob is used for selection of the two last digits.
5. IDENT button.
When pressed, the special identification pulse (SPIP) is sent for a duration of approximately 20 seconds.
6. XPDR switch.
Selects transponder 1 or 2 as active transponder. The one not selected will remain in standby. Normally transponder number 1 is selected on flights with odd flight numbers and number 2 on even flight numbers.
7. ALT RPTG switch.
OFF - The altitude reporting is selected off.
1 or 2 - The transponder will reply to interrogations requesting both code and altitude replies. Selected in the same manner as XPDR.
8. Readout of selected code.
Each digit can be set from 0 to 7.
9. ATC FAIL light (amber).
Illuminated when the selected transponder or its altitude information source has failed. TCAS fail is announced on the VSI.


The Traffic Alert and Collision Avoidance System (TCAS) is an independent airborne system designed to supplement the ground based Air Traffic Control system. The system continually interrogates transponders of aircraft in the vicinity. By processing the replies the system determines if a threat exists. Depending on the level of threat, either an aural alert will be given or an aural alert combined with evasive vertical guidance to ensure safe vertical separation from the conflicting traffic.

The TCAS computer is the heart of the system which controls the airspace surveillance, intruder and own aircraft tracking, threat detection and resolution and advisory generation. The computer contains a receiver/transmitter which interrogates transponders of other aircraft. The replies are processed together with other data, such as pressure and radio altimeter altitude, to control the collision avoidance logic parameters that determine the protection volume around the TCAS aircraft. If a tracked aircraft is a collision threat, the computer selects the best avoidance maneuver, and if the threat aircraft also is equipped with TCAS, this maneuver is coordinated via the Mode-S transponders.

The airspace which TCAS scans is up to 40 NM horizontally in front of the aircraft and 9900 ft vertically above and below. Up to 30 aircraft can be tracked tracked by TCAS simultaneously. The vertical scan may be adjusted by the pilot.

In MD80 the TCAS information is superimposed in the VSI (Vertical Speed Instrument). In many other aircraft types the information is presented on the ND.

VSI/TCAS The VERTICAL SPEED instrument with no TCAS information. The pointer indicates vertical speed in feet per minute.

The range of TCAS information - the small ring around the aircraft is 2 NM. The total distance forward is 6 NM and backward 4 NM.

VSI/TCAS The same instrument during TCAS test.
Restricted vertical speed. This area instructs the pilot what vertical speed regions is to be avoided during an RA.

FLY-TO vertical speed arc. This area instructs the pilot of the vertical speed to ensure safe altitude separation from conflicting traffic. TCAS expects adjustment in pitch attitude to attain the desired vertical rate within 5 seconds during an RA.

OTHER TRAFFIC - hollow cyan diamond.
Indicates other traffic, (the predicted flight paths do not penetrate the Collision Area) which are within range of the display.
PROXIMATE TRAFFIC - solid cyan diamond.
Indicates other traffic within 6 NM and within +/- 1200 ft vertically. Proximity traffic are shown to improve situation awareness.
TRAFFIC ADVISORY - solid amber circle.
Indicates intruder aircraft predicted to enter the Collision Area within 35-45 seconds.
RESOLUTION ADVISORY - solid red square.
Indicates intruder aircraft predicted to enter the Collision Area within 20-35 seconds.
the amber -03 and "arrow up" indicates an aircraft 300 feet below and climbing with a rate of more than 500 fpm.
The cyan -11 and "arrow down" indicates an aircraft 1100 feet below and descending with a rate of more than 500 fpm
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