View Full Version : 737-800 Descent Forcast Page

10th Jan 2002, 20:07
Does anyone know what the "box" uses to compute a descent profile if it doesn't have updated winds? Does it use the actual cruise altitude winds for the entire descent, a "standard wind data set", or something else?

I tried to find the manufacturer website, Smith Industries, but came up empty-handed. Does anyone know a good site for FMC related questions?

11th Jan 2002, 12:51
Scooby hi, I think current wind and then a linear decrease in wind strength down to end of descent (thats for FMC planning if you have left the forcast wind boxes blank.

I think the only real place for good gen on the FMC is from the book produced by Capt Bill Bulfer.
about $40. <a href="http://www.firstnethou.com." target="_blank">www.firstnethou.com.</a>

All the best

Cornish Jack
11th Jan 2002, 15:30
I have just had a look at the 744 FMC control laws for V NAV descent and I would think the 378 must be similar. Amongst the mass of variables which it uses, is ground speed, which has to be that at the time of activation, so , presumably, it's using instantaneous winds. The profile is predicated on a continuous idle thrust descent unless modified by vertical profile constraints. This would, again, be modified, as part of the control law calculation, for any input of forecast TAI or descent winds.
Hope this helps

14th Jan 2002, 03:18
My understanding is that the descent is initially calculated on actual wind at cruise level which decreases linearly at about 1-2 knots per thousand feet on descent. This is how the descent point will be calculated with no forcast winds entered on the descent forcast page. As the "beast" descends it will update the profile (VNAV path) based on actual winds. If there is a large variation in the actual wind compared to the estimated linear wind gradient then the "speed" at which the path descent was originally calculated will be sacrificed as I am sure you know. With the -800's, especially a heavy one, I have adopted the technique of entering a "false" tailwind component in the descent forcast page to get the aircraft on its way down a little sooner and at least keep the VNAV path without going into an overspeed disconnect. There will be better ways but I am pretty new on the machine and haven't had the benefit of anyones experience to learn from!

Incidently if you are flying an -800 into Sydney you can forget all this wizardry. ATC have a good look at your profile and will always chop off about 5-10 track miles when you are only 30 or miles from the airport. In an -800 at near max landing weight this becomes a real handful. Better to decline the vector I am learning.