hopharrigan, if both of your altimeters on the ground were set to the same QNH, then they should have read the same figure (within tolerance).
If they didn't they should have been written up. (Indeed, in Australia, ground altimeter cross checks are required in the AIP.)
As to whether or not glass cockpit altimeters have an in-built true altitude correction it should be obvious that
they do not.
The altimeter reads a pressure level as a
standard (as posted by ptero.) That's why it is called a "Pressure altimeter"
Imagine if one aircraft were to fly a true altitude, and another ("old") aircraft was flying a pressure altitude. Both are told to maintain 5000, on an ISA-15°C day. The old aircraft would be flying at 5000 feet. The "new" aircraft would be flying at 5,300 feet (5000' true) - obviously reducing separation with anyone at 6000. It is not that the technology isn't
capable of performing the correction, it is simply not standard.
How much does it matter? If you use jepps you will notice that the printed lowest safe altitudes allow 1000' terrain clearance below 5000', and 2000' terrain clearance above 5000'.
This is to allow for the effect of true altitude at cold temperatures. So you are reasonably protected en-route (provided you don't bust the lowest safe, of course
)
With radar vectors, you will be asked to descend to a pressure altitude, however the radar controller will have adjusted his radar lowest safe
to allow for true altitude clearance, so you are reasonably protected there.
With a precision approach you will be following a glide path down to the runway. As long as you are on slope, you will be protected from terrain, however the OM crossing height check will show a difference between the true altitude printed on the chart, and the pressure altitude read in the cockpit. If you wish to conduct a "proper" altimeter check you will have to have performed the correction. (I fly in ISA+ temps, and I perform the correction in order to check this - it is just a quick mental calculation as shown in my previous post.) If you are on glide slope, however you will have terrain protection anyway.
When you get to the minima the correction is only occuring over 200 or so feet (CAT1), and so it is only of the order of 20' or so - even at really cold temperatures. Technically you will be busting the minima (assuming you are not visual) but a 20' error is probably not going to kill you.
The only place you really have to be on the ball is for a non-precision approach, in pretty cold temps. With a minima at around 800' AGL the correction (ISA-30°, say) will be on the order of (.8 * 30 * 4) 96 feet, so flying to the
indicated (pressure) minima, will result in a bust of 100feet or so. Something a professional pilot should be aghast at.
It doesn't matter if you use a correction chart (probably the safest, as it is hard to make a mistake), use the true altitude correction on the Jep. CR-2 calculator, or do it in your head (using the formula in my previous post) but if you are conducting
non-precision approaches in cold temperatures then you
have to adjust the printed (chart) minima for true altitude.
[This message has been edited by Checkboard (edited 01 December 1999).]