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Guest KD Larsen

Airspeed bleeds off in cruise

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Just wanted to try a very heavy flight for a change, so I set everything up for JFK-JNB flight & everything went smoothly until I reached cruise (initially FL290 due to weight, opt was FL298 & max was between FL310 & FL320, ie. plenty of leeway). All of the sudden, the airspeed started go to slowly decrease, up to a point of where I found myself doing M.55 with a 10 degree up nose attitude. I set the altitude to FL280 with a V/S of -500 fpm to try and regain, but it only managed to stop the decelleration. I then set the altitude to FL260 and the V/S to -1200 fpm, which worked, though the airspeed was very slow in climbing. At FL260 I waited for the airspeed to reach a sensible level, before I put FL290 in & selected open climb. The aircraft accelerated to 300 kts & proceeded with climbing at around 400 fpm until FL290 at which point the airspeed settled at M.77. And then it started over, with the airspeed once again slowly bleeding off.The data: ZFW : 362426 lbsFuel : 236482 lbsMax Fuel: 236974 lbsI had tailwinds of approx 40 kts the whole time. I'm also using the updated FDE's posted in this forum.

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The only time I've ever seen this is when an external wx add-on had downloaded some bad data and it set the temp at flight level to something like 50 deg C. FSUIPC's limitations on change rate made it somewhat insidious since I had temp changes limited to one deg C per 5 sec or so. When airspeeds start going funky on me, OAT/TAT is one of the first things I check...in fact for a while I had a small watchdog program running via WideFS that would pause the sim if the OAT deviated more than 20 deg C above standard temp for that altitude.Winds shouldn't be an issue if you have the delta rates limited in FSUIPC as well...I have mine set to no more than 1 knot per 3 seconds to prevent suddent shears that can cause stalls and A-floors.RegardsBob ScottATP IMEL Gulfstream II-III-IV-V L-300Washington, DC

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Thanks for ideas of what to look for - I might try the route again tonight (I have 2 weeks off from now on due to christmas - time to knock my bodyclock unconcious!) and keep an eye out for that.I was flying online at the time, so it could theoretically have been SB that entered some bad wx data. What would be the best option if that was the case? Keeping in mind that I've also entered a 1 kt pr 3 secs limit (or something to that effect) in FSUIPC. I suppose using the "Request Wx" would fix it.

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Just a tag on to the previous post:I'm currently enroute JFK-JNB & so far I've had no problem (except for a VERY poor climb performance - I could still see autogen trees after I had passed Canarsie on a 31L departure :(). I did go into FSUIPC, and apparently I hadn't Allowed changes to FS's own weather & set the limit for windchanges to 5 kts/degrees pr second.

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