VNAV Descent Path on NGX

[sho69607 29]

I noticed the descent path in the NGX was quite rapid. I was enroute from KSEA to KGEG a very short flight (40 minutes) and I did not go above FL270. The descent started at about 115-120nm out and at first it went into 3000-4000vs for the first 10 minutes and I ended up at 4500FT about 25 miles from the airport! please tell me this isnt normal. Is it possible the FMC was programmed incorrectly, like the Cost index or something? why did this happen?

[sho69607 29]

anyone?

[Foghorn 4]

This isn't normal.......

[sho69607 29]

I know its not normal but what is going wrong?

[Foghorn 4]

Sorry......Tongue in cheek response above. Couldn't help myself....... No, it isn't normal. It sounds like the FMC isn't programmed correctly.......Good chance I would think the weights, probably fuel, is incorrect. Others may have better answers but it would make it easier if you would post some parameters you used such as route, fuel, etc....Fuel on board for the flight should have been around 15467 lbs for KSEA-KGEG with about 5974 lbs for the trip.....The remainder for contingency......

[sho69607 29]

I think I had the fuel at about 1/3 full. The ZFW was at 137.0 The route had very few waypoints. The departure SID, one vor waypoint, and then the star. But now I think about it, the real killer was the reserve fuel. I had something like 15% at touchdown.

[Foghorn 4]

Did you have SID / STAR programmed?ZFW could be out?? Off the top of my head my ZFW is 121.8 but as I use my other pc for FSX I'm not sure what my actual load is, ie. number of passengers and cargo....If you want good fuel figures for a route try http://fuel.aerotexas.com/ Very quick and easy.........

[sho69607 29]

yes the sid and star was programmed. However I did delete a few of the waypoints on the STAR as they were out of the way for the way I was coming from.

[Foghorn 4]

I'd be checking your FMC data and redo the flight. My first few flights I had a few anomolies but I know my data wouldn't have been anywhere near correct. It's the ol' GIGO, garbage in, garbage out with the FMC.......

[sho69607 29]

Ill try that hopefully this time..I wont be cruising through the high desert a few thousand feet off the ground!

[Foghorn 4]

Give it a go with the correct fuel and check the ZFW depending on your pass/cargo......You may find it works much better......

[RYR738 9]

yes the sid and star was programmed. However I did delete a few of the waypoints on the STAR as they were out of the way for the way I was coming from.

By deleting waypoints like that you cause VNAV to make a VNAV SPEED Descent instead of a VNAV PATH descent !!

[sho69607 29]

so even if they take me out of the way follow them anyways?

[RYR738 9]

so even if they take me out of the way follow them anyways?

It depends on the actual route !You might be able to adjust the route in the CDU but just deleting waypoints is not a good idea.VNAV calculates it's descent profile backwards starting from the End of descent (E/D) up to the Top of descent (T/D)

[sho69607 29]

Forgive me for being a noob. but the CDU is part of the FMC right?