Jump to content

Sign in to follow this  
Guest airborne1

FMC descent profile wrong?

Recommended Posts

Guest airborne1

Hi all, I am a bit confused with how the fmc profiles the aircraft's descent. Whenever I descend, either at the T/D location or before, using des now, I always require drag which I presume to be the deployment of the speed brake. What is peculiar is that the engines are still at 40-45% ng. Surely if the engines were at idle then the aircraft would not require drag and save lots of fuel.I use headphones for the sound and it is very noisy using the spoilers all the way down.Any help on this matter would be greatly appreciated.Keith Sandford.

Share this post


Link to post

First of all you dont need to deploy the speedbrakes hole the way to up. Pull them down to flight detend and you dont get the noise.Second: DRAQ REQUIRED is exactly wat you sad. You need to use the speedbrakes. The FMC descent profile is mod. by you. You need to enter the proper descent forecast, with wind dir. and spd and then the FMC is gona calc. were the T/D is starting. Just enter the FMC forecast page correctly and you wont have any troubles anymore.Because DRAQ REQUIRED means that you have entered a wrong value in the descent forecast page. And the descent IDLE thrust is at IDLE N2 not IDLE N1!! Good luck.

Share this post


Link to post
Guest boxjockey99

Keith This happens on the real aircraft and there is a simple solution(s)First you need to know what is happening. The FMC is calculating what is known as a 'Constant Descent Approach' (CDA) ie it is planning to not have to level off till it lands. This is mainly becuase it is more efficient and less noisy for both those aboard and those beneath you. So the FMC uses power as required to maintain both speed and V/S in order to meet its CDA. The little scale that appears on the right lower corner of the nav display (looks like a mini-glideslope display) shows you where you are in relation to the vertcial profile or you can go to the DES page where it will show you in numbers how HI or LO you are in the bottom left of the page.Right on to the solutions. First, make sure you set the MCP alt BEFORE the top of drop or you'll miss it. Set the MCP to the new level a sensible time ahead (say one waypoint but really you can do it any time once in the cruise.) This means that as soon as the aircraft gets close to the T/D point it can start slowing itself down and then descend on the profile. I notice it likes to decellerate about 2-3 miles BEFORE the T/D then ease itself down bang on the profile.Second, don't put spoilers out immidiately, let it settle down. The aircraft will initially think it needs drag as the engines take a little time to come back to idle, by putting the spoilers out immidiately it will think ah 45%n1 is ok and just keep that! So let it fight on for a few minutes ensuring you don't go too far off the profile on the little glide slope display on the ND!Finally and this is a quirk of the FMC and its being a computer. If it can only hold say 2 ft HI it will a) do it's best to maintain that but :( be unhappy about it because it isn't bang on. Becuause it isn't bang on the profile it will complain and ask for more drag in order to get back onto the profile. Simply watch the mini-glideslope display and if it looks about in the middle then it should be OK, just ignore the warning. I suspect in your case you are setting the MCP too late and so it is significantly off profile before the T/D even gets there! If all of this doesn't work and the engines stay at 45% n1 etc hit LVL CHG and set the speed window to the DES speed in the FMC. LVL CHG will automatically put the throttles to IDLE and the A/THR to ARM mode. The VNAV profile (glide slope indicator thingy) will still show so you can then decide to either to allow it to make up the descent as it is with the engines at IDLE or wind up the speed to chase it back to the middle if the IDLE isn't enough (if you wind up the speed the aircraft will pitch down to accellerate as A/THR is set to IDLE in LVL CHG). Once it gets back to the middle you can re-engage VNAV and let the aircraft sort itself out!Hope that lot helps, let me know how you get onKris

Share this post


Link to post

A few words here, The FMC is very complex and it will issue you a message to RESET THE MCP ALT at 5 miles to top of drop. It should be the one thing you pay attention to when you see your progress gettin close to TOD. As for the DRAG REQUIRED message on the upper half of a path descent, without getting to complex one can simply ignore the message (i.e. you can deploy the speedbrakes but it is not really going to be a issue unless you are within a few knots of the POLE or in rough windy conditions.) You can also use F4 to increase your speed because the AT is in the ARM mode. Now getting close to 10000' it's good to be close to target since the aircraft needs to DECEL by 10000' to 240 knots (10 knot buffer from 250). Remember also that a PATH descent does not keep the target speed (unless you are getting much too slow) but will keep the path regardless. Now with a speed descent the aircraft will forget all about the descent profile path and keep the target SPEED. It is important to know the difference and what is better to use if you have an idea of ATC restrictions etc. Ok time for bed ;-)[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/forum/betaimg.jpgAMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | WD SATA 80 GIG | 512 DDR 400 |

Share this post


Link to post
Guest airborne1

Thankyou all very much, to get advice from a real pilot is indeed an honour.Keith Sandford

Share this post


Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  
×
×
  • Create New...