Plain English Meaning

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.

Full 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.

Context Anchor

Checked after engine start during the required inspection of the navigation and flight instruments, especially before taxi or instrument flight.