(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

What a stupid phrase ! Sqwarking indicates 'transmitting' and standby indicates 'not transmitting' so what they are really telling you is to "transmit not transmit". Whats wrong with 'turn off your transponder'???

Here we go again, making everything more complicated than it has to be becasue we're British :p

EA:D

I echo CM's response, 7000/C. If you've got it, use it.

Beagle - being at a unit that interrogates every 4 seconds, it is entirely possible to watch a transponder change through all 4096 codes (OK a bit of poetic licence). This can get extremely confusing when the Code Callsign Conversion system is trying to play catch-up. Before you know it you have a C152 out of Elstree indicating that he is BAW1 routing via Dublin.

A military ORCAM ssytem, now that's a good idea. The major stumbling block is the lack of networked communications. Come the replacement of MSSR (allegedly due 2006) we may be able to do something about this; don't hold your breath. In the meantime, get those fingers on your knobs!

EA
 

Squawk Standby - 3 syllables, easy to understand and quick to say, especially as it's normally given in conjunction with "contact tower (freq)"

Turn off your transponder - 6 syllables and a real gob full when given with the contact tower part of the message. The idea of R/T useage is brevity, not who can transmit the longest sentence possible.

You'll be saying we should adopt that bloody ridiculous Americanism "Taxy into position and hold next!" (How many runway incursions did that cause when the good ol' CAA termed Holding points as 'positions' for a short while - enough said! :rolleyes: )

CM

As regards the 10-second scan rate ... I just got back from a delightful day out at Bourn, and on the way there and back, and while we bimbled around Bourn and Grafham Water, I reckon my transponder was flashing something like two to three times a SECOND.

Could have been all sorts, including 2Donkeys TCAS interrogating me.

What the controller sees depends to a large extent on what radar processing system is used.

What happens often depends on code callsign conversion.

Taking the 7700 case first. If one sets 7700 then an alarm goes off etc etc. However since many radar systems are programmed to know that 7700 is an aircraft in distress then when the 7700 disappears, the computer will freeze the target at a location so that SAR aircraft can be directed to that place. Thus the busy controller has to take time out to clear the false alarm.

On the normal codes, if the processing system has been given your callsign then it will display your callsign insted of the code. If the code changes or you select standby, then the computer will do a thing called coasting the target. This means that it will continue to paint the target a positions where it expects you to be. The symbol will change slightly to indicate this to the controller. After about 3 updates, the callsign will disappear. The reason for this is that not every transponder reply is correctly received and processed by the ATC system and thus it makes allowances for mised replies.

If someone else selects your code then the computer will remove your identification from the target because it knows that ther can not be two aircraft with the same callsign.....better to have no aircraft displayed than the wrong one.

I expect that in the USA, the continuity of the system ensures that every unit has your code and callsign programmed in advance so that the target with your name attached will simply coast whilr you fiddle with the codes......provided you don't select the 75,76,7700 codes which will stop the coast and set off the alarm.

regards,

DFC

Re: (Re)setting transponder codes
 

Bay,

PLEASE switch to standby before you rotate to another code, I've had numerous acft that were dialing in a new transponder code in flight cause a code for another acft that hasn't departed to activate and send a departure message through the NAS.

This forces more work on us because we then have to remove the flight plan that was in-advertantly activated and then enter a new proposed flight plan into the NAS so that the pilot has a flight plan on file when he calls for his IFR release.

Mike

Just as a matter of interest I thought that the new 'digital' transponders do away with this problem. You just tap in the four numbers and hey presto!

Am I wrong?

ICN

Thanks Chilli...
 

now that I think about it, I seem to recall being told squawk standy when I am approved to change to my destination frequency, ie from farnborough to White Waltham.

otherwise it's as you said, being told to squawk 7000.

Sky

I have to agree with W'Boy SATCO - if you have Mode C - select it.
I assume he speaks from an ATCO standpoint.

I fly in and out of Class D airspace controlled by NATS and there can be a multitude of 7000 squawks flying around the periphory some days. Whilst accepting that the Alt. readout will only be unverified against a 7000 squawk from an ATC standpoint, I would much prefer to have some idea of the altitude of potential conflicting traffic than not.

I was descending IMC out of 5,000 a few weeks ago (outside CA) with info on two radar contacts potentially conflicting but no Mode C.

It made me look for a hole in the clagg real fast:eek:

Is there any reason not to select Alt ?

Yes. One of the a/c I rent only has Mode A. Presumably it's better to use that than nothing at all?

I have heard some pilots say that they do not use it, for fear of being accused of infringing controlled airspace.

But as someone who spends a lot of time pottering around outer London, under the TMA, I do always use ALT as the discipline, reminds me to stay well under 2500ft :)

In basic PPL and IMC training (same instructor) I was taught to always switch to standby before changing squawk. However when doing IR training, a very well respected instructor (Eric at Stapleford), said very emphatically to keep mode A and C going while changing. He argued that, in CAS, the controller will not want to lose the SSR return even for one sweep. He also said that most radar processing kit (not the transponder) is set up to delay any emergency alarm caused by momentarily selecting 7x00. So I do not switch to standby when changing.

Keef:
I also saw lots of transponder interrogation flashes when we were flying around Bourn yesterday. There are Lakenheath, Cambridge (SSR operating, I believe, with controllers under training), Luton, Stansted and Cottesmore units all fairly close, but we all flew at 1200 feet altitude or less and I would have thought that we were under the radar horizon of all except Cambridge.
So does the TCAS transmission also trigger a flash on mode A&C transponders? I seem to remember that the transponder return pulse (ie aircraft transmission) is at a separate frequency (+ or - 64MHz ??) to avoid triggering responses from other transponders in the area. If I am roughly correct, wouldn't this mean that TCAS will not trigger a 'flash?'

The transponders in a Agusta A109 Power have an automatic standby function whereby as soon as you change the sqawkcode it automatically goes to standby mode for a few seconds whilst you finish twiddling... even if its in the transmit position. Sorry can't tell you what make it is...

I've always been told to set to standby manually whilst you twiddle. And when in Standby, you appear as a dot on the radar screen as opposed to a proper 'blip'.

As for mode C - dont a lot of the new TCAS gizmo's rely on that to work out how far above/below someone else you are...?

Isn't it funny how all the old wives tales about not using standby when changing come from instructors who never actually see a radar screen and that they can all be disproved by the people who actually see the response :D. alphaalpha's and rotorhorns latest (the dot instead of the blip - not true I'm afraid) re-inforce that quite nicely

Gotta laugh haven't you :cool:

CM

May I suggest you select standby first.
Then there is no chance of transmitting an unwanted code as you cycle through the box to the one you wanted to send.
Just like everything else related to flying safely - Unless you KNOW BETTER FOR A FACT, why rely on old-wives tales, mythology, bull!!!!, tradition or the belief that the kit will not let you down when you don't have to?
Be careful, be safe and do the sensible things.
Its just a thought I've been using for 30 ys of flying.

P.S. If you turn C off cos you might get noticed in CAS - you should be. The accompanying transponder code may just help to ensure the correct license gets pulled.

Stillin1 and Chilli:

Sorry, guys, I think you are a bit out of order suggesting that when a PPL follows instructors' advice, he is following an old wives tale. If you don't trust your instructor, who do you trust?

Of course there is bad advice and there is good advice and we should be prepared to learn from our experience and the experience and knowledge of others. I'm trying to do this from this thread.

I'm still confused as to the correct procedure so I looked up changing tx codes in MATS Pt 1 and the Air Pilot.

I could not find anything relevant in MATS.

The Air Pilot is not specific. ENR 1.6.2 covers SSR operating procedures but does not state that standby should or should not be used when changing code. It does say that the transponder should not be switched off or the code changed unless instructed by ATC. It also says that 'pilots are warned of the need for caution when selecting code 7000 cecause of the proximity of special codes...' These two points suggest that we should change code without switching to standby. [sorry, I couldn't pull the exact quotes out of the pdf file]

I did not do a very exhaustive search of the above publications.

So, I'm afraid I'm still not sure whether or not to switch to standby.

AA
 

Why are you not sure? Every ATCO here has said switch to standby. At the end of the day (TCAS excepted) the transponder is their for OUR benefit, not the instructors - so who you going to believe now? And if any instructor comes to you with one of these B***S**T old wives tales then please feel free to point him in my direction. So far the dross we've seen on this thread in this respect shows that they're talking out of their a***s!

The words about "showing caution" you suggest as meaning you don't need to select standby. As a counter argument I could equally say that selecting standby IS showing caution. It just goes to show that if you read something sometimes you can read into it what you wish. Or am I just banging my head against a brick wall here:confused:

CM

Chilli:

Thanks. Message received and I do bow to your professional knowledge. I was thinking this morning about what had been said on this thread. Although I was still uncertain, before flying today, I decided to change my SOP and switch to standby before changing code. The change was no big deal. This thread and your comments have now convinced me as to what may be better practice.

So, I'm sorted out, and thank to you and the other experts. As you say, many instructors are not giving clear advice on how to change squawk. Perhaps the Air Pilot should be more specific? Or would this be over the top?

I'm only a PPL, but it behoves me to fly to the highest standards possible, especially when IFR in CAS. What do the professionals do? I'm genuinely interested.

It seems this topic is a good example of PPrune's value.

Regards

As folks have said, it depends on the instructor. Those I've met in the UK "who know" seem consistently to say "standby before changing". So do the ATCOs.

I'd like to hear the view of some US ATCOs, because over there I got my knuckles rapped for selecting standby first!

I don't know whether TCAS causes the transponder light to flash - does anyone? From the rate mine was flashing at Bourn this week, there were a lot of interrogators in the vicinity. Can't have been ground-based SSR, so I concluded it must have been stuff up above.

But I have been known to be wrong ;)
Where is 2Donkeys when you need him?

Isn't TCAS passive....the radar signal from the SSR triggers your transponder, then nearby TCAS receives your reply..........? So assuming you're not being painted by any ground radar, then TCAS won't work....?? (dunno, just speculating?)

Standby is normal for the US as far as I'm aware.

Cheers
EA:)