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 how to operate an aircraft transponder and which code to set, and which code to set only if in dire need.
During or previous to a flight a pilot is given a transponder code by ATC over the radio, such as in the radio phrase 'Experimental 2909B, squawk 7000'. The pilot inputs these numbers in the box and his dot on the radar scope becomes correctly associated with his identity. Software can add more data to this dot as: call sign, flight plan 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 anymore these days), it is called Mode C veil in the US. Mode S also transmits altitude information like mode C.
Without the mode C reporting, ATC cannot see any altitude information on the radar screen and must rely on the altitude reports by the pilot. This information is obtained by a special encoder device connected to the aircraft static system and the transponder, it also reports altitude against a fixed barometric setting (1013,25 hPa or 29.92 inch) thus indicating flight levels.
All mode A/C, and S transponders include a 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 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 phrase: '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 phrase: 'Experimental 2909B, if you read this, 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).
The next link opens a manual suitable to print with instructions to use a transponder.
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 flight plan usually have special codes assigned (7000 a/c).
Transponder codes are four digit 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. Note: 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 accidentally had 7500 displayed on the radar screen.
Additionally, modern digital transponders (like Garmin) are operated by buttons to avoid this problem altogether, just enter the new code and it is transmitted the moment you press the fourth digit (not during the change).
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.
(*) Some 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.
These codes are used worldwide and as a pilot you will need to know them by heart:
Note: Some countries assign certain blocks linked to certain destinations, operations or to certain aircraft as a preassigned code (flights to some destinations always get a transponder code starting with predetermined numbers). Normally, the transponder codes are assigned randomly.