In RVSM operations, how many Altitude Reporting Transponders must be operational?

• A. One
• B. Two
• C. Three
• D. Four

Answer: (A)

Redundancy in altitude reporting is essential for RVSM. To maintain the precise 1000 ft vertical separation that RVSM requires, an aircraft must have two independent altitude-encoding transponders that are both operational in flight. Each transponder provides altitude data to air traffic control, and having two independent sources lets ATC cross-check the altitude and ensures continuity if one path fails. A single transponder would not meet the RVSM safety requirement because there would be no backup for altitude reporting. Three or four transponders aren’t required for RVSM, though additional units may exist for other reasons; the standard minimum is two.