Definition
A built-in inaccuracy of the pressure altimeter caused by the limitations of its mechanical design and the assumptions of the standard atmosphere model it is calibrated against. Even a perfectly functioning altimeter will display some error because the real atmosphere rarely matches standard conditions of pressure and temperature, and because the instrument's internal components (aneroid wafers, gears, and linkages) cannot respond with perfect precision.
Plain English
A small amount of error that every altimeter has, even when it's working correctly. It comes from the way the instrument is built and from the fact that real-world air is almost never exactly the same as the textbook conditions the altimeter assumes.
Context Anchor
Seen in instrument flying when learning how altimeters work, why indicated altitude may differ from true altitude, and why temperature and pressure changes matter.
Derivation
Inherent comes from the Latin inhaerere, meaning 'to stick in' or 'to be attached to.' Here it means the error is built into the instrument itself -- it can't be removed, only accounted for.
Why Pilots Care
These errors can cause the displayed altitude to differ from true height above the ground, directly affecting terrain clearance decisions and assigned altitude compliance.
Grounding Statement
A pressure altimeter does not directly measure height; it measures air pressure and converts that pressure into an altitude reading.
Intuition Check
Inherent does not mean “small” or “unimportant.” It means the error can exist naturally in the way the altimeter works, even when the instrument is operating normally.
Example Sentence 1
During the preflight check, the pilot noted that the altimeter read field elevation within 75 feet, an acceptable range given inherent altimeter error.
Example Sentence 2
During preflight planning the instructor explained how inherent altimeter error increases in nonstandard temperatures.