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?

I update the winds and the direct to point if i notice the speed is not matching what it should be, usually a good indicator something is set wrong:}

There's always Level Change.

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

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.

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.

Do your aircraft have Geometric Path? That makes for some 'interesting' path displays which some fail to understand.

PP

"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.:8
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...:ok:

I always understood that if there are no descent winds entered, it works on the cruise wind washing out to zero during the descent. I find it generally calculates descents fairly well if you put the last cruise wind for the top one, a halfway down descent wind and altitudes in from the computer flight plan, and make the lowest wind 2000' and what is reported from the ground. BUT, the most important thing is you don't just sit there and rely on it- all the way down you should be mentally calculating and adjusting for the wind component. It's only a computer with all the faults of the programmers. YOU are flying the plane. The 757 tends to take you down too early, adjusting the wind to increase the tailwind components by about 10 kts works for that aeroplane. The 737NG sometimes has a real calculation 'burp' and can suddenly start telling you you are 7000' high on the profile descent. That is why it is important that YOU decide, and just don't leave it to a computer.

Wind
 

And do you insert true or magnetic wind on decent forecast?

It is entered as true.

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.

I agree with Framer. That's exactly what I do and it seems to work fine as long as you remember to add data in the TAI on/off line if required.

Maggot

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.