Dont quote me on this, but I think UK CAA certification requires 3 altimeters for aircraft cleared to fly in instrument conditions, whilst FAA and most of the rest of the world only requires 2.
As for which one is correct: you check them both before takeoff, and hopefully they read pretty well the same. If they dont, the aircraft doesnt fly till the fault is rectified. If you have a failure in flight you will hopefully have some other cues. You have a vertical speed indicator, a radio altimeter, you may have 2 independent air data systems, which will tell you if they have failed or disagree with each other. You will know from the power and configuration of the aircraft whether it ought to be climbing or descending. You will probably have a GPS-derived height available for a rough cross-check.
Secondly, if you have a failure, it will probably be due to a blockage of the static air system (possibly by ice). So - is one of the pitot heaters not working? Otherwise if you deliberately climb or descend, and one of the altimeters moves, and the other doesnt, Bob's your Mum's very good friend. Alternatively, you can try the alternative static source, which in unpressurised aircraft will be cabin altitude. This again will give you a rough cross-check (may be a couple of hundred feet out due to the way the air presure varies inside and out).
There are many ways to skin tiddles!
Last edited by Non-PC Plod; 19th June 2008 at 09:47.
Reason: Further thoughts