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PilotGav

Descending with VNAV and ATC

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If ATC directs me to descend to, lets say FL 110, prior to the VNAV path directing a descent, I think I'm correct in assuming placing 11000 in the MCP altitude and then Descend Now/Execute in the FMC will begin my descent, m while still staying in VNAV mode.Once I level off at FL 110, and then get assigned my next altitude by ATC, what's the procedure to continue the descent?When I tried this yesterday, I was still in VNAV at 110, however, changing the MCP to 6000 didn't begin the next descent, and when I opened up the descent page in the FMC - descend now was no longer available.Am I doing something wrong? What is the proper procedure for descending according to ATC instruction, while staying in VNAV mode?Gavinhttp://www.interprom.com/gavhttp://www.precisionmanuals.com/images/forum/ng_driver.jpgSpellink doesn't count!!

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Guest tmetzinger

>VNAV doesn't disengage - it just doesn't continue the>desccent.>>Are you saying I shoudl"re" engage vnav even though it's>already lit?>>I'll try that either way.The problem may be:You descended "early" using DES NOW, and reached your MCP altitude constraint. The aircraft leveled off. Good. But if VNAV is still active, you're still "below" your calculated vertical path.When you get a new altitude clearance, and change the MCP, the aircraft doesn't resume the descent until you intercept the vertical path.You (and I) expect it to resume the 1000 FPM descent when the MCP constraint is removed. I just don't know if it would do this in real life or not.You could set up a VS descent at 1000 and keep an eye on the vertical profile guidance, switching back to VNAV when you intercept the calculated path. In essence, you're doing the same thing a DES NOW command would do.Best wishes,

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Guest boxjockey99

I think I have my head around this. Again my experience is of the classic FMC and EFIS BUT here's what we do:If we are cleared to descend NOW! we would go to the DES page on the FMC and select the CAPTURE prompt. I'm not sure if this is in the NG FMC so I'll check and get back to you. This CAPTURE mode will commence a descent at 1000fpm until it intercepts the planned VNAV Path at which point the VNAV mode will follow the VNAV path and increase its ROD. If you level prior to the FMC planned alt ie MCP set to FL110 while the FMC wants to be down to 5000ft QNH the system will not descend below MCP alt and will enter ALT ACQ mode and then ALT HOLD. When you get a new clearance you should reset the MCP alt and hit the VNAV button to re-engage VNAV it should try to get itself back onto the VNAV path. In some cases it may not engage if it deems the VNAV path to be unattaintable, in which case a message will come up on the FMC and you will hae to use either LVL CHG or V/S to descend with your clearance.HOpe that helps, inthe mean time I'll try it out and report back with any findingsKris

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>You (and I) expect it to resume the 1000 FPM descent when the>MCP constraint is removed. I just don't know if it would do>this in real life or not.Yes,it should resume the descent at a 1000 fpm desent until intercepting the idle descent VNAV PATH, but in this simulation it doesn't.Floyd

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1] In a SPD DES pressing DES NOW starts a SPD DES, it will not capture any path at any time (but will keep wpt restrictions thence resume) 2] For the FMC default DEScent (PATH DEScent) if one uses DES NOW it will maintain 1000 FPM until the original calculated PATH in encountered and then resumes a PTH descnet, nothing else to program in this case and most CERTAINLY IS 100% CORRECT in regards to modeling this behaviour. 3] An intermediate level off in VNAV will show on the FMA "FMC SPD/ALT HOLD", the VNAV button will not be lit! BUT it is controlled by the FMC, this is exactly like the real bird too! To resume a DEScent simply set the MCP and press VNAV. If you have waited too long you might not be able to re-captue the path. Using VNAV PTH in the real world when you know ATC will interfere is not a good plan, go LVL CH or SPD DES and save yourself the extra workload! 4] A final word, Gents, with the implementation of ALT INTV - VNAV ALT we will have to unlearn some of this behavior, as an example,a: You are climbing to your CRZ Altitude of FL 310 but are asked to hold at 10000. MCP is at 10000 and airplane levels off, what the FMA now shows is FMC SPD/VNAV ALT"**, simply redial in your CRZ ALT and hit the ALT INTV button! Just to give you fellas an idea of what to expect on the 800-900 ;-) ** VNAV ALT will not be the a default MODE on the 800-900 (it will remain FMC SPD/ALT HOLD in these cases, VNAV ALT must be selected from the PMDG/STYLES OPTIONS MENU.[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 |

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Guest tmetzinger

> 2] For the FMC default DEScent (PATH DEScent) if one uses DES>NOW it will maintain 1000 FPM until the original calculated>PATH in encountered and then resumes a PTH descnet, nothing>else to program in this case and most CERTAINLY IS 100%>CORRECT in regards to modeling this behaviour.>> 3] An intermediate level off in VNAV will show on the FMA>"FMC SPD/ALT HOLD", the VNAV button will not be lit! BUT it is>controlled by the FMC, this is exactly like the real bird too!>To resume a DEScent simply set the MCP and press VNAV. If you>have waited too long you might not be able to re-captue the>path. Using VNAV PTH in the real world when you know ATC will>interfere is not a good plan, go LVL CH or SPD DES and save>yourself the extra workload! I beleive that Gavin indicated that when he reached the MCP altitude that VNAV did NOT extinguish.I quote:"When I tried this yesterday, I was still in VNAV at 110, however, changing the MCP to 6000 didn't begin the next descent, and when I opened up the descent page in the FMC - descend now was no longer available."Either something isn't working right (sim error) or he wasn't in VNAV the way he thought he was (pilot error). Gavin, would you try this again? And when you level off at your MCP altitude, check two things.... The status of the VNAV button (lit/dark) and the Flight Mode Annunciator on the PFD.Based on what Randy said you should see the VNAV button dark and the FMA should show ALT HOLD in the pitch mode.

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It's important to remember too that at some point during the descent/approach, especially if ATC starts vectoring you or stepping you to intermediate altitudes that it's probably better to just revert to manual MCP modes like HDG SEL, LVL CHG, and VS. A lot of times this ends up being much simpler in the terminal environment than trying to make the automated modes do it...

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Guest boxjockey99

OK I tried this all out on the PMDG bird then I did it in the real sim earlier today and this is what happened:CLIMB-----VNAV set for cruise level of FL260 MCP set 10,000Levelled off at 10,000 using ALT ACQ then ALT HOLDVNAV button light out FMA = FMC SPD, LNAV, ALT HOLDReset MCP to 26,000 - ALT HOLD button light comes onselect VNAV button - ALT HOLD button light out VNAV light on climb commenced Same in real 737-300 simDESCENT-------VNAV set for 5000 in FMC, MCP set 10,000Prior to T/D using DES NOW function in DES page (same as CAPTURE on 300 model) Descent commenced at 1000fpm until intercepting the VNAV profileat (or just above) 10,000 entered ALT ACQ then ALT HOLDVANV button light - OUTFMA - FMC SPD, LNAV, ALT HOLDReset MCP to 5000, ALT HOLD button light ONPress VNAV and descent re-commenced. I discussed the possibility of going too far past the profile with my TRI, he said even in this situation the VNAV would still re-engage and try to recover the profile. In this situation however it is unlikely you would use VNAV when you see how far off the Vert profile you are. In this situation you are more likely to use LVL CHG in order to get speed control then wind this up to the barber pole and use the speedbrakes as appropriate.We actually did this in the sim and ended up 9999ft above the Vert profile after taking out several legs of the Stansted Lorel 1K STAR. Using VNAV all we got was a descent of about 2200fpm, by isung LVL CHG along with the speedbrakes with the speed window set at 315 kt (yes we had free speed below FL100) we managed 4500fpm and quickly recovered to make a standard configured approach. This is by no means ideal and there are other ways to recover this problem (namely going into the hold and shuttling down the stack) but if you are feeling brave this is one way to do it.Hope that helpsKris

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