Descend problems

[Guest pschartau]

Hello,The following scenerio has occured twice now while flying online:- while climbing, I decide that I want to cruise 2000ft higher than filed, for wind reasons- after getting the clearance for this, I change the cruise level in the FMC, hit the EXEC button, and then change the cleared altitude on the MCP to the new cruise alt- Everything seems to work fine- I enter descend forecast winds into the FMC, and I'm in a hurry, so rather than using the standard descend, while cruising I select a 0.81/330 speed descend, so that the TOD is recalculated- I get descend clearance a couple of miles before I reach the TOD- I select the cleared alt on the MCP and on the FMC descend page I hit "DESC NOW"Now here comes the problem:- Rather than doing a 1000fpm descend (as it should after hitting DESC NOW), the aircraft seems to behave as if it's in LVL CHG mode, flies 330 knots, and drops like a stone (5000 fpm)!!!Have I done something wrong or is there a problem with the aircraft?RegardsPhilipp

[gbalean 0]

Hi Philipp!Correct me if i'm wrong ;), but doesn't this 1000fpm initial descent only apply to a plan descent (I normally don't push the DESC NOW button)? In speed descent, the plane will put itself at whatever attitude necessary to keep the speed you entered, in this case 330 kts.Maybe I'm completely off target however :)Geoffrey Balean (YSCB)

[Guest pschartau]

Hi Geoff,thanks for your answer. I understood that for any descend, the aircraft descends on idle throttle, and choosing the pitch in order to maintain the selected FMC speed. However, in DESC NOW mode, the aircraft always descends at selected speed and 1000fpm, until it intersects the planned path, when it continues as before on idle thorttle.Is this correct? If yes, why doesn't it behave that way?ThanksPhilipp

[crvm 0]

"- after getting the clearance for this, I change the cruise level in the FMC, hit the EXEC button, and then change the cleared altitude on the MCP to the new cruise alt" Here is an easy way to perform this. 1] Have the CRZ page open on the FMC, get your cleance and then2] Re-set the MCP to your desire altitude. This will place the new altitude into the CDU's S/P.3] Line Select 1 left LSK and EXECute. The aircraft will now perform a "CRZ CLB" to the new altitude. "- I enter descend forecast winds into the FMC, and I'm in a hurry, so rather than using the standard descend, while cruising I select a 0.81/330 speed descend, so that the TOD is recalculated" And here is a major part of your problem. Descents are a bit complicated so it's best to have a good understanding on the various options offered to you. If you desire to keep a set speed (why would you? unless ATC -restrictions etc) you cannot use a PTH DES to maintain a steady speed, the target speed in for refernce prposes only the aircraft will not make any attempts to keep the speed unless you become some knots under the target or overspeed etc, you must choose a SPD DES. A path descent will keep the calculated path without regards to the descent target speed. A speed descent will keep the speed without regards to the calculated path. A DES NOW will descend at 1000FPM until the original calculated descent path is reached and start whatever descent you choose (If you did not choose a descent *type* the default is a PTH DES. So my advice to you is this, if you want to manually control the aircraft do so, there are many other ways to descend if one is in such a hurry without asking the AFDS to go along with your plans but if you do make sure you know how of each mode works and then having it do what you want is much more simple a task. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

[Guest pschartau]

Thank you for your tips! But...> A DES NOW will descend at 1000FPM until the original>calculated descent path is reached and start whatever descent>you choose (If you did not choose a descent *type* the default>is a PTH DES. That is my entire problem!!! The aircraft does NOT descend at 1000fpm, instead, while in VNAV, when pressing DES NOW, it behaves like LVL CHG mode - the throttles go idle, and at the same time the aircraft maintains the speed selected in the CDU descend page, which results in a 4000fpm or more descend!cheersPhil

[Guest tmetzinger]

>>That is my entire problem!!! The aircraft does NOT descend at>1000fpm, instead, while in VNAV, when pressing DES NOW, it>behaves like LVL CHG mode - the throttles go idle, and at the>same time the aircraft maintains the speed selected in the CDU>descend page, which results in a 4000fpm or more descend!>I think since you were "a few miles before TOD" that by the time you initiated the DES NOW function you were close enough to intercept the path. Remember that a fast descent means a steep path.DES NOW is generally used when you're many (tens to hundreds) of miles "early" but have been told to descend anyway.

[Guest pschartau]

Hi Tim,thanks for your answer, but that's not the solution. When I say "a few miles", I mean about 20 miles. Also, I can see the deviation from the descend path increase rapidly on the ND.cheersPhil

[Guest Zapper]

Hmmm..Does this problem only occur after carrying out a STEP Climb?Can I ask.. when you are having this problem what is the VNAV FMA saying? SPD or PTH?To be honest I avoid using DES NOW as it just stuffs up an otherwsie carefully planned profile. That said it does send you down the hill at 1000'/min and is used to intercept the VNAV PTH. Just make sure you aren't somehow ending up in a SPD DES. As always.. the FMAs tell the truth.Rgds,Brad Marsh

[Guest pschartau]

Nope, it occurs always when in SPD descend. I checked the manual, it says nowhere that the DES NOW mode is only for PATH descends.Surely, when in SPD descend mode, the CDU still plans some sort of path, otherwise it wouldn't be able to calculate where the TOD is. So it should descend at 1000fpm until intersecting that path and then continue at the planned speed and whatever descend rate that gives you!It works fine in ECON PATH descend though.cheersPhil

[Mats_J 2]

Hi Phil,Seems to be something amiss about this in the AOM. According to Bill Bulfer the DES NOW (CAPTURE) is only valid for a PATH DES. So I guess that's what is causing you problems here.Will report this to the team as well, as the DES NOW shouldn't be selectable if you are flying a SPD DES.Cheers,

[Guest pschartau]

Thanks, that clarifies it!cheersPhil

[crvm 0]

No sorry it DOES NOT. I was not at all clear on the differences and in fact sounded like the selection of DES NOW function was the same for both DES MODES, wrong. DES NOW is there for either a PTH and SPD DES.Only three things remove the DES NOW prompt -1] FMC already in a descent2] Active leg has a GP angle3] No hard altitude constraint between you and the next GP angle a:PATH DEScents with DES NOW - Will result in a 1000 FPM with TARGET SPEED until the calculated DES path is encountered, then PTH DES etc.b:SPD DEScents with DES NOW - Will result in no capture of the calculated decent path! Once you press DES NOW with SPD selected as the MODE it will go SPD DES immediately! So one can say there is not a *function* that DES NOW does with a SPD DES only that it starts your SPD DEScent NOW. NG OPSMNL 263,1165,1166,1328,1357 and many more REFs in manuals ;-)[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg

[Mats_J 2]

I guess I have to correct myself after getting some further information from the team.DES NOW is available for a SPD DES as well as for a PTH DES. It will however not try to capture any path, and will not descent with a 1000 fpm. DES NOW in SPD DES mode will throttle down to IDLE, then ARM for the A/T mode and VNAV SPD will be the pitch mode.Sorry folks... ;-)Cheers,

[Guest Ray CYYZ]

>> NG OPSMNL 263,1165,1166,1328,1357 and many more REFs in manuals ;-)Sure you didn't drop a page last time you took the books for a walk for serious reading? :-lolBy the way as I haven't seen it mentioned anywhere, but is the 800/900 running U10.5 now instead of the U10.4?Ray

[crvm 0]

Hehehe. Funny you mention this question Ray, I have brought this very thing up a few days back. I am not sure but I would at least like to see the VNAV restrictions from U10.5 added for less VNAV discons upon descending. There are a couple of other goodies, all I can do is ask.. [h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpg