How many Altitude Transponders are required for RVSM?

• A. 0
• B. 1
• C. 2
• D. 3

Answer: (B)

RVSM relies on high-integrity altitude information and continuous monitoring, so redundancy is built into the altitude-keeping and reporting systems. To meet this, an aircraft must carry two independent altitude-encoding transponders with separate power and wiring. Having two transponders ensures that if one unit or its data path fails, the other can continue to provide accurate altitude data to ATC and support the aircraft’s automatic altitude-keeping in RVSM airspace. This redundancy is essential because the reduced vertical separation in RVSM makes a single-point failure potentially unsafe. With one transponder, a failure could leave ATC with unreliable altitude data and could compromise RVSM compliance; zero transponders would mean the aircraft cannot report altitude at all. In practice, two transponders (primary and standby) are the standard requirement for RVSM.