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tamba765

Problems meeting speed restrictions in VNAV path descent

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When a crossing altitude is coupled with a slower speed restriction at a waypoint, complying with the altitude works fine, but I almost always need the speed brakes to meet the speed restriction. The wind direction doesn't matter and the aircraft is on the VNAV path descent profile.

This happened occassionally in the real aircraft, but not every time. Is anyone else experiencing this problem. Thanks.

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It is not unusual to use the spoilers to meet speed and crossing restrictions. A lot of times the autothrottle is disconnected because the engines tend to surge when at part throttle in climb or descent (FL CH). The best way to meet these speed/crossing restrictions is to disconnect the autothrottle and fly the green arc using V/S.

 

Billy Bluestar

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I realize all that. I'm saying that in the real world this doesn't happen all that often. I pull the thrust levers back to idle after they go to "hold", but the aircraft still doesn't meet the speed restriction. As I said before, I'm in VNAV PATH when this occurs, not FLCH. Disconnecting the autothrottle and going to V/S is not standard proceedure for any airline that I know of. Besides, disconnecting the autothrottle in a V/S climb is definitely not a good idea.

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Do you have the winds entered into the FMC? I've noticed that pretty much eliminates having to use spoilers save the occasional steep descent after a waypoint close to another.

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I realize all that. I'm saying that in the real world this doesn't happen all that often. I pull the thrust levers back to idle after they go to "hold", but the aircraft still doesn't meet the speed restriction. As I said before, I'm in VNAV PATH when this occurs, not FLCH. Disconnecting the autothrottle and going to V/S is not standard proceedure for any airline that I know of. Besides, disconnecting the autothrottle in a V/S climb is definitely not a good idea.

 

Well, I guess we'll just have to disagree.

 

Billy Bluestar

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Besides, disconnecting the autothrottle in a V/S climb is definitely not a good idea.

A V/S climb is never a good idea, regardless of the auto throttle. It can never match an IAS climb for efficiency, economy or speed and introduces significant safety issues.

 

I am not quite sure what you mean when say this does or that doesn't happen in the "real-world". In the real world, and in the sim, wind speed and direction always matter. If you don't tell the FMC what to expect, then it will make its plans based on zero speed winds. If you dont have zero speed winds, its plans will be wrong. Also in the real world, and in the sim, the use of spoilers, V/S, flying FMC predictions (green arc), manual throttle and even full manual control are all quite normal in managing descent profiles. As is skipping, ignoring, announcing "unable" or requesting a change for a given restriction so that all the others can be met more easily, efficiently, and/or safely.

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Where can you find descent wind speed?

If you are asking what the predictions are for planning purposes, look to your weather engine. If you don't use one already, I recommend investgating AS2012. You can get them directly from FSX but if memory servers, it is a pain in the butt. If you want to know where to enter the descent forcasts, have a look in your FMS manual 12_51

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Westjet uses 280 for CLIMB and 270 for DESCENT. Once I change those values in the FMC I've never had an issue making restrictions, unless a controller got lazy or the winds/ZFW are incredible.

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