When intercepting a course, a pilot must turn to join the course before it centers, or the aircraft will overshoot the course. The lateral distance required to complete the turn when the course is centered, or **lead**, is a function of turn radius and intercept angle.
![[estimating turn lead original.png]]
A 90° turn must be led by the radius of the turn; the lead for any angle less than 90° can be determined by the formula above, where:
- $r$ is the radius of turn
- $x$ is the intercept angle (degrees to turn on course)
[[Radius of Turn at Standard Rate]] is a function of velocity, so:
$
0.005v(1-\cos(x))=lead
$
Note that this formula **does not account for the effects of wind**; groundspeed, rather than true airspeed, can be used instead. The time required to roll-in and roll-out of the turn is also omitted for simplicity.
## Application
In practice, it is easiest to remember your aircraft's turn radius during various phases of flight, then apply a rough estimate for the degrees of turn required:
| intercept angle | lead expressed as % of radius |
| :-------------: | :---------------------------: |
| 90° | 100% |
| 60° | 50% |
| 45° | 29% |
| 30° | 13% |
To make effective use of this information, pilots should be thoroughly familiar with their aircraft's navigation displays and CDI scaling for various navigation systems and phases of flight.
### Example
For an aircraft flying at 120 KTAS in light winds:
| intercept angle | lead |
| :-------------: | :----: |
| 90° | 0.60nm |
| 60° | 0.30nm |
| 45° | 0.16nm |
| 30° | 0.08nm |
###
By Kevin Sakson