Squawk
 

Hello there,

I've always wonder what is the correct procedure or what is best regarding the use of the transponder when a new code is assigned in flight while under radar coverage. I mean is it best to select the transponder to the STBY mode before chanching to the new code or make the selection while in the "ON"? What's the effect on the screen of the controller?

Thanks,

The generally accepted practice is to select standy while changing digits. This is to prevent inadvertent selection of another code (which can cause false identification of flights which are not yet airborne) or the accidental selection of one of the emergency codes on the way past.

Having said that, the advent of push-button transponders in some light aircraft are beginning to make this obsolete as when you start entering another code it seems to stop squawking all by itself until all four digits are in.

If you want a fuller reply, then do a search on this subject and you will see a longer discussion.

1 - Turn off transponder.
2 - Select new code.
3 - Turn on transponder.

Anything less than the above results in the confusion already described.
If you are cycling through codes which involve (from or to), the number 7 (i.e. 4500 to 7000) there is a very high likelyhood that for just a second your transponder will transmit 7500 - hijack. This will instantly be displayed to on every radar which has SSR within at leat 60 miles, plus to the distress and diversion cell, and many military statitions - ground based and airborne.
Under current circumstances they all would be within their rights to think that you were trying to indicate that you were being threatened, and had a real problem.
It sounds bizzare, but I suggest you don't try it out for real.

At the risk of being shot down by the masses, exercise some caution with respect to placing your transponder to SBY whilst changing your Squawk, as on many a/c this will also turn off your TCAS - not a good idea! TCAS is one of the few bits of kit we have which has saved my life twice. Neither of them controller error!!!