VNAV not controlling speed

[steeleimaging 0]

I must be missing something. I have autothrottle on. I have a perfect route set in fmc.When I engage VNAV, it does not follow the plan.Thoughts?

[yos233 1]

Perhaps a picture. What are your exact steps after takeoff?

[martybuzz 2]

VNAV is for your vertical profile. LNAV is for your lateral navigation. When you arm VNAV before take off, does it illuminate? F/D (Flight Director) armed? Your FMA's (Flight Mode Annunciation) on your PFD should show blank, TOGA, TOGA with LNAV and VNAV in grey beneath, meaning they are armed. A/T (Autothrottle) armed?Buzz

[steeleimaging 0]

Hey Buzz... Thanks for the reply.Yes. VNAV illuminates before I throttle up. FD is armed. FMA has grey VNAV and LNAV beneath both TO/GA. AT is armed.I have a good flight plan, FMC speeds are set. VNAV in FMC is specified. TRAN and RESTRICTIONS are set. I just keep doing it over and over. with all of the above set... I throttle up to 70... press me TO/GA screw... and it just keeps accelerating way beyond my 250/12000 limit. It goes up to 290... then I start over. It doesn't even pay attention to the 250kias below 10k restriction.

[FLEX1978 373]

Hey Buzz... Thanks for the reply.Yes. VNAV illuminates before I throttle up. FD is armed. FMA has grey VNAV and LNAV beneath both TO/GA. AT is armed.I have a good flight plan, FMC speeds are set. VNAV in FMC is specified. TRAN and RESTRICTIONS are set. I just keep doing it over and over. with all of the above set... I throttle up to 70... press me TO/GA screw... and it just keeps accelerating way beyond my 250/12000 limit. It goes up to 290... then I start over. It doesn't even pay attention to the 250kias below 10k restriction.

A screenshot of your FMA would help us easily identify any errors.Regards

[Paul_Smith 92]

... It doesn't even pay attention to the 250kias below 10k restriction.

250 below 10k is a request not a demand. A loaded 747 can not climb safely below 250 and the FMC will not try to force it. ATCs will always grant minimum safe climb speed even if it is well above 250.

[steeleimaging 0]

Ok, here is my setup...

[Paul_Smith 92]

Ok, here is my setup...

Setup looks good, but can you also post a screen shot from when you think the VNAV is not doing what you think it should.

[steeleimaging 0]

You bet. After I get done with work, I am on it. Thanks for your help!

[steeleimaging 0]

OK. I know that this shows a "close' speed... but after I took the shot, the plane kept on acceleration to 250. I was hoping the engines would spool back and the plane would slow. But, it didn't.

[yos233 1]

You dont have the autopilot on. Click one of the three buttons in the upper-right corner. You have to engage the overall autopilot for VNAV and LNAV to work.

[steeleimaging 0]

I dont think that it actually right. Auto throttle shouldn't be dependent on the Autopilot.

[yos233 1]

Half right. All automatic features require one of the three autopilots on, no matter what. But, you can have the SPD or THR button pressed without pressing anything for lateral or vertical navigation. You can also have the VNAV button on (as you do in the picture), but you will need a steering feature of some type on, because VNAV needs to know where you are going to provide an accurate climb.In the FSX default CRJ, and maybe the 737, that was true. If speed hold was on, you didn't need the master autopilot. But you do in the 747.

[scandinavian13 5,192]

Since the MCP is currently using VNAV as its reference, what it is doing is hanging the aircraft on the engines, essentially. In more proper terms, the engines are run on climb thrust (as set before takeoff) and the speed is adjusted with pitch. Since the aircraft is clearly not on the autopilot and is definitely not on the FD cue, it is not on the correct speed. So, essentially, the AT is independent of the AP, but the current AT mode is reliant on the correct flight path angle to yield the proper result.

[Tabs 2,790]

Dave,I see two major mistakes in your screenshot:1. The airplane's flaps-up clean speed in your shot is precisely... 223. You're asking it to fly below clean speed at 210. It's smarter than to allow you to do that or you might end up stalling at higher angles of attack. The plane will not fly below clean speed with the flaps up in VNAV. Clean speed can be above 250 knots even if the airplane is heavy - this happens all the time in the real world and they get the 250/10000 restriction waved by ATC. What departure procedure are you doing that has a 210/12000 restriction anyway? That's pretty extreme and I've never seen or heard of that in the real world. 2. I think you're misunderstanding how VNAV works in the climb. VNAV SPD (unlike elements of the VNAV PTH mode used in descents) is primarily a "pitch for speed" mode, similar to FLCH. This means essentially that the A/T commands a fixed thrust setting (the calculated CLB-2 mode's N1 limit in your case) and then the flight director varies pitch commands in order to maintain the specified speed. It doesn't care about rate of climb, it's just going to pitch at whatever angle produces that speed - the required angle and resulting rate of climb will change as you get higher, becoming more and more shallow in order to maintain that same speed. In this shot you have the autopilot off and you're not manually following the FD - you're pitched below what it's commanding, which will result in you going faster than the VNAV speed target.