How does ATC manage lower IFR altitudes?

[kontiki]

Hi all,

I'm trying to understand what I've run into with IFR flying under FL180 with ATC.

Transponder reports pressure altitude. I use true altitude as my altitude reference, per FARs.

Sometimes ATC gets on the horn to ask me about my altitude. OK, I'm using a good baro correction; I'm within 100 ft of alt per my altimeter. ATC sees me 200ft off altitude.

Does ATC use some kind of barometric correction for lower altitude IFR aircraft, would they understand if I told them I'm using MSL and they are seeing pressure altitude?

Have I got it wrong? How can they assign altitudes to within 500 ft if were not all using the same system?

Thanks,

 

[Palmpilot]

My understanding is that ATC's computers do adjust the displayed altitude, using an altimeter setting appropriate for the area.

By the way, I believe we're supposed to be using indicated altitude (with a local altimeter setting in the Kohlsman window), not true altitude.

 

[TMetzinger]

ATC gives you an altimeter setting, you put that in the kollsman window on your altimeter. You keep the airplane at your assigned altitude (MSL) as read on your altimeter (which you've corrected for nonstandard pressure using the local altimeter setting). You do NOT apply any other correction.

Your transponder reports pressure altitude, and the ATC also applies the local altimeter setting to make the corrections. So if your systems are working correctly you should be close. If you're doing the above and ATC always thinks you're significantly off your assigned altitude, you should have your static system plumbing, altimeter, and transponder checked.

 

[BellyUpFish]

What instrument in your plane reads true altitude?

You need to be using indicated altitude.

 

[ejensen]

You have to use the setting supplied by ATC not something from a nearby AWOS or other data on your IPad, XMweather, etc. They want all aircraft in their sector reading the same altitude.

 

[ajstoner21]

As others have said, you should use indicated altitude, not true altitude. Though, You should very much be aware of your true altitude on IFR when flying in extreme cold temperatures. The following chart shows some altitude error to be aware of.