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Adam106

VNAV Speed Descent?

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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

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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).

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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

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

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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.

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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 !

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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

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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 Peace.gif

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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

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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?

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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?

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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

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How about using Speed Invervention?

Push (when VNAV engaged) –• IAS/MACH display alternately shows selected IAS/Mach and blanks• when IAS/MACH display is unblanked, FMC speed intervention isactive, FMC target speed is displayed, and IAS/MACH Selector may be used to set desired speed• when IAS/MACH display is blank, FMC computed target speed is activeand displayed on the airspeed indicator.
FCOM2 - 4.10.4

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I am no expert, but if you go through your legs starting from the end and apply the 4x rule to check if each of your waypoints is low enough, you should have plenty of space to slow down. For example, if a waypoint 12 miles from the airport says "/5000A", it is clear that you are probably going to overshoot, and you should change it to r "/3000B", and maybe put a lower speed if necessary . If you want to be realistic and consider this "cheating", you can always try sticking your hand out the window to increase drag... Henri

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In DES NOW VNAV goes in under-the-path mode. I.e. FMC SPD and -1000 fpm vertical speed until crossing the path OR if it transitions to approach phase level off until crossing the path. When on path (or to be precise near the path) VNAV will resume normally.

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How about using Speed Invervention? FCOM2 - 4.10.4
Speed intervention is only available for the Des when the Des page is active (Actually being in the DES flight phase). So, it is after the fact. I would like to plan for a speed target des while in cruise phase and have the FMC calculate and forcast the new TOD, ETA and path based on a changed speed durring the Des portion of the flight. The FCOM section 11 describes the ability to command manual speeds either in Mach or in CAS for a Vnav Manual Econ or Manual Speed descent before des is active.

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In DES NOW VNAV goes in under-the-path mode. I.e. FMC SPD and -1000 fpm vertical speed until crossing the path OR if it transitions to approach phase level off until crossing the path. When on path (or to be precise near the path) VNAV will resume normally.
Thanks for detailed reply, EMV but..It is a normal DES phase most of us usually use but if pushing DES NOW makes VNAV to DES VNAV SPD mode, (not PTH)as described on FCOM2, "DES NOW (VNAV SPD) does not attempt to capture the VNAV descent path."This means as long as I descent fast enough, there'll be no chance to meet calculated descent path.So, the question is.. which mode should be used to get back to the predicted path and eventually see VNAV PTH on PFD. p.s. Actually this is so little part of the flight but we all wanna do that all with FS world.. right?

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When you press DES NOW it goes to under-the-path mode and DES phase becomes active prematurely. What may happen however is that the active wind and temperature mixing model starts working, in which case you are actually over-the-path due to unforecasted tail winds or very large ISA deviations. The other thing is that you might be exceeding the FMC absolute limit speed for DES which is the lowest of 330 / M0.80...In this case VNAN goes into overspeed mode (i.e. VNAV SPD). (I hope we are not confusing DES NOW with resolving VNAV ALT with ALT INTV here...)

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Speed intervention is only available for the Des when the Des page is active (Actually being in the DES flight phase). So, it is after the fact. I would like to plan for a speed target des while in cruise phase and have the FMC calculate and forcast the new TOD, ETA and path based on a changed speed durring the Des portion of the flight. The FCOM section 11 describes the ability to command manual speeds either in Mach or in CAS for a Vnav Manual Econ or Manual Speed descent before des is active.
You should definitely be able to define your own speeds. Are you using the correct entry format? Are you overwriting the default with your own descent speeds? Eg. .78/280? Remember that the FMC is going to switch from Mach to IAS when they meet on the way down (around FL260 or so). Most crossing restrictions are below the altitude where you'd have IAS restrictions instead of a Mach restriction. Eg. Cross NORMY at or below 13,000, 270 knots.

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I just did another flight and this time instead of trying to enter the Mach number, I entered the CAS for that Mach number at given alt and it worked. For some reason the Des page pre des will only accept CAS as target speeds instead of Mach numbers. I'm gonna take a stab at this and say, it's because every CDP has to be for a constant airspeed and not a Mach number or else the CDP would not be a CDP and the descent path would get steeper at lower altitudes since if you targeted a mach number, your airspeed would increase as you started to descend. This makes sense, since the FMC in cruise, will try to predict a T/D based on the numbers in the DES page only on a Constant Descent Path... Wish it was clearer in the FCOM JB

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As long as you put them both it, It should work for Mach and IAS. Have you tried putting both in? .78/280

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