Transponder equipment requirements

A transponder answers radar; Mode C adds automatic pressure-altitude reporting, so ATC sees your altitude next to your code (TC AIM COM 8.1). The other modes: Mode A is the identity code only, Mode S adds a datalink, and ADS-B Out broadcasts your GPS position.

Where it is required. No one may operate an aircraft (other than a balloon or glider) in transponder airspace without a transponder and automatic pressure-altitude reporting equipment (CAR 605.35). Transponder airspace is (CAR 601.03):

• all Class A, B, and C airspace; and
• any Class D or E airspace designated as transponder airspace in the Designated Airspace Handbook.

This includes all Class E airspace from 10,000 ft ASL up to and including 12,500 ft ASL, within radar coverage (TC AIM COM 8.2).

Who is exempt. The requirement names two aircraft that are outside it: a balloon or a glider (CAR 605.35(1)). Nothing else is exempt by type, weight, or flight rules.

When it quits in flight. With an unserviceable transponder you may continue to the next aerodrome of intended landing; after that landing you may complete the planned flight schedule or go to a maintenance facility only if ATC authorizes it (CAR 605.35(2)(b), TC AIM COM 8.1).

When you have no working transponder before you go. ATC may authorize you into transponder airspace anyway, but the request must reach the ATC unit before the aircraft enters the airspace, and that includes taking off from an airport inside it (CAR 605.35(3), TC AIM COM 8.1). Approval needs aviation safety to be unaffected and may carry conditions.

Trap: thinking a transponder is needed everywhere below 12,500 ft ASL. The Class E band starts at 10,000 ft ASL and needs radar coverage; below that, only A/B/C and designated airspace require it. The second trap is asking ATC for the no-transponder authorization once you are already inside the airspace: it has to be arranged before entry.

Practice