Transponder and ATC

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Ravioli

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Looked around a bit first, and probably old territory, but here it comes:

My Mode C transponder (GTX327) uses an external altitude encoder in my EFIS. I adjust barometric pressure in my EFIS and it matches 'closely' to my standby altimeter (steam) when the baro is set the same. That's all fine.

The GTX327 always displays Pressure Alt, fed from the EFIS.

Technical questions:
Is the mode C sending the pressure alt, the corrected alt, or both to the ground station?
Is the ground station using making 'automagic' corrections?

I know that at Flight Levels it's 29.92 all day, but there is a question about the display difference on the VAF forum and I thought I'd ask hear since there are plenty of people on the scopes who probably know.

Thanks!
 
What told us all kinda of stuff but what the EFIS is...

GTX327 should transmit what it gets, which should be uncorrected pressure altitude
 
Looks to me like the ADS-B out message is supposed to be only pressure altitude (with lots of other non altitude data) also. So in theory both a mode C transponder and a separate ADS-B transmitter would be sending out the same pressure altitude.
 
Your Mode C is sending PA. The controller's scope automatically adjusts for it thru a baro feed allowing for an indicated altitude display. Some of the older systems (ARTS III A) do have to have a human type in the current altimeter into the computer but I think most of them are gone now.
 
I'm pretty sure the Mode C sends pressure altitude. ATC then corrects for it using the altimeter setting for your area. Which is why when they read back your altitude to you, it should match what your altimeter says within tolerance, otherwise you either have a problem with your Mode C, your altimeter, or your altimeter setting is incorrect, or a combination of some or all of the above three.
 
image.jpeg You can see the current altimeter (29.73) on the lower left in this pic. Computer continuously updates it. If it fails the controller can do a manual update as well.
 
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bnt83 said:
What told us all kinda of stuff but what the EFIS is...

GTX327 should transmit what it gets, which should be uncorrected pressure altitude
Click to expand...
Ha, yea, probably one of these:
EFIS
AcronymDefinition
EFISExterior Finish Insulation System (construction material)
EFISEnglish for International Students
EFISEuropean Federation of Immunological Societies (Berlin, Germany)
EFISElectronic Flight Instrument System
 
azure said:
I'm pretty sure the Mode C sends pressure altitude. ATC then corrects for it using the altimeter setting for your area. Which is why when they read back your altitude to you, it should match what your altimeter says within tolerance, otherwise you either have a problem with your Mode C, your altimeter, or your altimeter setting is incorrect, or a combination of some or all of the above three.
Click to expand...

The tolerance ATC uses to consider Mode C valid is if the difference between what the pilot reports and the Mode C readout is less than 300 feet it is considered valid and may be used for separation. I'm sure it's not what you meant, but twisting the knob until the "altitudes" "match" is probably not a good idea. "Comparing" them is, and if they are consistently off then it should be looked at
 
luvflyin said:
The tolerance ATC uses to consider Mode C valid is if the difference between what the pilot reports and the Mode C readout is less than 300 feet it is considered valid and may be used for separation. I'm sure it's not what you meant, but twisting the knob until the "altitudes" "match" is probably not a good idea. "Comparing" them is, and if they are consistently off then it should be looked at
Click to expand...
Correct, I didn't mean to fiddle with the altimeter setting to make them match, just compare and use as a guide to troubleshooting if they don't match. And 300 feet is what I had heard as well. Also, my understanding is that if the discrepancy is greater than that ATC is supposed to tell you to "stop altitude squawk".

Edit: there's a pretty good discussion here that indicates that the maximum allowed discrepancy is 125 feet, not 300. IIRC 300 is the just the threshold at which ATC considers your altitude squawk invalid, as you said. You should have it looked at - and corrected - if the discrepancy is >125 feet.
 
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I've also been told to "stop altitude squawk" when our transponder pressure altitude was randomly jumping from correct to several thousand feet off. I'm either guessing there is a built in error detection or the controller noticed the jumps. I bet the Fedex arrivals jumping a thousand feet after we set off their TCAS helped the controller. I've never been vectored out of their airspace as quick until then.

ATC went screw your FP, I'm getting you the heck out of my hair.
 
"Stop altitude squawk" is the phraseology and 300 feet is the cut off point where it's done. Bad Mode C's have led to unnecessary TCAS RA's that further led to actual loss of separation incidents. Not real close but losses nonetheless.
 
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