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hagley4930

777 cruise A\T fault

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Hello, my PMDG 777 will not hold speed in the cruise. The speed in VNAV decays until low airspeed protection comes in. Win 8.1 64-bit, up-to-date latest 777, Intel HD4600 graphics, Haswell 4770K, Asus Maximus VI Hero M/B, 16GB RAM. Saitek X52. I have searched extensively for an answer on the internet, including this forum, but to no avail. Help much appreciated, TIA. 

 

Ian J. Patterson

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Hi,

 

Few questions to try to find out the origin of the problem:

 

_ What are the indications on the FMA (SPD / LNAV / VNAV PATH)?

_ How do the engines thrusts react? Are they fixed / variating / responding to your throttle input)?

_ What option have you chosen for the A/T override in the FMC (Never / Always / Only in hold)?

_ Is that issue only in cruise or also during climb?

 

My guess is that there may be a conflict with your throttle preventing the A/T from working properly or something is wrong with VNAV mode.

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Another theory would be your WX program.  Are you using FSInn?  Send a screenshot of your EICAS, especially showing the TAT or OAT.

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^ Yep, would bet money on this being weather related.

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FSINN weather simulator ingestion has always created a greater than normal OAT at higher altitudes (Cruise).  This will cause the aircraft to operate at a higher density altitude than what is normal;   ultimately causing the engines to have no performance to them (decay of airspeed).

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I had the same problem where my altitude seemed to rise and fall by about 1000ft. The engines were erratic and if I touched the throttles the engines would either increase or decrease. I turned all A/P VNAV LNAV and autothrottles off and then re-engaged everything as well as making sure that my joystick throttle was in the idle position. This seemed to settle everything down as long as I did not touch the throttles.

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No weather prog running, but I'm using real-world weather (updates every 15 min.). I can't find a way so send or attach an image, despite having read the FAQs and help page. None of the methods described show up in my post. Help please:huh: .

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I remember when I first started flying the -200LR when she was released and I made a flight from some aiport to KORD, a short hop.   From there to KDEN and then KDEN back to KORD.  When taking off from KDEN I noticed that the airspeed wasn't right.   I checked it and eventually found out that despite the A/T being armed, I still needed to engage it properly after a turn around. 

 

Just check that, because the 777 is different from the other older Boeings.   Because of the protection built into it, you can find that something looks right for a while and then find that the aircraft is not doing what it is supposed to.   Also make sure that when you are in cruise your PFD modes reads LNAV / SPD / VNAV PTH.   This is the correct mode display to show that the flight computers are handling the aircraft's vertical and lateral navigation and also, importantly is handling the speed as per the vertical profile.  Very important bit that!

 

If yours are different, tell us what your modes are so we can have a look at it.   I also agree with the others here that your throttles may interfere.   Are you running FSUIPC mapped controls, separate throttle quadrant, etc?

 

Kind regards

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Yes, my A/P modes are SPD/LNAV/VNAV PTH, FMC in ACT ECON SPD. One difference is that, for my most recent flight, OMDB-EHAM, I  used real-world weather (updates every 15 min.). For some reason, previous 777 flights were set to user-defined weather. I have a registered FSUIPC, but use it mainly for wind smoothing and to fix control acceleration problems. My Saitek X52 is a HOTAS and, therefore, has a seperate throttle lever (just the one). Once airborne and with VNAV and LNAV plus A/P selected, I pull the hardware throttle back to idle to avoid problems. BTW, the real-world weather flight, with Auto-Cruise plus Auto-Step Climb time compression enabled, went off without a hitch. Computers, what can you do about them? 

Yours, even more confuzzled,

Ian J. Patterson

​p.s. Is REX a good weather program to use? I'm thinking of getting it and would greatly appreciate advice in order to avoid this very problem. 

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p.s. Is REX a good weather program to use? I'm thinking of getting it and would greatly appreciate advice in order to avoid this very problem. 

 

ASN.  REX is great, and has some great textures, but ASN has the weather radar function that can really change how you look at flight simming right now.

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