Definition
Two transponder reply formats used by Secondary Surveillance Radar. Mode A returns the four-digit identification code (squawk code) assigned by ATC. Mode C returns the same identification code along with the aircraft's pressure altitude, encoded in 100-foot increments referenced to 29.92 inches of mercury. Both modes operate by replying to interrogations transmitted from a ground-based SSR antenna.
Plain English
Mode A is the part of the transponder reply that tells controllers which aircraft you are. Mode C adds your altitude to that reply. Together they let ATC see your identity and height on their radar screen.
Context Anchor
Seen in procedure notes and equipment requirements for instrument departures, including RNAV departures, when radar identification and altitude reporting are required.
Derivation
SSR stands for Secondary Surveillance Radar — 'secondary' because, unlike primary radar which bounces a signal off the aircraft skin, SSR works by interrogating a transponder onboard the aircraft, which then replies. The 'modes' are simply different reply formats the transponder is programmed to send.
Why Pilots Care
Enables air traffic control to identify the aircraft and maintain safe vertical separation using accurate altitude data.
Intuition Check
Do not read “Mode A” and “Mode C” as general settings like “normal mode” or “advanced mode.” In this context, they are specific transponder reply types: Mode A for the code, Mode C for altitude reporting.
Example Sentence 1
Before takeoff, the pilot set the assigned squawk code and selected ALT so the transponder would reply in both Mode A and Mode C.
Example Sentence 2
Aircraft without Mode C capability cannot provide altitude information when interrogated by SSR Modes A and C.