737 NG Path Decent
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737 NG Path Decent
Hi everyone and Happy 2010!
Do you do updates of the next waypoint during decent to correct the decent path?
Some people say it's ok, some people say it's wrong.
The question is that even if you insert some forecast winds on the decent page, (maybe the problem is those wrong decent forcast winds) it gets high or low near the IAF.
I monitor the spot wind during decent and when I notice a big wind change, I simple do a "direct to" the next waypoint again and the path corrects.
Any one?
Do you do updates of the next waypoint during decent to correct the decent path?
Some people say it's ok, some people say it's wrong.
The question is that even if you insert some forecast winds on the decent page, (maybe the problem is those wrong decent forcast winds) it gets high or low near the IAF.
I monitor the spot wind during decent and when I notice a big wind change, I simple do a "direct to" the next waypoint again and the path corrects.
Any one?
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Can someone explain which winds are used and where on the 737? I understand the wind entered in the perf page is used in the climb and the cruise? If no wind is entered in the descent forecast page then will it work out on zero wind?
I have also heard that on the active leg i.e. the White waypoint rather than magenta, then the FMC will use the actual wind - can someone confirm if/when this is the case please?
Many thanks in advance
I have also heard that on the active leg i.e. the White waypoint rather than magenta, then the FMC will use the actual wind - can someone confirm if/when this is the case please?
Many thanks in advance
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For the Classic which I assume is the same in this regard as the NG: If you don't put any winds into the DES FORECAST page, then the FMC will use CRZ wind linearly interpolated to zero on the ground. However, if you modify the route during the descent the FMC will then use the current actual winds! So if you re-enter the top way point in the LEGS page, may find yourself getting above or below your profile. But if the actual winds change significantly again, the aircraft may have excess or not enough energy.
I have found that the best thing to do is just monitor the descent, see how it goes and make as few adjustments as possible taking into account sequence and traffic the subsequent impact on my profile!
Personally speaking and a point of personal interest on the Classic, I've not noticed much benefit of putting wind in the forecast page. One can reduce incidents of DRAG REQUIRED messages with fake tailwinds, but then the speed can also become irritatingly slow too. I prefer minimal input unless significant wind is forecast.
I have found that the best thing to do is just monitor the descent, see how it goes and make as few adjustments as possible taking into account sequence and traffic the subsequent impact on my profile!
Personally speaking and a point of personal interest on the Classic, I've not noticed much benefit of putting wind in the forecast page. One can reduce incidents of DRAG REQUIRED messages with fake tailwinds, but then the speed can also become irritatingly slow too. I prefer minimal input unless significant wind is forecast.
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It's a computer. A computer can only do as good a job as the information you put in (garbage in - garbage out). If it doesn't work out as expected, don't be too quick to blame the computer.
Complete the descent forecast page with the most accurate information available to you. TAI ON/Off is often overlooked and can have a big influence on the descent profile. ISA deviation is a tricky one, since Boeing says it should be the 'average ISA deviation during the descent'. Ideally you should have winds in there down to platform altitude. If you lowest forecast wind is at FL180 it will assume a linear reduction to zero between FL180 and ground level. Don't put the cruise wind in the descent forecast page. It's a waste of one of the entry options in the descent forecast page, and is better done in the route data on the legs pages or on the 'perf init' page by putting the top of descent cruise wind in there.
After you have done all that, monitor the descent and be ready to intervene if required. Actual winds different from the forecast winds (does happen), ATC speed constraints and/or short cuts can throw your VNAV path descent out. I think the biggest issue with using VNAV PATH descent is knowing when to abandon it.
A fake tailwind causes the A/C to descend too early and increases overall fuel burn. VNAV PATH is not a speed mode, so by definition the speed cannot be low. VNAV PATH works with a 'target speed'. If the speed gets sufficiently below this target speed, it will correct by increasing the thrust (or give a 'thrust required' scratch pad message). Don't put deliberate garbage in the computer, the result will be garbage from the computer.
Complete the descent forecast page with the most accurate information available to you. TAI ON/Off is often overlooked and can have a big influence on the descent profile. ISA deviation is a tricky one, since Boeing says it should be the 'average ISA deviation during the descent'. Ideally you should have winds in there down to platform altitude. If you lowest forecast wind is at FL180 it will assume a linear reduction to zero between FL180 and ground level. Don't put the cruise wind in the descent forecast page. It's a waste of one of the entry options in the descent forecast page, and is better done in the route data on the legs pages or on the 'perf init' page by putting the top of descent cruise wind in there.
After you have done all that, monitor the descent and be ready to intervene if required. Actual winds different from the forecast winds (does happen), ATC speed constraints and/or short cuts can throw your VNAV path descent out. I think the biggest issue with using VNAV PATH descent is knowing when to abandon it.
One can reduce incidents of DRAG REQUIRED messages with fake tailwinds, but then the speed can also become irritatingly slow too.
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"A fake tailwind causes the A/C to descend too early and increases overall fuel burn. VNAV PATH is not a speed mode, so by definition the speed cannot be low. VNAV PATH works with a 'target speed'. If the speed gets sufficiently below this target speed, it will correct by increasing the thrust (or give a 'thrust required' scratch pad message)."
While the above is true it does not apply to geometric descend.
I.e.: on STARs with more at or below fixes. Here thrust will be on in order to maintain FMC programmed speed, you can also OPEN the speed window, and stay in VNAV PATH and dial the desired speed...
While the above is true it does not apply to geometric descend.
I.e.: on STARs with more at or below fixes. Here thrust will be on in order to maintain FMC programmed speed, you can also OPEN the speed window, and stay in VNAV PATH and dial the desired speed...
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If no wind is entered in the descent forecast page then will it work out on zero wind?
Personally I find putting a 1000ft wind in the decent page instead of the higher forecast wind helps quite a lot. I think this is because it provides the linear change in wind to a lower level rather than it calculating it back to zero at ground level. eg I would have a 30,000ft, 20,000ft and a 1000ft wind in the decent forecast page.
Seems to work for me.
Seems to work for me.
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I have also heard that on the active leg i.e. the White waypoint rather than magenta, then the FMC will use the actual wind - can someone confirm if/when this is the case please?
From B757/767 FMC's, and there's no reason to doubt the same for B737, I was always told the actual cruise wind is used for the active leg and the next leg. i.e. the time & fuel data you see on Progress 1.
For descent winds I check the forecast and see if you will descend into a increasing head/tail wind. Because of the linear reduction from actual CRZ W/V to zero at rwy this effect will really throw VNAV PTH off its calculation. Perhaps then some help in Desc Fcst page is a good idea. If you are making a straight in from FL350 there's not too much chance to adjust, and a head wind at CRZ may turn into a tail on the way down and calm at Rwy. All good to bear in mind. If you are going to make an unknown ATC safari, with a 180 degree track change to finals, it's probably not worth it.
Best computer on the a/c is between the ears.
From B757/767 FMC's, and there's no reason to doubt the same for B737, I was always told the actual cruise wind is used for the active leg and the next leg. i.e. the time & fuel data you see on Progress 1.
For descent winds I check the forecast and see if you will descend into a increasing head/tail wind. Because of the linear reduction from actual CRZ W/V to zero at rwy this effect will really throw VNAV PTH off its calculation. Perhaps then some help in Desc Fcst page is a good idea. If you are making a straight in from FL350 there's not too much chance to adjust, and a head wind at CRZ may turn into a tail on the way down and calm at Rwy. All good to bear in mind. If you are going to make an unknown ATC safari, with a 180 degree track change to finals, it's probably not worth it.
Best computer on the a/c is between the ears.