Definition
The number of degrees before the desired heading at which the pilot begins rolling out of a turn so the aircraft arrives precisely on the target heading rather than overshooting it. A common rule of thumb is to begin the roll-out approximately one-half the bank angle before the desired heading (for example, lead by 10 degrees when using a 20-degree bank).
Plain English
Stop turning a little before you reach your target heading, because the airplane keeps swinging through the turn while you level the wings. The amount you stop early is the roll-out lead.
Context Anchor
Used during instrument turns to predetermined headings, when the pilot is watching the heading indicator and timing the rollout onto an assigned or selected heading.
Derivation
"Roll-out" refers to rolling the wings level out of a bank. "Lead" here means starting an action early so the result lands on time -- the same sense as leading a moving target. Together: the early start needed to finish the roll-out exactly on heading.
Why Pilots Care
Prevents heading overshoots that create navigation errors or require corrective turns during instrument approaches and en route navigation.
Grounding Statement
The airplane does not stop turning the instant you start leveling the wings, so the rollout must begin before the desired heading is reached.
Intuition Check
“Lead” does not mean a leader or another aircraft ahead of you here. It means an early start point before the heading you want.
Example Sentence 1
Turning right from 090 to 180 at a 20-degree bank, the pilot began the roll-out at about 170 to allow a 10-degree roll-out lead.
Example Sentence 2
At higher airspeeds the roll-out lead must be increased because the turn radius grows larger.