Which airspeed should be used when climbing with two engines over an obstacle?

When climbing with two engines over an obstacle, the appropriate airspeed to use is VX. This airspeed is the best angle of climb speed, which allows for the maximum altitude gain over the shortest horizontal distance. It is particularly important during a two-engine climb when navigating around obstacles, as it ensures that the aircraft can achieve the necessary height to clear these barriers. Utilizing VX enhances the climb gradient, making it effective for scenarios involving obstructions like buildings or terrain immediately after takeoff.

In contrast, VY, while it provides the best rate of climb over time, does not focus specifically on obstacle avoidance. VREF is typically the reference landing approach speed and is not relevant for climbing situations. VLOS refers to the "minimum landing speed," which also does not apply to a climb scenario. Hence, choosing VX is critical for maximizing safety and efficiency when ascending over obstacles with two engines.