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forshaw

Problems maintaining cruise alt via the FMC

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HI Folks; 

I have always used the PMDG 747 on short hops KBOS-KJFK and back (long story)and seldom from KJFK to KORD. I have never taken it above 28,000 feet until recently I took it to 31,500 Ft flying from KORD to KJFK and it didn't work well. 

 

Once I was cleared to the final climbing alt 28,000 to 31,500 the never stopped at 31,500 ft it just kept climbing not stop to 38,000 ft and down to 35 and them back to 380. My FMC is to 35,000 and the Auto Pilot panel is also set to 31,500 and still the aircraft kept climbing even when I took the AP off. I tried it on 3 occasions and the same thing. Never had this issue in the years I have been flying this aircraft under 29,000 feet. Have used other aircrafts for long hauls with no issue. I am not using SIDS and STARS but a Flt plan generated by FSX and then using the PLN convertor for Co Route load.

 

Cruising Alt on the FMC is at 31,500 and all "Waypoints" say 31,500

 

Never had an issue using this

 

Not sure what it is issue. Any help would be greatly appreciated.

 

I tried to attach a snap shot but there is no option to do so via the post options..

 

Thanks

Anthony Forshaw.

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I cannot answer you but I can give you reassurance that you are not alone.

It seems to be utterly impossible to get the FMC to control the plane properly above 30,000ft.

My 747 will climb past the set Flight level and then come back, go below then climb again and so on for about ten times. Even if the FMC says max. altitude

36000, it will never get there without stalling.

Same with the lateral navigation. It is fine to start with but if you change the heading manually ( or sometimes on its own)

it proceeds in a series of "S" moves crossing the course and back again and never again stays on track.

 

The only answer I have found is to stay below 30000ft don't alter any settings and accept that you will use more fuel.

It is just a useless bit of programming, I would say.

 

Ian Harrison

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I have to say that in over a thousand hours flying the PMDG 747-400 I've never experienced this problem: so I'm not sure that it is a "useless bit of programming".

 

 

 


Same with the lateral navigation. It is fine to start with but if you change the heading manually ( or sometimes on its own)
it proceeds in a series of "S" moves crossing the course and back again and never again stays on track.

 

Have you got turbulence turned on in FSX? If so, turn it off: it's well-established that the (not very realistic) FSX turbulence effects can play havoc with the autopilots on complex payware aircraft and result in this S-turn effect.

 

With regards the altitude capture issue:

 

- What weight are you at?

- What weather conditions? Are you using a 3rd party weather addon? What is the SAT indicating (PROG page 2)?

- What autopilot modes are you in?

 

 

 


I have never taken it above 28,000 feet until recently I took it to 31,500 Ft flying from KORD to KJFK and it didn't work well. 

 

Not that it should make any odds, but 31,500ft is an unusual cruising level: eastbound you would normally be at odd flight levels (310/330/350 etc) and evens westbound (300/320/340 etc).

 

Something else to try: the (free) Flight1 Registry Fixer -- sometimes your FSX/FS9 registry entries can become corrupted for whatever reason and this is known to cause issues with the PMDG 747. See if running that tool helps at all.

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OK. I unreservedly take my comment about programming back. That was just the frustration coming through. :wub:

I use OPUS for the weather and last time I looked Turbulence was off in FSX.(I will check again as funny things can happen).

The weight is controlled through the PMDG loader program and on that flight was 6 tonnes under MTOW.

I can't answer now what SAT was but will look next time.

 

What I find confusing is that I have just flown EGLL to FAOR at no more than FL290 (unrealistic, I know) but absolutely no problems,

no deviation, nothing, perfect approach and full auto-land.

 

On the FAOR to EGLL leg I started at FL290 and after 2 hours climbed to FL310 which was fine. The FMC told me another 2 hours

later that Optimum was about FL330 and max about FL348 as far as I can remember. I tried to climb to FL330 but it wouldn't and started

losing speed (A Ice was on) when I tried to return to FL290 that's when all the trouble started. Nothing would stabilise and I had to manually fly the plane

to land it.

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No problem ;-).

 

That is very strange. It would be interesting to know whether Anthony is using Opus weather also.

 

The SAT (as well as a general overview of the weather conditions & winds at the time) would be helpful as an error in the weather injection could cause a problem.

 

Which autopilot modes were you using for the climbs? LNAV/VNAV or V/S? What cost index?

 

I've occasionally noticed that if there's a large change in the wind during the latter stages of the climb it can hurt the climb rate significantly, especially if the aircraft is heavy, and it can take the aircraft a while to return to cruising speed, but it doesn't happen very often and I've never had a problem getting to an altitude or subsequently accelerating once I'm there. For what it's worth, the other day I came back to LHR from JFK -- straight up to FL350 and a subsequent step to FL390 later in the flight without a hitch (ASN weather), so it's strange that you're both experiencing the same problems.

 

I know you used the PMDG load manager on this occasion, but have you ever used an external load manager such as TOPCAT to load the aircraft?

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Hello Ian and Anthony,

 

Just to update you -- I've recently been in conversation with one of our VA members who has been experiencing the same problem.

 

He has managed to solve this by unticking the A/T and Elevator Override options in the PMDG menu.

 

Hopefully this may be of use!

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