SDBY when switching transponder codes?

rocketflyer84

rocketflyer84

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RocketFlyer84
I've heard differing instructions on the point of should the transponder be switched to standby when changing codes and am curious to hear input from others, especially a controller or anyone with knowledge of what ATC sees.

I've seen some that as a habit switch to standby, enter the new code and then switch back to ALT.

Others just adjust the code while in ALT and say to never switch off ALT while in the air unless ATC asks you to.

My training and the AIM says to avoid passing through codes like 7500 to avoid any unintended automatic alarms with the radar controller; however what's the impact of other codes being pinged back during the few seconds of setting a new code? This would seem most applicable to transponders with manual dials for each number.

I can see the activity light blinking while setting codes so, per my understanding, for those few radar pings I could be sending back some random other code, perhaps the code of another airplane in the same airspace (likely they're often assigned on nearby numbers). What does the radar controller see while this is going on? Does this impact anything on their screen? Can they even see this momentary cycling through codes?

Thanks for any insight!
 
If you flip through a normal code, the controller just sees a goofy code for a sweep or two. Since they expect this, and it's preferable to dropping off radar, it's not big deal.

Now if you flip through an emergency code inadvertently, the software goes nuts and it blot out a large portion of the display IIRC.
 
That just sounds like another step to forget.

I'm conscientious about the emergency codes but leave the TPX on ALT.
 
People have been leaving it in ALT for 50 years without it being a big deal. Just leave it in ALT, and make sure you don't go through 7500/7600/7700 (or 1200 in the DC SFRA or any security TFR). Of course, I'm spoiled by my Garmin GTX330, which doesn't start sending the new code until all four digits have entered and if you hit the wrong one, you can back up and start again (same for the GTX327 and G1000).
 
Set standby on the ground,once airborne it goes to alt. If atc gives me a change I leave it on alt.
 
Just change the code
 
I almost never use STBY anymore. It's either OFF or ALT.
 
The only time I touch my GTX330 is to enter the code. Never touch ALT/OFF etc. it's automatic.

For the old knob type, I've heard you can keep it on ALT even for ground ops and it makes no difference whatsoever. ATC radar will automatically filter out replys from transponders on the ground (unless they are using ASDE-X.)
 
As I recall I saw a discussion about this some 10-15 years ago. If I recall correctly some ATC type said it's not a good idea for your airplane to "disappear" from the scope leaving only a primary target.

Don't know if that statement was accurate but it ended my brief foray into this unnecessary extra step.
 
Ron Levy said:
If you happen to go to one of the major terminals where they have ASDE-X, you need to go active when you taxi, and keep it on until you park.
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... And the AIM has a relatively recent (2012) change recommending it being on for taxi on the ground as the default, although that does seem to be a bit of overkill.
 
Your aircraft isn't going to dissapear from the scope while dialing. If you're fast and just happened to dial it during the 4-5 second sweep or 12 sec center update, then you'll tag up with your new code. If your target gets interrogated while you are still dialing, then your tag will drop and show whatever code you selected during the dial. It's not a big deal.

If you somehow get pinged while you just happened to select one of the "special" non discrete codes, it'll register on as an alarm on the controller's scope for a split second. You might get a query from the controller. You might not. I had a guy once who had his transponder locked on emergency. I queried him, he responded with "negative emergency." He appoligized profusely for his transponder like he was going to get in trouble or something. I didn't care. No paperwork on my end and the few minutes that it was alarming was no big deal. MSAW alarms go off on a daily basis, so you get used to it.
 
dans2992 said:
The only time I touch my GTX330 is to enter the code. Never touch ALT/OFF etc. it's automatic.

For the old knob type, I've heard you can keep it on ALT even for ground ops and it makes no difference whatsoever. ATC radar will automatically filter out replys from transponders on the ground (unless they are using ASDE-X.)
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ATC radar may do that, but GA-type traffic watch systems do not. BTDT, on the receiving end, at 100 above DH still in the goo on an ILS. :eek: Then we broke out at 50 above and saw the Learjet at the hold short line matching up with the traffic marker a half mile ahead and 200 feet below. So, being a believer in that line about "Do not do to others what is hateful to you", I leave mine in STBY on the ground before I take the runway and after I clear, unless I'm where ground transponder operation is required for ASDE-X.
 
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Lousy traffic system then. The XAON PCAS is smart enough within it's limitations to not report things on the ground when first powered up. If it knew (as your Lear almost certainly does) that it's approaching the ground it should inhibit these. It's obviously NOT a compliant TCAS system which stops doing descent advisories at something like 1500' and shuts up completely when you are low enough to be in GPWS territory.
 
flyingron said:
Lousy traffic system then. The XAON PCAS is smart enough within it's limitations to not report things on the ground when first powered up.
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This was on approach, not when we first powered up.

If it knew (as your Lear almost certainly does) that it's approaching the ground it should inhibit these.
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Guess you haven't flown with too many light GA traffic watch systems -- they aren't smart enough to know height AGL.
 
Ron Levy said:
This was on approach, not when we first powered up.

Guess you haven't flown with too many light GA traffic watch systems -- they aren't smart enough to know height AGL.
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No unfortunately not because most of the suck badly so I'm not surprised. Went through this with the Garmin guys a couple of years ago. Short of a real TCAS they're not much better than the Xaon stuff (other than integration with the flight deck).
 
midlifeflyer said:
... And the AIM has a relatively recent (2012) change recommending it being on for taxi on the ground as the default, although that does seem to be a bit of overkill.
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See below:

FAA AIM 4-1-20 said:
3. Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems. IN ALL CASES, WHILE IN CONTROLLED AIRSPACE EACH PILOT OPERATING AN AIRCRAFT EQUIPPED WITH AN OPERABLE ATC TRANSPONDER MAINTAINED IN ACCORDANCE WITH 14 CFR SECTION 91.413 MUST OPERATE THE TRANSPONDER, INCLUDING MODE C IF INSTALLED, ON THE APPROPRIATE CODE OR AS ASSIGNED BY ATC. IN CLASS G AIRSPACE, THE TRANSPONDER SHOULD BE OPERATING WHILE AIRBORNE UNLESS OTHERWISE REQUESTED BY ATC.
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AIM 4-1-20 has your answer...the transponder is to be ON at all times when in airspace requiring one. The Air Safety Foundation has a series of videos titled "AskATC," in which controllers answer this and similar questions.

BTW, "I've been told...." is a red flag. Look it up yourself or ask for documentation to support what you've been told.

Bob Gardner
 
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Thanks for the responses. I do leave it on always. I fly a few different aircraft at my flight school, some have manual dial types while others are digital and automatically cycle on/off during takeoff.

Interestingly, when practicing touch and goes at a nearby airport with Class C airspace I've had the tower occasionally give the instruction "turn transponder to STBY when in the pattern".

Meanwhile 20 nm away at my home airport in Class D airspace the tower is all over you if get 500 ft off the ground after takeoff without a secondary radar return. Proud to say I follow my checklist and have never done this myself but seems like every time I fly out of there someone is being reminded to turn their transponder to ALT and last time ATC was reading the riot act to someone who took off without their transponder (from ATCs remarks I guess that call sign was a frequent offender).
 
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rocketflyer84 said:
Thanks for the responses. I do leave it on always. I fly a few different aircraft at my flight school, some have manual dial types while others are digital and automatically cycle on/off during takeoff.

Interestingly, when practicing touch and goes at a nearby airport with Class C airspace I've had the tower occasionally give the instruction "turn transponder to STBY when in the pattern".

Meanwhile 20 nm away at my home airport in Class D airspace the tower is all over you if get 500 ft off the ground after takeoff without a secondary radar return. Proud to say I follow my checklist and have never done this myself but seems like every time I fly out of there someone is being reminded to turn their transponder to ALT and last time ATC was reading the riot act to someone who took off without their transponder (from ATCs remarks I guess that call sign was a frequent offender).
Click to expand...

Bet that controller would come unglued if a flight of cubs came through negative transponder.
 
I never switch mine off Alt ever unless asked to do so which I only can remember doing at OSH. It comes on and off with the plane/avionics master.
 
bobmrg said:
AIM 4-1-20 has your answer...the transponder is to be ON at all times when in airspace requiring one.
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Let's be clear on this. When in "transponder required" airspace, you must per 91.215 have one and it must be on (the specific exceptions in 91.215 notwithstanding). In all other controlled airspace, per 91.215, if you have one it must be on, but there's no penalty for not having one, and the regs are silent about uncontrolled airspace. On the ground, by AIM recommendation (not regulatory mandate), it should (not must) be on "to ensure the aircraft is visible to ATC surveillance systems", but if there are no such systems on the airport, there is no reason to turn it on until you launch or leave it on after landing, and as I said above, there are reasons to leave it in STBY while on the ground at airports where there are no such systems.
 
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Henning said:
I never switch mine off Alt ever unless asked to do so which I only can remember doing at OSH. It comes on and off with the plane/avionics master.
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Mine goes standby to alt back to standby automatically.
 
So on the GTX330, what's the difference between GND and STBY?
 
Larry in TN said:
It still reports altitude but it also transmits data that shows that it is on the ground. All mode S transponders have that capability.
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The 330 reports nothing in GND mode, which is manually activated by pushing the STBY button or automatically activated by a timer a preset number of seconds after your speed drops below a set value (30 knots on mine). If the automatic mode is activated, it also shifts from GND mode to ALT mode (Modes 3A and 3C active) when you exceed that speed.
 
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