PMDG 747-400 Autopilot problems

N

Nareg Haladjian

Users Local time
Today 11:18 PM
Hi. I have encountered a problem on a recent flight with the PMDG 747-400 and had to stop the flight because it was driving me mad. This flight was from KJFK to FACT, which is an extremely long flight. I started up from Cold and Dark and entered all the FMC information. The problem was that the autopilot would not follow the FMC's set speeds in the LEGS page. Also, the cruise altitude was very low on the legs. It was set to 9882 feet even though I set FL320 as my cruise altitude.

The speed was set to 260 in the FMC and I had RNAV and LNAV enabled, along with left autopilot. The problem was that I was going 345 while flying at 10000 feet. I stopped the flight after that and tried it again from the beginning. I encountered the same problem and stopped the flight again. I was worried it wasn't going to be able to detect my approach and landing speed.

I was wondering if anyone could provide some help with this. Any suggestions could be helpful. Thanks :).

Extra flight information:
Departure Airport: KJFK
Departure Runway: 13L
Destination Airport: FACT
Selected runway: 01L (I believe)
Airplane: PMDG 747-400 with Rolls-Royce Engines
Plane was loaded with full fuel.
FMC double-checked before take-off
Took off with 10 degree flaps.
Used TO/GA mode for take-off (pushed N1 past 70%, then enabled TO/GA)
Altitude Bug set to 10000.
 
Hi Nareg,

as I understand: you've selected 10 000 altitude (altitude bug) on your MCP. The plane will not climb above the altitude selected on MCP, no matter what you've programmed in FMC/CDU. In order to climb further, you need to select a new altitude on MCP (let say your FL320 as in FMC) and click altitude intervention. BTW, as you are eastbound, you should have odd flight level (except for oceanic, which can give you even clearance).
Have you also set the speed on MCP? It is normal that the plane has to accelerate after 10 000 ft (there is usually a speed limit of 250 under 10 000) in order to be able to keep climb profile. If you want a hard speed limit (like 260), you need to set in on MCP as a speed intervention OR set appropriate speed restrictions on FMC/CDU.
All above can be, of course, done while on LNAV / VNAV.
Did you get any messages while programming CDU line UNABLE CRZ ALT? In such a case, program lower flight level and make step climbs later. If you don't and simply CLR that message, the FMS will not be able to calculate correct VNAV profile. You should always refer to OPT and MAX altitudes on VNAV page.

Also, due to a long route and high TOW, you may need to look at your climb profile as it probably be necessary to use step climb to you final cruise FL. How much fuel did you have? For that route, without considering winds, you would need about 377K lbs with reserves.

I hope I am being clear here.

Regards

Jan
 
Jan Betlach said:
Hi Nareg,

as I understand: you've selected 10 000 altitude (altitude bug) on your MCP. The plane will not climb above the altitude selected on MCP, no matter what you've programmed in FMC/CDU. In order to climb further, you need to select a new altitude on MCP (let say your FL320 as in FMC) and click altitude intervention. BTW, as you are eastbound, you should have odd flight level (except for oceanic, which can give you even clearance).
Have you also set the speed on MCP? It is normal that the plane has to accelerate after 10 000 ft (there is usually a speed limit of 250 under 10 000) in order to be able to keep climb profile. If you want a hard speed limit (like 260), you need to set in on MCP as a speed intervention OR set appropriate speed restrictions on FMC/CDU.
All above can be, of course, done while on LNAV / VNAV.
Did you get any messages while programming CDU line UNABLE CRZ ALT? In such a case, program lower flight level and make step climbs later. If you don't and simply CLR that message, the FMS will not be able to calculate correct VNAV profile. You should always refer to OPT and MAX altitudes on VNAV page.

Also, due to a long route and high TOW, you may need to look at your climb profile as it probably be necessary to use step climb to you final cruise FL. How much fuel did you have? For that route, without considering winds, you would need about 377K lbs with reserves.

I hope I am being clear here.

Regards

Jan
Click to expand...

Thank you for your reply. For the altitude, I originally entered FL410 in the FMC. It gave me a warning saying MAX ALT 347. I decided to change it to FL320. While climbing, I realized the 6762NM leg (assuming the cruise leg) was at 9882. I set the altitude on the MCP to 10000. When it reached 10000, its speed automatically went up to the point where it cannot go any higher. I have never encountered this problem before because on previous flights (normally from KLAX to KSFO)it listened to the FMC's speed and the cruise level.

But what I realized at the VNAV page of the FMC was this. (I will try and type it out how I saw it.)

CRZ ALT-------AT JFK
FL320-------5000A
ECON SPEED------- ERROR
334 / .841-------4560 LO
SPD TRANS------- TRANS ALT
279 / 10000-------180000
SPD / REST-------MAX ANGLE
---/---- -------290

The lines that I put in bold are ones that I don't understand. What am I supposed to enter for that? The second thing I don't understand is the error it is giving me. What does that mean?

I have set the fuel to full through PMDG option for this flight (382.6K fuel). Now my main concern is the error it is giving me. Thanks :D.

EDIT: I just tried the take off again and I got a FMC message saying UNABLE NXT ALT right after leaving the ground and enabling LNAV. What do I have to do for that?
 
Nareg,

First of all I do not understand what you mean by the "6762 NM leg". What was your KJFK - FACT route?
To enter an altitude/FL at MCP is usually just to comply with ATC altitude instruction. Otherwise (when flying offline) you can enter your cruise FL in MCP.
Your VNAV should be programmed before TO without any errors.

The error you've mentioned - UNABLE NXT ALT - is consistent with the error from the VNAV page you showed. There is an altitude restriction to be above 5000 ft at your next waypoint, which the aircraft (probably due to weight) cannot achieve. Therefore you get this error message and its description on the VNAV page. You can just delete the restriction at the appropriate waypoint (well in case there is not terrain obstacle at or before that waypoint which requires you to be above 5000 ft).
SPD / REST just give you the opportunity to enter custom speed restriction at an altitude. Usually you leave this empty (as you had it on the VNAV page).

Regards

Jan
 
Nareg Haladjian said:
Jan Betlach said:
Hi Nareg,

as I understand: you've selected 10 000 altitude (altitude bug) on your MCP. The plane will not climb above the altitude selected on MCP, no matter what you've programmed in FMC/CDU.
Click to expand...

I set the altitude on the MCP to 10000. When it reached 10000, its speed automatically went up to the point where it cannot go any higher.
Click to expand...

As Jan suggested, and you confirmed, it seems like you had 10000 set on the MCP. Once the aircraft reaches this, the fmc knows it has passed the 250k speed restriction, so the speed increases.

However no matter what is set in the fmc or if the vnav is on, the plane will not climb above 10000 until you change that on the MCP.

When I fly offline, (always) I just set the MCP Alt to my cruise level, and let the aircraft climb at it's own rate.
 
Hi all posts above are Absolutaly correct and one important tool it is manual . Pmdg 747 400 its realy nice plane to fly as long as you know all its capability and settings without that you will not enjoy it fully . With sugested cruise alt in FMC remember whats been said before you have to always chack what is set in MCP window. However if you lets say set your cruise first at FL 300 as your gross weight might be high for long flights and you want to make a step climb ,you can do it in two ways .
First change higher alt for example current FL 300 you dial new FL 320 in MCP window and press alt nob. It will change your cruise alt from FL300 to FL320 in fmc and plane will perform step climb . also you mention speed restriction as 267/10000 . this normaly happend with higher gross weight. It is only a sugestion to what minimum speed you should acelerate if you decide clean up flaps ,but if you need to stay 250/10000 you simply in vnav fmc place your own speed restriction ,perhaps due to ATC ... Remember this restriction only comes in play if you are using vnav speed . it meens when you press vnav nob the window next to your speed in MCP wont shows any number you simply using speed preset in FMC,however should you comand less/more speed without changing anything in FMC you simply if in VNAV press speed nob and in MCP windos for speed it will shows you speed then you can just dial speed you desire . Keep in mind your flaps settings . If you need any help please do ask Cheers Petr
 
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