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TCAS II and VFR Traffic

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Article Information
Category: Safety Nets Safety Nets
Content source: Skybrary Skybrary

Light aircraft, which constitute the bulk of Visual Flight Rules (VFR) traffic, are not normally fitted with Airborne Collision Avoidance System (ACAS), but increasingly, many are fitted with transponders. These are of course essential for flight in controlled airspace where SSR is a vital safety tool, and are also valuable to indicate certain emergency situations. Transponders are also extremely important for flight outside controlled airspace where their use considerably enhances safety by making VFR traffic visible to ACAS-equipped Instrument Flight Rules (IFR) traffic.

Effect of Transponder Setting

The effect of different transponder settings on TCAS II operation is illustrated on the table below:

Transponder Setting Effect Protection
OFF or SBY Intruder not detected No protection
ON TA without altitude Assists visual contact
ALT TA and RA Full TCAS Protection

To obtain maximum protection, transponders should always be switched 'ON' with 'ALT' set, unless other instructions are issued by the controller. This applies outside controlled airspace as well as inside.

Inside controlled airspace, the controller will advise what code to set; outside controlled airspace, code 2000 should be set except where another code is specified for VFR traffic.

RAs between Traffic Separated by 500 ft

When VFR and IFR traffic is separated by 500 ft, a "Monitor Vertical Speed" RA will be generated even though both aircraft are level at their correct altitude; this alerts the IFR aircraft to the proximity of the VFR traffic. However, deviation from level flight by the VFR aircraft is common, and will generate a "Climb" or "Descend" RA as appropriate if the separation becomes less than 350 ft.

TCAS and the Aerodrome Traffic Pattern

Although traffic separation in the aerodrome traffic pattern is normally provided by the ATCO, TCAS can be effective in alerting the pilot of a potential collision risk.

Further Reading