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Redhornet

Unable to stay at cruise target speed

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I don't know if this is a known bug on a couple of flights I have had I am unable to maintain the cruise speed at FL390.  Target cruise is set to Mach 0.84 but the airspeed drops to around 222 knts.  The only way I can maintain the correct mach speed is to stay in CLB mode not cruise.  I know my weights and fuel are fine im using Topcat along with PFPX, my wieght on this particular flight was a ZFW of around 177.246kg with 29, 385kg of fuel so I was not over weight and the cargo was balanced correctly.  My cruise N1 was 88% and it won't hold mach 0.84 yet the clb n1 is 101.3% and will hold the airspeed.  Anyone have any ideas on this?

 

Regards

 

Jason

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Post a screenshot of the PFD & upper EICAS.  Also what is the SAT & TAT?


Rob Prest

 

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Unfortunately I can't post a pic of either as I have now finished the flight.  I was using Activesky 2012 as my weather addon.  The purple marked showed that the speed should have been -.84 but stayed steady at around 222 kts.   If this happens again I'll let you know.

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Okay this it seems is not due to the PMDG 777, by what I can tell it's do do with FSX.  I have the same problem with the PDMG 737MGX.  I decided to close down Activesky 2012 to see if that made a difference and it didn't.  I changed the weather settings to weather themes, clear skies yet it clears it for like 5 secs then reverts back to user defined settings, which i check it's shows no winds but in the sim it showing a wind of 003/17.  How can there be wind when the wind setting is clear in the menus.  I used FSUIPC to clear the settings and again I get this weather bug, the outside temperature revers to +38 degress and even in a light load and light fule the aircraft is struggling to climb above 30000ft for it's optimum altitude of 370.  Any ideas what is causing this as I have no other programs running except FSX.  Okay so I just closed FSX down restarted it with a default flight and the weather is off.  So maybe it has to do with Activesky.  Hmm this is wierd.  Looks like i'm gonna have to investigate a bit further.

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Flying online with FSInn? Known issue on all aircraft if you don't disable FSInn weather.


Richie Walsh

 

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No mate don't use FSInn.  Just doing another test flight from LEMD - EGGP 737NGX 700.  Maybe to do with Activesky 2012 after all atm I'm climbing through FL290 and the aircraft is holding the correct climb speed nicely, lets see what happens through to cruise fl 360.  OAT is currently -18c at fl 300.  I'm beginning to think that due to the last flight where the outside temp was showing +38c was what was causing the plane to struggle to climb and hold it's airspeed.  Time to check with Hifisim see if there are any issues reported atm.  If i can find how to obtain my forum details that is.

 

Okay here's an update.  Currently in Spanish Airspace FL360 outside temp is -21c and cruising nicely at mach 0.79.  So no problems.  Gonna report this to hifi sim and see what they say.

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I'm beginning to think that due to the last flight where the outside temp was showing +38c was what was causing the plane to struggle to climb and hold it's airspeed.

 

Hi, Redhornet,

 

I don't know what's causing it and don't have Active Sky, but an OAT of +38o at any conceivable cruise altitude is definitely very far off and will definitely cause climb and speed performance issues, as Rob and Richie are suggesting.

 

Mike


 

                    bUmq4nJ.jpg?2

 

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Yeah that's what I thought.  So I'm gonna report this issue to HiFisim.com as it's not the first flight where I have had this issue.

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Your problems seem to have been those 38C at cruise FL.

 

But for additional info.....

 

Years ago BOEING changed the cruise thrust limit from CRZ to CLB on the real aircraft!

What I have heard is that it was a bit underpowered at CRZ thrust and if you would loose a bit of speed (changing winds/turbulence) it would struggle to get back to its target speed.

 

I guess that is why you have the option in the FMC settings to make CLB your default thrust limit when at cruise FL, which I did.


Rob Robson

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I think this might be up to the operator, as an option, I know BA use CLB, but have seen others use CRZ.


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