In RVSM airspace, how many Altitude Reporting Transponders must be installed and how many must be operational?

• A. One transponder, one must be operational
• B. Two transponders, both must be operational
• C. Two transponders, at least one must be operational
• D. Three transponders, two must be operational

Answer: (C)

RVSM operations rely on redundancy in altitude reporting to keep precise vertical separation. Two independent altitude reporting transponders are required so that if one path or unit fails, the other can still transmit accurate altitude to ATC. The option that two transponders must be installed with at least one being operational reflects this redundancy idea: you have a backup source available, ensuring altitude reporting can continue even if one unit is down. In practice, for full RVSM capability, both transponders should be working, but the minimum described here aligns with having a spare path available while maintaining continued altitude reporting. Having only one transponder wouldn’t provide redundancy, and requiring three or both to be always operational goes beyond the stated requirement.