Losing airspeed in flight

[Guest troytroy46]

I having some problems while in flight, while in cruise i seem to lose airspeed (or wont hold airspeed that set in FMC for that desire flight level example 84.3 at FL340 ) the Vnav switches off and FL CH comes on why does this happen i seem to program the FMC correctly and i use FS Build for planning routes and active sky for my weather any help or advice would be gratefulMichael Troy

[downscc 1,327]

Next time it happens, check your TAT. I suspect that a unrealistically high temp is the culprit. Even with ActiveSky, I've had it happen in locations remote from reporting stations. I hope you report back with your findings, this has come up every couple of weeks in the forum and I still haven't seen a thread continue to resolution.

[Guest gentrrm]

I am currently experiencing the same situation on a flight from KSFO to EDDF (Frankfurt). I have FL340 selected in FMC and my CRZ speed was around 305. I am somewhere over Iceland and my altitude is 31000 and falling and speed is 200 and falling. I use Active Sky and FSNav, but FSNav is not activated. In the time that it took me to find this post (I had seen it a few days ago) and try to figure out what TAT was, the situation had corrected itself and now thinks are back to normal. If you suspect unrealistic high temps, how can it be corrected? Also, explain what TAT is and where to find it.Thanks,Rick.

[Guest keiron]

I cross the Atlantic quite frequently and the same thing happens to me every time, usually when i get about half way, the winds will rapidly shift from tail winds to headwinds (and visa versa)..bare in mind i usually get 70kt+ average head/tail winds when crossing the Atlantic.. and seconds later i find myself loosing so much airspeed the stall alarm is on and i have to switch to Con power just to get the airspeed back up quicker!It's really annoying but i cant fly without Activesky 6 so i just put up with it time and time again! I've also experienced these violent wind shifts over the pacific as well which cause the same flight problems. It doesnt matter what route i take,(NATS, normal flightplans,flightware route)..it always happens about half way across the Atlantic and also in the Shannon centre airspace.As for the TAT, i was flying th LDS 767 yesterday and was approaching Vegas coming in from Hawiaii then at Fl330 my TAT switched from -18 to +8 and my airspeed went nuts..that was odd.I'd be interested if anybody has come across all this and if there is a solution to all this , maybe Activesky6 settings needs tweaking??Or educate me if i'm doing something terribly wrong cheers

[downscc 1,327]

There's information in the ActiveSky forum next door, try: Force Constant Route Aloft Temp. Also saw discussions on updates to solve this and other oceanic wx stuff... check it out.

[Guest SKireyev]

This has to do with a number of things, from wind direction as someone mentioned to change in temperature, to change in atmospheric pressure. The PMDG engine parameters and model is so realistic that the performance of the model is seriously affected when the sim throws it some of its weather curveballs. As someone mentioned, checking Force Constant Temp and Force Constant Pressure (options are side by side) will take care of most of the issues. The issue really has to do with the sparsity of weather reporting stations over the Atlantic, so when you switch from one station to the next, where in real life a change would have been very gradual, in the sim the change can happen in a matter of seconds.I recall a flight from Sydney to London (stopover in Hong Kong), where I lost nearly 8000 of altitude as a result of this anomaly. This was a result of not having a weather radar, and flying in head first into a front carrying CB up to FL360. I've never seen the wings flex so much.Cheers,

[Guest gentrrm]

The problem may very well be the fact that the PMDG are too realistic for FS to handle. I'm currently on my return flight from EDDF to KSFO and again the same drop in alt. and speed is affecting my flight. The only differance between this flight and the last one is that I didn't use ActiveSky. With ActiveSky not is use, checking Force Constant Temp and Force Constant Pressure won't have any effect. Is this somethink that we just have to live with or is there some setting(s) within PMDG or FS that will help.Cheers,Rick

[downscc 1,327]

You have control over wx in FS.. will that work for you?

[Guest SKireyev]

I don't understand why not use the options given by ActiveSky. We know its a limitation, we know there's a work around. Sure, having same temp and pressure throughout the cruise is less than realistic, but then so is having the engine fluctuate. I use the two options, and have never had that issue ever since. Considering I make 4-6 trips from KBOS to EGLL every month, that's a pretty reliable track record.Cheers,

[falcon999 24]

You can have the best of both worlds... from another thread I just responded to concerning using FSUIPC to assist in smoothing:I use 1knt every three seconds and 1 degree every three seconds to ensure the changes are very gradual and the sim can respond. If your still having problems after using this configuration, change the time to something like 1knt and 1 degree every five seconds. Try different settings until you find one that works best for you.PS: I have also played around with using both AS and FSUIPC smoothing and have had good results.George

[Guest SKireyev]

>You can have the best of both worlds... from another thread I>just responded to concerning using FSUIPC to assist in>smoothing:>>I use 1knt every three seconds and 1 degree every three>seconds to ensure the changes are very gradual and the sim can>respond. If your still having problems after using this>configuration, change the time to something like 1knt and 1>degree every five seconds. Try different settings until you>find one that works best for you.>>PS: I have also played around with using both AS and FSUIPC>smoothing and have had good results.>>George>>>Thanks George! Good catch! That is yet another good way (I need to set it up again following a complete re-install of the sim a couple of weeks ago). But I think you do have to have a registered version of the FSUIPC.

[falcon999 24]

Good point Sergey, yes you need the registered version of FSUIPC.

[Guest gentrrm]

Apparently, I misunderstood your earlier response. I was under the assumption that ActiveSky was the culprit and if ActiveSky was used, settings within ActiveSky could be checked which in turn would correct the problem. Again, I was assuming that if ActiveSky wasn't used then the problem wouldn't occur. I ditched my 747 somewhere over Iceland and restarted the flight again...this time using ActiveSky. I made the recommended adjustments in ActiveSky and this seems to have corrected the problem. I'm just now at the point where I ditched the aircraft on the proceeding flight and alt. and speed have held steady the whole flight. Problem solved...case closed.Thanks for all the help,Rick