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Guest autothrottle

Problem with speed

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Guest autothrottle

Hello,I have a little problem with the speed during a Vnav Path descent.The flight is from LFPG to LFST and the aircraft is the 737-600.About 30 miles from de T/D I program the STAR and I insert speed and altitude constraints in the FMC (just one : ANDLO 220/5500)During all the descent I have a tail wind of 270/25.So I insert in the Forecast page (before the T/D): 10000 fts 270/60 to avoid a "over speed disconnect".But during the descent speed increases nevertheless and I have to use the spoilers at 10000 fts.Do you have other councils to avoid a vnav disconnect ?Or there is no other solutions that use of speed brake ? Thanks in advance.Daniel.

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Guest ahmedem

A very high percentage of descents in the NG will require the use of the speed brake especially during decel. I've flown IRL in NGs many times and the speed brakes have always been used during descents.If you run a search there's been a load of topics on this subject but to summarise most people, including myself will normally not use VNAV during descents instead opting for V/S and FLCH modes.

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Guest tomahawk_pa38

I thought like this a few weeks back. Since then I have practised and now use VNAV almost all of the time. I do increase the speed on the Descent page to /300 and now I never get 'des path unachievable' or 'Overspeed disconnect' messages. I watch the speed carefully and as soon as it starts to head towards 300kts I put the spoilers up.

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Guest EDCS

I think the vnav decent problem is one problem PMDG has to fix with the next step (BBJ?). If posibble, I plan my decent earlier and use vnav speed. If not, you have to monitor speed and wind direction and use the spd brk.Best wishesStefan

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Guest boxjockey99

DanielI completely understand the problem you're having, thankfully the old classic wing isn't as slippery so we don't suffer from it too much but here's some ideas for you to ponder. Firstly, you're putting the STAR in too late!, Consider that the FMC is calculating the routing as best it can between the route discontinuity from the end of the route to the destination airfield (ie a straight line). It has calculated its descent profile based on that straight line distance. If you are putting the STAR in 30 miles before the T/D you are all of a sudden changing the goalposts and the descent path must be reworked to take into the account the new STAR and the various alt/speed constraints it requires. This may mean that you are already past your T/D for that routing and so you end up high on the profile. As a result the VNAV PTH mode will dive for the profile and the speed will build regardless of the target speed and soon you'll end up with a VNAV disconnect. Tail winds will exacerbate the problem and in this scenario you need to be ahead of the game. The normal plan for us on the -300 is to set up an expected approach in the box prior to take off so the box has something to work with even if the runway is not correct. Normally I put in the approach that would give me the shortest distance route or take an educated guess based on the weather reports and local knowledge. In the cruise about 120nm from T/D we'll have got the latest weather from the VOLMET stations so we'll know the wind and so the probable runway. This is when you firm up the approach plan and start putting in any contraints that you might want to impose eg speeds for the intercept on the LOC etc. The STAR should put the 240/FL100 restriction in the descent page but some don't so check that its there or the box will assume it can do high speed all the way down! To the practiced pilot the setup will take about 5 mins to firm up and then you can brief your chum about what you're planning. This is usually around 80 miles from T/D. A quick nervous wee a letch at the cabin crew should have you at around 20nm which is just enough time to talk to your fans in the back (the passengers) and then request descent. The magic box should be able to handle that and you should find that the system holds its own.There is an old saying with aircraft.... never go anywhere in an aircraft that your mind hasn't been 5 minutes before. In this case the rule is more like never go anywhere that your mind hasn't been 120 miles before! The key to the 737 is preparation, be ahead of the game and ahead of the FMC as it will bite you in the #### if you let it fly YOU! Hope that helps, I'm gonna go and give it all a try and if I have any problems I'll get back to you.All the best Kris

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Guest EDCS

Kris,also thanks for this helpfull information.Two weeks ago I spend

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Guest autothrottle

Hello,Thank you very much for all your answers guy !I take notes of all your councils.Thanks.Daniel.

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Stefan,I'm not sure that PMDG are interested ni solving the real world problems with the NG for Boeing. ;-) If you know how to master the VNAV PATH descent you will not have any problems. Yes you have to monitor your speeds and that includes any wind shifts as well. As Randy would have put it; "Remember tha VNAV PATH sacrifies SPEED for PATH. And VNAV SPD sacrifies PATH for SPEED." You still have command even tough the Otto is doing it's job. If you are hot - Speed brakes out.


Mats Johansson
PMDG Flight Test Dept
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Guest EDCS

Hi Mats,why not. Sometime Boeing tells us, the real one works like the PMDG:)Wish you and all others nice Easter Vacation.GreetingsStefan

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Guest GeoffC

I always set a DES speed of around 280 - 290 well before TOD. Occassionally I put out the spoiler but never disconnect VNAV anymore.

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Guest autothrottle

Just another question.Does the 737 need 1Nm to loose 10 kts or 1Nm for 5 Kts ?Thanks in advance.

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