Changing squawk on transponder...

[SmolaTheMedevacGuy]

I've got two questions:

1. Do you switch the transponder mode to SBY, when changing the squawk? Some say you should, some say it does not matter (as for me, I usually do not bother). So should we, or not?

2. One of the arguments for switching to SBY is that when you set 77XX (where XX may be whatever you want), you set off an alarm. Is that true, or only 7700 does make the radar screen red?

Greetings
Smola

[expedite_climb]

Light a/c concencus is yes turn it to sby incase you select 7700, or 7600 or 7500. It is only these that make the warnings flash up. E.g. have flown lgw departures with a 77xx squawk before.

Airliners (757 / 767 at least), leave it on, as turning it off, even momentarily turns the tcas off completely - this could be fatal. Just be careful what you are dialling up.

[BlueEagle]

An ATCO once told me never go to STBY as, if you do, they will lose you on their screen momentarily until the new squawk comes up but, given the idiosyncratic behaviou of computers, you may just not show up on his screen again, then he really has a problem! I believe this to be applicable to SSR, perhaps some ATCOs would comment?

[Captain Stable]

I concur with e_c.

77xx (where xx<>00) does not set off the alarms at ATC units for miles around. So don't worry too much about that. If you don't want to go to standby, set the least significant digits first, thus enduring you don't go through 7700 or whatever just as it's being interrogated.

On more advanced transponders (not the silly little things you get in a PA28 from your flying club) the transponder will put itself into standby for a few seconds the instant one of the digits is changed.

[renard]

On RMU's we are told not to switch to standby as the machine does it automatically when the dials are turned.

77XX will not set the bells off. Codes are generated for routes, and if you fly to the Channel Islands from Southern Britain, you get a squak of something like 7772 - makes you careful when you set it.

[Checkboard]

*I have been told* :) that modern transponders fitted to airlines inhibit transponder replies for some short time after a code change, so that intermediate codes won't be sent while twirling the dials. Hence STBY isn't required in these cases.

[Onan the Clumsy]

I was told this and used to do it, but the real world finally intervened and if they give me a code, I just dial it in.

Sometimes if I get a handoff, its supposed to be xxxx -> 1200 -> yyyy, but as I'm going to pick up a new code, I just leave it on the xxxx and maybe call in as "N12345 over such and such, level three thousand five xxxx"

One thing I was told (by a reliable source) which surprised me was to use STBY on the taxi out to let the vacuum tube warm up before selecting ALT. I laughed this off, but my source insisted that he'd read an article on that very model, and as it was his a/c, it felt listening to him was the least I could do.

The fancy new ones are all solid state, but if the old ones really do contain a tube, it'd explain why I continually hear "N12345 I've lost your transponder - again"

[Miserlou]

On a slight sidetrack.
If a controller in Germany tells you to squawk Alpha xxxx. Don't do it!

Well do it of course, the numbers bit, but he didn't mean Alpha.

Well he did mean Alpha but only in as much as if you don't have Charlie, which of course, you do, so its only to confuse you.

Are you with me?

So to recap, if you didn't have a choice it wouldn't make a difference but if you do, and do as requested, you've done it wrong!

I'll get me coat!

[5milesbaby]

If you are anywhere in the world and are told to squalk A****, then the controller is using the correct phraseology for a Squalk change. If you have a Mode C to suppliment, then leave it on too, a controller cannot tell you to deselect this unless it is discovered to be corrupt. Remember that the 4 number bit is the Mode A bit.

It is true that by switching to standby in some areas, the radar information associated with the return can be lost and not fully regained without re-identification. You are not 'foreground' therefore not bright green, and just a dull grey blending into the black behind. On a side note too, when selecting your numbers, don't hit the ident button. If you haven't got all four of yours correctly selected, and waited a few seconds for radar to receive the new selection, you may inadvertently process the flight details of an a/c using a similar squalk. Confused? Should see the controller who gets the 'ghost' flight details.

[SOPS]

So what is Mode S?

[Miserlou]

So what if he doesn't want to see your Alt. readout? Can they remove that themselves?

Genuinely curious. I know there are some clutter reduction widgits but I've never actually been to a civil radar centre. Shame on me.

I suspect that this is a case of technology outgrowing the legislation. I'm sure there are a few in these forums who have never seen a transponder without mode C.

[5milesbaby]

Mode S is a totally new system not used yet by anybody (properly). It involves full interactions between pilots and ATC without the use of comms RTF.

At Swanwick we are able to remove anything we want including the callsign (why???????), however, understandably most removals aren't permitted. This applies for both foreground and background tracks. The former being the traffic working/worked/to work a sector, the latter being traffic a sector is uninterested in.

[witchdoctor]

I had always understood mode S to be used in TCAS equipped aircraft which allows two systems to 'talk' to each other so that if resolution advisories are given they do not conflict - i.e one to climb, the other to descend, but never to give advisories in the same sense such as for both to climb.

When first learning to fly I, I was advised by different instructors to either switch to standby whilst changing, or to dial it straight in as there is a delay built into the system. However, as all this was in light aircraft I'm still none the wiser. Best advice I did get I suppose was to avoid dialling through '7' altogether unless it is in the squawk.:confused:

[Tinstaafl]

Since I don't have a clue whether or not the TXDR in the a/c I fly are fully transistored or have a valve somewhere then I switch to STBY after start.

I also don't know if it auto-STBY during code changes so I manually select STBY during code changes. That guards against inadvertant 7x00 alarm bells. It's also easier/faster to get the code set if it's entered in the order given, instead of of out-of-sequence as one person has suggested.

The argument about not using STBY when entering the code due to ATC issues falls apart if there are TXDRs that use STBY automatically.

[swish266]

ICAO doc 4444 PANS-RAC and doc 8668 PANS-OPS do not specify selection of "stby" mode while changing sqawk codes.
As far as I am concerned all operators should have adopted the above when designing their respective SOP's.

[ManaAdaSystem]

I have never bothered to switch to STBY before changing the code, and I have never received any complaints. Never had a u/s transponder either.
Mr. Jeppesen says that in some corners of the world, any code starting with 75, 76, 77, may trigger an alarm.

[skeptic]

It may well be true that changing a squawk resets the transponder to sby, but how do you know? Certainly less complex ones (ie GA sets) do not, so how is it safe to just assume that yours will? Surely good airmanship dictates that you go to sby before changing squawk, unless you have read the spec on the unit - and who has ever seen one of those?

The argument that says just dial it up because you would disappear off the screens if you go to sby manually is spurious - because youll disappear anyway if the thing goes to sby on its own, wont you? Surely better safe than sorry?

[BlueEagle]

I think it might help if we had a bit more ATC and Engineer input here.

I have never heard before of any transponder that automatically went into stby when the numbers are changed, equally I have been told that by going to stby one can disappear from the screen and computers being computers, one may not always reappear as required. As it was during a visit to LATCC that we, as a group of pilots, were specifically asked NOT to select stby I have never done it since, (over twenty years now).

Any ATCOs and Radio Engineers care to comment please?

[Agaricus bisporus]

So you disappear from the screen for 3 seconds. So what? They knew who and where you were, they told you to change squawk so if you drop off the screen (if the primary is that poor) for an instant before popping back up in another guise it is hardly a surprise to ATC, is it?

That argument sounds perilously like cobblers to me. I'd be far more concerned by accidentally passing 7*00 and setting off all the alarms, so I'll continue to select SBY before changing squawk as good airmanship seems to require.

Whats the problem with that?

[Captain Stable]

The problem is that in more advanced systems, by selecting Standby you disable TCAS. If you simply twirl the numbers, the system puts itself into Standby without disabling TCAS - it still operates.

If, for some reason, you get distracted by another (more urgent) job, there is a danger that you may forget to put the Txpdr back to ALT/RA and not only are you then bereft of TCAS advisories, but so is anyone else into whose path you may blunder.

On the more simple systems - the type of King Silver Crown gear you find in little piston-engine puddlejumpers for example - the level of sophistication requires more care to operate. But applying that level of care to systems with more sophistication holds additional dangers.

[M.Mouse]

I hate to disappoint but a good friend of mine DID trigger an alarm when changing squawk by momentarily and accidentally selecting one of the emergency codes.

[spudskier]

I was made aware by ATC themselves that 77XX WILL set off alarms(bells, lights, whatever) as will any transmission on 121.5

[Captain Stable]

I don't know about transmissions on 121.5 or 343 but 77xx squawks are regularly assigned to routine traffic.

I suspect your ATC sources are mistaken, and that the information several ATCO's have given me is correct - that it is only 7700, 7600 or 7500 that set off alarms.

[Zlin526]

WTF is a transponder? My trusty steed hasnt even got a starter!

[BlueEagle]

Think you may have missed the point there. Not everywhere has Primary all the time, TMAs and dense European areas maybe but once you get out on airways in the more remote parts it is SSR only and if you drop off the screen and DON'T come back due some computer glitch, then you are going to cause problems.

The reason I stopped going to stby was because the controllers at LATCC asked us not to, to have continued after that would have been bad airmanship, don't you agree?

[avioniker]

Switching to standby is a matter of personal preferance and nothing more. If you're afraid of the 7's then don't dial them in.

Follow the instructions in the book that came with the thing or are written in your company's operations manual. Theoretically someone who knows something about your transponder wrote the instructions.

Whether or not the transponder goes momentarily to standby when you change a digit is fully a function of manufacturer's design and shouldn't be a concern of the operator. I've tested and certified myriad types and it seems that no two operate the same. Some go to standby when you change one digit, some transmit the new code as soon as a digit is changed, and a couple transmit the new code two or more seconds after the code has been changed. The transponders all have to meet FAA/JAA TSO-C74b [Mode A] or any class of TSO-C74c [Mode A with altitude reporting capability] as appropriate, or the appropriate class of TSO-C112 [Mode S] standards and are built to do so.

Mode S is the name given to a type of transponder (a unit that responds automatically to an interrogation) that may also act as an interrogator (a unit that automatically initiates an interrogation). It incorporates the features of Mode A, Mode C, and may interrogate other aircraft when needed. It identifies the aircraft being interrogated as well as the interrogating aircraft by an assigned code (in the US it's the N number). The S stands for "selective interrogation".

It's used in conjunction with TCAS to determine the relative altitude to your aircraft and, with the TCAS processor, the potential threat and Closest Point of Approach (CPA). If the target is determined to be a threat by TCAS the Mode S increases the frequency of target interrogation as the CPA is neared.

If anyone wants more information I'll be glad to cut and paste some stuff for you.

:}

[Tinstaafl]

seems that no two operate the same. Some go to standby when you change one digit, some transmit the new code as soon as a digit is changed, and a couple transmit the new code two or more seconds after the code has been changed.


Hence my position about selecting STBY before changing. I wouldn't have a clue about the TXDR's behaviour while changing code. There's certainly no information supplied in any documentation I've seen for any of the a/c I've flown. Maybe in the engineering documentation...

[avioniker]

Most simply put: when operating a piece of equipment it's always best to do exactly what the manual says and usually ONLY what the manual says. Don't add steps.
With regard to going to standby, if I were conducting an evaluation or giving a test, I would reduce the evaluee's score for performing extra procedures not in the manual or handbook.
Selecting standby, any time other than when when the unit is malfunctioning or the operator is directed by ATC, would be one such time.

[roundwego]

Early generation transponders had no inbuilt transmit inhibitor when changing channels therefore all pilots were instructed to switch to standby when changing.

Most modern transponders will inhibit transmission until a number of seconds after a digit has been changed. This delay is increased if the new selection is one of the emergency codes. If the a/c flight manual doesn't give any info (which most don't) check with your maintenance org or the equipment manufacturer. KNOW YOUR EQUIPMENT but if in doubt, switch to standby.

What a boring subject - suprised this generated so much discussion.

[av8boy]

Sadly, I don't think this is boring... Guess I've got to get a life...

Quick observation having to do with the US regs and the AIM: 91.215 says that when you're in airspace requiring a transponder, the transponder has to be "operated." Ok. Cryptic.

So, looking at the AIM (4-19(e)(1)) we find:

*********
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.
**********

Of course, the AIM is NOT regulatory. Still, I get the impression that the FAA's position on this is that you shouldn't go to standby when changing codes. If they WERE suggesting standby, then there'd be no need to remind us about the potential for momentary excitement should we run through any of these codes (assuming, of course, that this was carefully thought through!).

I, for one, have seen "momentary false alarms" come from inadvertent emergency codes, and although it got my attention, it pretty quickly became obvious that the pilot was simply cycling through the code on his or her way to the assigned code (and if in doubt, I asked...). I've come to expect this from time to time, in the same way that I've come to expect to see a pilot cycle through a code assigned to an aircraft that was just about to depart--resulting in the computer tagging the aircraft I was identifying with the data block of the aircraft which had not yet departed. Perhaps this would mean that I would have to suspend the track of the departure until he/she really DOES depart, but again, not the end of the world. In the long run I'd prefer to see that secondary target associated with the primary as the code was being changed.

For instance, assume you're VFR squawking 1200 with Mode C and you call me up for some radar service. As I dig up a code for you I'm looking for your 1200 code where you said you were at. If you go to standby to enter the code, then your secondary target and altitude go away. In most cases this is probably not an issue. However, if I see a target start changing codes, there is the possibility I can help both of us out. It helps me because I can start to plan--obviously you're not identified until we dance the dance, but I've got a pretty good clue that I'm looking at you. This helps YOU because I may be working or observing traffic near you and as I see you come out of 1200 I can say something like "possible VFR traffic twelve o'clock less than a mile opposite direction altitude indicates 3200." If you go to standby, I don't know whether you are my customer or whether the Mode 3/A on a VFR target has simply gone away (as is so often the case). In that case, I can see the guy that's going to hit you, but I can't see you (well, maybe a primary, but you get the drift).

Of course, these are just MY thoughts. Give an old guy a break. I'm retired.

Dave

[ratsarrse]

Bendix King have published pilot guides for their products online (pdf format).
http://www.bendixking.com/static/catalog/index.jsp

Doesn't answer the question at hand, but might be useful for people who rent aircraft and want to get some more info on the kit installed.

[flymeboy]

On the A320/A321 the transponder automatically holds the last squawk for 20 seconds after you turn the dial, then it changes to the new one. So you do not need to set it to SBY to chage it. Horray!

[Jetstream Rider]

Blue Eagle - I am surprised. On a similar visit to LATCC the D+D controller told me to select sby first to avoid the setting off of the alarms for the usual codes, but also to avoid the 'others' which set off alarms which we as pilots are usually unaware of. (such as 0030 for 'London FIR lost', (I might have the number wrong)).

He said that if 7700 is recieved, it might be the last thing a pilot is able to do in an emergency, so they still call the services out. He said that it had happened and left them in a real dilemma.

I can see airmanship points on both sides of the arguments however.

Seems the answer is not at all clear and depends on the fit in your aircraft.