For RVSM, how many altitude reporting transponders must be installed and what is the operational requirement?

• A. Two transponders with at least one operational
• B. One transponder that must always be operational
• C. Three transponders all must be operational
• D. Two transponders both must be operational

Answer: (A)

The key idea is redundancy for altitude reporting in RVSM airspace. Aircraft must have two independent altitude‑reporting transponders to ensure that altitude data continues to be available even if one unit fails.

The best answer reflects that two transponders must be installed, and at least one of them must be operational. This setup provides a backup: if one transponder is inoperative, the remaining functioning transponder can still report altitude to ATC, allowing continued operations in RVSM while the faulty unit is repaired. If both transponders are inoperative, the aircraft would not meet RVSM requirements and would need to leave RVSM airspace or have repairs completed before continuing.

So, two transponders installed with at least one actively reporting altitude aligns with the emphasis on redundancy and maintaining altitude data visibility to controllers.