Changing transponder code

[Talkdownman]

Don't forget that many modern transponders self-mute anyway as soon as the first digit is selected thus obviating the need to select Standby. Probably worth finding out how one's transponder behaves.

Originally Posted by Hants Eaglet

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.

Well, well...what a surprise...

[mrmum]

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.

Indeed, as aviation mythology is debunked on PPrune, we do need to keep dreaming up new stuff to retain our status as "we know everything-it's got to be my way sky-gods" to our students.
On the other hand, maybe some of us try to improve our knowledge and be as well-informed as we can be. Have a look at the top of column two, of page two, of this Eurocontrol publication,
http://www.skybrary.aero/bookshelf/books/714.pdf
which admittedly is from March 2009 and you'll see;

To avoid transmitting an emergency code while changing selections, always switch temporarily to SBY until the new code has been set in the display.

This does only seem to apply to older, traditional type transponders, rather than the more modern, digital, push-button models. Pretty sure this wasn't written by any instructors though.

[A and C]

Mrmum

It all rather depends on how long you intend to take changing the code, even my slowest students get the code set with enough time to not upset the radar system.

I have no doubt that turning the transponder to standby will totaly avoid the problem but it also leaves you invisible to traffic systems for a short time so it is time for you to do a risk assessment on this, you can:-

A/turn off the transponder to avoid the remote chance that you will trip an alarm for a second, the result of this action is the remote chance that an aircraft is near he won't get any warning and might hit you.

B/ leave the transponder on and keep some traffic protection.

On balance I will recomend option B to my students, ATC getting hacked off with me I can sort on the phone, what I can't fix is the results from a mid-air. ( no matter how unlikely).

[Rod1]

So you are just outside the London restrictions, it is the opening ceremony of the Olympics and your aircraft has just squawked “hijack” for a short time.

Wonder what happens next?

Rod1

[A and C]

ROD1

Rod you have been reading too much Tom Clancy, the most likely thing to happen is you will get intercepted by a military helicopter if you are heading towards the games but if you are not probably just a call from ATC to check on you. However you will have a much better chance of not hitting all the other traffic that will have been funnelled around the outside of the zone.

If any of you think that the southern UK is going to become a free fire zone I should have a long hard think about how you write the R.o E. to avoid killing people on the ground............. the military will have.

[huv]

Just asked ATC

AOPA (US) just issued a short movie about this (Dec 30th for you members). It it password protected so I cannot link to it, but the message is: ATC asks pilots NOT to switch the transponder mode to standby unless specifically asked to do so. ATC expects to lose the numeric code during code cycling, but do not want to lose track and altitude. No mention is made of old or new transponder types.

At an instructor's meeting in my flying club (in Denmark) this came up last year since we had strong and different opinions about it.

I brought the question to the chief of ATC controllers at our airport, and he initially said that he could not care less, then added that he was leaning towards always keep the active mode. I then presented him to a selection of the horror stories I - and some of you - have heard about what could happen if we struck some of the tricky squawk codes during cycling, without selecting standby. He just shaked his head, never having experienced or heard of ATC problems in that respect. So now we teach our PPL students to always leave the transponder in active mode.

Personally I still catch myself selecting standby when cycling old transponders with dial type code selectors. Old habits ...

[yakker]

During a visit to Swanwick Control Centre the emergency codes would show up as codes were changed from commercial aircraft. They were on screen for only a second and the controllers did not see it as a problem, so why should we?

[Skylark58]

I reckon I can switch to Standby, reset to the new code and switch back to Alt in about 10 seconds. What is the sweep rate of a typical ground radar?

[RTN11]

Going back to my ATPLs, I think a typical radar does a 360 sweep about every 6 seconds. You also have to consider TCAS - if you go off and then on it could set alarm bells ringing for someone coming onto an approach.

[A and C]

RTN11

Please could you expand on your last post ?

[GeeWhizz]

This will become quite complicated. There are many rules for the uses or SSR. Most SSRs found at many airfields across the UK that provide a Lower Airspace Radar Service generally has a 10-12RPM. Its slower than PSR and its uses are slightly different.

Sticking with SSR though. For changing the code its not really going matter if an emergency code flicks on for a revolution then off to the code requested. It happens all of the time. During the Olympics you wont get any visits from helicopters or fighter aeroplanes after such a very short period, but do double check that once set it is not set to one of the emergency codes.

I don't change to stby mode before changing as it is so quick, but it really doesn't matter for two reasons. Providing PSR is available, radar identification can be maintained so no need for either SSR number. But firstly, if you are requesting a service that requires radar identification then this should be done in good time: before entering the restricted zone or CAS or whatever. Mode 3A still needs to be validated, and mode C verified or switched off.

Second, if remaining on the 7000 you're mode C may not be verified at all by the ground unit. It's not required unless they issue a squawk in which case every attempt should be made to validate the 3A and verify C (or switch it off if out of limits). There are airfield specific conspicuity codes out there used daily, possibly everyone here has been issued a common Basic Service code (?). Other units will recognise these codes (they are all published somewhere in the AIP I think) but will treat them as unknown aircraft (i.e. 7000 squawks) with the benefit of knowing who to telephone to arrange a deal regarding reducing standard separation.

If you are asked to change squawks during a handover it is accepted that for the next few SSR revolutions mode C may not be visible. The first unit that issued you the code has verified your mode C and its not about to read incorrectly when you change the 3A. But then also you might be on a 'cleared' course/alt, or in reciept of a service that requires you to tell the ground unit you are doing something different, or request a change to what you are doing. Thence he drop out of mode C doesn't matter after the initial verification on identification (unless it fails, which will be very obvious on the ground, and is then switched off - a time when it is utmost important to tell ATC what you are doing vertically!).

Of course some of this changes if PSR isn't available... in which case switching to Stby to change code means being lost on the screen for 15 seconds - not nice in a busy place.

[A and C]

So to put all the technical information above into context there is no need to set stby when changing transponder codes, in fact it is a disadvantage to do so.

So another bit of aviation folk lore is busted ! It makes you wonder who makes up this stuff , how much more utter rubbish is practiced in the name of airmanship ? And how much cockpit time is spent away from important tasks like lookout just to keep black art brigade happy?

[Moli]

To standby or not?

In the UK, It's not a myth that need busting. Modern transponders are perfectly okay to change code without placing in standby. However the old rotary dial through ones if not placed in standby do cause alarms to go off. In D&D every time someone dials through one of the emergency codes alarms do in fact go off and the "emergency" squawk flashes up on a screen. If controllers are already dealing with other emergencies, that false "emergency" distracts them from what they are doing.
As for Swanwick controllers having problems with squawks changing, of course they don't as I can't imagine there are too many airliners around with the old dial type transponders plus virtually all the Swanwick CAT is operating on ORCAM squawks and therefore retain the same squawk throughout.
If you have an old dial transponder, switch to standby.

Moli

[A and C]

Moli

Sounds to me like Myth! the radar head sweeps at 10-12 RPM so your transponder will reply to that radar at 5 second intervals so what are the chances of a pilot tuning the transponder through an emegency code during the time that his transponder is being interogated by a radar (remember the interogation pulse is fractions of a second).

[ShyTorque]

Reading the associated manual helps, if you can find one. I used to change to STBY years ago. On the more modern mode S transponder I use these days the output is suppressed for a short period whilst changing codes so I don't select STBY first, although I avoid scrolling past the distress codes.

[BackPacker]

Does anyone know which rotary dial transponders do a temporary suppression, and which do not?

My club has about 13 aircraft, with various types of transponders. All were recently swapped for Mode-S transponders, but this was a like-for-like swap. So some now have a Mode-S push-button transponder (where obviously this issue doesn't exist) and some have a Mode-S rotary-knob transponder. Can I safely assume that these transponders do a temporary suppression? I'd hate to have to trawl through 13 different manuals to find out.

[Spitoon]

Sounds to me like Myth! the radar head sweeps at 10-12 RPM so your transponder will reply to that radar at 5 second intervals so what are the chances of a pilot tuning the transponder through an emegency code during the time that his transponder is being interogated by a radar (remember the interogation pulse is fractions of a second).

Unfortunately - or fortunately, depending on your perspective. - the technology used in radar systems (the ground bits anyway) has moved on a bit. A modern system tracks each target and will make some 'assumptions' if the code returned by a target's transponder changes. Equally, the picture displayed to a controller may well be formed from data collected by a number of sensor heads.

At the same time, both ground and airborne parts of the system have been modified to overcome problems caused by pilots and controllers not following simple instructions or thinking they know better than the people who built the equipment. Such modifications on some systems include things like the suppression of mode A data (either sending it or displaying it) when it changes until the data is again consistent and not displaying an emergency code until there is confidence that it has been selected intentionally, for example, it has been sent for two sweeps or received by two separate sensors.

The bottom line is that some combinations of equipment benefit from switching the transponder to STBY before changing the code, although most now can cope with not doing so. As Shy says, RTFM if you can and do what it says.

[Moli]

A and C

I bow to your superior knowledge. However having spent 3 years as a controller in D&D I can assure you that dialed through codes do set off alarms in D&D and they do appear on the aircraft in emergency D&D display. You can chose to dismiss my first hand knowledge as myth if you wish but those with older transponders with rotary adjustment will continue to cause false alarms at D&D if they do not select standby when dialling through the 7500 7600 7700 codes.

Moli

[A and C]

Moli

With other radar users on the forum not having this problem and some of them asking for transponders NOT to be set to Stby when changing codes I can only assume that it is the way that the D&D radar is set up

My guess is that the problem is not one of scrolling through the codes so much as the way D&D radar date is proccessed when it comes from the radar heads, my guess is that you are getting the data "raw" and without the two sweep delay.

The other issue is pilot training, your tyro pilot is told to change the code, he starts to do this, suddenly remembers what his instructor said and having already got an emergency code selected dithers a bit and then selects Stby by that time the damage is done and your (hair triggerd) radar set picks this up. I have seen this happen with students and if the guy had just selected the correct code without delay no ones alarm would have tripped.

[Moli]

As my old Gran used to say to me...You can lead a horse to water

Moli