VNAV... level off at FL100? why?

[Guest jawbreaker]

hii'm getting more and more satisfied with the 737NG. looks amazing and flys great. i think i can live with that "rocket-climb" of 5000 ft/min (even at clb-2 mode) and the pretty hard VNAV capture and ALT AQ. (hold your sickbags ready) :-) :-Pbut i'm curious why the plane levels off when descending through FS100. sure, it has to slow down to restriction speed, but campared to the DF737 (which simply takes back thrust and keeps V/S to slow down), i wondered if this is a normal behaviour. i never realized that in a real plane. do they really stay at FL100 until reaching 250? and isn't that dangerous, maintaining an altitude for some time even though ATC ordered a descent from - let's say - FL140 to 8000 ft.?any comments welcome,thanx,stu

[Guest tmetzinger]

>but i'm curious why the plane levels off when descending>through FS100. sure, it has to slow down to restriction speed,>but campared to the DF737 (which simply takes back thrust and>keeps V/S to slow down), i wondered if this is a normal>behaviour. i never realized that in a real plane. do they>really stay at FL100 until reaching 250? and isn't that>dangerous, maintaining an altitude for some time even though>ATC ordered a descent from - let's say - FL140 to 8000 ft.?Now before anybody blasts this gentleman for mentioning the DF airplane, can we get an answer to the question? I've never noticed a level off to decelerate during descent as a passenger in real-life operations.-Tim Metzinger

[Mats_J 2]

I think you have a decceleration step during descend at VNAV. Especially on VNAV SPD DES because you are running engines at idle and nothing more to cut to decrease thrust with. Comparing the PMDG to DF is a bit unfair to the former. ;-) Though I really like the DF it's not "as real as it could get"The main reason why you won't feel this in real life if probably because real pilots manage the DES stage by other means than VNAV, due to ATC restrictions, unpredictable de-routes etc, etc. As I understand it V/S gets you a more hands on control, and can plan your descend to be shallow enough to enable a slow down without need to level off. It increases the work load for the crew though, but gives a smoother ride for your passengers still nausious (SP?) from the launch! ;-)Just my thoughts on the subject,

[Guest amerton]

Hi Stu,This is correct. The FMC will not let the aircraft descend in VNAV below 10000 (or whatever airport speed restriction altitude is set) if its speed is not a few knots within asr.Within the idle segment (in general before asr alt), the throttles are aso held back (there are "different idle levels" - search the forum on this subject). There is a shallow decel segment before asr alt, but the AC will level out at 10000 to bleed speed if it still is too fast.Keeping VS to slow down is equivalent, you have to decrease VS to bleed speed, that will result in a shallower decel path...(of course if you mean DF cuts back on the throttles and bleeds speed at VS xyz *while shooting though the asr alt*, then I'd say it is not modelled accurately.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest jawbreaker]

thanks guysi see, so due to ATC it's pretty unusual to have a FL330 to 4000 ft. descent. seems quite logic to me.oh, and believe me, i didn't want to bring in another product here and make you angry...:-) it's just the plane i flew until i got the 737NG, so it's my only reference. but its definately not compareble. let me say, that the PMDG 737NG has the BEST programmed ILS capture i've ever seen on FS. so don't blame me...greets,stu

[Guest amerton]

No problem !As other 737s exist, it's not a problem to relate "things" to other products ;-)Fair enough....737 Pilots refer to their aircraft, non ATP guys refer to their simulator experience.What would be a problem would be statments like :It's not the same than product xyz, SO IT'S WRONG !Asking is fine, though ;-)anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com