A transponder is the aircraft's half of a conversation with the surveillance system: the ground's secondary surveillance radar (SSR) asks, the transponder answers, and the answer paints the flight (identity and level) on the controller's screen and into the collision-avoidance logic of every ACAS-equipped aircraft nearby. Modern Mode S transponders add a unique aircraft address and a data link to that exchange; the procedures below speak the classic language of Mode A, the identity reply, and Mode C, the altitude reply. The rules of the air module met the emergency codes in passing; this lesson owns them, together with the operating procedures for the transponder and the strict discipline of the airborne collision avoidance system.
Operating the transponder
The baseline rule is total availability: the pilot of a transponder-equipped aircraft shall operate the transponder at all times during flight, regardless of whether the aircraft is inside or outside airspace where SSR is used for air traffic services, because the answers feed not only the ground but the ACAS units of other aircraft, which protect everyone precisely where no radar service exists. In the absence of any ATC direction or regional air navigation agreement, the pilot operates the transponder on Mode A, code 2000. Mode C, the mode that adds the aircraft's pressure altitude to the reply, is operated continuously alongside, unless ATC directs otherwise: an altitude-less return halves the controller's picture and blinds ACAS vertically. And the IDENT feature is activated only when ATC requests "SQUAWK IDENT": the button momentarily highlights the aircraft's return, and an uninvited push highlights the wrong aircraft at the wrong moment.
Three codes carry emergencies, and this lesson is where the course's earlier pointers land. Set:
7700 — for a state of emergency.
7600 — for a communication failure. Its 2025 companion 7601, the visual-diversion code for IFR flights, belongs to the lost-communications procedure taught in the rules of the air module.
7500 — for unlawful interference: the discreet code the unlawful-interference rules promised, transmitted so the ground learns of a hijacking even when the crew can say nothing.
The mnemonic order never changes: seventy-five, taken alive; seventy-six, radio fix; seventy-seven, going to heaven. Crude, but it sticks.
When the transponder fails
A transponder failure in flight makes the aircraft invisible to secondary radar and to the ACAS of every aircraft around it: the controller loses the identity and level tags and must fall back on primary radar or procedural separation, and other aircraft lose their collision-avoidance protection against the flight. ATC is informed at once, and the flight can normally continue in accordance with its flight plan under the degraded service; continuing into, or next departing into, transponder-mandatory airspace requires the repair or a specific permission. When the transponder fails before departure and no repair or replacement is possible at that aerodrome, the primary action is to inform ATS as soon as possible, preferably before the flight plan is submitted (the plan is annotated accordingly), and the flight proceeds, subject to any permission required, as directly as possible to the nearest suitable aerodrome where the repair can be made.
ACAS: the last safety net
The airborne collision avoidance system (ACAS), TCAS in its common implementation, interrogates the transponders of surrounding aircraft, computes closure, and warns the crew. Its main reason for existing is stated plainly in the rules: to serve as a last-resort safety net against mid-air collision, independent of the ground ATC system — the layer that still works when every layer above it has failed. Its warnings come in two stages:
Traffic advisory (TA) — an attention-getter announcing traffic that may become a threat.
Resolution advisory (RA) — a computed vertical escape command (climb, descend, or hold vertical speed), coordinated, where both aircraft carry ACAS, so that the two machines choose complementary manoeuvres.