Look out the front. The last known aircraft without a front window was the Spirit of St Louis, and even then he had a periscope.
My apologies, I did not mean to be so abrupt. What I should have said was:
<quote> Why are you potentially going flying with an instrument that allegedly gives purpoted erroneous information, such a theoretical scenario must surely be in contravention of some important rule or law or Air Navigation Order or similar.
The appropriate Certificate of Airworthiness (or other equivalent approval) will list a Minimum Equipment List, listing all appropriate instrumentation to fly in accordance with said C of A (or other equivalent approval).
I am sure that an accurate altimeter is on the MEL. </quote>
That having been said, for good old VFR flight, a front window should still suffice.
DT