Unusual flight control problem

[Summoner 0]

I was attempting to fly a cargo flight from Bangkok to Tokyo today in FSX, when I ran into an odd flight control issue that I have never seen before (though I certainly can't say that I have seen them all). Please keep in mind that this was only my 5th flight with the PMDG 747 (another awesome product, btw!), so the problem could very well have been me. Let me describe what happened...

 

As I was approaching my initial cruise altitude of FL330, I noticed that the IAS was about twenty knots slower than the 331 knots the FMC had calculated for the climb (it got a little bit closer when the FMC switched over to Mach, but it was still slow). I thought that was a little odd, but I wasn't in danger of stalling the aircraft so I let it be. What started ringing alarm bells in my head (and in the cockpit) was when the autopilot blew right through FL330 and kept climbing. I turned LNAV off and switched to LVL CHG to get back to FL330, and the autopilot very slowly stopped its climb and began a descent back to the correct altitude. I started poking around to find out what was wrong, when I noticed on the EICAS STATS page that the elevator was deflected full nose-down. I immediately checked the trim setting, and it was in the green arc, set to 4.8 units. I disconnected the autopilot and bumped the elevator, which immediately reset the elevator to neutral and gave me a huge climb due to the trim. I manually re-stabilized the aircraft at FL330 and trimmed out the elevator needed to maintain altitude, then re-engaged the autopilot. The autopilot immediately re-trimmed to 4.8 units, which sent my altitude crazy again (and disconnected the autopilot) until it got back to FL330, but with full nose-down elevator.

 

I returned to Bangkok, and kept an eye on the trim and elevator status the whole way back. The entire descent, approach, and landing, the autopilot kept the trim set to 4.8 units, and the elevator gradually decreased its nose-down deflection as the speed decreased.

 

I am just wondering what may have gone wrong for the autopilot to always use the same trim setting regardless of airspeed, and deflect the elevator to compensate. Is it something that I may have done (or neglected to do)? The trim setting used by the autopilot is not the one that I set for takeoff, which was 6.0 units. Could it be some kind of malfunction with my hardware? I did see an FSX pop up dialog message when I started the simulator asking if I wanted to run an .exe file that I didn't recognize, so I said no, but that turns out to be something related to my X52 Pro joystick, which I reinstalled the drivers for yesterday. The joystick appeared to be functioning normally throughout the flight, though. I also did not have any failures set to occur in the PMDG 747.

 

I am not sure when I will have time to try to reproduce this issue. Since I am no longer going to be flying for a few hours, I am going to be working on chores around the house. I am curious to know if anyone has experienced a similar event though, and any ideas regarding what I can do to prevent it from happening again.

[PHXMD-11 86]

 

 

I returned to Bangkok

 

Very sound judgement.

 

 

 

any ideas regarding what I can do to prevent it from happening again.

 

Are you loading the aircraft via the PMDG load manager or the default FSX manager? If it's the latter, there may be problems with the aircraft.

[Summoner 0]

Summoner, on 21 Sept 2013 - 12:33 PM, said:

 

I returned to Bangkok

 

Very sound judgement.

 

Well, it was that or continue the flight with an unknown problem with a flight control that could potentially get worse, and since I am not paid to be a test pilot, the choice was an easy one.

 

All of the fuel and payload were loaded via the PMDG 747 load manager. I read the warnings about using the built-in FSX payload and fuel utilities, and since I prefer to have my aircraft functioning properly, I make it a point to use the utilities provided by PMDG.

 

On a side note, I had the opportunity to perform a properly modeled 747 fuel dump on this flight, which was something of a positive. Maybe I am a little bit odd for thinking that, I dunno. 

Edited September 22, 2013 by Summoner

[gregh747 0]

Were you able to reproduce the problem. My PMDG- 747-400 is doing a similar thing.

It will not follow the flight director in a similar way as you described and then bust through the cruise altitude and keep climbing. Pressing FLCH starts it to return to the FMC and MCP cruise altitude setting, and then it can just just climbing again on its own. I also found that V/S set to negative (decent) and it will continue to climb. Further along in the flight it seems to settle down and start responding holding VNAV and following altitude commands as it should.

There has been a few (but rare) posts but no resolution I have found so far I have seen.

I have also noticed on the stats page the elevator right up one end, as you described, only by switching off AP, does the elevator return to normal, but you have to re-trim a lot after disconnecting the AP. As mentioned in other forums I used the PMDG load manager first.

[Martin Connor 0]

Can I ask if you have both just updated your Airac Data?  I only ask as I am having weird problems as well, and just installed airac 1309.

[Martin Connor 0]

After my recent experience, I'd look closely at the loading of the aircraft.  Badly distributed cargo perhaps?