The previous articles discussed mode S transponders only. But still, a lot of aircraft are flying with the familiar mode A/C transponder. We continue here by explaining howto operate an aircraft transponder and which code to set, and which code to set only if in dire need.

Operating Transponders

During or previous to a flight a pilot is given a code by ATC over the radio, such as in the radio phrase 'Experimental 2909B, squawk 7000'. The pilot inputs these numbers and his dot on the radar scope becomes correctly associated with his identity. Software can add more data to this dot as: call sign, flightplan information and more.

Around controlled (meaning busy) airspace there is often a requirement that aircraft be equipped with an altitude reporting mode A/C or mode S transponder (in fact, mode A transponders are not even manufactured these days anymore), it is called Mode C veil in the US. Mode S transponders also transmit altitude information.
Without the altitude reporting, ATC cannot see any altitude information and must rely on the altitude as reported by the pilot. This altitude information is obtained by a altitude encoder device connected to the aircraft static system and the transponder, it reports altitude against a fixed barometric setting (1013,25 hPa or 29.92 inch).

Ident
All mode A/C, and S transponders include an so called 'Ident' button, which activates a special bit in the reply. When SSR equipment receives the Ident bit, it will result in the aircraft's blip being highly visible on the radar display. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot (Radio frase: 'Experimental 2909B, squawk 0060 and Ident').

Ident can also be used in case of a possible aircraft radio failure to check if the failure is one way and whether the pilot can still transmit or receive but not both (Radio frase: 'Experimental 2909B, if you can hear me, please Ident'). After which (if he heard this call from ATC) the pilot must set code 7600 on his transponder to indicate a communications failure (in this case a transmitter failure).

Transponder codes
A discrete transponder code is assigned by air traffic controllers to uniquely identify an aircraft. This allows easy identity of the aircraft on the radar screen. VFR traffic with no flightplan usually have special codes assigned.

Transponder codes are four-digit octal numbers (decimal numbers 0 - 7, binary 3 bit digits: 000 to 111 octal), these were developed to indicate the status of the aircraft. The dials on a transponder read from zero to seven inclusive. Thus the lowest possible code is 0000 and the highest is 7777. Care must be taken not to set any emergency code during a code change!

For example, when changing from 0500 to 7000 (VFR code): you might turn the first knob to a 7 (thus 7500), and at this time the second knob still is set to 5. This would momentarily have the transponder replying the hijack code (7500), which might lead to a lot more attention than desired. I would recommend to set the transponder in 'standby mode' before changing the codes. It will cause a temporarily loss of target info on the radar screen, but it's safer than having bells going off in ATC centers and F16s scramble for you because you accidently had 7500 displayed on the radar screen.
Additionally, modern digital transponders (see Garmin) are operated by buttons to avoid this problem altogether, just enter the new code and it is transmitted the moment you press the last digit (not during the change).

Routine codes
There are other codes known as 'conspicuity or routine codes' which are not necessarily unique to a particular aircraft, but they have their own meaning and are used to transmit certain status information about the aircraft to ATC, possibly when the aircraft is not in radio contact or flying VFR in uncontrolled airspace.

• 1200: Visual Flight Rules (VFR) flight, this is the standard code used in North American airspace when no other has been assigned (*)
• 7000: VFR standard code for most European airspace when no other code has been assigned (*)
• 7004: Aerobatic and display code
• 0001: Military code for high speed uncontrolled (non-ATC directed) flight (US)
• 7001: Sudden military climb out from low-level operations (UK)
• 2000: The code to be squawked when entering a secondary surveillance radar (SSR) area from a non-SSR area (used as a VFR squawk code in some European countries)
• 0000: Military escort (in the US), suspected transponder failure (in the UK)
• 7777: Military interception (US/FAA)

(*) Some transponders have a button which, when pressed, outputs a preselected code. For U.S. built transponders like Garmin this code is preset to 1200 but can that be changed to 7000 when you want to use this transponder in Europe.

Universal Emergency Codes:

• 7700: Emergency
• 7600: Lost Communications
• 7500: Hijack (Unlawful Interference)

Note: some countries assign certain blocks linked to certain destinations (flights to some destination always get a transponder code starting with certain numbers). Normally, the transponder codes are assigned randomly.