Definition
A failure or abnormal operation of one or more flight instruments such that the displayed information is missing, frozen, erratic, or incorrect, leading the pilot to receive false or unreliable indications of aircraft state.
Plain English
An instrument is not working properly and is showing wrong, stuck, or jumpy readings — or no reading at all.
Context Anchor
Encountered during preflight checks, normal instrument scanning, and upset prevention and recovery training, especially when a bad indication could distract the pilot or lead to a wrong control input.
Derivation
From Latin malus (bad) and functio (performance). A malfunction is literally a 'bad performance' — the instrument is still operating, but not correctly. This is important because a malfunction can be more dangerous than a complete failure, since the pilot may not immediately realize the indication is wrong.
Why Pilots Care
A pilot who continues to trust faulty instruments during an upset can quickly enter a spin or lose control.
Intuition Check
An instrument malfunction does not always mean the instrument is completely dead. It can also mean the instrument is still showing information, but that information is wrong or unreliable.
Example Sentence 1
The pilot suspected an instrument malfunction when the airspeed indicator stayed pegged at zero during the takeoff roll.
Example Sentence 2
The instructor simulated an instrument malfunction to train the student to ignore the faulty turn coordinator and rely on the horizon.