What should be pressed on the autopilot after tuning the LOC frequency for LOC/LDA?

When tuning the Localizer (LOC) frequency for an Instrument Landing System (ILS) or Localizer Performance with Vertical Guidance (LPV) approach, the appropriate action is to press the NAV button on the autopilot. Engaging this mode allows the autopilot to capture and track the localizer signal, which provides lateral guidance for the approach.

The NAV mode is specifically designed to utilize navigation signals from both VOR and LOC sources effectively. By selecting NAV after the LOC frequency has been tuned, the autopilot will follow the course determined by the localizer, helping to ensure the aircraft is properly aligned with the runway. This is essential for a precise approach, particularly in IFR conditions.

While other modes like APP (Approach) could be relevant in some situations, pressing NAV after tuning the LOC frequency is the standard procedure to ensure that the autopilot is set to respond to the localizer's lateral guidance correctly.