Which transponder mode is required in controlled airspace?

Which transponder mode is required in controlled airspace?

• A. Mode A/C or Mode S
• B. Mode A only
• C. Mode C only
• D. Mode S only

Answer: (A)

In controlled airspace, air traffic control needs to know both who you are and how high you are. That requires a transponder that reports altitude. A transponder that only uses Mode A gives identity (squawk) but no altitude, so it doesn’t meet the requirement. A transponder that provides altitude can be either a Mode A with C (often spoken as Mode A/C, meaning the transponder reports altitude as well as identity) or Mode S, which also includes altitude information plus more data. So, the best answer recognizes that any altitude-reporting configuration satisfies the rule: either Mode A with C or Mode S. The other options fall short because they don’t guarantee altitude reporting (Mode A only), or they imply an incomplete/less common setup (Mode C only), or are not as inclusive as the needed combination.

In controlled airspace, air traffic control needs to know both who you are and how high you are. That requires a transponder that reports altitude. A transponder that only uses Mode A gives identity (squawk) but no altitude, so it doesn’t meet the requirement. A transponder that provides altitude can be either a Mode A with C (often spoken as Mode A/C, meaning the transponder reports altitude as well as identity) or Mode S, which also includes altitude information plus more data.

So, the best answer recognizes that any altitude-reporting configuration satisfies the rule: either Mode A with C or Mode S. The other options fall short because they don’t guarantee altitude reporting (Mode A only), or they imply an incomplete/less common setup (Mode C only), or are not as inclusive as the needed combination.