How do LNAV and VNAV work in a flight simulator?

LNAV (lateral navigation) guides an aircraft along the active FMS flight-plan route, while VNAV (vertical navigation) manages a calculated climb, cruise and descent profile. In real-world aviation and flight simulators, they generate flight-director commands; with the autopilot engaged, those commands steer and pitch the aircraft automatically.

LNAV versus VNAV: what does each mode control?

LNAV controls lateral guidance; VNAV controls vertical guidance and, in many jets, coordinates speed or thrust targets.

Neither mode is the autopilot itself. With only the flight director switched on, LNAV and VNAV display commands for the pilot to follow manually. When the autopilot is engaged, it follows those commands. An autothrottle may manage thrust, but its exact relationship with VNAV depends on the aircraft.

How do LNAV and VNAV use the FMS flight plan?

Both modes depend on a valid, active route and the aircraft-specific data needed to calculate guidance.

1. Initialise the FMS. Enter or confirm the aircraft position where the simulated system requires it.
2. Build and activate the route. Add the departure, waypoints, airway legs, arrival and approach as required. Remove genuine route discontinuities rather than blindly deleting every discontinuity displayed.
3. Enter performance data. VNAV may require weight, cruise altitude, speeds, reserves, cost index or other performance entries. The exact fields vary by aircraft.
4. Set the cleared altitude. The altitude selected on the mode-control panel normally acts as a boundary. VNAV cannot climb or descend through it merely because the FMS profile continues beyond it.
5. Arm or engage the modes. LNAV may engage immediately or remain armed until the aircraft can intercept the active leg. VNAV similarly changes between climb, path, speed and altitude-related submodes.
6. Read the flight-mode annunciator. The annunciator, not the illuminated button or magenta route alone, tells you which modes are armed and which are actively controlling the aircraft.

If the add-on uses a conventional scratchpad FMC, our worked route-entry and performance example for an FS2004 FMC demonstrates the basic programming sequence. Modern aircraft differ in their keys and pages, but the dependency on an activated route remains the same.

What changes between aircraft and simulator add-ons?

Mode names and the depth of VNAV simulation differ substantially between aircraft.

• Boeing-style systems commonly provide dedicated LNAV and VNAV controls, with submodes shown on the flight-mode annunciator.
• Airbus aircraft normally use managed navigation, climb and descent terminology instead of cockpit buttons labelled LNAV and VNAV, although the underlying lateral and vertical functions are comparable.
• General-aviation GPS units may calculate a target descent profile without coupling it to the autopilot. The pilot might still need vertical speed or flight-level-change mode.
• Basic simulator aircraft may simplify VNAV, omit parts of the performance model or provide advisory guidance only. An imported simulator flight plan may also exist in the world map without being loaded into an add-on aircraft's independent FMS.

Why will LNAV or VNAV not engage?

Most engagement failures come from route state, capture geometry, missing performance data or another active autopilot mode.

• No active route: the route may be entered but not executed or activated.
• A discontinuity or vector leg: LNAV has no continuous leg to follow. A vector segment may require heading mode until cleared towards the next waypoint.
• Poor intercept geometry: LNAV can remain armed when the active leg is too far away or behind the aircraft. Use heading mode to establish a sensible intercept.
• Incomplete VNAV data: missing cruise altitude, weight or performance entries can prevent a valid profile from being calculated.
• Selected altitude blocks the profile: VNAV will level at the panel altitude even when the FMS predicts a higher climb or lower descent.
• The wrong mode is active: altitude hold, vertical speed, heading select or approach modes may have replaced LNAV or VNAV. Confirm the annunciator after every selection.
• Limited aircraft modelling: some default or older aircraft display VNAV information but cannot fly the complete path automatically.

For Boeing-specific symptoms and corrective actions, use our 737 LNAV/VNAV fault checklist.

Why does VNAV not descend at top of descent?

VNAV will descend only when its profile is valid, the correct mode is active and the selected altitude permits a descent.

Before the calculated top of descent, set the lower altitude only when appropriate for the simulated clearance. Lowering the selector does not necessarily command an immediate descent: VNAV may wait for the calculated point, while some systems require an altitude-intervention or early-descent command. Our step-by-step 737 descent setup explains the usual sequence and transition towards the approach.

Are LNAV and VNAV the same as an ILS?

No. LNAV and VNAV normally follow FMS-computed guidance, whereas an ILS approach uses radio-based localiser and glideslope signals.

For an ILS, the aircraft normally transitions from LNAV or heading guidance to localiser capture, then from a descent mode to glideslope capture. Selecting approach mode at the wrong time, using the wrong navigation source or expecting VNAV to capture the glideslope are common simulator mistakes. See how simulated ILS glideslope guidance is generated and captured for that phase.

An approach chart containing an LNAV/VNAV minima line does not simply mean that pressing both cockpit buttons makes the approach valid. It refers to an approved combination of lateral and vertical guidance, often involving barometric VNAV or satellite augmentation depending on the procedure and equipment. Barometric VNAV also relies on the correct altimeter setting and may be subject to temperature restrictions that a simplified simulator aircraft does not reproduce.