Hi CA
There are two functions that help capture the Localiser. One is LOC CONVERGENCE FUNCTION and the other is ENHANCED LOC CAPTURE FUNCTION. They have different requirements. They are given in Aircraft Systems, Auto Flight- Fight Guidance- AP/FD Common modes- Approach. Enhanced Loc Capture enables the capture to start earlier to prevent overshoot. The salient features below:
The LOC* mode engagement in the pre-capture zone is possible, when :
‐ The LOC deviation is more than 2.3 dots
‐ The FMS is in GPS PRIMARY
‐ The difference between the track and the QFU is between 25 ° and 115 °
‐ The guidance roll order is such that LOC* will capture the LOC beam without overshoot.
When the LOC deviation becomes lower than 2.3 dots, LOC* mode no longer uses the FMS
position data for guidance, but the actual LOC beam deviation to complete the capture of the
axis.