Depends very much on the transponder. I don't doubt it was a problem with the very old transponders. Many of the more modern rotary dial transponders suppressed the return for a while after a change to avoid broadcasting a half-set code. With the modern push button electronic kit, the code is not changed until the 4th digit is entered, of course.
What's the reason given _not_ to turn off the transponder for a moment or two? Does it really matter if you don't appear for a scan or two, especially as ATC have probably just asked you to change the code anyway and may be expecting it to happen?
Ancient transponders (like I was presented with last week) - always a good idea to go to the Standby position before touching any selector. It doesn't hurt to have "Alt" deselected for the short time that it takes to set the new Squawk.
It used to be that you were never given a Squawk starting with a "7" unless you had some sort of problem. Before I retired from professional flying about five years ago, I was even being given Squawks such as "7742".
Since re-entering the world of light aeroplanes I have also been given some interesting Squawks. One unit in the south of England (is it Bournemouth?) have given me "7377" on a couple of occasions. That could possibly be quite fun with a bit of finger trouble on an ancient transponder!
Interesting definition of “ancient transponders”. The KT76a was still on sale in the US brand new up to about 18 months ago – may still be. I would guess that there are more KT76’s out there in the world than any other unit by some way.
Surely common sense will dictate if your unit needs standby or not.
Surely in classes of airspace where a transponder is a legal requirement, it would actually be illegal to switch to SBY to change code?
I leave it on ALT, and make sure I don't go through 7700, either by starting the change with the last number, thereby worst case is a brief squawk of 770X, or knowing which way the numbers turn so you never go through 7700.
In the early stages of my PPL training with a retro transponder, the instructor changed the code, briefly passing through 7700, 7600 and 7500, we got a pretty quick call from ATC checking all was ok as they had alarms ringing.
Old equipment - in the aircraft and on the ground - could do unpredictable things if the transponder was not set to STBY before changing a code. As others have mentioned, with modern equipment there are no problems in normal operations.
Quote:
Surely in classes of airspace where a transponder is a legal requirement, it would actually be illegal to switch to SBY to change code?
Old equipment - in the aircraft and on the ground - could do unpredictable things if the transponder was not set to STBY before changing a code
Out of interest, how old is old? I'm assuming something would be in the manual for the equipment about this, however this is not a document the average pilot is ever likely to get hold of.
With the rotary selectors, you need to select standby first. ATC will expect this and know you may fade for a few paints, however, your label may not disappear entirely if the radar has a "coast" function or there is also primary radar coverage. The problem with not selecting standby is that in busy airspace, as you scroll through the digits, you may scroll through the code assigned to another aircraft which will give ATC a dupe. I.e. two different targets with the same callsigns.
Sorry folks. I'm not able to post a link but I believe it is under "Watch AOPA live" on the AOPA USA website. I believe it is not mandated but recommended. D.O.
Ahhh, you noticed that I specifically dodged the issue. Who can be sure - it's down to a combination of airborne and ground equipment? For practical purposes, if your transponder does not use thermionic valves I wouldn't worry about switching to STBY unless there is some evidence/experience to suggest that it causes 'problems'. Problems may not simply be triggering emergency code alerts or duplicating code-callsign pairings in the radar system.
What's annoying to controllers is the inadvertent false selection of any of the emergency squawks, which creates a distraction on screen as most radars flash the code energetically as an attention getter, not very nice when you're busy and concentrating hard.
The solution of course is to make the conspicuity code 1200, like the sensible Americans. Shows that the original selection of 7000 wasn't thought out properly in the first place.
I once suggested to NATS management that it might be sensible to seek a change, this was rejected out of hand, by a manager who had minimal operational experience and couldn't be bothered.
One thing that all above posts seem to failed to pick up on is that the Radar ground stations have a short time suppression of some codes to avoid the alarms going off if you should pass through them when changing code.
As said above all the new transponders dont transmit untill the last number is selected.
I think that the "turn to standby" when changing code is likely to have been dreamed up instructors sitting about on bad weather days who seem to like to try to turn flying into a black art.
The solution of course is to make the conspicuity code 1200, like the sensible Americans. Shows that the original selection of 7000 wasn't thought out properly in the first place.
Indeed, IIRC conspicuity in the UK used to be 4321, before it became 7000 in the early nineties, one of the early euro-harmonisations, was changed so as not to upset Paris Centre who used it for IFR traffic in their FIR.
The solution of course is to make the conspicuity code 1200, like the sensible Americans. Shows that the original selection of 7000 wasn't thought out properly in the first place.
The problem is not just the emergency codes. If you leave the wrong code selected and it belongs to an IFR flight on the ATS route network, radar systems that detect it may match it up with the a flight in their database, with the potential of a lot of confusion.
