Changing Squack Codes
 

Just a quick question for those that might know. If ATC asks me to change the code in my transponder, do I have to ident? I've picked this up somewhere and was just curious if it was an acceptable procedure or more of a bother.

Thanks in advance...

gulf227

When you are instructed to change the SSR code, you change the code and recycle (switch to standby then switch back to Mode C). There is no requirement to ident, this is for two readons 1) If ATC wanted you to ident, they would have told you, and 2) By you changing your SSR code to one that you have been instructed to, is in itself a method of identification.

ATC...

Switch to standby, change the code, THEN switch to Mode C please.

fweeep is right thought. If I ask somebody to "ident" it is because I need it for identification. If somebody else is "ident"-ing unasked at the same time (even in an adjacent sector) I have to do it again or do something else (like a 30 degree turn), so it is in everybody's interest to minimise the identing that goes on.

We see a lot of spurious (?) "ident's" on our system (EUROCATS 2000), often wonder if some pilot has his elbow on the button or if it is a glitch.

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"Station calling Centre, grow a head..."

On a similar note, why do some pilots ident on final, ie when they are hovering over the threshold and the one ahead is slow to vacate. Do think that we have forgotten them and seeing them ident will make us give them landing clearance??

On a similar line - why do so many GA pilots switch the transponder off when asked to squawk 7000, or change to 7000 and switch the mode C off? Are they afraid that someone is going to catch them doing something wrong!

We've been working with SSR only for quite a few weeks now (wonderful Watchman support!)and it really makes you wonder just who is lurking around out there.

fweeeep & karrank...

just want to know...... what will show up on your radar screen if you do not switch to 'stby' when changing code. thanks

When a pilot does not switcg to standby when changing code you see him selecting numbers as the radar sweeps past his a/c. I know this from the amount of airbus operators who fly out of LL on 2000!

Man Fu,

Two things could happen, firstly the SSR part of the antenae may generate a tage for each code as you cycle through the numbers (which makes none readable with certain settings) and secondly, some transponders appear not to change code unless they are re-cycled. Often a pilot will say "but we are sqawking xyz, but the previous code shows untill they actually do recycle.

...not to mention inadvertant selection of 75, 76 and 7700!

Make my screen go all flashy and it wakes me up with the alarm!!! (Yr callsign also disappears and reads 'SOS')

Here we go again.

Most modern transponders do not require you to go to standby when changing code.

There was a lengthy discusion about this a couple of months back, I'm not going to reiterate it. Do a search.
http://www.pprune.org/ubb/NonCGI/For...ML/000274.html

CPB

[This message has been edited by Capt Pit Bull (edited 04 August 2000).]

I've seen plenty of spurious codes come up when aircraft are changing ssr, even with "modern" aircraft. A major glitch in our EUROCATS 2000 (Australia) is feared by procedural controllers using the same system as radar ATC. If somebody squawks a code assigned to an off-radar aircraft (even monentarily) the aircraft's symbol can (and has) move to that position (ie, may no longer be visible to the ATC controlling it,) with obvious negative consequences, untill somebody notices BAW9 doing circuits at Scone....

Stand-by, change ssr, select mode C, please.

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"Station calling Centre, grow a head..."

Sorry I thought I was helping... I DO however first select STBY (usually right after takeoff :) :) :) :)) then change the code, mostily to prevent any stray 7700/7500 codes!

Thanks for the discussion!


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FLY SAFE!!!

Karrank,

Our manual is unambiguous - don't set standby when airborne unless specifically instructed by ATC.

If this causes untenable problems for ATC, it needs addressing.

The concern for us is the integrity of TCAS. If an intruder has transponder at standby, then our TCAS is blind to him. Even worse, if we are at standby, our TCAS does not function at all (i.e. can't track any intruders), and has to go back to square 1 tracking all the intruders when the transponder is back on.

Correct me if I'm wrong on this next bit - you clearly know more about looking at radar screens then I do. From an SSR point of view, if I'm wearing the wrong code (temporally) at least you can still see a blip, so you know there is an aircraft where I am. You can still see my altitude. Depending on the radar type you can still see the speed value / vector, and presumably your STCA still works. In otherwords, my incorrect code is inconvenient, but not a total show stopper. So we could say perhaps ATC is 'X'% effective instead of 100%. But our TCAS is still working (100%).

If I set standby though, surely you can't see me at all? (accepted - unless you have raw data coverage). I.E. your effectiveness is degraded. Maybe its not as bad as the wrong code, but your effectiveness has got to be less than 100%. Shall we say 'Y'%

As a controller, you are better placed to put a value on the relative hassle of (no data) versus (wrong code but other data present).

Onviously it all boils down to the relative degradation in performance.

Normal operation (i.e. no code change req'd):
ATC efficiency = 100 %
TCAS efficiency = 100%

New code, we don't set standby:
ATC efficiency = X% (<100%)
TCAS efficiency = 100%

New code, we do set standby:
ATC efficiency = Y% (<100%)
TCAS efficiency = 0%

I guess what we really need is transponders to have a mode which inhibits them from replying to Mode A but still allows replying to Mode C. We could set that when changing code. That would solve everything:

- No code confusion.
- You still get a blip + altitude and speed tag whilst I change codes.
- TCAS doesn't even interrogate Mode A so will work quite happily.

Unfortunately that isn't going to happen. In the mean time, remember that TCAS is designed to be an independant safety net. It won't work if we turn it off!

On the possibility of accidentally setting 77,76,7500. It does not take a great deal of effort to avoid turning though these settings.

The bottom line with this is that we operate the equipment the way the manufacturers tell us too. If it looks like that method is degrading the ATC system, then the solution is to get the regulators to tell the manufacturers to modify the kit or the operating techniques.

CPB

Well Pit Bull.......

If there is an operational reason (guess you HAVE to follow the manual) for not selecting standby I guess it's a balance thing, and for ATC it is not really a huge thing. We get far more nuisance 75/76/77 from merging tracks symbols (different levels... usually) than from switching codes.

Other than the circumstance I mention above, losing the radar track for a bit is no problem with EUROCATS 2000, coz the data-block keeps flying across the screen as directed by the flightplan + what the system remembers from your radar progress. Best bit about the silly machine.