How does the FMS VNAV mode help with vertical navigation, and what limitations should pilots monitor?

• A. VNAV provides vertical guidance and speed constraints; monitor actual performance versus expected performance and adjust for deviations.
• B. VNAV provides lateral navigation only.
• C. VNAV is used only during takeoff.
• D. VNAV ignores airspeed constraints.

Answer: (B)

VNAV is designed to manage the aircraft’s vertical profile along the flight plan, using the vertical constraints (altitudes) and any speed restrictions to generate a managed climb, cruise descent, or descent to an approach. It calculates and follows a vertical path, and it can apply target speeds to meet those altitude constraints or optimize climb/descent performance. This helps reduce pilot workload by letting the FMS steer the aircraft through level-offs, gradual descents, and step-downs while keeping the intended flight path.

However, pilots must monitor how the actual performance stacks up against the computed profile. Winds, weight, temperature, airspace restrictions, or ATC changes can cause deviations from the plan, so you should be ready to intervene if the aircraft is not on the intended vertical path or is not meeting speed constraints. VNAV is active throughout the flight, not just during takeoff, and it does not ignore airspeed constraints; it uses them to shape the vertical path and maintain compliance with the flight plan.