Definition
Inaccuracies in indicated altitude caused by mechanical imperfections in the altimeter or by atmospheric conditions that differ from the standard atmosphere the instrument is calibrated to. Common sources include nonstandard pressure (the altimeter setting), nonstandard temperature, and instrument scale or mechanical error. When uncorrected, these errors cause the altimeter to display an altitude that does not match the aircraft's true height above sea level or above the ground.
Plain English
The altimeter doesn't always show the exact right number. The air it measures changes with weather and temperature, and the instrument itself isn't perfect, so the altitude shown can be off from what the aircraft is actually flying.
Context Anchor
Seen in instrument flying when setting, checking, and relying on the altimeter for altitude control, terrain clearance, and assigned altitudes.
Derivation
Altimeter comes from Latin roots meaning “height” and “measure.” Error comes from a root meaning “to wander” or “go astray.” Together, the phrase points to a height-measuring instrument whose reading has wandered away from the correct value.
Why Pilots Care
Uncorrected altimeter errors can result in terrain clearance violations or loss of separation from other traffic.
Analogy
An altimeter error is like a bathroom scale that reads a few pounds high or low. The number looks exact, but if the scale is off, the reading is not the real value.
Grounding Statement
An altimeter can be working normally and still read wrong if the air pressure or temperature around the airplane does not match what the instrument is set up to assume.
Intuition Check
Altimeter errors do not always mean the pilot made a mistake or the instrument is broken. They mean the altitude shown may differ from the real or intended altitude for several possible reasons.
Example Sentence 1
Before departing into colder-than-standard air, the pilot reviewed altimeter errors and added a temperature correction to the minimum crossing altitude.
Example Sentence 2
Changing pressure systems ahead required the crew to anticipate altimeter errors during the approach.