View Full Version : Transponder code selection

8th Jan 2004, 02:09
Hi all, a quick question, what is the reccomended procedure for dailing in an ATC 'squawk' code. I have seen some pilots select the transponder to Standby , dial in the code and return the transponder to ON/mode 'C', I was taught to select the code once it is given by ATC, because turning the transponder to STBY and back ON gives some kind of alert , don't know what kind , maybe an ATC'er could shed some light.

8th Jan 2004, 07:39
Placing the mode selector in standby ensures that there is no chance of an incorrect code being diplayed on the controllers display as you cycle through the numbers.

Example - Current code 7000, new code 1234. If one changes the 7 to 1 but take some time, the transponder may transmit 1000 which could be assigned to another aircraft.

Thus the recomended practice has been to turn the transponder to standby while selecting a new code.

However, in standby, no mode C is transmitted and this has safety implications for both the TCAS system and for ATC conflict alert.

Consequently, current practice is to leave the transponder operating while changing the code but;

a. Don't hang about when changing; and
b. Avoid cycling through any of the special purpose codes.

From an ATC point of view, when a transponder response is lost, the system will plot the aircraft in the expected position for the next few updates but without any mode C readout. If you happen to be on a code assigned to another aircraft as the transponder responds, the display may drop your callsign and that of the other aircraft but the mode C will remain. IN some systems, this will generate a low level warning regarding a duplicate squalk but provided you quickly move off that squalk by the next response, the warning will be cancelled.

As a guide, most enroute radars turn at about 10 RPM these days so for a single radar head, your transponder will be interrogated every 6 seconds. Approach radars are usually 15RPM (4 second updates) or better.



Captain Stable
10th Jan 2004, 01:06
DFC, your answer is fine for a basic TXPDR.

However, modern TXPDRs in up-to-date aircraft are rather more sophisticated than that, and switch themselves to standby immediately one of the selector controls is moved, and remains in standby for a second or two (I forget exactly how long) thereafter.

Furthermore, leaving it at ALT/RA/TA setting (depending upon aircraft type) leaves the TCAS system operative and monitoring other aircraft. If you select SBY you lose that protection. Not a good idea.

14th Jan 2004, 01:01
In the 320 and the ATC selection is at ON/AUTO, then the transponder will continue sqawking the last selection untill all four numbers of the next code are completely entered. Therefore the system never operates at standby in normal use and altitude reporting / TCAS protection remains active at all times.

14th Jan 2004, 08:00
Brakes Hot -- That is true, only when a keypad-xponder is installed. A320s can be delivered with the dial-xponder as well, likewise other aircraft can be delivered with the keypad-xponder. However the buyer wants it :).

West Coast
15th Jan 2004, 11:59
Your idea would work if your txponder is only being interrogated by one site at a time. Not sure how it is on your side, but there are multiple radars looking at you at any given time. Especially in a terminal environment.

Captain Stable
16th Jan 2004, 00:43
Correct, West Coast - the TXPDR is being interrogated many, many times - multiple radar heads, plus all the other TCAS-equipped aircraft in range.

16th Jan 2004, 01:19
Indeed, the transponder will be interrogated by several radar heads at any one time in most terminal areas for code - All the ACC ones plut the APP radars from every airfield in range.

However, without getting into sort boxes etc, for an ACC sector, the information received from several heads will be compared and the best response displayed to the controller i.e. the response from a single head. Thus in general, the controller will see an update every 6 seconds or so (screen refresh rate etc may delay this slightly).

For an airfield, in general a single head is used but as I said these rotate faster.

For the airfields and ACC sectors not interested in your flight, it will be ignored or perhaps even filtered out.

As for TCAS - does TCAS use the code select element or simply the source of a signal and Mode C?



West Coast
16th Jan 2004, 05:49
Good question, I think it would only source what it needs. The TCAS aspect and changing to standby is one I haven't thought about. With the newer models its a mute point as it delays the new code till complete but older acft may be a different story.

16th Jan 2004, 16:47
I reckon there is no problem if the CDU has a tuning page. Just punch in the numbers in the scratch pade and move them up to the active line. And it is faster than dialing, too.

18th Jan 2004, 05:11
standard practice in Australian GA and RPT is to select STBY first enter new code then return to swarking MODE C. You will find most people are taught this. Plus once you get use to doing this it become a natural habit of turning it to STBY first

Captain Stable
18th Jan 2004, 20:28
A natural habit indeed, azzamang, but as is pointed out, one which is not necessarily desirable.

West Coast
20th Jan 2004, 01:08
Switching to standby is not without its own problems. At least in the states on some of the older ATC equipment. The ARTS data blocks on the controllers screens is associated not with primary radar but with txponder data. If you turn to stby and for some reason don't turn it back on, the computer starts looking for you. After a short time frame the data block starts coasting on the screen and may end up dropping all data on the aircraft.