VNAV Speed Descent?

[Adam106 3]

Should the FMC Descent page have an option for a 'SPEED' descent? Apart from the speed intervene button how do we get VNAV to enter a VNAV SPEED descent?I was under the impression the des page should have options for PATH and SPEED descents - PATH being the default, with SPEED being selectable at button 5R? The FCOM v2 seems to agree. Adam Turley

[Buzz313th 0]

Anyone? JB

[nrcrate 40]

There was a RW pilot asking about this a while back. Not sure if he got an answer or not.

[emvaos 13]

We modelled the advanced version : common VNAV. Accordingly VNAV will go into VNAV SPD automatically once over-path, seriously over-speed (i.e. approx -5 max allowed) or with SPD INTV (but not in approach phase).

[Buzz313th 0]

Maybe the first service patch will address this, as there is now way right now to use the FMC to plan a descent with a speed target. Trying to set a speed descent for M.75 prior to descent and the FMC legs page still shows an economy descent with each fix crossing speed forcast at 267 kts before the restriction. Shouldn't the crossing speeds on the legs page show the descent target speed of M.75 at each fix? JB

[emvaos 13]

Yes up to switchover altitude. PS : VNAV SPD has nothing to do with this though.

[Buzz313th 0]

So with the advanced version, how do we plan a T/D based on a descent speed target at idle thrust? Correct me if I'm wrong, but if time can't be made up to meet a schedule in cruise, don't they try to make up the time through a faster descent? And if so, wouldn't the airlines wanna save as much fuel as possible and have the T/D for this "Speed Descent" based off of idle thrust? JB

Yes up to switchover altitude. PS : VNAV SPD has nothing to do with this though.

Well it doesn't. When I try to plan for a speed target of any Mach number in the descent page, the legs page still shows crossing speeds of 267 kts after the T/D and the aircraft will also fly the entire descent at 267 until the restriction altitude or fix.

[emvaos 13]

So with the advanced version, how do we plan a T/D based on a descent speed target at idle thrust? Correct me if I'm wrong, but if time can't be made up to meet a schedule in cruise, don't they try to make up the time through a faster descent? And if so, wouldn't the airlines wanna save as much fuel as possible and have the T/D for this "Speed Descent" based off of idle thrust? JB

You don't. It does it automatically. That's what VNAV does ! If your procedure is of a CDA type (constant descend approach) even better. That's why VNAV SPD (manually selected) is an ancient relic and no other Boeing plane has it. Welcome to the '90s !

[Buzz313th 0]

So I have to overshoot my T/D before I can get the FMC to give me any ETA forcasts based off the higher target descent speed? That makes no sense. JB

[emvaos 13]

No no no.... The "idle" part of VNAV PTH is ideally a level change down based on predictions, entered speed and weather forecast. It will design an idle path all the way to first restriction including decelarations... Try it....Trust me

[Buzz313th 0]

I tried it, but I am unable to plan for a "Speed Descent" with a target speed. Every single Vnav Descent defaults to Vnav Path even when I set my target speed to a mach number prior to descent. The Des page will even say "ACT M.75 DEC" but the FMC still plans and forcasts for an economy descent at 267kts. Personally, I think you guys left out the ability to define, plan and execute a Vnav descent based on a speed target. You think you can put it on the list for an upcoming service pack please? JB

[Av-Fan 0]

See following link: http://www.smartcockpit.com/data/pdfs/plane/boeing/B737/instructor/B737-FMC_U10.6_Changes.pdf

[Spin737 102]

I tried it, but I am unable to plan for a "Speed Descent" with a target speed. Every single Vnav Descent defaults to Vnav Path even when I set my target speed to a mach number prior to descent. The Des page will even say "ACT M.75 DEC" but the FMC still plans and forcasts for an economy descent at 267kts. Personally, I think you guys left out the ability to define, plan and execute a Vnav descent based on a speed target. You think you can put it on the list for an upcoming service pack please? JB

On the DESC page, can you define the speed? E.g. .78/280? If you can, what's the problem?

[namida 0]

I was under the impression the des page should have options for PATH and SPEED descentsAdam Turley

I'm quite indeed with this feeling. I's another question but posting here seems helpful for otheres interested in DES function.When I push DES NOW and descending with VNAV SPD, what is proper prodecure to get back to the path?

With a VNAV speed descent planned, VNAV starts an idle thrust early descent.VNAV does not attempt to capture the VNAV descent path.

FCOM2, 11.31.37 Early descent is opposite situation to late descent, but DES NOW (VNAV SPD) does not attempt to capture the VNAV descent path as mentioned above.With other jets I've experienced so many times it sinks below VNAV path I have been curious until read this section, But this is a different question. Should I just turn VNAV off and on or use V/S until captures descent path and VNAV again?

[Buzz313th 0]

On the DESC page, can you define the speed? E.g. .78/280? If you can, what's the problem?

Before the Des page is active I am unable to define a mach number for a speed. When I try, the page title references IAS in Kts and not the Mach number I am defining as the target speed. After the Des is active, the Des page title changes to reflect my selected Mach number, but the rest of the FMC ignores the "Mach" Speed Target and the plane descends following the scedule for econ descent at default speed. JB