(Re)setting transponder codes
 

I heard from someone that the following is a "standard procedure" for setting transponder codes:
1. Switch it to Standby
2. Twist in the new squawk code
3. Switch it back to On or Alt

The reason for this was to prevent accidental squawks of 7x00 codes.

I had never heard of this before. The only advice I've been given is to twist in the new squawk code backwards (i.e. from right to left), which at least in the US where the VFR code is 1200, reduces the possibility of accidentally squawking 7x00.

So I'm curious... What do you do? Am I alone in not having heard of this "standard procedure"?

Yup, normally only change the code with the transponder in the standby position.

The reason os not only that one could cycle through 75,76,7700 but also the fact that you could cycle through a code assigned to another aircraft.

For example you are given a code of 0002. If you cycle through 0001 on the way to 0002 and another aircraft is already assigned 0001 by ATC then there can be momentary identification problems.

I know that the time it takes is very small but there is a distinct posibility.

The method I recomend is;

Write down the assigned code
Select Standby
Dial in the code
Confirm the code against the written record
Return to ALT

Regards,

DFC

It is good practice to switch any transmitting device to Standby when changing frequency or information transmitted.

There is nothing worse than seeing a 7500 squawk, even for a few seconds. Doesn't do mine or anyother controllers heart any good!!!!!!:(

Do controllers have hearts then? :-)

Sorry, joke, fine upstanding body of people who deserve lots more money, honest. :-)

Ian

Mr S - shame on you ;) (makes mental note to ignore certain C182 next time it calls :D)

CM

Oh no...grovel grovel grovel - best go non radio for a while...so any clues as to your unit?

Ian

Seems to be an "American" thing - when I was there, I was told by a usually reliable CFII that I should NOT switch to standby while changing squawk.

I still "standby" in Europe - I can imagine the reaction of the ATCO as the transponder wanders across the range of available codes!

In the UK I've been taught to always select standby before changing the transponder codes. Not to do so seems very bad practice. Surely the SSR readout on the radar would be all over the place as you change the code? As well as the risk of passing through a 7xxx code, you stand the risk of passing through a code allocated to another aircraft. This may well upset various TCAS and Radar systems, whose software developers may not have considered that two returns with the same code is valid, so long as it only occurs momentarily.

UK 1 - US 0.

I was told off by an examiner for switching to standby when changing transponder codes recently. He claimed that there's a delay build into the transponders which prevents them sending spurious codes. No idea if he was right, though - should probably check on the King website.

Urban Myths
 

The 'delay story' is one of those myths put around by people to try and cover up their laziness ;)

As the secondary head interrogates your transponder it will display exactly what you have selected at that moment - I see it every day when people are changing squawks. Select standby, and you get a clean change.

CM

As far as I am concerned it is standard practice to switch to standby before changing sqwarks. I have never flown with anybody who doesn't do so and it makes good common sense all things considered.

The only draw back is you have to remember to switch back to ON. Something I forgot during by BCPL GFT, luckily the examiner either didn't notice or at least chose not to notice. It's not something you do twice after that.

I also thought that a 75 76 or 7700 sqwark remained on the radar screen even if you change sqwarks afterwards. Although ATC Babes post sugest otherwise. Can anyone clarify?

Also presumably there must be a small delay otherwise the radar would show constantly changing numbers as an aircraft (that hasn't changed to standby) changes sqwarks?

Interesting discussion - I wasn't really trying to start anything; I just wanted to find out how common this procedure is and why I didn't know about it.

It is looking like this might be a "different in the US, different in the UK" type thing. I've searched the FAA FARs and AIM and they make no mention of switching to Standby. The AIM is where I thought I'd find this sort of info, but it is pretty silent. The only relevant thing I could find after a quick search is:

From Chapter 4, Section 4-1-19 of the AIM
e. Code Changes
1. When making routine code changes, pilots should avoid inadvertent selection of Codes 7500, 7600 or 7700 thereby causing momentary false alarms at automated ground facilities. For example, when switching from Code 2700 to Code 7200, switch first to 2200 then to 7200, NOT to 7700 and then 7200. This procedure applies to nondiscrete Code 7500 and all discrete codes in the 7600 and 7700 series (i.e. 7600-7677, 7700-7777) which will trigger special indicators in automated facilities. Only nondiscrete Code 7500 will be decoded as the hijack code.

Searching the Bendix-King site yielded nothing of interest.

I believe (but stand to be corrected), that radar antennae sweep every 10 seconds, so presumably if one changed the transponder code within less than 10 seconds, the changing code would not show up on the radar screen. Would this not explain the delay that has been mentioned?

Depends on the transponder fit. Some models have an "Emerg" function will will transmit 7700 as well as the assigned code. It is these that will keep the code / callsign displayed as well as the emergency. For the majority of us though it's a case of you display what you select - if you deselect 7700 then it doesn't show anymore.

1) How do you know what part of the sweep you were in and 2) You are not being interrogated by one SSR, but possibly by several at different times - why do you think the interrogation light blinks irregularly. It's true the average scan rate is 10 seconds - some may be slightly faster, some may be slower.

CM

Standby. From a controller's perspective, the 'lazy' method is one of those irritating little things.

PS. SSR update rates can be as high as once every 4 seconds.

Perhaps if the UK SSR system was as pilot-friendly as the US system, this wouldn't matter! It seesm that every time you change frequency (certainly on military frequencies) you have to change the SSR code in the UK....

I was told that changing squawks without selecting 'standby' was fine as it takes 10-20 seconds after changing codes for the new code to be transmitted - or perhaps that really meant for the new code to be registered by the radar heads which were around 10-20 years ago?

Beags

When we have a unified ATC system outside of controlled airspace instead of lots of different units then we too will be the same as the US - until that time the system we have is the best you'll get I'm afraid as it shows who's working what. It's the old 'dosh' problem I'm afraid.

Once upon a time what you said would be true (I once worked with a radar with a 15 second scan rate) but those days are long gone and I refer the honourable gentleman to my previous answers.

CM

Question for the ATC's among us...
 

when we leave a frequecy, particularly FIS frequencies like Farnborough, ATC say words to the effect of "call next station, squawk standby" where apon I duly turn the knob to standy and dial in 7000 on the transponder until I have a new squawk.

so my question is

1) can you still see me if I am on standby ( from the posts so far I assume not )

and

2) If I set the transponder to 7000 and turn it to 'alt', because its a local flight or I am passing along the edge of you cover ( not talking about MATZ's here), am I making myself more visible and being a help or am I cluttering up your screen and being a pain in the @ss.

Sky

Like the UK, Oz training also tends towards selecting Standby before changing code.

I find I don't even think about it, it's just habit.

No - we can't see you when you're squawking standby. The example you quote is interesting because what the ATC unit is saying (if that is what they are saying) is wrong. You should be being told to squawk 7000.

"Squawk Standby" should be used when you're joining the circuit and it means exactly that - the standby switch and not, as many PPL's assume (incorrectly), 7000 but still emitting.

As for your second point - it should be 7000 with mode 'C' (if you can) unless you're in the circuit. Have a read of Redhill and Gatwick.........

CM