I know there's been lots of discussion about this, but can I have a simple yes or no please. In VNAV descent should the aircraft decelerate according to the speed settings on the LEGS page? If I set a constraint of say 200/3000 should I expect this speed to happen automatically.John

Make BOTH *BIG* (200/3000 manually entered). But do not expect this aircraft to both DESCEND and SLOW at the same time. you must help it slow and planning is a major part of descending. If you are humming along at 250 knots and @7000' but only a few miles ahead you put a restriction of 200/3000 do not expect VNAV to perform wonders. Know what the limitations are and work within these at all times for you planning.[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 |

So I take it that is basically a yes.John

The answer is yes if you enter a speed constraint, however, the NG is horrible at slowing in VNAV in my opinion. Best thing to do is to plan a descent ahead of the FMC calculated TOD to help slow down when necessary. It's not like it won't descend on schedule and slow at 10,000, it's just that I find it hard to slow on final for Vref. I've heard the NG is like this in real life, and the spoilers are worthless. Last night I had to drop the gear just to slow down from 180 knots on final so I could extend flaps beyond 5.Chris

I am wondering if we could get a VNAV Descent tutorial for the problems we are having.I would love to hear ways of planning the descent. JeffG

Hi jeffGIt wont do this:250/10000-------| | |200/9000--------|---------><----Time?------>Will do this:250/10000-------| | | | |200/9000--------------------------|<---------Enough Time-------------|Hope the diagrame works.Regards

My diagram didn't come out right.Regards

Here's an example to demonstrate my problems.Egll 27L to Lfpg 27L MID SFD DPE DPE2E ILS27L MERUE Transition using the Lfpg SIDs and STARs from the NAVDATA siteCruise level FL270, Fair Weather, Fuel 60% 0 60%, Payload 37582737-800ALT constraint at SOKUM FL130 achievedSpeed constraint at CRL36 250 ignoredALT constraint at MERUE FL110 achievedSpeed and ALT at CRL10 250/FL110 achievedCRL03 VNAV flicks off with Descent Path unachievableAccording to my chart you should stay at 4000 till the ILS so this behaviour was not out of order. I'll try this again with different constraints. But I still don't understand why the constraint at CRL36 was ignored. Only the CRL10 constraint was on the DESCENT page. Are speed constraints acted on if there is no ALT constraint at the same waypoint?I managed the rest of the landing using V/S and SPEED then an Autoland but the Spoilers didn't deploy, Oh well.Just before CRL03 on the Magenta Line were two green circles, what were these?John

" Are speed constraints acted on if there is no ALT constraint at the same waypoint? No. In the real world YES you CAN have a speed res without an altitude restraint. I think this needs to be on the FIX ME LIST. Please understand that your route contains MANY restrictions something not good for a VNAV descent. The FMC will calculate from wpt to wpt. The AFS will dwsiconnect VNAV if the path exceeds 7 degees above 10000' and 6 below. Even in the real world one would not want to use VNAV in restrictive situations. Have you tried a SPD DEScent on this route? It might be a better choice. If the altitudes are clearances to each altitude set the MCP to the altitude to help the aircraft loose the excess energy on the way down. With VNAV ALT option active VNAV will stayed engaged. [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 |

There are several ways of conducting a VNAV desecent. But the main thing is IRL we have our heads ALWAYS ahead of the aeroplane. A rough guide for almost any descent is called the 'Three Times Profile' This is a very simple way to make sure your descent isn't going to leave you hot and high or low and slow.. and guess what.. the big secret?Multiply your Altitude in thousands of feet by three and this will give you the rough distance you need to get down.So if you are at 12000' then 12 x 3 = 36 miles .. Most guys use a bit of a buffer as well and add 10 miles so while you are learning to fly this rocket, say 10 miles.. so at 12000' we would want to be about 46 miles... hard huh?! :)To get a little more technical:The aeroplane declerates about 10kts per nautical mile in level flight at flight idle and a little less in LVL CHG descent. So if you are doing say 300kts at 25000' and you want to know how far down the track you will be at 250kts at 10000' assuming you will do a 300kt descent to 10000' then a level deceleration ....you have 15000' to descend .. so 15x3=45 miles to when you get to 10000' at 300kts and then a further 5 miles to slow down 50kts .. So you will reach your target speed/alt combination in about 50 miles .. make sense?Most blokes also add about 1 mile for every 10 kts of tailwind.I think that is enough for now but I am glad to help further if ne1 is interested. Energy managment is teh biggest challeng any pilot faces when transitioning to a jet and teh 737 can be a slippery customer if you are not on your toes. As alway in aviation it is best to get good advice and then stick to it!Also have a look at the suggested DES Profiles on the Geezer Guide.Cheers,

Just a note for the guy talking about spoilers being useless. Below 230kt they are considered to be much less effective as brakes so at 250kt they are freat but below 230 they loose a lot of their effectiveness. The 300 will go down and slow down but only to a point. The Vnav profile will decellerate by raising the nose to achieve 500fpm (min descent for ATC purposes) and allow the speed to wash off. Once it is at the desired speed it will continue down pitching for the path that it has calculated. My training capt described VNAV in a simple (ish way) so this is how he see's the classic version of VNAV:VNAV PATH is the most efficient way to descend, however you must remember it can do only 2 things!1) descend at 1000fpm to intercept the path2) close the thrust levers and glide, pitching for the VNAV path profile regardless of speed... so it will probably accellerate past the magenta bug, this is normal.VNAV SPEED is less usefull as it ignores the profile completely, SPEED can do only one thing!1) close the thrust levers and glide pitching for 293ktSo in the 300 with Update 5 of the FMC we are in a dilema as we cannot have VNAV path when in HDG SEL. we only get VNAV SPEED in HDG SEL which is a pain in the bottom! Our only recourse is to mimic VNAV PATH using V/S mode and descend at 1000fpm till we come within 700ish feet of the planned VNAV path then engage LVL CHG and use the speed window to maintain the profile just as VNAV PATH would do. This isn't ideal and I believe it was addressed in the U10 version of the FMC so NG shouldn't be affected. HOWEVER the VNAV modes remain the same so they only do what I have stated above! Hope that helps get it straight in your head, I know it helps me as it removes the mystery behind whatr VNAV is doing in the descent!Kris