In ACT mode with ALT on, what information is transmitted?

• A. The transponder code and altitude information
• B. Aircraft altitude only
• C. The transponder code only
• D. None

Answer: (A)

When the transponder is in ACT mode with ALT on, the reply sent to ATC includes both pieces of information: the assigned transponder code (Mode A) and the aircraft’s altitude (Mode C). The code identifies the aircraft on radar, while the altitude data provides vertical position, which helps ATC maintain safe separation. With ALT on, the altitude bit is enabled, so the transponder broadcasts the altitude along with the squawk code in its response. If ALT weren’t on, the altitude wouldn’t be transmitted, leaving only the code.