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Yet another FMC Q

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Guest Traffic

Sorry if this has been asked a billion times, I just couldn

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Guest boxjockey99

RobEven the real thing does this but in the real world we can always take an educated guess as to which runway we will be using. The problem you are encountering can be solved another way though if you really don't want to start guessing the runway in use. Once you have the destination airfield in as the end of the route then you need to specify what altitude you want to be at that airfield by means of a hard height or at least give the FMC a hard height somewhere on the descent path this effectively gives it something to work out. The problem is that an airfield alone doen't put in the relevant heights and therefore the FMC doesn't know what alt it needs to descend to. So you might try putting say a 3000ft hard height at a waypoint close to the airfield, this will end up giving the message 'NO DES PATH AFTER XXXXX' where XXXXX is the point you put the hard height in. Or simply put the airfield elevation (or rather 200ft above it) in as a hard height next to the destination airfield! Hope that answers your question, any more feel free to askKris

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Guest tomahawk_pa38

I have reduced the number of Des Unacheivable messages by increasing my descent speed to /300 in the Descent Page, probably because this slightly faster speed is easier to maintain during the descent path. You will see in lots of other threads on this subject that the 737 is a bit of a beast to control going downhill !! and initially I deliberately put in hard height constraints thinking that this will help - it doesn't ! Thanks to advice in this forum I have now almost mastered the art of VNav albeit by using the little cheat above (if it is a cheat - perhaps someone will confirm if 300 kts is a realistic decent speed). There's still lots about the fmc/auto throttle etc that I don't understand - but the fun is in the learning for me. If I don't know which runway I am going to land on - though the wind direction is the obvious way of guessing, then I put in the ILS intercept height as my final altitude - seems to work Ok.

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Guest boxjockey99

DaveOn the 300 with a cost index of 35 in the FMC the descent speed is 293kt till FL100 then 240kt below. So 300kt is not out of the ordinary. In actual fact we often end up at 320 - 330kt in the descent to regain the profile having been kept high by manchester radar. The NG has a much more critical wing and so is more 'slippery' in the descent. The answer is that you need to plan ahead in order to get your descent profile right. If there are descent restrictions then you need to descend earlier in order to meet them although this really isn't an issue in MSFS.Hope that helpsKris

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Guest Traffic

Thanks for the replies. Greatly appreciated.I guess its not too hard to just put in any Runway for now, as all can be changed later.Cheers.

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Guest glenb

One thing that can be done is to choose a lower cruise alt in the cruise page. Enter the desired altitude in the MCP. It shows up in the FMC scratch pad and can be up loaded to L1, the cruise alt.It may not be true to actual conditions but you can choose a runway, approach, transitions and/or a STAR in the DEP/APR page prior to TOD. Take care of the discontinuities. If you hard enter the altitudes for the approach fixes from the approach plates in the LOG page with your speeds you can get a very smooth decent with the deceleration for flaps and speed reduction even using a RNAV approach to near touchdown using only the FMC with no modifications in the MCP. The 900 seems to require more drag than the 600.If the runway you choose is not the correct one it can be changed prior to approach.Remember to select the flap setting in the approach ref page.I think hard entering altitudes on the approach is the thing that makes it work.USA approach plates are athttp://avn.faa.gov/digital_tpp.asp?ver=050...5&end=2-17-2005Good luck.

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Guest Traffic

Thanks Glen. I'll try that.

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