Proper TCAS actually sends out an interrogation pulse
to pick up the local transponder targets - cheap TCAS
relies on picking up the responses from secondary
radar interrogations in the vicinity.

I've got Skywatch in my beast and its amazing how
much traffic it makes you aware of that you simply
do not see regardless of the quality of your external
scan...

I was taught to go to standby as well, but my transponder
buffers an updated squark code until complete before
changing the old one.

-- Andrew

SSR will respond to a TCAS interrogation.

Some of the more new transponders (Bendix King KT70/71 etc) will auomatically go to sby when changing codes.

Like most bits of kit in any machine,its good to be trained to the specific kit fitted.Aeroplanes with WW2 avionics need their pilots to be trained by WW2 veterans.
If theres achance of a duff squawk from an old transponder I'm sure it should be used in a way that it won't do the wrong thing.
Modern kit eg Garmin327 only transmits a new code on pressing fourth digit
General discussion and training on say a transponder,GPS,radio etc is of limited use.I'm sure it should be type specific.
So,if you change planes,get trained on the actual kit in the new ones.In general modern avionics can dramatically reduce pilot workload when handled to the instruments designed capability and not just operated to a limited perceived capability.Put another way,its all in the manual!! .and the manual probably has lots of pages because the instrument has lots of functions over and above the ones that you thought..

During the fist 25 hrs of my PPL in Canda, I was never once advised to switch off the Xpdr to change codes (and I was flying with a moderately young instructor).

In the UK, on about the first flight as a student, I was told to go to standby to change codes.

20 yrs ago, I used to work on avionics devpt (with special responsibility for IFF aka SSR) - to the best of my knowledge, there is no 'suppression' of code changes while the dials on your Narco / Bendix, etc are being twiddled.

ATC Babe (or 10W, if EGPK is easier for most) - if a bunch of PPRuNers were to grovel nicely, what are the chances of a look around your workplace (as I've only really done superficial tours at miltary airfields over the last 20 years). Would someone like Sandy be the best person to talk to?? We could maybe even have a Diet Coke or 2 at the Hilton afterwards??!! ;) ;)

Looks like a landslide vote here for selecting standby - it's a pity there's no formal instruction that you can stick in front of some of the "old wives" - maybe we can help with that as we're looking for ideas from you guys to help reduce airspace infringements. Not quite the same subject, but close!

"When to use Mode C?" has given us lots of comment from pilots & controllers - the answer has to be: always unless you're in the circuit or you're told to switch it off by ATC.

TCAS might be your final savour - if your mode C is off then the automatic anti collision function cannot give the TCAS equipped pilot avoiding action. And it doesn't need an ATC radar to make it function, it's a stand alone piece of kit which just needs 2 mode Cs to produce a safe result!

By the way, if you do blunder into controlled airspace, I suspect the boys in blue are less likely to be sympathetic if your mode C was OFF, causing more traffic disruption etc.

Why not have a look at "Your Say" on www.flyontrack.co.uk and give us your thoughts.....

I always select mode C unless told otherwise by ATC....in fact what is the point in mode A unless your transponder doesn't have mode C ?....probably because I learned in an airport under a mode C veil and everytime I fogot to switch the TXpdr on, I got the 'XXXX not receiving your mode C'....and the obligatory 'roger,I'll recycle it ':D

Cheers
EA:)

I would second the tentative request to spend half a day shadowing and ATCO and getting a squizz at a radar screen myself. Yes, it might make my flying better, but it would just be interesting as well.

I make a great cup of tea????? :)

Circuit Basher - you were probably not advised to select Standby on your transponder whilst flight training in Canada because the Canadian AIP clearly states -

"Do not select "STANDBY" whilst changing codes as this will cause the target to be lost on the ATC radar screens"

Perhaps it's an airspace thing, in the congested UK there's presumably a lot more primary radar coverage so contacts are not as readily "lost" as in the wide open spaces of North America where p'raps there's more reliance on SSR. Any ATCOs from t'other side of the Atlantic care to state their preference?

I guess which ever way you change codes, it pays to do it carefully...
http://www.bayarea.com/mld/bayarea/3947879.htm