Definition
In magnetic compass turns, lead or lag refers to the predictable error in the compass indication caused by magnetic dip when turning to or from a northerly or southerly heading in the Northern Hemisphere. The compass lags behind the actual heading when turning to or from north, and leads the actual heading when turning to or from south. The amount of lead or lag is approximately equal to the latitude of the aircraft, plus half the bank angle. To roll out on the desired heading, the pilot must stop the turn before the compass reaches the target heading (when turning to south) or after it has passed the target heading (when turning to north).
Plain English
When you turn the airplane using the magnetic compass to find your new heading, the compass doesn't read correctly during the turn. Heading north, it reads behind where you actually are. Heading south, it reads ahead. So you have to stop turning either before or after the compass shows the heading you want.
Context Anchor
Used when making turns by reference to the magnetic compass, especially when no reliable heading indicator is available or when checking compass behavior during instrument flying.
Derivation
Lead' comes from Old English 'lædan,' meaning to go in front. 'Lag' comes from a 16th-century word meaning to fall behind. In compass turns, the compass card literally goes in front of (leads) or falls behind (lags) the aircraft's actual heading, so the everyday meanings carry over directly.
Why Pilots Care
Correct compensation prevents overshooting or undershooting headings and maintains accurate course control without gyroscopic instruments.
Analogy
It is like following a person whose watch is a little fast or slow. The watch is still useful, but you must know whether it is running ahead or behind before you trust it exactly.
Grounding Statement
In a turn, the magnetic compass can swing ahead of or behind the airplane’s actual heading instead of tracking it perfectly.
Intuition Check
Do not assume the compass always points to the exact heading during a turn. In this context, lead means the compass indication is ahead of the airplane’s actual heading, and lag means it is behind it.
Example Sentence 1
Turning from east to north at 30 degrees of latitude, the pilot rolled out after the compass passed north, allowing for the expected lag.
Example Sentence 2
Turning through south, continue past the indicated heading because the compass leads and will settle on the target only after rollout.