Definition
Inherent inaccuracies in the readings of pitot-static and other flight instruments caused by their mechanical design, the physics of how they sense pressure or motion, and installation factors on the airframe. These errors are predictable and must be accounted for when interpreting indicated values, particularly airspeed and altitude.
Plain English
Small but expected inaccuracies in what an instrument shows, caused by the way the instrument is built and mounted on the airplane. The needle isn't lying on purpose — the instrument simply has built-in limits to how accurately it can measure what it's measuring.
Context Anchor
Seen when learning the different types of airspeed and how indicated airspeed is corrected to get a more accurate value.
Derivation
Instrument comes from a Latin word meaning a tool or device. Error comes from a Latin word meaning to wander or stray. Together, instrument errors are places where the instrument’s reading strays from the value it should show.
Why Pilots Care
These errors must be known and applied as corrections so pilots obtain accurate performance numbers for safe takeoff, climb, and landing decisions.
Intuition Check
Instrument errors do not mean the pilot made a mistake reading the instrument. They mean the instrument itself may be showing a slightly imperfect reading.
Example Sentence 1
The pilot referenced the airspeed correction card to account for instrument errors at low speeds.
Example Sentence 2
During the preflight briefing the instructor showed how instrument errors combine with other corrections to produce true airspeed.