Transponders

[little red train]

I've always put box to standby when changing through Squak codes, starting a new coarse at a new school, my instructor in the debrief says he, and the examiner, would prefer to see me just change codes, and not bother switching to standby.

his view is I'll still be on radar, and you won't lose my blip..

what happens in a rare case If you get one response on 75/6/7 ect?

What would you like me to do?

Thanks in advance.

[information_alpha]

our air traffic monitor in the tower is a direct feed from the radar. quite often i get an alert to say sos, radio fail etc squawk being detected. when you take the height filter off and wind it out to a bigger range there is nothing there and the alert disappears really quickly. I have been told this is due to pilots dialing through their squawks without being on standby. as a tower controller nothing is done - cant speak as a radar controller though!

[Standard_Departure]

You will find a variation of opinions on this subject, however, I have seen aircraft changing codes, and a variety of codes display on the radar before it gets to the allocated code.

In my opinion, to recycle, switch to standby, change the code, switch it on again.

Just an opinion from an ATC

[HEATHROW DIRECTOR]

Tell your instructor/ examiner they're talking out of their hats (not electric ones!). Day after day we see odd targets appear because of this problem. Yesterday, momentarily, there appeared to be a Manx a/c in the White Waltham circuit. What your instructor may not appreciate is that many codes are paired with callsigns in ATC computers. If you momentarily stop on one of those codes, up comes the callsign on our radars. If an ATC unit has asked you to squawk the fact that your SSR is lost for a few seconds is immaterial - and expected. It doesn't mean we "lose" you..

[zonoma]

IF you must leave it on, make sure you are NOT going to spin thro' an emergency code. D+D will be very grateful as their alarms won't sound.

On the subject of squalks, anyone had any problems with the allocation of 77** to EGLL ORCAM? Seems to be a part on SFD radar where coverage can be confused and these new ones come up with any of the three we don't wanna see. Filed on and collecting dust. Any comments??

[wobblyprop]

during my PPL, as HD says and I agree, i was told to switch to standby when changing codes. Good habit to get into i think.

Just for interest i took an RAF pilot friend of mine up recently. We were given a sqwak and he started to change it without going to standby. I asked him why. He said flying as fast as they do they didn't have time for standby as well as chaning codes - Fair point i think.

[Rad1]

A lot will depend on the SSR processing software. Good software which has been written to take account of what many will consider as bad practice probably won't have any problems. On the other hand, poor software could do any number of apparently unpredictable things if tx'der codes are 'cycled through' by an aircraft.


PS - Same principle applies to radar as cars - just because it costs a lot of money doesn't mean it works well.

[HugMonster]

This question came up on this board a short while ago. It was pointed out then that quite a few more modern transponders (King Digital Silver Crown and Collins Proline Digital) actually switch themselves to standby briefly while you change codes.

From a pilot's point of view, I would far prefer the TXPDR to stay ON, since the "blip" IS lost from TCAS whilst on standby.

The danger of setting off an emergency alarm is small, since they will ONLY respond to 7500 , 7600 7600 codes, not any other endings, and the SSR equipment needs to scan the equipment, and interrogate it and get a response in the microsecond whilst turning the knob that you have it on 00. I would, however, recommend that it be turned the other way rather than through 00 if you leave it switched on.

Please, please, though, make sure you ALWAYS squawk Mode C. It assists TCAS lots, and stops ATC units having to give us avoiding action against you under a RAS when you're probably thousands of feet below us!

[1261]

It takes a couple of seconds to switch to standby, so why not do it?

Certainly for APP Radar it's an annoyance rather than a major issue. The fact that your SSR is lost for a few seconds makes no odds.

The TCAS point is interesting, though, and seems very sensible. Does anyone else have a view on this??

[Lew Ton]

Pilot friends of mine always prefer the transponder to remain on, with mode C, to continue the protection of TCAS. In all the years I've been stuck in front of a radar I don't ever remember seeing more than a (rare) fleeting glipse of an emergency code as someone has dialed through it. I think there has been more times when someone has 7700 selected instead of 7000! :) :)

Mind you, this has been on steam-driven radar. What happens at TC, and more importantly D&D, when someone dials through an emergency code? Do bells ring?

Interestingly, we sometimes have departures that are allocated 7701 or some other 770x and I have seen (very occasionally) the code come up as SOS for the first paint until the radar sorts its bits out. Again, this has not been a problem to me, what say others?

[beaglepup]

Today, as in the past, we had 777x on two or three departures

[little red train]

Thanks for the advice guys,

I was told it was the Examinors and therfore my Instructors Prefrence, to keep the box on, and from a TCAS point of view I can see why.

As HugMonster points out very well, setting a 1 at the end the box, would have the desired effect of not giving radar grief.

what urks me somewhat is our, and most Transponders I have seen are Standy, On, Alt. There is a strong temptation to set it to "ON" (cause thats how a want it :)) than any other less descriptive setting. wouldn't "ON" being the full whack and a "Mode A" help a bit for everyone giving Height Readout? a bit of a design re-think could help I feel.

Thanks again.

[fill-your-boots]

Looks like you might have signed off from this topic but just in case you come back to it here is another reason why you SHOULD turn to standby while rolling through the codes.
The ATC computer that looks after all the flight plans both long term stored (ieBAW123 airborne 9AM every day) and one offs is linked in to the UK radar systems and is constantly on the lookout for the transponder codes that it has allocated to a paticular flight.
The allocations are at random but lets say it has given squawk 7654 to BAW123 on this particular occasion. If you happen to stop your scrolling on 7654 for a few seconds then the computer will recognise the squawk and activate the flight plan for BAW123.
It generates a whole lot of work for us to then cancel the flight plan and reinput it for the real BAW123 when it gets airborne.
There are various protection perameters designed to stop this but sod's law is much more powerful and often capable of overcoming the protection!
As an ATCer and PPL I would always recommend puttihg the 'box to stanby whilst you change codes.



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cleared-to-go. what slot time?

[Lew Ton]

FYB, quite right, I'd forgotten that.

[HugMonster]

FYB, I appreciate that it may be thoroughly inconvenient for a filed plan to be activated by a transponder that might have been switched to standby instead...

But it is a little more than inconvenient if an airborne TCAS system loses sight of opposing traffic.

Sorry, but in my book, safety comes rather a long way above inconvenience. I still suggest people leave them on, and squawk Mode C (Alt reporting) at all times.

A rather better answer is to suggest that people DON'T pause for a few seconds in setting their transponders!

[10W]

F-Y-B

What you say is generally true, however we need to have a realistic view of the frequency of your false activations and the potential effects on safety with the differing methods of changing squawk.

The Auto Departure Message function requires the code to be shown for two consecutive sweeps. That would mean the code having to be 'stationary' for between 3 and 6 seconds assuming that it is changed and recieved exactly when the first sweep takes place. I think it would be extremely uncommon for this to happen as most pilots I know, me included, tend to rattle through the numbers in quick succession. It may result in wrong codes being picked up, but the speed of change would normally mean that the same code isn't seen twice. Can you give a ball park figure as to how often it occurs, and further, how many are because of this problem and not down to other cockpit finger trouble ??

For the occasions where the system is beaten, I agree with Huggy, safety must take preference over inconvenience every time. Perhaps if the problem is a large one (and I know not if it is or isn't), changing the parameters to 3 sweeps might alleviate a large proportion of the difficulties ??



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10 West
UK ATC'er
[email protected]

[fill-your-boots]

Frequency of occurance in my area of responsibility maybe twice a year so in the UK as a whole I would guess at ten or a dozen. Not a lot but I would wager that occurances between scrolling PPL's and TCAS traffic are less.

Here is the RT scenario that generates.
"GABCD Request join zone at Blah Blah Blah"
"GABCD FIS at the moment the EGXX QNH is 1013 squawk 4321"
PPL gives a read-back then starts scrolling meanwhile ATCO says
"GABCD cleared to enter the zone via Blah Blah etc"
PPL stops scrolling in order to scribble down clearance on knee board and transponder is left on the code for mythical BAW123.
I have been on the giving and receiving end of this and it does happen.

Question for the boffins out there does TCAS not work when an A/C is squawking standby - after all it is not off just transmitting 0000. I'd be interested to know.

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cleared-to-go. what slot time?

[Capt Pit Bull]

Squawking Standby is a bit of a misnomer really, since you are not Squawking anything.

Turning your own transponder to standby not only makes you invisible to other aircraft TCAS', but also completely disables your own TCAS. When your transponder is put back to active, your TCAS has to start from scratch again with its tracking.

Consider this - when you get given a new squawk, this generally means that you are getting handed over. This in itself is a risk factor for an airprox, so it is the last time you should want your transponder or TCAS turned off.

I would suggest that if you fly a TCAS equipped aircraft, then almost certainly (unless the manufacturer dictates otherwise) it will be safer overall to change code without going to standby. If you are TCAS equipped, the odds are that your transponder won't send the new code until you've finished setting it (by one means or another).

If you are not TCAS equipped, then get the manual for the transponder itself (not just the half page in the aircraft manual) and see how it behaves when a new code is being input.

As an extra line of defence it is not very difficult to avoid setting an emergency code enroute to the new code.

CPB

[RATBOY]

would it be reasonable to insert some logic into the flight plan management software to look for the squak code a few more times or over a longer period that 2 scans or so? I can't believe a change from a max of 6 seconds (for terminal SSR, enroute/long range can be longer )to say 12 to even 20 seconds is going to hurt the feature of automatic flight plan activation. A decrease in the time sensitivity should putthe false alarm/activation rate down to almost nil.

[Late Downwind]

Working in the London TMA and as a former pilot I can see both sides to this argument! However 240 knots = 1 nm per 15 seconds. That could make a great difference in the TMA, and that's without "No speed restriction" thrown in. In the TMA it's often "almost exactly 3 miles" 'cos we're so busy. So the "waiting for a few more sweeps" thing won't work so well. A miss idented a/c is wasted RT and thinking time. Plus there's all the other problems associated with the auto DM of the wrong aircraft and loss of callsign conversion etc. Is the TCAS thing so much of an issue? Do you pilot types not trust us or wot? (or are you THAT bad at keeping to your assigned SIDs, levels, headings etc?)

[OK the above was a bit harsh perhaps!]

[This message has been edited by Late Downwind (edited 27 April 2001).]

[RogerOut]

Late Downwind I must check later to see the replies to your post :) BTW I do agree - TCAS is an additional safety feature not the primary. But I don't think Hug will agree with you. Good luck :)

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RogerOut
I Keep Mine Hidden

[HugMonster]

Dead right. I don't agree.

And if you want to know why I don't, go take a look at the statistics on level busts.

TCAS is a required peice of kit for most public transport aircraft now. If it's there, it's there for safety. Let's not marginalise its safety potential for the sake of convenience.

Let's face it - you guys are sitting warm and comfortable, and you won't get hurt by a midair. We will.

Nuff said?

I do, however, agree on the "wait a few more sweeps" part. Don't think that will help much. But if time is a consideration, perhaps you shouldn't be allowing us to blat around at Mach Lickettysplit if you can't identify us...?

[This message has been edited by HugMonster (edited 27 April 2001).]

[zonoma]

Hug, try explaining to me how we are not hurt by a mid-air on our frequency?? (and I know the obvious 'cause you dont die bit')

[5milesbaby]

Hug, there are occasions where the Squalk allocated to an a/c by another agency like Maastrict/Brest etc is already given to an a/c in the uk. Looks great when a Loganair or Scottish Island hopper is screaming in from the south at FL390, destination Benbecula :)
Other occasions are when a Squalk is passed down the line to a joining a/c, which is 2 hours late, the time at which the main computer re-allocates the Squalk, so another has to be found as the a/c is just airbourne....

[Mooney]

little red train,

TO answer your question!! While on your IRT ( in a Seneca etc) Put the transponder to STD BY mode while you change Squark!

In our airline we just change the squark code with the transponder in TA/RA for all the reasons Hug Monster mentions.

Err Hugmonster- nice line! ;

"Dead right. I don't agree."

[RogerOut]

Hug - why does ATC need to identify a/c so they can fly at Mach Lickeyspitt (is that what you called it?) Please explain, I'm only a controller. Stop digging if the dirt is falling back in.......

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RogerOut
I Keep Mine Hidden

[HugMonster]

I don't understand most of the latest replies to this thread.

Put simply, I don't trust every aircraft in the sky to be where it's supposed to be. Whether that's the controller's or another pilot's fault, I don't care.

TCAS is a required piece of kit for most aircraft nowadays. It's very useful at times. Let's not marginalise its usefulness.

Leave transponders on, Mode C (for anyone that's unsure, that's the "Alt" setting) at all times. Anything else, and TCAS is useless. If that's inconvenient to ATC, then I'm sorry, and it's a shame, but tough. Safety comes before inconvenience. Get your guys to come up with software wizardry to sort it out so you don't need to reinput FPLs or whatever.

And zonoma? OK, you may get PTSD in the event of a midair on your frequency. We get dead. Want to trade?

[Serco Stuffer]

Engineeringy type answer.

Auto dm can be adjusted, 2 strikes is normal within 10nm of an airfield but could be more or less but less would be a bit daft.

Its 20nm at Manchester due radar coverage. So you have to be twiddling with your weapon (old terms)near to your departure airfield for the computer thing to get confused - it knows where you are due the flight plan.

In the open U/FIR it can be worse.