Why VNAV Doesn't Work When You Turn On the Autopilot

You pressed VNAV and nothing happened. Or it engaged for half a second and then kicked off. Or it's been engaged for ten minutes and the aircraft is just sitting at cruise altitude refusing to descend, while the top of descent marker disappears behind you.

VNAV problems are maddening because the button is lit, the mode is supposedly active, and the aircraft just won't do what you expect. The good news is that it's almost always one of a handful of causes.

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Half the fixes below happen in the FMC. Practise those pages hands-on in our free 737 CDU Trainer — Airbus pilots, use the A320 MCDU Trainer.

Boeing: PMDG 737 and 777

VNAV on a Boeing does two things: it manages your climb profile after takeoff, and it manages your descent profile on arrival. Most problems happen during descent, because that's where the system needs more from you.

The PERF INIT page isn't complete

This is the number one hidden cause and it catches everyone at least once. On the PMDG 737, if the reserve fuel field on the PERF INIT page is left blank, VNAV will not engage at all. The FMC can't compute a profile without knowing your fuel reserves.

Go to the CDU, press PERF INIT, and check that every field has a value — especially ZFW, reserves, cost index, and cruise altitude. If any of them are blank or show dashes, fill them in and press EXEC. That orange EXEC light needs to be dark before VNAV will work. Any pending executions block it.

The MCP altitude window is wrong

This is the "VNAV won't descend" problem, and it confuses everyone the first time. Here's the rule: VNAV will never descend the aircraft below the altitude set in the MCP window. Period. It's a hard limit.

So if you're at FL370, the MCP still shows 37000, and you want VNAV to follow the descent profile down to FL240 for a STAR constraint — it won't. It'll just sit there at FL370 because you haven't given it permission to go lower. You need to dial FL240 (or lower) into the MCP altitude window before reaching the top of descent.

Think of the MCP altitude as a floor that VNAV can't break through. The FMC has the whole descent path planned, but it needs you to keep lowering that floor as you descend. ATC clears you to FL240? Dial it in. They clear you to 10,000? Dial it in. VNAV handles the how, but you control the how far.

No altitude constraints in the flight plan

Open the LEGS page and look at the right side — the SPD/ALT column. If the waypoints on your STAR don't have altitude or speed restrictions next to them, VNAV has nothing to build a descent profile from. It's just guessing.

This usually means the SID/STAR wasn't loaded properly, or you loaded a route without selecting an arrival procedure. Go to DEP/ARR on the CDU, select your STAR and approach transition, and the constraints should populate automatically.

VNAV PATH vs VNAV SPD — it's not broken, it switched modes

Check the FMA at the top of the PFD. If it says VNAV PATH, the aircraft is following the computed geometric descent path — altitude takes priority. If it says VNAV SPD, the aircraft has switched to speed-priority mode, which means it's pitching to hold a target speed rather than following the planned path.

VNAV SPD usually kicks in when you're too high or too fast for the path. The FMC is saying: "I can't follow the planned descent angle at this speed, so I'm going to prioritise not overspeeding instead." If you also see DRAG REQUIRED on the CDU, the FMC is telling you to deploy speedbrakes. It needs more drag than idle thrust alone can provide.

This isn't a malfunction. It's the system doing exactly what it should when the energy state is wrong. Deploy the speedbrakes, let the aircraft get back on profile, and VNAV PATH will re-engage.

Both flight directors need to be on

On the PMDG 737, both FD switches (left and right) should be in the ON position. If only one is on, VNAV may not behave as expected. It's easy to miss during startup — the switches are small and they don't look obviously wrong when only one is flipped.

You changed the approach setup during descent

Known PMDG quirk: if you modify the landing configuration on the APPROACH REF page (flap setting, approach speed) while VNAV is actively managing a descent, it can disconnect. Set your landing config before starting the descent, or at least before you're on the profile. Making changes mid-descent confuses the path computation.

Airbus: Fenix A320 and FlyByWire A32NX

The Airbus doesn't have a VNAV button. Vertical navigation is managed through the FCU altitude knob and the concept of managed vs selected modes — same push/pull philosophy as lateral navigation.

Push vs pull on the ALT knob

This is the single biggest source of confusion for Airbus beginners.

• Push the ALT knob = managed descent (DES mode). The FMGC follows the computed descent profile, respecting all altitude and speed constraints from the STAR. This is the Airbus equivalent of VNAV.
• Pull the ALT knob = selected mode (OP DES or V/S). The aircraft descends to whatever altitude you've dialled in the FCU window, ignoring all STAR constraints. This is like pressing FLCH or V/S on a Boeing.

The mistake: pulling when you should push. If you pull the knob at top of descent, you get Open Descent — the aircraft goes to idle thrust and dives for the selected altitude with no regard for crossing restrictions. ATC will not be impressed.

Managed descent needs NAV mode active

Here's one that's easy to overlook: managed descent (DES mode) only works properly when the aircraft is also in NAV mode laterally — meaning the FMGC is flying the flight plan. If you're in selected heading mode (HDG pulled), managed descent won't engage or will behave unexpectedly.

The logic makes sense: the FMGC needs to know where you're going to calculate when to descend. If it doesn't control the lateral path, it can't predict when you'll reach the waypoints with altitude constraints.

Descending before top of descent

If you push the ALT knob before the aircraft reaches the computed TOD, the Airbus doesn't just start an idle descent like you might expect. Instead, it descends at a constant vertical speed to intercept the planned descent path from above. This can look weird — the aircraft descends slowly, then levels off briefly, then follows the profile normally.

It's not broken. It's intercepting the path. But if you were expecting an immmediate idle descent to your target altitude, it'll seem like nothing is happening.

Quick Checklist

When VNAV or managed descent won't work, check these:

1. Boeing: is the PERF INIT page complete? ZFW, reserves, cost index, cruise alt — all filled in, EXEC pressed
2. Is the MCP/FCU altitude set correctly? It must be at or below your next target altitude
3. Are there altitude constraints in the STAR? Check LEGS or F-PLN page
4. Boeing: VNAV PATH or VNAV SPD? If SPD — deploy speedbrakes, you're too high
5. Boeing: both flight directors ON?
6. Airbus: did you push or pull the ALT knob? Push = managed, pull = selected
7. Airbus: are you in NAV mode laterally? Check the FMA

Most of the time it's item 1, 2, or 6. The PERF INIT page is incomplete, the altitude window is too high, or you pulled when you should have pushed.

Hardware That Helps

An autopilot panel makes altitude management much more intuitive — physically dialling altitudes and pressing mode buttons beats clicking around the MCP with a mouse, especially during a busy descent. The Honeycomb Bravo Throttle Quadrant includes a built-in autopilot section with ALT, VS, HDG, NAV, and IAS controls. Dialling in a new altitude is a quick twist instead of a scroll-wheel battle on screen.

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For the full deep dive on descent planning and energy management, see the VNAV descent guide. If your lateral navigation isn't working either, check the LNAV troubleshooting guide.