Definition
The period after electrical or vacuum power is applied during which a gyroscopic instrument's rotor accelerates from rest to its normal operating speed. Until the rotor reaches that speed, the instrument's indications are unreliable and should not be used for reference.
Plain English
The short time it takes for the spinning wheel inside a gyro instrument to come up to full speed after the engine starts. Until it's fully spinning, what the instrument shows can't be trusted.
Context Anchor
Checked after engine start during the required inspection of the navigation and flight instruments, especially before taxi or instrument flight.
Derivation
“Gyro” is shortened from “gyroscope,” from Greek roots meaning “turning” and “to look at.” That helps because a gyro instrument works by using a fast-turning internal wheel to provide a steady reference.
Why Pilots Care
Using a gyroscopic instrument before it has reached full speed produces unreliable attitude or heading information that can lead to disorientation.
Analogy
It is like a toy top: while it is still slow, it wobbles; once it is spinning fast enough, it becomes steady.
Intuition Check
Gyro spin up does not mean the pilot manually spins anything. It means the instrument’s internal gyro reaches operating speed after power or vacuum is available.
Example Sentence 1
After engine start, allow a few minutes for gyro spin up before checking the attitude and heading indicators against known references.
Example Sentence 2
The heading indicator remained unusable during the initial gyro spin up period following power application.