VNAV

VNAV, short for vertical navigation, is a flight management system capability that automates a vessel’s vertical profile along a flight plan. It works in conjunction with LNAV (lateral navigation) to provide a coordinated, fully managed navigation path from climb to cruise to descent and approach. VNAV aims to meet altitude and speed constraints while optimizing fuel efficiency, flight time, and performance.

VNAV uses data such as aircraft weight and balance, performance models, engine settings, winds aloft, air traffic constraints, and published altitude restrictions. It calculates a vertical path that includes climb gradients, cruise altitude, and descent profiles. The system can adjust vertical speed, flight path angle, or thrust mode to stay on the planned profile and to comply with constraints at waypoints and step-down fixes.

In operation, pilots can select managed VNAV mode, allowing the FMS/autopilot to fly the prescribed vertical profile automatically, or use selected mode to manually control vertical speed or flight path while still benefiting from the underlying constraints and data. VNAV commonly manages climbs to assigned cruise levels, descent to target altitudes, and descent planning for arrivals and approaches, integrating with speed constraints and procedures.

Limitations and considerations include dependence on accurate data and constraints, weather, wind, and sensor inputs. Inaccurate data or degraded performance can lead to suboptimal profiles. Pilots monitor VNAV performance and can interrupt or override with other modes if necessary, ensuring safe and compliant operation.