Fix Boeing 737 LNAV or VNAV not working by checking FMC route activation, PERF INIT, MCP altitude, active legs, intercept geometry and FMA modes.
LNAV or VNAV usually fails in a Boeing 737 simulator because the FMC route is incomplete, the active leg is invalid, required performance data is missing, or the mode’s engagement conditions are not met. Check the FMA, not just the button lights: it shows what the autopilot is actually commanding.
In flight simulators generally, the exact logic depends on the aircraft. A detailed 737 add-on may reproduce Boeing engagement criteria closely, while a simplified aircraft may provide basic GPS tracking but little or no true VNAV. Confirm that the model actually supports both modes before treating their absence as a fault.
Fix LNAV and VNAV in this order
Work from the flight-mode annunciator and FMC rather than repeatedly pressing the LNAV and VNAV buttons.
- Read the FMA. The annunciations across the top of the primary flight display identify the active and armed autothrottle, roll and pitch modes. Detailed 737s normally show active modes in green and armed modes in white.
- Confirm aircraft capability. If LNAV or VNAV never appears on the FMA, check what that particular 737 model implements. A keyboard assignment labelled VNAV does not add VNAV logic to an aircraft that lacks it.
- Validate the FMC route. The route must be activated and executed, with a sensible active leg ahead of the aircraft. Remove unintended discontinuities and confirm that the FMC has a valid position. Our 737 FMC route-programming walkthrough covers activation, discontinuities and performance entries in detail.
- Complete PERF INIT. VNAV needs the required performance fields for that model. These typically include weight data, reserves, cost index and cruise altitude. Execute any pending modification after entering the values.
- Set the MCP altitude correctly. Select an altitude above the aircraft for a climb or below it for a descent. VNAV treats the MCP altitude as a limit and will not climb or descend through it.
- Create a valid LNAV intercept. If the aircraft is well away from the route, use heading select to approach the active leg at a sensible angle. Make a waypoint ahead of the aircraft active if the FMC is trying to return to a leg behind you.
- Verify the result on the FMA. Do not assume a mode engaged because its switch responded. The FMA must change to
LNAV,VNAV SPD,VNAV PTHor another expected mode.
Why won't LNAV engage?
LNAV will not engage when the FMC lacks a usable lateral route or the aircraft is not positioned to capture the active leg.
- The route was entered but not activated or executed.
- A
DISCONTINUITYexists at or immediately after the active leg. - The active waypoint or leg is behind the aircraft.
- The aircraft is too far from the route, or its intercept geometry is unsuitable for that model.
- The FMC position is invalid because the simulated inertial-reference system has not been aligned.
- An incompatible roll mode, such as a captured localiser, is already controlling the aircraft.
Detailed 737s may allow LNAV to be armed before take-off only when the runway and first departure leg meet the model’s geometry requirements. It can remain armed until after lift-off; that is not a failure. If it refuses to capture in flight, use heading select, establish a clean intercept towards a leg ahead, then arm LNAV again.
Why won't VNAV engage or start descent?
VNAV usually refuses to engage because PERF INIT is incomplete, or it remains level because the MCP altitude does not authorise a climb or descent.
Before take-off, inspect the CDU for boxed performance fields and scratchpad messages. An imported route does not automatically provide valid weight, cost-index, thrust and cruise-altitude data. The exact mandatory fields vary between 737 simulations.
During climb, the selected MCP altitude must be above the aircraft. During cruise, VNAV will not begin descending merely because a lower altitude exists in the FMC route; the MCP must also be set to a lower cleared altitude. If the aircraft has not reached the calculated top of descent, remaining level is normal.
If you pass the descent point while the MCP is still set to cruise altitude, the aircraft may become high on profile after descent is authorised. It can then show speed-related guidance or request drag rather than immediately recovering the path. Our step-by-step 737 descent and approach setup explains MCP altitude selection, path recovery and when to use LVL CHG or vertical speed instead.
FMA indications that identify the fault
The FMA distinguishes a genuine mode failure from normal armed, path and altitude-limiting states.
| FMA indication | Meaning and action |
|---|---|
LNAV | LNAV is active and tracking the FMC’s active lateral leg. |
HDG SEL active with LNAV armed | LNAV is waiting for acceptable capture geometry. Check the active leg and fly a better intercept. |
VNAV SPD | VNAV is using pitch to maintain its target speed. This is normal during many climbs and when recovering towards a vertical profile. |
VNAV PTH | The aircraft is following the calculated FMC vertical path. |
VNAV ALT | The aircraft has reached the MCP-selected altitude while VNAV remains engaged. Set the newly cleared altitude and use altitude intervention where the model requires it. |
ALT HOLD | Another vertical mode is holding altitude; VNAV is not controlling vertical flight. Check the MCP target and FMC data before reselecting VNAV. |
Does CMD have to be on for LNAV and VNAV?
No. LNAV and VNAV are flight-guidance modes and can command the flight-director bars while the autopilot is disconnected. Turn on the appropriate flight directors; engaging CMD simply makes the autopilot follow their commands.
Autothrottle is separate as well. VNAV may still calculate pitch and path guidance with autothrottle unavailable, but the pilot must then manage thrust manually. Poor manual thrust control can make VNAV appear unable to hold speed or path.
Can LNAV and VNAV fly the whole approach?
No, not every approach uses LNAV and VNAV all the way to the runway. An ILS normally requires localiser and approach modes for lateral and glideslope capture, while a supported RNAV procedure may use LNAV laterally and VNAV path guidance vertically.
Procedure coding and aircraft capability matter: some RNAV approaches provide lateral guidance only, and simplified 737s may not model vertical approach guidance. See our explanation of GPS and RNAV approach guidance, active legs and vertical limitations before assuming the APP button should capture an FMC path.
Immediate mode cancellations and simulator faults
A mode that appears briefly on the FMA and then disappears is more likely being cancelled by another command than blocked by incomplete FMC data.
- Check for duplicate keyboard, joystick and hardware assignments sending heading, altitude-hold, vertical-speed or approach commands.
- Do not press mutually exclusive MCP modes in rapid succession; the last valid selection normally replaces the earlier one.
- Rebuild a short route manually if an imported plan contains duplicate waypoints, vectors or unresolved discontinuities.
- Start from a standard aircraft panel state if the problem began after loading a saved flight.
- Temporarily remove optional avionics or aircraft modifications that may conflict with the 737 model.
If LNAV and VNAV work in a clean flight with a short manually entered route, the aircraft logic is functioning. Add the original route, controller profile and optional modifications back one at a time to identify the conflicting element.