How many independent altimeters are required for RVSM operations?

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

Answer: (B)

Two independent altimeters are required because RVSM hinges on having reliable, redundant altitude information when vertical separation is reduced. With two separate, independent altitude measurement systems, a single failure or error in one system won’t leave the aircraft without accurate altitude data. The two altimeters are designed to be independent (different sensing paths and power/static sources) so a common cause can’t take both out at once. They both feed the cockpit indications and the altitude reporting to ATC, and in RVSM airspace the crew cross-checks that the readings agree within tolerance. If they don’t, altitude data can’t be trusted for RVSM operations, which is why the minimum requirement is two. Having only one would remove essential redundancy, and having three or four goes beyond the standard requirement.