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Richard McDonald Woods

Constant surging in CRZ

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Flying the 747-400 with RR engines, I am getting a constant surge in altitude and N1. Took off from EGLL for KSFO at 377.4 tonnes and 331 POB. Climb and step climbs were smooth and appropriate for the TOW. But in cruise I was trying to maintain FL270 (when max was FL300+) and commanded speed was M.829. The speed kept dropping to M.821 and the altitude to FL26.86, the engines would then surge and we climbed to FL27.16. This occurred many times, so I tried a step climb, when allowed, to FL310. Step climb was smooth but surging started again once level. Had to cancel the flight.Regards, Richard McDonald WoodsWinchester, UK


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

Win10-64, P3Dv4.5, PMDG 748 & 777, ASP4 + ASCA, vPilot, Navigraph, PFPX, ChasePlane, Orbx 

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Yes I've got this problem too.. I turned off all weather, no gusts, turbulence nothing.. I wasn't running anything like ASX. There were even no winds at altitude.. I have a video of this happening if you're interested in seeing it.. It gets quite bad after a while.. In one video I have been crusing about 5 to 10 minutes or so.. and it's started to get really bad.. Then it got so bad I set the MCP to 34000, activated V/S and set that to +200 to try to calm it down. Eventually it did and I climbed steadily. After that was under control, I then reset the MCP to 33000 and changed V/S to -100 and waited for it to recapture the path. At this point it was fine. The second file is the moments right after I have gone level at 33000 and I've just set VNAV back in command. VNAV PATH was just lit, it's only just gone out, what you're seeing is how it still bobs.. eventually it gets as the first video.They're quite big files though.. 26MB each.. if you're interested add me to MSN or something and I can send them over?craig_read@hotmail.comCheersCraig


Craig Read, EGLL

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Ryan,Ah! Your question rings a bell. I was using ASX.Cheers, R


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

Win10-64, P3Dv4.5, PMDG 748 & 777, ASP4 + ASCA, vPilot, Navigraph, PFPX, ChasePlane, Orbx 

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So I set off on the same trip this morning, hoping to be able to report that, without ASX, my aircraft would not keep surging at CRZ level. Indeed it appeared to settle for a few minutes on levelling out at FL270, but then resumed its surging as before.Promise that I didn't have ASX on!Cheers, Richard


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

Win10-64, P3Dv4.5, PMDG 748 & 777, ASP4 + ASCA, vPilot, Navigraph, PFPX, ChasePlane, Orbx 

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I studied this for awhile while at cruise at FL410 in the 400X. I don't have ASX (yet). I noticed that the pressure altitude and IAS would shift values, thus causing the power and pitch changes. I was using default FSX weather with updates, but there is no correlation between the update cycle and the shifts.I turned the FS lat/lon/alt/ias info on (SHIFT+Z) and watched. The absolute altitude remained the same during the pressure altitude changes. This implies that it is the air model causing the problem.I turned off FSX weather (Clear, no WX), which is effectively std atmosphere conditions. The shifts stopped. I believe FSX is causing the problem with it's shifts in the air model as you transition different wx reporting locations. Since we fly pressure altitude at flight levels, any sudden change (no matter how minor) in barometric pressure will be interpreted as a change in pressure altitude and the IAS.I solved this problem in FS9 with constraining pressure changes in ActiveSky or FSUIPC (registered version). I don't remember which, but I am going to update these addons today to see if the same fix will work in FSX.This is definitely not a bug in the PMDG.


Dan Downs KCRP

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Holy cow, did I ever jump from the pan into the fire! Installing ASX opened a new can of worms, they mention another SP in their forum but until they fix their problems I think the best way to fly FSX is with static weather.


Dan Downs KCRP

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Dan,When I did my video I was using the Clear Skies theme in FSX and still got the oscillation... Is this what you're talking about or have I missed a setting? Would you mind explaining how to get it to stabilise a bit further.. I'm not all that familiar with detailed setup in FSX..Thanks..Craig


Craig Read, EGLL

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When I selected Clear Skies (Clears all Weather) from the Weather dialog box everything that I was experiencing settled down, after it reached equilibrium that is. It had to seek a new IAS and ALT to match std atmosphere but once it got there it was smooth from then on. I did notice that the auto-throttle needs a little tuning, e.g., it is always overshooting and undershooting the setpoint but this was a variation of maybe +/- 3 kts. Should be better but it was pretty smooth compared to what ASX did to me. Goodness.


Dan Downs KCRP

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I was wrong about the auto-throttle, seems that I've got a wx situation that is stable and she's smoothin'-a-cruisin' just like you'd like her to. I'm using ASX and FSUIPC4, tho I still had air model problems until I brought up the FSX Wx dialog box, looked at the current temp/press at FL410 and okayed it. I think I stopped wx updates... still trying to find a way to be okay with FSX weather but the 400X is fine.


Dan Downs KCRP

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Guest gilbnx

I noticed the same problems.I saw that FSInn weather update (VATSIM) shows +15c at FL260 which is unrealistic. I turned it off and let FS update the weather.I managed to get a normal climb to my cruise (which didn't work before when I used FSInn weather). My new problem was the oscillation.. It tried to reach FL330 but then it went up and down - each time at about 1500-2000 fpm from FL335 to FL325 or so... I had to disengage autopilot and stabilize it myself. Still after that it could not maintain altitude and at some point started to drop at 4000fpm even though VNAV was engaged and weather update was only based on FS (no add-on weather update).I'm starting to give up on 744X and thinking of moving back to FS9 until all the bugs are being sorted out. I can't even make one successful long haul flight so far...Gil.

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>I noticed the same problems.>I saw that FSInn weather update (VATSIM) shows +15c at FL260>which is unrealistic. I turned it off and let FS update the>weather.This isn't a PMDG issue either. The issue with FSInn and Simconnect producing incorrect TATs is a well known one that affects many other aircraft when climbing to upper levels. It's been discussed on the FSInn Support forum and on the LevelD forum as well but the root problem appears to be with FSX and Simconnect at the moment.I too am getting the power surges though and since 99% of my flying is online I do have to use either FS RW weather or ASX. I've tried both and get the same issue with both so it looks like FSX is the culprit. It does however seem to me that the 747X is far more sensitive to these wind and pressure changes than other aircraft though, that may simply be because the sound quality is so high that it's more noticeable but it certainly hops around a lot more than my other add-ons.Having said that - I love this beastie :)


Bill Casey

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Guest gilbnx

Ok.. On today's flight - No VATSIM, No outside weather besides the one comes from FSX internally - climb to cruise altitude was OK.The most annoying part is the Oscillation at cruise altitude. It starts with few 100s fpm and as time passes, it goes up to 1500fpm.I had to disengage A/P, stabilize the aircraft manually and re-engage A/P (still, some oscillation happened after I re-engaged).I was well below the OPT alt, outside temp was about -40c at FL330.BTW, on my descent using VNAV, we tried to maintain 12000ft on the arrival and same thing happened (few handreds fpm oscillation) so it's not so much the high altitude or temprature as far as I can see...I looked at some old posts from 2005 (old versions of course) and noticed that users suffered the same symptoms in the past; some of them related that to the load manager which I'm using as well. Is there a chance that there is a connection here? Weight and balance?Gil.

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Hey,I'm currently cruising at FL350.. and there is no oscillation now at all.. I think it could be an FSX instability when leaving the simulation to go to windows. I used to do this fairly regularly with the FS9 version, but the FSX version seems to be quite sensitive to it. When loading the simulation in FSX and not leaving the sim once, I had a completely trouble free ride. Right now I've loaded it in a window and have it set not to pause on task switch so the A/P is always doing it's job. Again.. doesn't appear to be oscillating anymore.. Perhaps an instability in the FSX system?CheersCraig


Craig Read, EGLL

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Guest gilbnx

Craig,I too using the FSX in a window with no pause on task switch.What is your load? how did you load the A/C? what is your CG% value?Do you know your GW at FL350?Thanks,Gil.

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