Simviation Forums

Hello all, I'm new to these boards but I've been into flight sims for years (since MSFS fit on a single 3.5" floppy disk, and came with a REAL manual!).  I have a question about altimeters.  I know that the altimeter judges your distance from sea level based on the atmospheric pressure, and that you know what to set your altimeter to according to the ATIS report.  The question is, once you set your altimeter to the departure airport, what happens next (as far as the altimeter is concerned)?  Suppose the atmospheric pressure is at 30 inches when I depart, but it is 29 at my destination, would I need to reset my altimeter to match the ATIS report at the destination airport?  Also, since the altimeter measures from sea level, unless you're in a seaplane you would need to add the elevation of the runway to your altimeter to get your distance from ground, correct?

Thanks,
Mike

Before taking off and landing you ring up ATC and ask for the atmospheric pressure in their area. This will set your altimeter to read the runway at 0 feet. Irrelivent of sea level.

There's something called Transition altitude and Transition level ( in U.S the TA 18.000ft) Once you're above that you'll reset the altimeter to standard  29.92 inches  or 1013hPA.
There's always a 1000ft cushion between the TA and TL so TL can vary (depending on air pressure)

Same thing when you descend...when passing thru the transition LEVEL you'll change barometric pressure from STD to the airport's reported QNH.
And there's no reason to adjust your altimeter so it reads zero on the runway since the approach charts are marked after QNH (aswell as QFE in italics)

When ground adjusting your altimeter for local pressure you use the known altitude above sea level as a cross check of your setting...that's why you see all those small airports that have field height printed in big letters on the hangar or tower.

Conversely, you can rough set your altimeter by correcting displayed height until it reads field height. Assuming the altimeter is properly calibrated the resulting pressure in the window will be very close to actual local pressure.

Great, thanks for the info, that cleared a lot up!