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Guy014

Autopilot cannot maintain heading in strong winds

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All - I was doing a flight in the 737-800 winglet variant from KMDW > KLAS. Halfway through the flight I was getting 70 knot headwinds at FL380. The flight was conducted on VATSIM with Active Sky weather. I was under MCP SPD , LNAV , ALT HOLD the whole way (Southwest flight) and at one point the autopilot couldn't keep its heading. It began banking steeply to one side and didn't correct. The flight director showed the proper action to take to correct and the control wheel wasn't correcting either. No control surfaces were interfered with and the autopilot was on the entire time. Why couldn't the autopilot correct or maintain heading in these strong winds? I'll also mention they were varying between +/- 5 degrees and +/- 5 knots very quickly. I took control and got back on track when I engaged LNAV and the autopilot again. For a little while it was on heading and it began to lose control again. So I took her manually, flew her for a few miles and the winds settled down to a steady 70 knots. The rest of the flight went unaffected with the autopilot on. As well, no warnings or cautions appeared. Any help would be appreciated.


Erik L.

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Hi,Strange, I already experienced winds > 90kn, with no AP disco. Maybe in the options of the NGX ?


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The manual for PMDG does recommend adjusting the weather behaviour settings in FSX. You should also enable the 'smoothing' option in FSUIPC if you are running it. Both have caused me to have unexpected and unrealistic reactions to the weather, like continuous adjustment turns along a track, or constant buffet alerts at higher flight levels even though there's little wind reported. The NGX is a bit 'too' smart for the rather basic weather. If you're running ASE the same probably applies. I run REX2 and I had to set the smoothing in FSUIPC to sort my problem out.As to running in LNAV + ALT Hold, I found problems doing this. I had a buffet alert and it disconnected CMD and tried to throw itself out of the sky. I had a chat with a friend who is an Easyjet pilot and he said to use VNAV. The VNAV allows the FMC to manage the climb and cruise, adjusting power accordingly and compensating. When you use ALT and SPD hold, you're effectively 'forcing' the FMC to 'push' against the weather, rather than, quite literally, going with the flow.


Richard Williams

VFR pilot

VATSim UK S2

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Guys, I think this is a problem with activesky and pmdg. Sometimes you get unexplainable banking.Right now on iphone so can't really help, but do a search for activesky and pmdg, should be fairly easy to find topic(s).

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Fsuipc is known to aleviate the problem, especially if you prevent variance and turbulence.

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Guys, I think this is a problem with activesky and pmdg. Sometimes you get unexplainable banking.Right now on iphone so can't really help, but do a search for activesky and pmdg, should be fairly easy to find topic(s).
I had this problem last night with a 82 kt tail wind around Seattle and using Open Cloud weather - even made the weather into no winds - but took a while to settle down. Switching between LNAV, HDG hold, manual flying etc. Eventually landed with assistance from VATIMS Seattle ctr.Denis EGCC

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As to running in LNAV + ALT Hold, I found problems doing this. I had a buffet alert and it disconnected CMD and tried to throw itself out of the sky. I had a chat with a friend who is an Easyjet pilot and he said to use VNAV. The VNAV allows the FMC to manage the climb and cruise, adjusting power accordingly and compensating. When you use ALT and SPD hold, you're effectively 'forcing' the FMC to 'push' against the weather, rather than, quite literally, going with the flow.
Once you are at cruise, it should make no difference whether you are VNAV, or SPD hold.They are basically the same as far as the A/C is concerned. IE: if the FMC decides to run atmach .78, and you get to cruise holding that speed, and then switch to SPD hold, you should noticeno difference. The speed will be exactly the same for each mode, and there should be nochange in the behavior of the aircraft. Unless you change altitude, the FMC will keep thesame planned mach speeds for the entire flight. If you are using SPD hold at cruise, it's goingto adjust throttle no differently than if you were in VNAV. So you should notice no difference at all.Both are maintaining the same exact speed.Like some have mentioned, the quirk in question is sim weather program related.Myself, I run regular ole FSX weather, with winds aloft. I don't use FSUIPC at all.I routinely see strong winds, often over 100 knots, and I don't see this problem at all.The only effect I notice is ground speeds going up or down, depending if a head wind,or tail wind. If you have a strong headwind, you never go "with the flow" no matterwhich mode you use. :|VNAV will plan speeds, but the method the aircraft uses to maintain that plannedspeed is the same with either mode. It's merely adjusting the throttle, and the amountof thrust needed will be the same with either mode. You should see no differenceat all between the two modes.

Mark Keith

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Using Active Sky 2012, and FSUIPC, neither seems to fully fix the issue.What settings are you using Word Not Allowed?


Jason Aerts

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Once you are at cruise, it should make no difference whether you are VNAV, or SPD hold.They are basically the same as far as the A/C is concerned. IE: if the FMC decides to run atmach .78, and you get to cruise holding that speed, and then switch to SPD hold, you should noticeno difference. The speed will be exactly the same for each mode, and there should be nochange in the behavior of the aircraft. Unless you change altitude, the FMC will keep thesame planned mach speeds for the entire flight. If you are using SPD hold at cruise, it's goingto adjust throttle no differently than if you were in VNAV. So you should notice no difference at all.Both are maintaining the same exact speed.Like some have mentioned, the quirk in question is sim weather program related.Myself, I run regular ole FSX weather, with winds aloft. I don't use FSUIPC at all.I routinely see strong winds, often over 100 knots, and I don't see this problem at all.The only effect I notice is ground speeds going up or down, depending if a head wind,or tail wind. If you have a strong headwind, you never go "with the flow" no matterwhich mode you use. :|VNAV will plan speeds, but the method the aircraft uses to maintain that plannedspeed is the same with either mode. It's merely adjusting the throttle, and the amountof thrust needed will be the same with either mode. You should see no differenceat all between the two modes.
Quite true, but the buffet alert affected me during my climb to cruise level... Sorry I think I didn't make that clear in my post!

Richard Williams

VFR pilot

VATSim UK S2

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I haven't seen this even once using AS2012 with default settings for what it's worth...There are several large threads in this forum on how to configure weather settings optimally - look for those. Paul Deemer I believe is the author.


Ryan Maziarz
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An old topic to be sure.  I experience the same thing recently on a flight over the Canadian Rockies - using regular FSX Downloaded WX.  Got a wind off the left nose of 67-74 kts.  LNAV and HDG both doing S-Turns…and over banking the aircraft even though I had set the max angle of bank to 10º.  

 

Good to know that this is a known issues with solutions (for me it looks like I'll be playing wit FSUIPC)

 

That said, my solution was to turn off all lateral AP functions (left VNAV alone) and hand flew it. Kind of fun actually.  With the HSI dialed down to 5 Nm you can use it to help you "set the crab" - not perfect but I got to the waypoints.

 

The one thing I noticed is that 60 kts seems to be the "magic number" - when the wind speed went to less than 60 (57 kts) everything went back to normal…I switched back to LNAV

 

…the fun hand flying stopped and I was back to sipping my coffee and monitoring the instruments.  :wink:

 

OK so it says I need to put my name in my signature or this will get delete–fair enough.  Not sure which will match PMDG's records (I think I may have used my wife's credit card)  We'll go with:

 

William Kotheimer…I can't imagine there are any other with that last name  :smile:

 

 


R/ Hangar 14 

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Wow, old topic and this subject has gone through much evolution since 2012.  The wind velocity is not the problem, it's the unrealistic variations that FSX is famous for. FSUIPC paid version can dampen those excursions but the best solution is to go with (my recommendation) Active Sky Next.  ASN, with turbulence and other such things dialed back, will provide you a realistic feel and actual enroute winds.

 

The name isn't there for product verification, it does serve to keep things friendlier around here if we put our names on what we say.


Dan Downs KCRP

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I know ASN has much more stable winds aloft than any of the other wx programs, which includes FSX itself back

when it was working on a semi reliable basis. Always the same problem, and this was way before the NGX..

Some stations would have large differences, and as you passed them, the winds would change back and forth.

So you would be merrily at cruise, and then wham.. Overspeed.. Finally get things under control, and wham..

Changed again, and now too slow.. Back and forth in some cases. Enough to make a pilot want to jump out of the

seat and raid the galley for one of those little bottles of JD..   lol

Don't get any of that winds aloft mayhem with ASN.  I bought ASN mainly to get radar, but that was a nice bonus.

I had been using one of the other programs before that.. Good clouds, but angry winds aloft.  :mad:


Mark Keith

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