Problem with VNAV

[belial_p 0]

Hi,Ive been using the 744 for quite a while now and for the past days, I have been confronted to something I dont understand. After takeoff, when I activate the AP, the plane usually follows the plotted course without any issue until I land. Now I have re-installed my FSX on a dedicated drive and thus re-installeld the PMDG 747-400. Everything works smoothly except for that. Before take off, I arm FD, A/T, set V2 speed in the the IAS, set the rwy heading and the alt in the MCP. I set the thrust to 70%, hit the TO/GA button and takeoff, then I usually activate the AP at around 1000-1500ft and LNAV/VNAV go green on the PFD. But those days when I do so, the plane seems to start following the course right until it reaches the planned cruise altitude for the flight, instead of gently reducing thrust and stabilizing the plane at the altitude given in the MCP, the plane keeps going. And going, it just wont stop climbing. The only solution I found to the issue, is to disengage AP, rearm VNAV and start AP again, then it finally follows the pollted course at a normal altitude. I have checked all my wayppoints carefully and they all seem to be normal. As usual.I use FSC 8 to design my plans but i usually enter them manually into the FMC. I have done that for quite some time without any problem until now. Am I missing something? I have had that on three different flight plans so far. BTW, the descent and landing go without any problem its just that the plane doesnt take its planned ALT when it should on climbing. Can someone help me plz? Im sure Im doing somethign wrong cause it worked like a charm before. :( I have re read the manual and cant figure out where I mess up.PS : I re-installed the plane twice without any result.Regards,Peter

[Shmiffy 0]

Hi,Ive been using the 744 for quite a while now and for the past days, I have been confronted to something I dont understand. After takeoff, when I activate the AP, the plane usually follows the plotted course without any issue until I land. Now I have re-installed my FSX on a dedicated drive and thus re-installeld the PMDG 747-400. Everything works smoothly except for that. Before take off, I arm FD, A/T, set V2 speed in the the IAS, set the rwy heading and the alt in the MCP. I set the thrust to 70%, hit the TO/GA button and takeoff, then I usually activate the AP at around 1000-1500ft and LNAV/VNAV go green on the PFD. But those days when I do so, the plane seems to start following the course right until it reaches the planned cruise altitude for the flight, instead of gently reducing thrust and stabilizing the plane at the altitude given in the MCP, the plane keeps going. And going, it just wont stop climbing. The only solution I found to the issue, is to disengage AP, rearm VNAV and start AP again, then it finally follows the pollted course at a normal altitude. I have checked all my wayppoints carefully and they all seem to be normal. As usual.I use FSC 8 to design my plans but i usually enter them manually into the FMC. I have done that for quite some time without any problem until now. Am I missing something? I have had that on three different flight plans so far. BTW, the descent and landing go without any problem its just that the plane doesnt take its planned ALT when it should on climbing. Can someone help me plz? Im sure Im doing somethign wrong cause it worked like a charm before. :( I have re read the manual and cant figure out where I mess up.PS : I re-installed the plane twice without any result.Regards,Peter

Hi Peter:Reading your post i think i know what you are trying to achieve, but i may be wrong so please correct me if i am heading off on a tangent.My understanding is that in your PERF page you have set a Cruise Altitude, (lets assume FL310) and after departure you want the MCP to constrain that climb to FL120 or some such height?To achieve this there are two methods. I will walk you through the method i would use, which is loosly based on the real world BAW SOP's.Having set the route up and the FMC up correctly (you seem to know what you are doing with that so i wont teach you to suck eggs) you can ARM LNAV, VNAV, and AutoThrottle prior to push, you may get an UNABLE NEXT ALTITUDE annunciate on the FMC, it's basically the aircraft deciding that you are facing the wrong way and not moving fast enough to climb 4000 feet in the next 30 miles. Ignore it.Once you have completed your checks, pushed, taxied, and so on. You can line up, increase power and set TOGA so that you see THR REF annunciate in the PFD. Above 400' AGL, you can engage the AP, and LNAV & VNAV Shall annunciate their roll mode. At this stage, the aircraft will climb based on the calculated VNav Path. If you have any hard or soft restrictions on your legs pages ( /2000A or /2000B or /2000 or /1000A3000B for example) the aircraft will adhere to those where it can. Likewise, with an altitude restriction on the MCP, it should respect that, and climb no higher than the restriction in the MCP ALT Window.When you want to climb higher, or are cleared higher, you can push in the MCP Altitude Adjust Knob, and the Aircraft will continue its climb to the new level set.If it is not performing as expected when carrying out the above, then you have an installation or PC Problem i would suspect.CheersPaulEdit: If the aircraft does appear to want to continue climbing past the limiting altitude that you have set in the MCP, double check that you are not in V/S Mode, and just hit the VNav button again. You should see VNav Path or VNav Alt annunciate on the PFD.

[basa 0]

Peter, a couple of screenshots of the situation would be very helpful.

[belial_p 0]

Ok, thanks for the help Paul I'm gonna look into that. BTW, yes, I am limiting the CRZ ALT of the FMC by using the MCP. Thats how I've always done since I dont master it yet and cant understand why it keeps setting plans much higher than what I ask as a cruise alt. Maybe, cause of the airways imposed alts? I have no clue but so far it never prevented me from flying at the lvl I wanted. As for the screenies Jose. I am working on it ! :(Also, I faced the same issue with both 400 and 400F planes, Im a bit confused as I have been flying those for quite some time without problem so far.Anyway, I will let you know the results of my test. :)

[belial_p 0]

Ok here are the different states for A/T, ROLL and PITCH:At rotating point : HOLD | LNAV (green) | TO/GA (green) VNAV (white)If I understood the manual correctly, thats perfectly normal.During climb to cruise alt I get the following states, which also seems normal to me. The plane is adjusting pitch roll and thrust to keep to the course at the programmed speed it seems :THR REF | LNAV | VNAV SPD (all green)Right before I reach T/C the PFD still shows the same states. Upon reaching the cruise alt, the different states change to what I believe they should be :Yet it keeps climbing... (I think all indicators show it should be cruising by now, including the CRZ sign in the 2nd EICAS.)And climbing...Regards,Peter

[des182001 1]

When you set the Altitude of 30,000 on the MCP did you push the Knob after?editI see that Paul mentioned this above....

[basa 0]

The aircraft is not able to follow the speed (M0.849) because is climbing at an excessive ROC for that altitude and for that fuel load, and the A/P is not able to follow the F/D bars.Seems like your elevators are blocked in the up position. :(

[Cuddly1956 1]

If you load the 747 almost empty it will overshoot the altitudes - it's a verrry powerful plane. If you're light just use v/s and a large derate to control it.John Ellison

[belial_p 0]

When you set the Altitude of 30,000 on the MCP did you push the Knob after?editI see that Paul mentioned this above....

Well I only press that knob to change alt. Usually when I define it from the ground its at 0 and so no I dont press it. But I will try pressing it after that. Thanks.

The aircraft is not able to follow the speed (M0.849) because is climbing at an excessive ROC for that altitude and for that fuel load, and the A/P is not able to follow the F/D bars.Seems like your elevators are blocked in the up position. :(

Aha, and how do I fix that?

If you load the 747 almost empty it will overshoot the altitudes - it's a verrry powerful plane. If you're light just use v/s and a large derate to control it.John Ellison

I set it to 1/3 full via the PMDG fuel manager. Thats my usual configuration as I dont usually fly more than 2-3 hours in a row, so I should have had that before. hmmm weird. :( Anyway, thanks for the answers. :( Regards,Peter

[des182001 1]

1/3 fuel should be fine as I do that for short flights and have no issues. What's your cost index set to?

[belial_p 0]

1/3 fuel should be fine as I do that for short flights and have no issues. What's your cost index set to?

I figured I'd better post all my FMC config :So, to answer your question my cost index is 200. :)Hmm for this flight I had 2/3 full, but the problem exists when i use 1/3 as well I'm sure.Regards,Peter

[Shmiffy 0]

Hi Peter:Nothing looks too far out of whack with your FMC Setup. If you are an American i can understand why you would be using Lb's as your weight setting, but if you are European it might be easier for you to use Kg's. That's settable in the PMDG Options.200 is a very high cost index, that doesn't appear to make too much of a difference in speed. At BAV we use the same settings as BAW. 0 for the Climb, 90 for the Cruise and end up with about .84 as a standard cruise. For trips from the west coast US to UK we sometimes use 0 as a CI in the cruise as well. Also you could set your Step Climbs to 2000 when in RVSM Airspace rather than ICAO (which is 4000 If I recall Correctly).Fuel on arrival of 178.2 seems high. Real world, i beleive BAW are happy with about 10Tonnes (36000 Lb's ??) on arrival at LHR with Gatwick as an Alternate. That would help slow down your landing speeds a lot. I usually arrive with about 10-15000 Kg on board and see landing speeds with Flaps 25 of about 140 ish Knots.Specifics relating to your problem though, i can see nothing obvious. Everything appears to be set correctly, so provided your MCP and Autoflight Functions are setup correctly, theres no real reason to see the problems you are seeing.CheersPaulOh- One last thing, rather than setting the altimeters to standard pressure by turning the Altimeter Set knob, you can just press in the STD button. In Europe we use Milibars rather than Hpa too ;)Edit: Another item CRZ on the upper Eicas is merely the current Thrust Mode, Although that said, it does indeed activate in the cruise. For BAW in the real world, they default to CLB thrust in Cruise, and you may occasionally want to switch to CON thrust with particularly strong tail winds.

[basa 0]

In Europe we use Milibars rather than Hpa too ;)

Ooops, He is using Inches of mercury. Hpa (Hecto Pascals) is the new name that, stupidly, someone gave to what always was called Milibars...Same thing with Newtons and Kilos. Who knows his weight in Newtons ??And about the elevators being blocked, I remember someone in this forum (months ago) having that problem caused by his joystick.

[Shmiffy 0]

hehehe. :( Thou shalt not post before coffee first thing in the morning hehehe. Thanks for picking that up! :( CheersP

[Paul_Smith 92]

I figured I'd better post all my FMC config :

Can you post your flight plan (with any height restrictions) as well please./Paul.