How do you determine the mechanical altimeter error in an aircraft?

• A. GPS altitude comparison
• B. Set field elevation in the altimeter and compare the Kollsman window setting with the current local altimeter setting
• C. Check fuel quantity
• D. Use the clock and logbook to estimate

Answer: (B)

Mechanical altimeter error is found by checking how the instrument responds to a known altitude with the correct pressure reference. On the ground you know the field elevation. Set the altimeter so that it would indicate that field elevation when you’re at the airport. Then look at the Kollsman window and compare that pressure setting with the current local altimeter setting (the local QNH). If the two pressures don’t match, the altimeter isn’t calibrated correctly and its indication is off. The difference between the known field elevation and what the altimeter shows under the proper pressure setting is the mechanical error.

GPS altitude isn’t a substitute for this pressure-based check, and fuel quantity or clock/logbook records don’t verify the pressure sensor’s accuracy.