747-400X Winds Aloft Entry

[AIRIFR 5]

Xavier

I just don't believe it. We are going to have to agree to disagree.

Michael

 

Michael

no problem, enjoy your flights.

Xavier

[scandinavian13 5,192]

I just don't believe it.

 

You don't have to. It's not correct.

 

Using my a route I fly occasionally, the math is as follows:

JYO-BCB (175nm), 110 knots, 15 knot headwind

Results in a 95 knot ground speed and a time en route of 1:50.

 

BCB-JYO (175nm), 110 knots, 15 knot tailwind

Results in a 125 knot ground speed and a time en route of 1:24 minutes.

 

Total time of 3:14.

 

Using an average method: 3:10.

 

While times are similar, they are not the exact same. The simple averaging is not taking into account the increased exposure to wind on the outbound leg, and the decreased exposure on the inbound leg. While this isn't the exact same as legs of differing length, the same distance with the direct head to tailwind shift has the same effect because the effective distance traveled is greater and lesser.

[AIRIFR 5]

You don't have to. It's not correct.

 

Using my a route I fly occasionally, the math is as follows:

JYO-BCB (175nm), 110 knots, 15 knot headwind

Results in a 95 knot ground speed and a time en route of 1:50.

 

BCB-JYO (175nm), 110 knots, 15 knot tailwind

Results in a 125 knot ground speed and a time en route of 1:24 minutes.

 

Total time of 3:14.

 

Using an average method: 3:10.

 

While times are similar, they are not the exact same. The simple averaging is not taking into account the increased exposure to wind on the outbound leg, and the decreased exposure on the inbound leg. While this isn't the exact same as legs of differing length, the same distance with the direct head to tailwind shift has the same effect because the effective distance traveled is greater and lesser.

 

You're right about that it is like talking about CP or PNR but it wasnt about return trip, it was about on single trip, one way.avg on one way is the same as each single entries, it wouldnt make sense to avg the winds on a forth and back trip wouldnt it? ASE does the winds avg on your single trip from the flight pln your imported.

 

That is why I didnt understand why you were asking this question earlier.

So if Michael was talking about return trip also but looking at his post I dont think he was, then Michael disregard my posts.

[scandinavian13 5,192]

You're right about that it is like talking about CP or PNR but it wasnt about return trip, it was about on single trip, one way.avg on one way is the same as each single entries, it wouldnt make sense to avg the winds on a forth and back trip wouldnt it? ASE does the winds avg on your single trip from the flight pln your imported.

 

That is why I didnt understand why you were asking this question earlier.

So if Michael was talking about return trip also but looking at his post I dont think he was, then Michael disregard my posts.

 

Re-read my last paragraph...

It's a slightly different concept, but with wind held constant, your effective exposure to the wind given a fixed distance between two points is different because of your direction related to the wind. I know that might be a little difficult to wrap your head around, but it's true, and a similar concept applies to different leg lengths.

 

Say I have 5 points:

AAA, BBB, CCC, DDD, EEE

 

The distances between the points are:

AAA 50 BBB 100 CCC 75 DDD 50 EEE (total: 275)

 

If the wind is (direct headwind for simplicity):

AAA 50

BBB 40

CCC 50

DDD 30

EEE 50

(Average: 44)

 

Assuming I'm moving 300 knots:

Using the average alone, that's 1:06 en route.

Using the leg exposure length, that's 1:04 en route.

 

Over much longer distances, that is much more significant. This is PPL Cross Country route planning stuff.

[AIRIFR 5]

Re-read my last paragraph...

It's a slightly different concept, but with wind held constant, your effective exposure to the wind given a fixed distance between two points is different because of your direction related to the wind. I know that might be a little difficult to wrap your head around, but it's true, and a similar concept applies to different leg lengths.

 

Say I have 5 points:

AAA, BBB, CCC, DDD, EEE

 

The distances between the points are:

AAA 50 BBB 100 CCC 75 DDD 50 EEE (total: 275)

 

If the wind is (direct headwind for simplicity):

AAA 50

BBB 40

CCC 50

DDD 30

EEE 50

(Average: 44)

 

Assuming I'm moving 300 knots:

Using the average alone, that's 1:06 en route.

Using the leg exposure length, that's 1:04 en route.

 

Over much longer distances, that is much more significant. This is PPL Cross Country route planning stuff.

 

Im not discussing the figures, nor PPL x/c planning, this is true what you're talking about but Im talking about the avg wind in itself.

 

The avg wind is computed from each wpt wind, it is not just made up and then the wpts are separate source, avg wind and all different wpts wnds where it comes from are the same, to have an average you need multiple items and that's for everything not just for what we're talking about, you cant make an avg from nothing, so the avg wnd is "all wpts wnds together in one" (you know what I mean) so entering each wpts or entering the avg wind which comes from each waypoints, is the same.

To have an egg you need hen, or to have hen you need egg lol, to have an avg wnd you need wpts wnds.

 

So for example a KSFO-YSSY about 6570nm with about 20 wpts, you would take each enroute wpts wnds/temps and calculate each fuel leg separetly then add them up or you will take the avg to calculate directly the fuel for the 6570nm?

[AIRIFR 5]

 

Xavier

EDDF-KLAX has 31 cruise waypoints of which 13 or 42% are in Europe or up the North Sea before turning to the northwest at the top of Great Britain and you have not even covered a 1000 mile out of 5300 mile flight. Any average is going to be heavily influenced by what the winds are in Europe and over the North Sea. Any flights out of LFPG, EDDF, EHAM to the U.S. west coast are the examples I was writing about. To say that the FMC does not know the difference between an average direction and speed and each waypoint direction and speed and the effect on ETA and fuel burn does a disservice to the FMC. Why do you think you can make entries by waypoints. It would be easier just to make one average entry than 30-45 entries for each waypoint. I have tried it both ways and when using an average I have missed the ETA by 25-30 minutes on 10-12 hour flights. I just completed a 9 hour flight from EHAM to KSEA entering winds by waypoint and the ETA at T/C was 1740Z and the ETA at T/D was 1741Z. One minute late on a 540 minute flight.

Michael Cubine

Michael

Curious to know how did you calculate your flight fuel for flight to Seattle from Amsterdam? And what flight time did you calculate?

How much fuel did you plan at landing and how much actually left at landing?

Xavier

[pcubine 143]

Michael

Curious to know how did you calculate your flight fuel for flight to Seattle from Amsterdam? And what flight time did you calculate?

How much fuel did you plan at landing and how much actually left at landing?

Xavier

 

Xavier

This flight was done in the MD-11 Cargo plane. From experience I know EHAM-KSEA, EHAM-CYVR takes about 200000 lbs. of fuel to do the flight and have reseserves for the altenate if there is no wind. The average head wind component was 31 knots. So I increased the fuel to 212500 lls, made the wind entries by waypoint and the PROGRESS page indicated around 37000 lbs left at KSEA and 28000 lbs left at CYVR which was the alternate. I was satisfied with the reserves but I was getting an INSUFFICIENT FUEL warning on the CDU so I went into page 2 of the FLT-PLAN INITand changed the default 10% to 5% for the RTE RSV % and then the warning message went away.

 

I let the FMS calculare the flight time. I just knew it was going to be another long one.

 

The fuel left at landing was indicated in paragraph one and since it was one minute late the calculated and actual fuel at landing were the were the same

[JRBarrett 3,070]

 

The avg wind is computed from each wpt wind, it is not just made up and then the wpts are separate source, avg wind and all different wpts wnds where it comes from are the same, to have an average you need multiple items and that's for everything not just for what we're talking about, you cant make an avg from nothing, so the avg wnd is "all wpts wnds together in one" (you know what I mean) so entering each wpts or entering the avg wind which comes from each waypoints, is the same.

?

 

No, it is definitely NOT the same... To expand on Scandinavian13's example, (using a slow aircraft over a relatively short route), I present the following example. This is based on typical figures for a Cessna 172, flying a total trip length of 200 nm, consisting of four 50 nm segments. Assume that the wind direction and velocity for each segment remains constant, and changes instantly to the new value at each new waypoint.

 

The aircraft maintains a constant true heading of 270 degrees, and a constant airspeed of 110 knots. Fuel burn is 9 gallons per hour.

 

All calculations were done using the FlyByE6B iPad app, but you should get the same results with any aviation calculator, or even an actual E6B "whiz wheel".

 

Segment 1: Wind 185@30

GS: 103 Time: 29:03 Burn: 4.4

 

Segment 2: Wind 270@40

GS: 70 Time: 42:51 Burn: 6.4

 

Segment 3: Wind 330@10

GS: 105 Time: 28:40 Burn: 4.3

 

Segment 4: Wind 355@25

GS: 105 Time: 28:35 Burn 4.3

 

Average GS: 96, Total Time: 2:09 Burn: 19.4

 

Now, the arithmetical average of the above 4 wind directions and velocities gives 285@26 Running that "average wind" over the total trip length of 200 nm gives a GS of 85 knots, a time enroute of 2:22 and a fuel burn of 21.3, which amounts to 10 knot error in ground speed, a 13 minute over-estimation of time enroute, and a 2 gallon over-estimation of fuel burn.

 

And this is over a (relatively speaking) very short total trip length, flown in a perfectly straight line, at low airspeed and fuel consumption.

 

Imagine the cumulative error over a 6000 mile trip at 470 knots TAS and 20,000+ lbs/hr fuel burn rate if you were to treat the "average" winds for 30 waypoints as being the "same" as the discrete actual winds for each of those 30 waypoints.

 

The (understandable) mistake you're making is in thinking that you can derive the "average" wind for the trip by simple arithmetic... i.e. by adding up all the individual waypoint wind directions and velocities, and dividing by the total number of waypoints. That may be how ASE or other FS weather programs do it - I don't know for sure - but that is not how it would be done in actual practice - The algorithm to calculate the accurate "average wind direction and velocity" for multiple waypoints over a long distance requires the use of integral calculus and statistical functions.

 

If you've taken a class in statistics, and understand how to use the Standard Deviation function, you can get pretty close to calculating the true average for multiple waypoints that way, (without calculus) but at best, the resulting estimation of time enroute will not be anywhere near as accurate as entering the actual winds for each waypoint... especially on a very long flight with winds changing significantly over the course of the total route.

[AIRIFR 5]

The average head wind component was 31 knots.

 

Did you write AVERAGE?

yeah...thought so...

 

No, it is definitely NOT the same... To expand on Scandinavian13's example, (using a slow aircraft over a relatively short route), I present the following example. This is based on typical figures for a Cessna 172, flying a total trip length of 200 nm, consisting of four 50 nm segments. Assume that the wind direction and velocity for each segment remains constant, and changes instantly to the new value at each new waypoint.

 

The aircraft maintains a constant true heading of 270 degrees, and a constant airspeed of 110 knots. Fuel burn is 9 gallons per hour.

 

All calculations were done using the FlyByE6B iPad app, but you should get the same results with any aviation calculator, or even an actual E6B "whiz wheel".

 

Segment 1: Wind 185@30

GS: 103 Time: 29:03 Burn: 4.4

 

Segment 2: Wind 270@40

GS: 70 Time: 42:51 Burn: 6.4

 

Segment 3: Wind 330@10

GS: 105 Time: 28:40 Burn: 4.3

 

Segment 4: Wind 355@25

GS: 105 Time: 28:35 Burn 4.3

 

Average GS: 96, Total Time: 2:09 Burn: 19.4

 

Now, the arithmetical average of the above 4 wind directions and velocities gives 285@26 Running that "average wind" over the total trip length of 200 nm gives a GS of 85 knots, a time enroute of 2:22 and a fuel burn of 21.3, which amounts to 10 knot error in ground speed, a 13 minute over-estimation of time enroute, and a 2 gallon over-estimation of fuel burn.

 

And this is over a (relatively speaking) very short total trip length, flown in a perfectly straight line, at low airspeed and fuel consumption.

 

Imagine the cumulative error over a 6000 mile trip at 470 knots TAS and 20,000+ lbs/hr fuel burn rate if you were to treat the "average" winds for 30 waypoints as being the "same" as the discrete actual winds for each of those 30 waypoints.

 

The (understandable) mistake you're making is in thinking that you can derive the "average" wind for the trip by simple arithmetic... i.e. by adding up all the individual waypoint wind directions and velocities, and dividing by the total number of waypoints. That may be how ASE or other FS weather programs do it - I don't know for sure - but that is not how it would be done in actual practice - The algorithm to calculate the accurate "average wind direction and velocity" for multiple waypoints over a long distance requires the use of integral calculus and statistical functions.

 

If you've taken a class in statistics, and understand how to use the Standard Deviation function, you can get pretty close to calculating the true average for multiple waypoints that way, (without calculus) but at best, the resulting estimation of time enroute will not be anywhere near as accurate as entering the actual winds for each waypoint... especially on a very long flight with winds changing significantly over the course of the total route.

 

Aviation is no statistic when it comes to planning, it is estimations and will always be.

U can use historical data/statistic for weather prediction,

You dont plan a slow aircraft flight over short distance at low altitude the same as you do for a jet over 6,000nm at high altitude.

 

Have you guys used for example, long range 5,000nm-8,000nm .85 cruise chart?, Im talking about this one for 747-400, same stuff for other jets.

 

How would you enter the chart and what would be the first variable needed to start with? Distance

Second? Wind

So on this KSFO-YSSY example, you would look up 6570nm then go up the chart to wnd REF line then use what wnd figure? yeah AVERAGE wnd figure for the ALL 6570nm, then continuing on for the rest.

 

And also in the Enroute Fuel/Time Ground to Air miles conversion chart for a distance of 4,000nm for example, what wnd component they're talking about for this 4,000nm distance in the chart? AVERAGE, you got me?

 

This isnt C172 flights over 200nm we're here for right? this is PMDG forum so I guess most of people here are about JETs flying over long distances especially if on this 747-400 subforum.So why keep talking about 172 which has nothing to do with 747-400 in regards to how you plan the fuel required for the flight, the time enroute etc.

 

And your example about winds suddenly changing over wpts, you know quite well that that doesnt exist ! so give example about some true and about a JET flight over 2,000nm for example not a 172 over 200nm to see how you go, Im sure you looked at pressure charts before.

 

And

"...Imagine the cumulative error over a 6000 mile trip at 470 knots TAS and 20,000+ lbs/hr fuel burn rate if you were to treat the "average" winds for 30 waypoints as being the "same" as the discrete actual winds for each of those 30 waypoints...."

 

Of course there will cumulative error if you're taking your 172 example over 6000nm from 200nm.you must compare what's comparable, you cannot apply this example over 6000 with this method especially if you're not gonna fly this 172 on 6000nm trip .

 

Well give me a 747 example, and talking about planning, plan that 6,000nm trip at 470kts and 20,000lbs/hr with 30 waypoints winds and post your outcome and you'll tell us what winds you used

 

 

So after you've done your planning with your charts you've got your winds to put in FMC then you get an ETA/fuel at destination which will saty pretty constant depending how good your planning was/accurate wnds aloft prediction was, and the average will give you an ETA/fuel at destination too but wont be constant only to settle to ABOUT same figure as with each individual wpts near TOD.If you fly a 2,00nm (example) with 15 wpts and wnds at each waypoints are pretty much same like 10-20degrees differences and 5-10kt difference, entering the avg will give you an eta/fuel pretty constant and consistant to individually entered ones.If you have some head then some tail larger differences, the result wont be as steady.

And ABOUT or CLOSE TO is OK in aviation, it is not down to 1min or 1lb of fuel over long distance, not even on 200nm trip on 172.I'm not talking about challenges/air time rally/precision contests etc, it is of course different matter.

 

This why we do checkpoints to track progress, this is nature of aviation

[pcubine 143]

by adding up all the individual waypoint wind directions and velocities, and dividing by the total number of waypoints. That may be how ASE or other FS weather programs do it - I don't know for sure

Jim

That is how ASE and AS2012 derive an average. I verified that in the Avsim HiFi forum over a year ago with Jim Skoda who was HiFi's technical guy in the forum at the time.

Michael Cubine

[AIRIFR 5]

:p0504:

[pcubine 143]

ASE takes all waypoint WV and average them, so entering them each or entering the averaged one is exactely the same as far as fmc prediction is

Xavier

This information comes from the following flight – PMDG 737-800WL at FL340 KJFK-KLAX at 1425Z. The KJFK-KLAX route that I used has 29 waypoints without a SID or STAR. The shortest leg is 4 miles and the longest is 194 miles. AS2012 was the weather engine. It indicated an average wind of 254/59 at FL340 plus the NavLog listed winds by waypoint. The necessary data was entered into the FMC along with the average wind. The Progress Page indicated an ETA of 1946Z at 1406Z for an enroute time of 5:40 hrs. Then the winds were entered by waypoint and the Progress Page showed an ETA of 1932Z at 1413Z for an an enroute time of 5:17 hrs. 23 minute less on relative short flight of 2180 miles.

Michael Cubine

[AIRIFR 5]

The necessary data was entered into the FMC along with the average wind. The Progress Page indicated an ETA of 1946Z at 1406Z for an enroute time of 5:40 hrs. Then the winds were entered by waypoint and the Progress Page showed an ETA of 1932Z at 1413Z for an an enroute time of 5:17 hrs. 23 minute less on relative short flight of 2180 miles.

Michael Cubine

 

You didnt tell when the data was entered. on the ground? or just after TOC? show the actual pictures showing what you're saying, this is just saying for me, so this post is useless.

 

Go on your KJFK-KLAX flight and just after the TOC take the pics to show us, if you dont wanna do that then dont say anything...

 

Bye..

[scandinavian13 5,192]

You dont plan a slow aircraft flight over short distance at low altitude the same as you do for a jet over 6,000nm at high altitude.

 

It's actually remarkably similar. Planning for the jet is a lot more in depth, but the concepts are exactly the same. You may not see it as the crew, but the information down at dispatch is remarkably granular.

 

The concept being argued here is a simple average versus a weighted average. In order to accurately calculate the wind for your route, the flight planning tools used by dispatchers use the latter. The reason has been explained several times here: regular, simple averages cannot account for everything. What about my winter flight from IAD to SEA? If I'm avoiding the jet stream for the first 500nm with a 30 knot headwind, and then have a 170 knot headwind for 1500nm, a regular average isn't going to cut it. Regular average for that would be 100 knots. The weighted average would be 130 knots (assuming a wind reporting stations at 100nm intervals).

 

I have a feeling that since a few people aren't picking up on this that I need to just do a manual trip calculation. If I can grab some dispatch docs today at work, I'll use that as a framework because I wouldn't want people thinking I'm applying little plane logic to big planes.

 

 

 

Let's get it straight, here, though:

Nobody is claiming that the cruise wind average on the relevant page of the FMC is wholly inaccurate (it's better than nothing at the very least).

We're saying that the weather planning features in software people are using (notably Active Sky), provides values that do not compensate for the wind exposure factor (based on leg length).

[AIRIFR 5]

If I'm avoiding the jet stream for the first 500nm with a 30 knot headwind, and then have a 170 knot headwind for 1500nm, a regular average isn't going to cut it. Regular average for that would be 100 knots. The weighted average would be 130 knots (assuming a wind reporting stations at 100nm intervals).

 

If you avoid the jet stream for 500nm it is up to you, ASE is no dispatch, the tool we have is just that.Of course down the dispatch office you weigh the average, but ASE is just giving you the avg "blindly", then you use it wisely.ASE will still give you accurate average based on the conveniently spaced sations and your loaded flightplan and on your loaded flightplan ASE see the wpts and how far they are from each other.