A Mode C transponder sends your pressure altitude to air traffic control, in 100-foot steps. The easiest check is the transponder's selector: if it has an ALT position, the unit reports altitude.
For a firm answer, ask an avionics shop to test it. The rules in 14 CFR 91.215 say where you need it.
A transponder replies to radar with a four-digit code. Mode C adds your altitude. The FAA rule describes the equipment as automatic pressure altitude reporting equipment with a Mode C capability that replies to Mode C interrogations by transmitting pressure altitude in 100-foot increments.
A Mode S transponder meets the rule too, as the regulation accepts Mode S capability as an alternative to Mode A with Mode C.
Section 91.215(b) requires an operable transponder with Mode C in these places, unless ATC authorizes or directs otherwise:
The regulation includes an exception for aircraft that never had an engine-driven electrical system, and for balloons and gliders, which can operate in some of these areas under conditions listed in the rule.
Section 91.215(c) says that in the airspace described above, or in all controlled airspace, a pilot of an aircraft with an operable transponder must operate it, including Mode C if installed, and reply on the code ATC assigns, unless ATC directs otherwise.
Under 91.215(d), a request to operate without a transponder, or with an inoperative one, goes to the ATC facility with jurisdiction over the airspace. For an aircraft that is not equipped with a transponder, the request must be made at least one hour before the proposed operation. For an inoperative transponder on the way to a destination or a repair station, the request can be made at any time.
Transponder rules sit beside the ADS-B Out requirements in 14 CFR 91.225. Our guide to flying a private plane into Canada covers ADS-B for cross-border trips.
If your transponder has an ALT position, it very likely has Mode C. Confirm with an avionics shop, and read 14 CFR 91.215 for the airspace where it's required.
It reports your pressure altitude to air traffic control, in 100-foot increments, along with your four-digit code.
Check whether the selector has an ALT position. Then confirm with an avionics shop or by asking ATC whether they see your altitude.
Under 14 CFR 91.215, in Class A, B and C airspace, within 30 nautical miles of the airports listed in appendix D of part 91 (the Mode C veil) up to 10,000 feet MSL, above Class B and C ceilings up to 10,000 feet, and at and above 10,000 feet MSL over the 48 states and DC (except at and below 2,500 feet AGL), among other areas.
Yes. The rule accepts a Mode S capability as an alternative to Mode A with Mode C.
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