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alex98

FMC ECON Mach with wind

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Hello,

 

First, sorry if it has already been discussed, I found nothing via the search function.

 

I have noticed that the ECON cruise mach is unsensitive to headwind or tailwind.

Did I miss anything in the FMC setup or this particular functionnality was left aside during NGX development? And if it was, what were the reasons?

 

Happy flying!

 

Alex

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This has to do with the cost index. What value did you use?

 

It's possible, I'm not 100% sure, that ECON SPEED depends on COST INDEX exclusively and does not care about the winds. I'll try to confirm this.

 

EDIT: I was wrong, have a read here:

 

For example, in the presence of a strong
tailwind, the ECON speed will be reduced in
order to maximize the advantage gained from
the tailwind during the cruise. Conversely, the
ECON speed will be increased when flying into
a headwind in cruise to minimize the penalty
associated with the headwind

 

So yes, winds should have an effect on the ECON SPD.

 

In the case of cost index 0, that is, fuel cost is infinitely high with respect to time cost, I can imagine that wind speed WON'T affect the ECON SPD, which will be the speed that gives the max range.

 

With cost index MAX; the same applies. ECON SPD will be the speed of least duration of flight, that is, max operating speed (this is regardless of wind)

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I use CI 30.

What I meant was, no matter which cost index I use, the computed ECON Mach remains the same regardless of the wind component.

 

For example, let say you fly with a CI 30. With no significant wind, your ECON Mach will be around .786M, but with 100 Kts headwind, it will increase to something like .795M - I dont know the real figure - and with a 100 Kts tailwind, it will decrease in the same proportion. This is how I understand the real FMC works.

Edited by alex98

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I have noticed that the ECON cruise mach is unsensitive to headwind or tailwind.

 

How are you testing this?

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How are you testing this?

 

Hi Kyle,

 

In Boeing's material "Fuel conservation strategies: Cost Index explained", you read (p.27, figure 3) that ECON speed is optimized for wind conditions.

Basically, when setting up the PERF INIT page, I tried different values for CRZ WIND and there was no change whatsoever to the ECON Mach speed.

Apparently the NGX FMC does not replicate this feature or I missed something while studying it.

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p.27, figure 3

 

 

Yes, in fact, let me copy the image here. I wasn't completly right about wind not affecting the ECON Speed at the extreme values of the CI. For CI=0 and a headwind, it will increase the speed above that of still air. You could theoretically reduce the speed when having a tailwind to maximize range, but that could put you in the second flight regime which is not advisable (close to stall etc). I bet this value of 0.773 is a tad higher than max range so as to have some speed stability as well.

 

One thing is the "blackboard theory of the Breguet equations, range, endurance etc." and then there's the operational practice in which safety factors come into play.

pWRiKzl.jpg

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Alpha Floor, some additional remarks to complete your post:

1. I can not say if those values in the picture are from the 737NG. Anyway the same principle applies to every aircraft flying with a Cost Index.

2. LRC is a fixed value regardless of the wind. This is one of the advantages of flying with a Cost Index. More flexible, therefore more economic.

3. VNAV climb speeds are affected by weight, temperature and wind. VNAV descent speeds OTH are only affected by weight and therefore are fixed (ie for a CI 30, VNAV descent speed is always 273 KIAS).

 

:Cuppa:

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Alpha Floor, some additional remarks to complete your post:

1. I can not say if those values in the picture are from the 737NG. Anyway the same principle applies to every aircraft flying with a Cost Index.

2. LRC is a fixed value regardless of the wind. This is one of the advantages of flying with a Cost Index. More flexible, therefore more economic.

3. VNAV climb speeds are affected by weight, temperature and wind. VNAV descent speeds OTH are only affected by weight and therefore are fixed (ie for a CI 30, VNAV descent speed is always 273 KIAS).

 

:Cuppa:

 

Hi Alex,

 

1. Even if the values are not for the 737, the relations between them still hold. 

 

2. LRC is not a fixed value, or at least it shouldn't be (I don't know how PMDG implemented it). The LRC is faster than the speed of Maximum Range Cruise (MRC) and provides a 1% range reduction with respect to this MRC. MRC is CI=0, as you can see on the table, there's an effect of the wind. The advantage of flying at LRC rather than MRC; is that the aircraft is more speed stable at LRC. A disturbance in speed at MRC will be more difficult to "counteract" than at LRC.

 

3. CI also has an effect on VNAV Descent Speeds. For CI=0 the target speed will be the one that provides maximum Lift to Drag ratio. For CI=Max on the other hand, the speed will be Vmo/Mmo. See fig. 3 on the second page.

 

What are your thoughts on this?

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The more you can "glide" on idle power, the more economy. For lower CI usually around 270 kias, vs the 290+ Of the higher CIs. That said though some altitude constraints between fixes demand a higher airspeed.

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Hi Alex,

 

1. Even if the values are not for the 737, the relations between them still hold. 

 

2. LRC is not a fixed value, or at least it shouldn't be (I don't know how PMDG implemented it). The LRC is faster than the speed of Maximum Range Cruise (MRC) and provides a 1% range reduction with respect to this MRC. MRC is CI=0, as you can see on the table, there's an effect of the wind. The advantage of flying at LRC rather than MRC; is that the aircraft is more speed stable at LRC. A disturbance in speed at MRC will be more difficult to "counteract" than at LRC.

 

3. CI also has an effect on VNAV Descent Speeds. For CI=0 the target speed will be the one that provides maximum Lift to Drag ratio. For CI=Max on the other hand, the speed will be Vmo/Mmo. See fig. 3 on the second page.

 

What are your thoughts on this?

 

Hi Alpha Floor,

 

2. Yes, I agree with that. I was refering to the fact that LRC on the NG (which approximates CI35 according to Boeing) remains unchanged whatever the wind component you encounter. 100kts headwind or 100kts tailwind will give you the same IAS. CI ECON speed is an all other ball game, due to the fact that it takes wind component into consideration (faster in headwind, slower in tailwind). Apparently, this is not modeled in the NGX.

By the way, MRC approximates CI 0. It is not CI 0. :wink:  At least that is the way I understand it. :P

 

3. Yes again :smile: . CI has an effect on VNAV descent speed. The higher the CI, the higher the descent speed. From memory, CI 0 will give you less than 250 KIAS* while CI 45 will give you 290 KIAS. WInd and temperature have no effect on these speeds, only the CI.

VNAV climb speeds OTH are temp and wind sensitive (faster in headwind, slower if above ISA).

 

*Here the NGX simulates older FMC (U10.6 and earlier) which means that the VNAV descent speed is never slower than 250 KIAS. On the newer FMC (U10.7 and on) CI 0 gives you 240 KIAS... Ask the Ryanair guys about it :fool:

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By the way, MRC approximates CI 0. It is not CI 0.

 

Yeah, in practice you don't get a "real" MRC. The plane won't fly slow enough for a "real, theoretical" MRC. But what is true is that the maximum range you will "get" is for a CI=0.

 

 

 


One thing is the "blackboard theory of the Breguet equations, range, endurance etc." and then there's the operational practice in which safety factors come into play.

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All the bean counters running the airlines these days, the best thing to hear over atc is the controller telling you to maintain a high descent speed instead of struggling in a vnav descent if your carrier uses a small cost index number. - David Lee

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