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).
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.
References
- CARs 605.35
- CARs 601.03
- TC AIM COM 8.1
- TC AIM COM 8.2