Which transponder capability is required as part of RVSM equipment?

• A. Mode C capable SSR transponder
• B. Mode S only transponder
• C. No transponder required
• D. TIS transponder

Answer: (A)

RVSM surveillance depends on aircraft reporting accurate altitude to ATC, so the transponder must actively encode altitude in its replies. This is provided by a Mode C capable SSR transponder, which sends pressure altitude data to radar systems used in RVSM airspace. The requirement isn’t about having a Mode S-only unit, a traffic-information transponder, or no transponder at all—the essential capability for RVSM is altitude encoding, which Mode C delivers. (Aircraft typically need two independent transponders with this capability for redundancy, but the key capability being asked is altitude encoding.)