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Old 16th Sep 2005, 18:41   #1 (permalink)
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Question Transponder to standby when changing squawk?

Here's a dilemna for us all... does anybody have the definitive answer, yes or no, or is it down to personal judgement?

A while ago, this thread http://www.pprune.org/forums/showthr...hreadid=131936 was started by someone asking the same question as me. Various answers with various good reasoning resulted, but no definitive answer.

First of all, from an ATCO's point of view, would you guys rather that pilots of aircraft with the "turn-dial" type transponders - ie. most of the GA world - turned xponders to standby when you give us new squawks? Do we simply disappear off your SSR screens if the unit is on standby, or do some/all of you have a primary radar overlay? How true is the myth that your radar heads have to complete three sweeps over the aircraft squawking an emergency code before the alarms/warnings are activated?

Secondly, from the pilot's point of view who has a TCAS system fitted and relies upon other aircraft to have their transponder active for it to work, what implications does it have when a pilot turns their transponder to standby for a few seconds?

Incidentally, I had the opportunity the other day of asking a CAA Staff Examiner for his interpretation of the UK AIP which states (more or less) that pilots of aircraft in flight shall not turn a transponder off or change codes without ATC telling them to do so. His response was that a couple of years ago, he set off the D&D alarms by cycling through 7700 with the transponder alive, and has ever since set the unit to standby while changing codes. He also advised me that some units (but not necessarily all) take a few seconds upon recycling a code to actually transmit the new one.

Let's hear the reaction from Air Traffic Controllers and pilots of TCAS equipped aircraft.

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Old 16th Sep 2005, 18:54   #2 (permalink)
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...or even worse cycle through 7500. You could wind up getting shot down if you do that!

ALWAYS turn to standby before changing codes. You don't always need to communicate with an ATC unit when setting or changing a code, selecting 7000 (conspicuity) or 7004 if carrying out aerobatics are a couple of examples.

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Old 16th Sep 2005, 19:00   #3 (permalink)
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Yep, do the standby thing! If you scroll through the codes to get to your assigned one, you can set off the emergency codes and we can't easily get the BIG RED DATA BLOCKS off our screens, such is the uselessness (new word) of our equipment.
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Old 17th Sep 2005, 11:25   #4 (permalink)
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Please turn the transponder to standby, put in the squawk and then turn it back on. We get so many false emergency indications from people dialling through codes.

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Old 17th Sep 2005, 12:30   #5 (permalink)
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How can I scroll through the mentioned squaks, when I do have a push button transponder setting?
Never in my flight life had a problem with this subject!
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Old 17th Sep 2005, 13:34   #6 (permalink)
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Read your manual and decide appropriately.
Most modern transponders cope OK with being left in Mode C.
For example, on the Bendix King KT 76C after entering a code with the numeric keys it waits 5 seconds before transmitting the new code.
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Old 17th Sep 2005, 20:06   #7 (permalink)
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agreed, pleaaaaaaassssse go to standby when changing squawk!

Even you military and PPL types. We get a load of incorrect corellations between flight plan and radar data due C172 pilot stopping mid twist to answer the R/T. GBAEW converts as BAW225 and the systems try to track the flight. Controllers know which is their aircraft but the systems aren't so clever (maybe perveptive is a better word here).

Can be a serious problem close to the boundary.
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Old 17th Sep 2005, 20:30   #8 (permalink)
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I fly an old military aircraft that is seriously overdue an avionics update but probably won't get it for lack of money / time remaining in service. For 20-something years I selected SBY before changing squawk. Then a few years ago I was told to leave it on, because of TCAS and separation being done against me when I show on SSR only - I think that was the rationale.

I do take care not to scroll through the emergency numbers.

But back to the opening post - what is the policy - can anyone quote a definitive document?

I would have trouble challenging the position of my trg staff without a policy document to quote.

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Old 17th Sep 2005, 20:49   #9 (permalink)
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Not being able to get rid of BIG RED DATA BOCKS doesn't seem quite as serious to me as a mid air collision because TCAS is selected off.

My company policy tells me in very clear wording to NEVER turn off my transponder but to think twice when turning the dials, and I couldn't agree more!


Last edited by Ziggy; 17th Sep 2005 at 21:40.
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Old 20th Sep 2005, 23:59   #10 (permalink)
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Transponders & Standby

Know your unit!

Most modern transponder, ie. Mode S, AUTOMATICALLY goes to STBY upon entering a new code.

I have come across several incidents where people with older units have set off D&D London's alarm bell because they did not go to STBY and cycled through an emergency code.

If in doubt, always go to STBY!
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Old 22nd Sep 2005, 11:50   #11 (permalink)
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How long does it take you to change the figures? Can't be more than 20 secs, and if a mid-air were to happen in that time, the separation up to that point must have been noticed.

In any event, you are still showing on radar while it is switched off, so you should get a warning
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Old 22nd Sep 2005, 13:13   #12 (permalink)
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I should never need TCAS if ATC do their job.........that's not the point.
TCAS is my last line of defense, the last safety in the chain of events and I refuse to turn it off.

Last edited by Ziggy; 23rd Sep 2005 at 10:57.
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Old 23rd Sep 2005, 09:12   #13 (permalink)
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I seem to remember reading the manual for a GTX330 (mode S txpdr) recently, and it says that the new code is not transmitted until the fourth digit is entered on the push buttons.

I was always taught to select standby first, though, for the dial type txpdrs.
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Old 23rd Sep 2005, 12:57   #14 (permalink)
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In the U.S., I very seldom see an "accidental" 75, 76, or 77 code alarm caused by someone switching transponder codes. Maybe one every few months? Must be differences in ground equipment and software.

Also, nothing in my Narco user's manual about switching to standby while changing codes. And I never have nor do.....
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Old 23rd Sep 2005, 19:27   #15 (permalink)
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Believe me. If I had a dollar for the number of times airplanes from a certain flying school to the south east of Winnipeg had set off our datablocks (as well the the aural alarm) because they have dialed through codes (and not just 75, 76 and 77) rather than standby.........well, I'd be buying everybody beer in January.
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Old 26th Sep 2005, 05:41   #16 (permalink)
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"How long does it take you to change the figures? Can't be more than 20 secs, and if a mid-air were to happen in that time, the separation up to that point must have been noticed"

You may be doing a fine job seperating me from your traffic, but the TCAS is showing me the traffic you may not be seperating me from. In the LA area the vast majority of TA/RA's are on VFR traffic that I'm not being seperated from. The controllers there do an absolutely superb job of keeping me away from IFR traffic that they may not have time to point out a VFR aircraft to me, the TCAS aids in that. Hitting a VFR aircraft hurt just as much as hitting an IFR one. That said, never turn off the txponder.
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