Transponder codes

Transponder replies to radar with 4-digit squawk. Five examinable codes:

Unlawful interference: unlawful seizure of aircraft in flight (TC AIM GEN 5.1).

VFR codes (TC AIM COM 8.4):

• Climbing: keep 1200 until departing 12,500 ft ASL, then select 1400.
• Descending: select 1200 on reaching 12,500 ft ASL.
• Use Mode C (altitude reporting) whenever transponder capable.
• Leaving airspace with special assigned code: revert to appropriate VFR code unless ATC assigns new one.

7700:

• Unable to reach ATC immediately: squawk 7700 first, then establish communication ASAP.
• Operate transponder as ATC directs (TC AIM COM 8.6).

7600:

• Communication failure: squawk 7600 to alert ATC.
• Does not relieve IFR communication-failure procedures (TC AIM COM 8.7).
• VFR radio failure in Class B, C or D airspace: set it if equipped (CAR 602.138(b)).
• Squawk it before phoning ATC (TC AIM COM 1.7).

7500 confirmation (TC AIM COM 8.8):

• Controller says "CONFIRM SQUAWK SEVEN FIVE ZERO ZERO" when doubting Code 7500 was selected on purpose.
• ATC does not assign 7500 unless the pilot reports unlawful interference in progress.

• After 7500, aircraft changes to 7700 or transmits "TRANSPONDER SEVEN SEVEN ZERO ZERO": aircraft threatened by grave and imminent danger, needs immediate assistance.

Trap: mixing up 7500 / 7600 / 7700, or reading 1200 as emergency code (just normal VFR). Silence does not re-assign 7500; it stands as intentional.

Practice