If the altimeter reading doesn't match expected altitude during flight, what is a likely cause?

• A. The local pressure setting on the altimeter is incorrect
• B. The airspeed indicator is malfunctioning
• C. The compass is misaligned
• D. The attitude indicator is unlevel

Answer: (A)

Altitude is determined by the ambient pressure, but the altimeter converts that pressure to altitude using a locally adjusted pressure setting. If that setting is wrong, the same atmospheric pressure will be translated into an incorrect altitude, so the reading won’t match what you expect. The fix is to update the altimeter with the current local pressure setting (the QNH) from reliable weather data or ATC and re-check against a known reference such as field elevation or GPS altitude. The other instruments mentioned don’t directly affect how the altimeter converts pressure to altitude—the airspeed indicator, compass, and attitude indicator relate to other flight measurements. Temperature and static-system issues can also cause altitude discrepancies, but the most common and direct cause here is an incorrect altimeter setting.