Aviation In Canada

Thursday, September 01, 2005

Changing Transponder Codes

Here's something that's kind of interesting about transponder operation. Changing codes. When assigned a code while airborne, many pilots will select "Stnadby", change the code, then select "On" or "Alt" again. Did you know that this isn't the preferred method from the other side of the scopes?

I know, most pilots will tell me they do this to ensure they don't inadvertently select any of the emergency codes. But the reason this is bad is that it causes ATC to lose the target on radar while the code change is taking place. The last thing we want, if we're trying to confirm which target is you on radar, is to stop seeing you on radar.

The soon-to-be vanquished AIP Canada actually has a passage about it in RAC 1.9.1, para 3, which states, "Do not select "STANDBY" while changing codes as this will cause the target to be lost on ATC radar screens."

In the same paragraph, they give advice on how to prevent inadvertent selection of codes. Their example is changing from 1700 to 7100 by changing the second digit first, making it 1100, then changing the first one to show 7100. This requires a little thought as to which codes could be "moved through" in the process. Another idea is to change the last digit first, which is often not a zero.

Whatever method you choose, it would make a controller on the other end happier not to lose your target when changing codes. Oh, yes, don't operate the IDENT feature when you change a code. Changing the code is enough. The ident feature should not be used except when directed by ATC. Some pilots seem to like being cool and calling in on departure like this: "Good afternoon, Center, Airline 123 with you off AirportX through 1,900 with the flash." Save it, please. Wait until instructed to use it. What if, just before you called in, someone else was told to squawk ident? Then two targets are flashing at the same time, and technically, neither one can be considered radar identified.