Transponders and ACAS

Transmit the right surveillance information

A code communicates a condition; it does not fly a procedure.

Normal operation and emergency codes

Operate a serviceable transponder during flight as required by SERA and the published procedures, with pressure-altitude reporting when equipped unless directed otherwise. Select the assigned code; in the absence of instructions use the applicable regional or State code, rather than assuming one worldwide VFR code. IDENT is used when requested, not as an informal acknowledgment of every transmission.

7700 indicates a general emergency, 7600 communications failure and 7500 unlawful interference. Current SERA adds 7601 for an IFR flight with communications failure whose PIC decides to continue in VMC and land at the nearest suitable aerodrome. The dedicated failure procedure determines when that branch applies.

Unserviceability

Advise ATC promptly of an in-flight failure and follow the coordinated instructions; restrictions, rerouting or loss of access to transponder-required airspace can follow. Before departure with an unserviceable transponder where repair is unavailable, obtain the required permission/arrangement and comply with the applicable conditions; do not simply depart through mandatory airspace.

Mode C reports pressure altitude on the standard reference, not QNH corrected height. Mode S supports selective addressing and additional data. A Mode S identity mismatch or wrong code should be corrected through the prescribed checks and ATC coordination.

TA prepares; RA requires a response

ACAS is an independent collision-avoidance safety net.

Advisories have different purposes

A traffic advisory draws attention to possible conflicting traffic and prepares the crew for a possible RA. Do not manoeuvre solely on a TA; use it for awareness and visual acquisition without unsafe fixation. An RA provides vertical guidance to reduce collision risk and requires prompt correct response unless doing so would jeopardise safety.

Follow an RA even if it conflicts with an ATC instruction. Do not manoeuvre opposite to an RA, and follow strengthened or reversal guidance promptly. Visual identification can be mistaken; do not disregard a valid RA merely because a nearby aircraft seems to have been seen. Stall warning, windshear and terrain-avoidance warnings can require a higher-priority safety response under the applicable procedures.

Tell ATC and restore the clearance

Notify ATC as soon as practicable using the prescribed TCAS RA report when deviating from a clearance or responding to a conflicting RA. After clear of conflict, return promptly to the cleared flight path unless instructed otherwise and report the return or resumed clearance.

The purpose is safe short-term avoidance, not an unauthorised long-term change of level. SERA’s ACAS obligations and the aircraft/operator procedures govern the response; the existence of historical PANS descriptive wording is not a discretion to ignore current mandatory requirements.

Can you apply the rules?

Four original questions. Choose from the facts, then review the explanation.