PMDG 747 Autopilot won't maintain altitude

[dom8984 0]

I've returned to the PMDG 747 after a while enjoying other aircraft, but am experiencing a problem I have not seen before. Have spent the last day and a half reading these forums, it seems I'm not the only person who has experienced this issue. But none of the threads I read seem to identify a cause and fix.

 

I tried flights from London to Johannesburg and back. On both flights everything is fine for the first half of the flight, using VNAV and LNAV to fly the route. Then around the middle of the flight the autopilot suddenly fails to maintain altitude. It'll go into a steep climb of +2000ft/min and then into a steep descent, overshooting the selected altitude and then going into the steep climb again. The process is repeated until I disengage the autopilot, but if I reengage it continues, even if I select ALT HOLD instead of VNAV.

 

Also in the early part of the descent to the destination airport, I try to use V/S mode to descend but the aircraft won't maintain the selected descent rate. It goes from dropping uncontrollably out of the sky to almost levelling off, even when still well above the target altitude. The issue does seem to get better descending below about 25,000ft.

 

I use ASN weather but have tried deactivating this and clearing weather and the issue persists. I don't think it's related to weather. I load the aircraft using the PMDG loader and fuel via the in-sim PMDG menu. Can't think of anything I might be doing wrong - it's simply almost as if the aircraft becomes unstable as fuel is burnt off.

 

Is there a definitive fix for this, or can anyone offer any pointers?

 

Thanks,

 

Dominic

[scandinavian13 5,192]

Is there a definitive fix for this, or can anyone offer any pointers?

 

Full names - first and last - in the forum, please.

 

As far as the issue goes, do you have a hardware trim axis assigned?

[kevinh 409]

I tried flights from London to Johannesburg and back. On both flights everything is fine for the first half of the flight, using VNAV and LNAV to fly the route. Then around the middle of the flight the autopilot suddenly fails to maintain altitude. It'll go into a steep climb of +2000ft/min and then into a steep descent, overshooting the selected altitude and then going into the steep climb again. The process is repeated until I disengage the autopilot, but if I reengage it continues, even if I select ALT HOLD instead of VNAV.

Before the 747 started misbehaving did you use any FSX menu items that replace the FSX view with a blank screen with window, such as the FSX map or display options? Doing this will freeze the simulation and the 747 doesn't like this (I assume it's to do with communication with the external software that is part of the 747 simulation). When you close the menu and return to the flight things get a bit out of sync. VNAV in particular gets messed up and the engines can lose thrust leading to loss of altitude.

 

There is no fix except to avoid using FSX options menus and the FSX map when flying the 747. If it happens, reselecting the currect AP modes can help sort things out.

[dom8984 0]

Hi,

 

Thanks to both for your replies.

 

No change of views in the way you describe, so I don't think that's causing my problem.

 

I do however have a tab on my Saitek yoke set via FSUIPC to control the trim. Could this be causing a problem?

 

Thanks,

 

Dominic Rhodes

[scandinavian13 5,192]

I do however have a tab on my Saitek yoke set via FSUIPC to control the trim. Could this be causing a problem?

 

Could be. Get the plane off the ground, put it on the AP and then unplug your yoke. If the issue disappears, it's your yoke, or FSUIPC.

[dom8984 0]

Would deactivating the yoke as a control input in FSUIPC have the same effect? Pulling out USB devices in the middle of a flight has caused my FSX to crash in the past.

 

Otherwise I'll try your suggestion.

 

Thanks,

 

Dominic Rhodes

[scandinavian13 5,192]

Would deactivating the yoke as a control input in FSUIPC have the same effect? Pulling out USB devices in the middle of a flight has caused my FSX to crash in the past.

 

Eliminating the trim axis from FSUIPC would address the potential issue with FSUIPC. Check to ensure there is no axis assigned in the FSX options as well.

[dom8984 0]

Perhaps that was it. Have completed two flights now without experiencing the issue again. I will test more next week.

 

Thanks very much.

 

Dominic Rhodes

[dom8984 0]

Tested again this week, no problems. Seems resetting the controller calibrations in FSUIPC cured this issue. Thanks again!