Transponder Standby Mode

[MarineGrunt]

What is the purpose of transponder standby mode? I was tought that after start it is placed on standby to prepare the unit for transmission, or to "warm up."

Is this correct? If so, what exactly needs to "warm-up" and how long does it take?

If this is not correct, I'm interested to find out the real answer...

 

[avbug]

Standby keeps the unit powered up, without replying to interrogation. One should go to standby whenever the avionics are powered up and one isn't squawking a code. In standby, mode A, C, and S are disabled; the unit is powered, but is not replying to any interrogations.

One should go to standby any time one is changing codes, to prevent passing through any emergency or assigned codes. Go to standby to set in the new code, and then switch back normal (or on) with the new code. This includes when told to squawk VFR; change to standby before setting the transponder to 1200, and then back to normal (or on).

 

[Flyboy758]

One should go to standby any time one is changing codes, to prevent passing through any emergency or assigned codes. Go to standby to set in the new code, and then switch back normal (or on) with the new code. This includes when told to squawk VFR; change to standby before setting the transponder to 1200, and then back to normal (or on). [/B]

I've never heard that before, but it's an excellent idea.

How exactly does the Mode C function work? I've heard the transponder is connected to the pitot/static system therefore allowing it to read your altitude?

 

[LAZYB]

One should go to standby any time one is changing codes, to prevent passing through any emergency or assigned codes. Go to standby to set in the new code, and then switch back normal (or on) with the new code. This includes when told to squawk VFR; change to standby before setting the transponder to 1200, and then back to normal (or on).

I know that was an old school procedure when I was learning to fly.

FAA has some thoughts on that procedure. Can't remember if it's in the AIM or where. When a transpoder is req'd you don't turn if off or to standby when changing codes.

 

[dmspilot00]

FAA has some thoughts on that procedure. Can't remember if it's in the AIM or where. When a transpoder is req'd you don't turn if off or to standby when changing codes.

I was also not taught to turn it to standby in the air either. My first instructor never even used standby, it was either ALT or OFF. Other instructors I have had since then do use standby.

A pilot friend of mine turned the transponder in a rental to SBY once while changing the code in the air. When he turned it back to ALT, it stopped working. It had to be sent to an avionics repair shop, I think. Something to keep in mind when you're flying old equipment, especially since it seems the transponder is often the oldest thing in the radio stack. While I'm on the topic, I had an ident button get stuck flush once, ATC finally pointed it out after 20 minutes, but there was no way to un-ident it

The AIM doesn't say to turn the transponder to standby, and while it doesn't specifically say you shouldn't, it does give advice on changing the transponder code without accidentally tuning in an emergency code.

 

[MarineGrunt]

So the transpsonder doesn't need time to warm-up on standby?

 

[bobbysamd]

Transponder usage

I, too, learned that you should never turn the transponder to standby unless instructed by ATC. You should just be careful not to pass through any emergency codes when changing codes.

You leave it on standby until just before takeoff and switch it back to standby after clearing the runway after landing.

 

[avbug]

There is no good reason not to go to standby when switching codes. This is something that's often taught, but ask the person who is teaching you why they think you shouldn't go to stdby when switching codes. Dollars for donuts, they don't have a clue.

That's a bit of a pet peeve...things like that which are taught, and you'll always find that the teachers are passing it on because that's what they heard, ad infinitum. Nobody ever seems to know why.

There's no good reason why. Go to standby, make the switch, and come back up.

If a transponder quit working when going from standby to on, it had nothing to do with the act...it was purely coincidental. There is nothing in the circuitry, or function of the transponder which could possibly be responsible for this. Purely coincidence.

More likely there was a failure in the conrol head, but this failure would occur when switching digits, not when passing from standby to on, or visa versa. With respect to power, the transponder doesn't know the difference.

I once flew with someone to check him out in a Cessna 210. He didn't want to bring the gear up; he said it had a much higher chance of failing if he was cycling it, and that he'd just fly the airplane with the gear down. Raise it only when flying cross country. Don't use it, it might fail. That's a similiar mentality to failing to go to standby when switching transponder codes. You are no more likely to cause the transponder to stop replying while going to standby and back, than you are by sneezing in the cockpit.

The transponder should be warmed up on standby. "Warm-up" isn't really necessary with most digital equipment; put the transponder to standby after powering up your avionics and leave it there until taking the runway.

Someone asked if the transponder works with the pitot-static system. It contains it's own pressure reference, always referenced to 29.92. It may be part of the altimeter (an encoding altimeter), or separate from the altimeter, as a remote mounted blind encoder. This is what provides the Mode C function to your transponder reply, when interrogated.

 

[dmspilot00]

I wasn't trying to imply the transponder would break if you went to standby and back. I was merely telling as interesting story.

If it is a good idea to switch to standby then why doesn't the FAA suggest it? In fact they seem to suggest not to. What is a good reason to go to standby?

 

[Vector4fun]

Speaking from an ATC standpoint, don't bother putting the Xponder in standby mode just to change codes. If you have enough manual dexterity to turn to the new code in under 10 seconds or so, it's not going to do a thing to screw with the ATC system.

Now, getting the WRONG code set can be a problem. Leaving a discrete code set after being told to squawk VFR is a problem. Mode-C readouts 200-2500' off is a BIG problem, but I can't ever remember seeing panties bunched because somebody went through 7700 in .48 seconds. Doubtful the transponder even replied in that small time frame.

70 kt finals followed by a 4500' rollout, THAT's a problem. Turning ATC down and turning the ball game UP for 200 miles, another BIG problem. Taxiing without a clearance, no-no. Forgetting to cancel IFR till 4 hours later at the restaurant=bad news. Waiting till you're in position with a LR31 on 2 mile final to tell the controller you want two minutes behind the MD-80, huge problem..........................................


Sorry folks, overdue for a short vacation....

 

[Ralgha]

A controller from Seattle Center told me a couple weeks ago that if you have the wrong code set in your transponder, and it happens to be the same code as one assigned to another airplane that hasn't yet departed, the ATC computer system will dispatch that other flight as soon as your code gets painted by a radar, thus royally jacking things up.

Therefore, it is feasable, though unlikely that if you leave the transponder on while changing the code, that a radar could paint you at the exact wrong moment, and catch a valid code from your transponder-in-transistion and dispatch another flight.

 

[BeachBum]

So the transpsonder doesn't need time to warm-up on standby?

According to a Cessna 150 POH, the King KT 76A transponder must be in SBY mode for 20 seconds of warm-up time when initially powered up.

 

[Vector4fun]

A controller from Seattle Center told me a couple weeks ago that if you have the wrong code set in your transponder, and it happens to be the same code as one assigned to another airplane that hasn't yet departed, the ATC computer system will dispatch that other flight as soon as your code gets painted by a radar, thus royally jacking things up.

That's true, sort of. But terminal radar systems, (ARTS), and I'm sure HOST in the center is similar, won't use just one reply on the wrong code to auto-acquire and start a track. ARTS will wait untill it sees the incorrect code 3-4 times in a row before that happens. That's about 15-18 seconds, which means it really never happens because someone was just changing codes. It DOES happen when someone sets the wrong code, and then leaves it set there.

 

[flyifrvfr]

There is no good reason not to go to standby when switching codes. This is something that's often taught, but ask the person who is teaching you why they think you shouldn't go to stdby when switching codes. Dollars for donuts, they don't have a clue.

If you fly in the Baltimore/Washington area which is covered by an ADIZ,the last thing you want to do is turn your transponder to standby while in the air.I was told by the FAA,"If the rader goes through one sweep while you are not on the correct code or not sqawking altitude,you will be violated."

 

[BellyFlyer]

Transponder Operation

What do you all make of this paragraph from 91.215?

(c) Transponder-on operation. While in the airspace as specified in paragraph (b) of this section or in all controlled airspace, each person operating an aircraft equipped with an operable ATC transponder maintained in accordance with Sec. 91.413 of this part shall operate the transponder, including Mode C equipment if installed, and shall reply on the appropriate code or as assigned by ATC.

Seems like it's saying you SHALL operate the transponder (not switch it to standby). Having said that, it probably doesn't make any practical difference if the xpdr is in STBY for a couple of seconds while you switch codes.

 

[nosehair]

Yeah, Grunt, it takes a little time to "warm up". Just what is warming up, I don't know, but as I understand these little GA transponder boxes, everyone I have operated, the reply light come on when you first turn from "Off" to "SBY, and stays on for a 20-30 second period, then goes out. This is the warm-up time. The light going out indicates it is warm and ready to reply.

 

[midlifeflyer]

This question comes up from time to time. The last time it did on another board, the general consensus among ATC participants was =do not= put it in STBY when switching.

 

[Checks]

The reason I would teach not to put the transponder to stby is because it could break.

The weakest part of those old airplane transponders is the knob. I had one break off on me once.
Just change your code while squaking altitude.

 

[Ralgha]

If you fly in the Baltimore/Washington area which is covered by an ADIZ,the last thing you want to do is turn your transponder to standby while in the air.I was told by the FAA,"If the rader goes through one sweep while you are not on the correct code or not sqawking altitude,you will be violated."

Then if they tell you to change transponder codes while you're inside the ADIZ, you're screwed, because you're almost certainly going to get painted at least once before you get the correct code set in whether you go to standby or not.

Maybe they just never tell you to change codes in there, I've never flown inside the DC ADIZ. I flew around in one around Portland once, but it was kinda pointless of them to have it, there's so many hills around that you could get pretty close to El Prez before the radar could even see you if you wanted.

 

[great cornholio]

Hey Vector

Whats up with ATC tellin us to switch sqawk codes in flight sometimes? Like just get a hand off then they are like Colgan sqawk 4513. Seems like it happens more often when we get big short cuts, or is that just my imagination?