Definition
An operating mode of a remote indicating compass system (such as a slaved gyro/HSI) in which the directional gyro runs independently of the magnetic flux detector. Heading is no longer automatically corrected to magnetic north by the slaving circuit; the pilot must manually set and periodically reset the heading indicator to match the magnetic compass.
Plain English
A setting where the heading indicator is left to spin on its own without being kept aligned to magnetic north automatically. The pilot has to set it by hand and check it from time to time because it will slowly drift off.
Context Anchor
Seen in discussions of remote indicating compass systems, especially when selecting between normal magnetic-slaved operation and manual gyro operation.
Derivation
Free' here means released or uncoupled — the gyro is no longer tied to (slaved to) the magnetic sensor. It is free to run on its own, the way an older non-slaved directional gyro always did.
Why Pilots Care
Prevents large heading errors caused by magnetic anomalies, turning errors, or acceleration errors that would otherwise affect a slaved system.
Intuition Check
“Free” does not mean unrestricted or better. Here it means the gyro is free from automatic magnetic correction, so the pilot takes responsibility for keeping the heading display accurate.
Example Sentence 1
When the slaving system failed in flight, the pilot switched the HSI to free gyro mode and reset it against the magnetic compass every fifteen minutes.
Example Sentence 2
After twenty minutes in free gyro mode the heading had drifted five degrees, so the pilot cross-checked the magnetic compass and reset it.