Definition
A radial used to begin a turn before reaching the radial or course you intend to intercept, so that the aircraft rolls out established on the desired course rather than overshooting it. The lead radial is selected based on aircraft groundspeed, angle of bank, and the angle of intercept, and is most commonly used when transitioning from one airway or course to another via a VOR or DME arc.
Plain English
It's the point on your navigation display where you start the turn early so you finish the turn lined up exactly on the new course, instead of flying through it.
Context Anchor
Seen in instrument flying when navigating by VOR and planning a turn onto another course, especially when leaving a curved route around a navigation station.
Derivation
"Lead" here means "to go before" -- the same sense as a quarterback leading a receiver by throwing where the receiver will be, not where they are. The lead radial is the radial you use to start the turn before the target course arrives.
Why Pilots Care
Starting the turn at the lead radial produces a smooth, on-course intercept and avoids the need for large corrections that can destabilize the approach.
Grounding Statement
Because an airplane keeps moving forward while it turns, the turn must begin before the exact line the pilot wants to end up on.
Intuition Check
Lead does not mean the main radial or the final radial you intend to fly. Here, lead means an advance radial used as the cue to start turning early.
Example Sentence 1
Approaching the end of the DME arc, the pilot watched for the lead radial and began the turn to intercept the final approach course.
Example Sentence 2
ATC cleared the aircraft direct to the VOR; the lead radial appeared on the RMI and the turn was initiated.