Engagement of CLB mode requires which engaged lateral mode?

The engagement of the CLB (Climb) mode on the A320 requires the NAV (Navigation) mode to be engaged as the lateral guidance. This is integral to the aircraft's flight management system and autopilot operation.

When the NAV mode is engaged, the aircraft follows the flight plan that has been programmed into the Flight Management Computer (FMC). This ensures that the aircraft climbs properly on the designated flight path and adheres to any associated lateral navigation requirements such as waypoints or airways.

Using the NAV mode provides continuous and precise lateral navigation throughout the climb, which is crucial for maintaining air traffic compliance and safety, particularly during initial climb phases when altitude and lateral navigation are critical.

In contrast, the other lateral modes such as HDG (Heading), RWY TRK (Runway Track), and GA TRK (Go-Around Track) do not provide the necessary dynamic navigation capabilities needed to support the CLB mode since they do not follow the predetermined flight path specified in the waypoint-based navigation designed for the aircraft's trajectory during ascent. Thus, the necessity of having NAV mode engaged for CLB ensures that the flight enters the climb phase with the correct navigation references.