In an FMS, what is the difference between LNAV and VNAV modes?

• A. LNAV provides lateral navigation guidance; VNAV manages vertical profile and altitude constraints to follow a climb/descent path.
• B. LNAV provides vertical guidance; VNAV provides lateral guidance.
• C. LNAV is used for autopilot in takeoff only; VNAV is used for approach only.
• D. LNAV and VNAV are unrelated to flight management.

Answer: (A)

LNAV and VNAV split guidance into two dimensions of the flight path in an FMS: lateral movement and vertical profile. LNAV provides the horizontal path, steering the aircraft along the planned route from waypoint to waypoint in the horizontal plane. VNAV handles the vertical component, following altitude constraints and a climb/descent path to meet those constraints.

In practice, LNAV keeps the aircraft on the intended course and route, while VNAV computes and follows the required climb or descent, adjusting the aircraft’s vertical speed or rate as needed to achieve the target altitudes at the right points. Both modes can operate together, giving you a full flight path: LNAV for the sideways tracking and VNAV for the up-and-down profile.

The other options mix up roles: vertical guidance is VNAV’s job, not LNAV; LNAV isn’t limited to takeoff and VNAV to approaches; and LNAV and VNAV are not unrelated—they’re complementary parts of the FMS guidance system.