Hard alt constraints

[Autothrottle]

Hello,

I have question about hard alt constraints on the 737 PMDG.

VNAV is not always very keen on too many restrictions during descend, especially hard constraints as the FMC calculates from point to point. Restrictions to close to each other may lead to a disconnect as it need some time to descend.

If you have hard constraints which are too hard for VNAV especially below FL100 where speed doesn't have to exceed 250 kts,
which mode of descent is the most appropriate ?
Because if you use V/S speed can exceed 250kts.
Can you ask for a holding point if you are too high on the descent plan ?

Thanks.

Daniel.

[Nexus]

If you're in VNAV PATH, the aircraft is slaved to the vertical path to meet the alt restriction...it will prioritize path over airspeed so to speak. You want to descend but must control the airspeed, so extend the inflight spoilers

[Autothrottle]

Hello,

I've maybe made an error when I programmed the constraints in the FMC.

http://62.210.160.29/ImgAlbums3/Images1 ... 791186.jpg

I use the AMFOU4R star.

At AMFOU i'm at FL200, at ARMUS FL120 and at SINRA I'm at FL060.

http://62.210.160.29/ImgAlbums3/Images1 ... 314231.jpg

At SINRA I'm at FL060 and I programme a speed constraint too ( 220kts)
At DRAMO I programmed a alt constraint 3000fts.

Is that correct ?

[Nexus]

Is that correct, NOPE!

Ask yourself this, is it possible that in 4nm (distance between SINRA and DRAMO) you can loose 3000ft and at the same time decrease airspeed with some 20kts?

Of course you won't make it. You have simply interpret the charts wrong.

Yes at SINRA you should be at FL 060, but at DRAMO you would NOT put a hard alt constraint. Instead the 3000ft hard constraint should be set at the FAF for runway 04R. The FAF for 04R is 9,4nm from the Localizer (ident: NA 110.70mhz). To do this correctly, keep the 6000ft constraint at SINRA, but do NOT manually enter a constraint a DRAMO, instead go to the FIX page and type in NA...in the RAD/DIS line, simply create a fix 9.4nm from the NA...copy the newly created fix into the scratchpad, go to LEGS page and insert it right before the RW04R waypoint.

Great, we now have the FAF in our legs page, only thing left is to set the 3000ft alt. constraint, I also added the speed constraint, but that's not really important, I won't be flying the approach in VNAV anyway

[Autothrottle]

Thanks for your help Nexus  ;)

Excuse me but I still have some questions (I know I am annoying  ;) )

How do you make to reveal the green circle on ND and to what does it corresponding ?

Can you advise me for the approach or descent, from when can I pass in LVL CHG during the approach to Nice ?

Thanks.

Daniel.

[Nexus]

the green circle is a function of the FIX page, as I wrote in my previous reply )
Go to the FIX page, and type in NA.

Now below you have 3 lines where you can enter a RADial and DIStance. We will disregard the radial input for now, but we must tell that to the FMC so we put a SLASH "/" infront of the distance input, which is 9.4

So correct would be /9.4

Okay now you have the green circle, but no actual waypoint where the circle crosses your flight plan. Now on the FIX page, click the LSK where we have the /9.4, if you did this right, you'll have a waypoint in the scratch pad, this wpt is where the circle is cutting though the magenta flightpath
So all that remains is to enter the waypoint in the LEGS page, DONE!  ;)

In fact I did not have to use LVL CHG much (just completed a flight from Paris to Nice), I descended a bit earlier than the T/D calculated by the FMC, reduced the VNAV target speed and below 10.000ft I switched to LVL CHG (speed 220), when vertical speed was stabilized I switched to V/S mode...that's how you do it, smooth transitions from 3 different descend modes  ;D

[Autothrottle]

Thanks a lot for your help Nexus  :)

Daniel.