Jump to content

Sign in to follow this  
Guest Cantuezel

"Unable to achieve (FMC) descend path"

Recommended Posts

Guest Cantuezel

Hi,I just flew a leg of about 550km with AP and FMC. At the FMC calculated T/D point the plane reduced to descend speed (280 IAS) and at first everything looked well.But after a while (the throttles were already at idle-position) the IAS (of course) increased more and more and suddenly there came the message..."Drag needed".So I extended flight-spoilers but even with them I was unable to achieve 270-280 IAS. Then after some more minutes there came the message on the FMC..."Unable to achieve descend path" (or so)From that point on I had to fly myself manually without FMC`s VNAF and the pre-calculated Descend path.My question: What did I wrong? It is not the first time with PMDG for me that I am much too fast even with the help of flight-spoilers. Is there a better way, so that I (or the AP) am capable to follow the FMC`s calculated descend path?Many thanks in advance...GreetingsSusan

Share this post


Link to post

whats new, i ve been flying this ac since 2003 and it happens to me too, just the way it is in real life i suppose.

Share this post


Link to post
Guest Cantuezel

Hi,well I really don`t think that in *real life* the cockpit crew pograms FMC and descend paths etc. when they all the time (specially when the enroute FL > 320) have to disengage the VNAF and the pre-calculated descend path when approaching an airport and continue manually...lol...:) :)Cheerio...Susan

Share this post


Link to post

Yea well i dunno, it happens to me and i just live with it, i also have the problem with speed on descent, the VNAV disconnets and i have to manually slow the ac down, 9 times out of 10 this happens.

Share this post


Link to post

If you're flying with winds aloft you have to input the data into the forecast subpage of the descent page.Hope this helps.Jeff


Jeff Hepburn

Share this post


Link to post

>well I really don`t think that in *real life* the cockpit crew>pograms FMC and descend paths etc. when they all the time>(specially when the enroute FL > 320) have to disengage the>VNAF and the pre-calculated descend path when approaching an>airport and continue manually...lol...:) :)They don't "program" descent path, it is programmed for them when they enter STARs, etc. The VNAV is precomputed for them. And yes, in real life they often have to depart this path because of ATC's actions. And as somone else here stated - if you did not key in predicted winds during descent you will often end up with too steep path and "drag required" message.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpg


Michael J.

Share this post


Link to post

hiwhat data do i need to input in the FMC for winds aloft,where do i get this info from, i dont use any third party weather programs.

Share this post


Link to post
Guest jase439

You'll need to get that information from the advanced weather dialog in FS.

Share this post


Link to post
Guest jase439

This is usually the result of programming bad constraints into the FMC or trying to come in "fast and heavy". If you have a steep descent profile coming up, level the plane, extend the spoilers and create drag. Below 250 knots you can drop a notch or two of flaps to help with this. Too often people make the mistake of coming in with 80% fuel, booking it down the descent profile at 2500-3000 fpm at 300 knots, and wondering why the spoilers aren't helping much. F = MAIf you find yourself in this boat, your best bet is to do an altitude intervention, level the aircraft, spool back the throttles, and slow the airplane, and then continue your descent in a controlled manner. Get your airspeed under control BEFORE you descend.

Share this post


Link to post

>>F = MA :-lolThis is why I always enjoy reading your posts...they actually make me use that thing between my ears for a brief second!


Jeff Hepburn

Share this post


Link to post
Guest gremel

AH yes.....Basic physic's 101 class or was it Aerodynamics class 101...Either way, they were both interesting..Argh..Regards,jack

Share this post


Link to post

>where do i get this info from, i dont use any third party>weather programs.you know, if you don't use any 3-rd party weather you can do the trick and assume that you ALWAYS have a tail wind and come up with some imaginary number, say 30 kt tail wind, or even more. Then you will never have the problem of a too steep descent, rather you may have to stretch but seems people here are most offended by the "not enough drag" issue. Just a thought how you could use ficticious winds to help you with your VNAV.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpg


Michael J.

Share this post


Link to post

Michael,This is an interesting idea you bring up. I know for a fact that this is used in real life as well as the FMC is known to be a bit quirky about the DES path and the NG is a slippery bird. Pilots tend to be on the conservative side with the head wind and tend to bias towards tail winds to be "on the safe side" of the DRAG REQUIRED message and to get some room to manouvre. ;-)Cheers,


Mats Johansson
PMDG Flight Test Dept
Boeing777_Banner_BetaTeam.jpg

| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

Share this post


Link to post

Susan,The difference between a VNAV PTH descent and a VNAV SPD descent is in a VNAV PTH descent the airplane pitches to stay on the path and disregards FMC target airspeed. In a VNAV SPD descent the airplane pitches to maintain target speed and ignores the path.When flying a VNAV PTH descent, the speed shown in the FMC is just a target speed for planning purposes. The autopilot will always pitch to maintain the path descent and disregard target speed. The Airspeed may increase up to VMO where then VNAV will disconnect. Airspeed increasing above target speed is generally OK as long as ATC doesn't hit you with a speed reduction. If airspeed is on the low side, then at about 15 knots below target speed the autothottles will add thrust until airspeed is back to target speed. Going over target speed is not a big deal but reducing speed to 240 knots at 10,000 feet is and this is where the speed brake will most likely be needed. Above 10,000 feet during a VNAV PTH descent the FMC will permit descending faster than target speed. It will not permit the airplane to fly faster than the 250 knots FAR rule and this is why VNAV disconnects at about 255 knots in a descent below 10,000 feet. Use the speed brake below 10,000 feet to maintain FMC default target speed of 240 knots. Unforcasted descent winds cause airspeed to go above or below FMC target speed. While still in CRUISE go to the FMC DESCENT page, push FORECAST, and insert the known winds at lower altitudes. This will help the FMC predict the PATH descent and if the forecast winds you put in the FMC are fairly accurate then the airplane will stay pretty close to FMC target speed during the descent.Physics and common sense also come in the play here. An altitude constraint during descent can not be so close to the airplane where by it is physically impossible for the airplane to accomplish it. During preflight check all FMC altitude and airspeed constraints to make sure they are physically practical for the airplane to do. Generally the airplane is capable of a 3 to 1 idle thrust descent in calm winds. A heavier airplane will glide farther than a light airplane.Also, be aware there is a VNAV "bug". If you are flying in a VNAV Pth descent, check the IVSI every now an then and if you see a magenta target bug this is not right. To get rid of the bug cycle the VNAV button OFF then back ON and the IVSI magenta bug should disappear and VNAV PTH descent will work properly.Good luck.Floyd


John Floyd

Share this post


Link to post
Guest Cantuezel

Thanks everyone for your help! :)@Floyd,thanks very much for your help with the diffenet VNav SPeed modes...yes, I forgot to enter winds, so I`ll try it again tomorrow...But I didn`t understand what you meant with the "VNAV bug". What is the "IVSI", what kind of cockpit instrument is that and where can I find it on the PMDG 737 cockpit? Or did you mean the "EHSI" instrument?Thanks again...:)GreetingsSusan

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  
  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    2%
    $640.00 of $25,000.00 Donate Now
×
×
  • Create New...