Definition
A lead radial is a VOR radial used as an early cue to begin the turn from one airway or course onto another, allowing the aircraft to roll out smoothly on the new course rather than overshooting it. The pilot starts the turn when the lead radial is reached, not when the new course is reached, because the aircraft needs distance and time to complete the turn at its current groundspeed and bank angle.
Plain English
When you need to turn from one route onto another, you don't wait until you're on top of the new route to start turning -- you'd fly straight past it. Instead, you pick a radial a little before the turn point and start the turn there. That earlier radial is your cue to begin turning so you roll out neatly on the new course.
Context Anchor
Seen in instrument flying when using VOR navigation, especially when joining, leaving, or turning between published courses.
Derivation
Lead' here carries its everyday meaning of 'going ahead of' or 'getting in front of' something -- the same sense as leading a target when shooting or leading a runner with a pass. The lead radial sits ahead of the actual course change so the turn can be started early.
Why Pilots Care
Allows a smooth, precise course intercept that keeps the approach stabilized and reduces pilot workload during critical phases of flight.
Analogy
It is like seeing a highway sign before your exit. You do not wait until the ramp is directly beside you to move over; the sign gives you time to set up and make the turn smoothly.
Intuition Check
Do not read “lead” as meaning the main or most important radial. Here, a lead radial is an early radial used as a cue to start a turn before the desired course is reached.
Example Sentence 1
Approaching the intersection at 180 knots, the pilot used the 045 radial as a lead radial and began the right turn early to roll out smoothly on the new airway.
Example Sentence 2
By intercepting the lead radials outbound from the holding fix, the aircraft rolled out on the airway without any overshoot.