VNAV Will not adhere to FMC speeds on descent

[drempel1 0]

Hi Guys

Not sure why VNAV will not follow the FMC descent speeds on assigned fix's. I have followed the appropriate instructions and tried different weight configurations with no results. I am new to PMDG and could use some advice. My work around is to enable speed and manually enter the speeds but from watching the tutorials I assumed VNAV would look after speeds from the FMC. My error?

 

Cheers

[scandinavian13 5,192]

My error?

 

Hi Cheers,

It could be. What is your cost index? If it is higher than a "normal" cost index, the aircraft will descend at a faster rate to maintain a higher airspeed. With higher descent speeds, it's difficult for the aircraft to reduce speed prior to the waypoint.

[drempel1 0]

Hi There

Interesting point. I have been using "50" as a Cost Index for the FMC. Not really sure of what normal is. I thought the higher the number the faster the flight etc. I will experiment with different valus to see if VNAV maintains the FMC descent speeds

Thanks for replying so quickly.

 

Cheers

[scandinavian13 5,192]

50 shouldn't be too bad, actually. Try lower values to see what you're getting. If it's the same, we can at least rule the CI issue out.

[sheepfu 3]

Also, try inputting the winds at various flight levels in your path. That helps the FMC calculate more effectively.

[pschlute 37]

There are two different FMC descents....Path and Speed. The default is Path. In a Path descent the a/c will follow a patth from TOD which in normal conditions will keep you at normal descent speeds. If your wind is different and you havnt entered into the FMC then it can over/underspeed. Remember in PATH its objective is to follow a path...NOT to maintain a speed. It is your job to use thrust/spoiles to keep speed correct.

 

You can of course change VNAV DES to SPEED. I this case the FMC will do as asked....however this may result in you being early or late for the atlitude restiction. Again it is up to you to monitor and take control of the situation.

 

The FMC will only do what is asked of it..... a specified PATH descent combined with a strong tailwind cannot equal "no overspeed" unless you have correctly programmed the FMC and can intervene.

 

Peter