Sign in to follow this  
Guest pschartau

Descend problems

Recommended Posts

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

Share this post


Link to post
Help AVSIM continue to serve you!
Please donate today!

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)

Share this post


Link to post

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

Share this post


Link to post

"- 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

Share this post


Link to post

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

Share this post


Link to post

>>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.

Share this post


Link to post

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

Share this post


Link to post

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

Share this post


Link to post

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

Share this post


Link to post

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,

Share this post


Link to post

Thanks, that clarifies it!cheersPhil

Share this post


Link to post

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

Share this post


Link to post

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,

Share this post


Link to post

>> 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

Share this post


Link to post

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

Share this post


Link to post

Haha, thats too funny, thats exactly why I asked ... everyone complains about Vnav in our sims yet the very same problem exists in the real plane :-lolRay

Share this post


Link to post

While we are discussing the topic of descents, I have a question about the 800/900 that perhaps a member of PMDG or the Beta Team can comment on.Using the 600/700 in VNAV PATH DES, with winds entered in the forecast page, you will still occasionally get a DRAG REQUIRED or be under speed and manual application of thrust is necessary.Now, I can correct the DRAG REQUIRED problem with the application of the speed brakes. But, I am unable to manually add thrust to correct the under speed situation.One other question

Share this post


Link to post

Hi Murf,I'll try to answer your questions. Though beware I might be wrong (It happens once in a while)! ;-)>Now, I can correct the DRAG REQUIRED problem with the>application of the speed brakes. But, I am unable to manually>add thrust to correct the under speed situation.This is something that I would like to see implemented in later models. A possibility to adjust the thrust when A/T is in ARM position. It is quite intricate though as there is no feedback from FS to the throttle levers (via servos) in the sim.I think the NG A/T will adjust the throttles when the speed drops low enough to compensate the lack of user thrust.>One other question

Share this post


Link to post

>Hi Murf,>>I'll try to answer your questions. Though beware I might be>wrong (It happens once in a while)! ;-)It's been known to happen with me on rare occasions as well.>>Now, I can correct the DRAG REQUIRED problem with the>>application of the speed brakes. But, I am unable to>manually>>add thrust to correct the under speed situation.>>This is something that I would like to see implemented in>later models. A possibility to adjust the thrust when A/T is>in ARM position. It is quite intricate though as there is no>feedback from FS to the throttle levers (via servos) in the>sim.It would certainly be nice if CH or another manufacturer would come out with a product which did move the throttle lever(s) accordingly. However, I find with my CH yoke that wehn I disconnect the Auto Throttle on approach the N1 remains as it was last set by the auto throttle. Once I touch the throttle N1 spools up or down to where the lever was positioned.It would be nice if the manual intervention could work like this. Don't know if this is possible but it sure would be nice to have this feature available.>I think the NG A/T will adjust the throttles when the speed>drops low enough to compensate the lack of user thrust.Must kick in at quite a low speed as I have not yet seen it happen, but then I am usually not that far below target.>>One other question

Share this post


Link to post

>>>Now, I can correct the DRAG REQUIRED problem with the>>>application of the speed brakes. But, I am unable to>>manually>>>add thrust to correct the under speed situation.>>>>This is something that I would like to see implemented in>>later models. A possibility to adjust the thrust when A/T is>>in ARM position. >>I think the NG A/T will adjust the throttles when the speed>>drops low enough to compensate the lack of user thrust.>>Must kick in at quite a low speed as I have not yet seen it>happen, but then I am usually not that far below target.Gentlemen -- Its funny this should come up, because I have been meaning to ask this question for weeks (only in a different way).1. My observations...(all these items are VNAV PTH)On approach but above 10,000ft the airplane will not add thrust (as far as I have seen) to compensate when speed drops below target descent speed. Is this correct? It seems like the plane would add power to correct.2. The above observation leads me to the following question...Initial Cleanance: Delta 425 descend at pilots discretion, cross LOMAC at 14-thousand feet, maintain 280 knots.Next clearance (before LOMAC): Delta 425, increase speed 300 knots.QUESTION...If I wanted to use the newly implemented speed intervention during a VNAV PTH descent to increase speed temporarily to 300 knots, then would the autopilot increase power to accomplish this increase without changing path angle?Hope this wasn't too confusing :)Thanks!

Share this post


Link to post

Hi Jeff>1. My observations...(all these items are VNAV PTH)>On approach but above 10,000ft the airplane will not add>thrust (as far as I have seen) to compensate when speed drops>below target descent speed. Is this correct? It seems like>the plane would add power to correct.Let's remember that a VNAV PATH DES will sacrifice speed for path. Meaning you as the pilot has to manage the speed. As this is not possible with the technology today the A/T will try to compensate for very low speeds during this descent. AT least that's what I've been experiencing.>2. The above observation leads me to the following>question...>>Initial Cleanance: Delta 425 descend at pilots discretion,>cross LOMAC at 14-thousand feet, maintain 280 knots.>>Next clearance (before LOMAC): Delta 425, increase speed 300>knots.The above situation looks a bit tricky. If I opted to go for a SPD DES I wouldn't easily know I would make 14000 feet at LOMAC. And with a PATH DES I wouldn't have full control of the speed. (Remember PTH DES doesn't give a dang about speed).V/S and SPD could probably be an option here. Adjusting V/S to get the green banana on LOMAC provided I set my MCP alt to 14000. And SPEED set to 280. Adjusting for the next clearance would mean alter the speed to 300, but that would change the green banana so I would have to realign it to LOMAC once again. If not able to do that I would have to state that would be unable to comply with ATC request.Any thoughts?>QUESTION...>If I wanted to use the newly implemented speed intervention>during a VNAV PTH descent to increase speed temporarily to 300>knots, then would the autopilot increase power to accomplish>this increase without changing path angle?I guess, as I haven't tested this scenario yet. That a SPD INTV would revert PTH DES to a SPD DES and then go with your speeds.>Hope this wasn't too confusing :)Well, I hope I wasn't too confusing answering! ;-)Cheers,

Share this post


Link to post

Try setting your Vnav Path speed to something more realistic. I'll bet your not even cruising at .810. I would never plan a path decent at this kind of speed in using Vnav Path descent because theres no room for error if forcast winds are off. The airplanes autopilot may pitch down to increase airspeed and once it gets close to the maximum speed, Vnav will disengage. A more reasonable airspeed might be .79/320 if you gota go fast. Going fast at this point in your flight waste fuel anyway unless it is ATC request.Floyd

Share this post


Link to post

@ZubartWas that response directed towards me or the oringinal poster?

Share this post


Link to post

>(Remember PTH DES doesn't give a dang about speed).Thanks for the reply Mats,I disagree with the above statement in some cases. Below 10,000 feet if the aicraft falls below target speed, the autopilot will add power. I am just curious to know what is correct in the real-deal. I can modify my easily modify my practices accordingly. Thanks for the reply!

Share this post


Link to post

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this