A couple of questoins regarding decending:1) If i'm not cleared to decend before the FMC's decent profiile kicks in, and therefore leave the MCP altitude at my cruise altitude, will the FMC automatically start the decent as soon as I decrese the MCP altitude?2) Why is it that when I DO decrese the MCP altitude DURING a decent, the VNAV mode most shuts off after a few seconds? Speed?Gavinhttp://www.interprom.com/gavhttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

Hey Gavin,1. Yes. It will descend as fast as it can, while maintaining the selected descent speed, to try and 'catch up' with the descent profile. If it's having trouble catching it you can increase the descent rate by using copious amounts of speed brake. 2. You'll have to explain the situation a bit more so I can be sure of the cause. Is this happening while still descending e.g. your out of FL200 for FL150 and ATC clears you to continue further to FL80 so you change the MCP at to reflect this and then VNAV cuts out?Or is it your already level at an intermediate alt (e.g. FL150) then ATC clears you further after which VNAV dies?Do you get an MCP alerts on the FMC?The second situation is the most likley scenario in my experience. This happens when the FMC calculates that it won't be able to meet the next alt constraint so it disengages VNAV. If this is the case you should get a message on the FMC saying something like 'UNABLE NEXT ALT'. If this is what's happpening then to fix it simply go into LVL CHG mode until the pink diamond on the vertical deviation scale on the ND is close to the middle the re engage VNAV. Don't be scared to use the speed brakes, many people think they don't/shouldn't be used in flight...not true :)You asked if VNAV might have disengaged due to speed. This, AFAIK, only happens in an overspeed situation. You'll get a 'OVERSPEED DISCONNECT' MCP if this happens.Hope this helps, let us know if you have anymore problems :)CheersDavidP.S. If FS9 ATC isn't descending you on time use the cruise alt change option from the ATC window. It's the same as giving a real world ATCO a hint e.g. 'Crossing XXX, we'd like to begin our descent soon Sir' ;) Just put the request into the FS9 ATC about 2-3NM before the TOD, any earlier and it'll start b!tching at you.

Thanks for the indepth and very helpful reply, David!I especially appreciate the PS. When I ask for an early decent from FS ATC, should I ask for a specific altitude, or will it just llow me to begin my decent without specifying?Gavinhttp://www.interprom.com/gavhttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

Hey Gavin,Technically you'll still be considered in the cruise mode by FS ATC (even though you'll be in descent mode in the aircraft) so you have to ask for a specific altitude, although it can go as low as you need.What I normally do is to try and request the next alt restriction e.g. I'f I'm at FL200 and I've got a restriction at FL150, I'll ask for a decrease of 5000 ft. Once I'm getting near the waypoint with this restriction I'll ask for the next one (say FL80, so a descent of 7000ft) an so on until FS ATC starts to give me descent instructions.You can, however, simply ask for a 10,000ft decrese at the beginning of your descent and then another after that and so on. This could possibly cause a bit of a problem however. As an example I'll use the same scenario as above. You're at FL200 and coming upto the TOD so you ask for a 10,000ft descent and are cleared to FL100. This is all good and you start your descent but on the way you have crossing restriction of FL150. Now depending on the situation you may arrive at FL150 before the actual waypoint with the restriction so you'll be flying level at FL150 for a time. The distance you fly level could be quite a while depending on the individual situation. During this time ATC will probably start b!tching at you to expidite your descent and, as you probably know, could cancel your IFR plan if you ignore them long enough. This is the reason I only ask for a descent to my next restriction, if I arrive at FL150 too early it won't be a problem as that's as far as I'm cleared.Also, while I'm here, when ATC starts giving you desent instructions you should probably come out of VNAV and continue the descent in VS or LVL CHG mode. Especially if going off route as the route they send you might be shorter than the programmed route.Hope this helps :)CheersDavidP.s. If you want to descend more than 10,000ft it's no problem to ask for two decreses in a row. E.g. If your at FL300 and your crossing restriction is FL150 that's a descent of 15,000ft which isn't available from the menu. To overcome this simply ask first for a descent of, for simplicity, 10,000ft. After ATC has come back you can the ask for another descent for the rest of the way (in this case another 5000ft) You'll probably not be allowed to descend all that way in the one go, ATC will probably come back to you saying something like 'Descend and maintain FL 220 expect FL150'Who says P.S.'s are supposed to be small :)

Awesone.Now if they'd just stop vectoring me six states away from final approach, I'd enjoy FS ATC more ;)Gavinhttp://www.interprom.com/gavhttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

I know that feeling! The only thing that annoys me more is when the send you 70nm from the airport, you do everthing your asked and then in return they cancel your IFR plan. You haven't heard the air turn blue until you've heard a pi$$ed off Glaswegian with a cancelled IFR flight plan! :)Glad I could help :)CheersDavid

"1) If i'm not cleared to decend before the FMC's decent profiile kicks in, and therefore leave the MCP altitude at my cruise altitude, will the FMC automatically start the decent as soon as I decrese the MCP altitude?"At TOD, the A/P should switch to MCP SPD | ALT HOLD (or at least it should on the real aircraft)I believe you will have to select a lower altitude and reselect VNAV. Cheers.Ian.

"1. Yes. It will descend as fast as it can, while maintaining the selected descent speed," Sorry but that is only true for a SPD DES not a PATH DES where the target spd is reference only, the FMS makes no attempt to keep the speed only the path. Your speed can get (or reach Vmo/Mmo) a few knots prior to the poll the FMS will kick off VNAV for LVL CH. Remember that a VNAV PTH DES sacrifices speed for path, a SPD DES sacrifies PTH for speed ;-)[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

Sorry for the mix up Randy, I was thinking of LVL CHG. If I'm miles off the VNAV descent profile I'll use LVL CHG to dive down and catch it and then engage VNAV again.Thanks for clearing it up :)CheersDavid