Definition
The inherent inaccuracy in a flight instrument's indication caused by mechanical imperfections, design limitations, or environmental factors, resulting in a difference between the value the instrument displays and the true value being measured.
Plain English
Every instrument is a little bit off. Instrument error is the small, built-in difference between what the instrument shows and what is actually true.
Context Anchor
Seen during instrument cross-check, when a pilot compares instrument readings and decides whether one indication can be trusted.
Derivation
From Latin instrumentum (a tool) and error (a wandering or straying). The word 'error' here does not mean a mistake the pilot made -- it means the instrument itself wanders slightly from the true value.
Why Pilots Care
Pilots must know that no instrument is perfectly accurate. Recognizing instrument error during a cross-check helps the pilot avoid chasing a single faulty reading and instead rely on the overall picture from multiple instruments.
Analogy
It is like a bathroom scale that always reads two pounds heavy. The scale still works, but its reading is not exactly the real weight.
Intuition Check
Do not read error here as only a pilot mistake. Instrument error means the instrument reading itself is off from the real condition.
Example Sentence 1
During the preflight check, the pilot noted the altimeter showed a small instrument error of 20 feet compared to the known field elevation.
Example Sentence 2
A small instrument error appeared between the attitude indicator and the turn coordinator during the cross-check.