LRC Speed vs Headwind
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LRC Speed vs Headwind
I understand that at LRC with a headwind you will fly at a higher speed to get long range……….. But how much speed vs. headwind,
All I find on the forums and tech books is “fly at slightly higher speed” Could someone maybe put down some hard numbers or at least some good rules of thumb.
Thanks
THE MULLEYE
All I find on the forums and tech books is “fly at slightly higher speed” Could someone maybe put down some hard numbers or at least some good rules of thumb.
Thanks
THE MULLEYE
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Now that's a good one !
I don't think anyone can give you "hard numbers" or rules of thumb for that, unless of course you find an FMC geek, or an engineer from Boeing...
Hopes this helps : On a 737-800W, U 10.6 FMC, and a cost index of let say 40 (and in ACT ECON CRZ ) in zero wind your CRZ Mach No. will be around .787-.790! In a 100 kt HW it will probably increase up to .795-.80. the most that I have seen was .805 in a 160kt HW. TAS wise the increase is close to 10 kt....
My company never uses LRC (unless we're getting low on fuel for arrival,
but to be honest it doesn't change much for the last hour of cruise).
Not exactly what you were looking for but hope it helps !
I don't think anyone can give you "hard numbers" or rules of thumb for that, unless of course you find an FMC geek, or an engineer from Boeing...
Hopes this helps : On a 737-800W, U 10.6 FMC, and a cost index of let say 40 (and in ACT ECON CRZ ) in zero wind your CRZ Mach No. will be around .787-.790! In a 100 kt HW it will probably increase up to .795-.80. the most that I have seen was .805 in a 160kt HW. TAS wise the increase is close to 10 kt....
My company never uses LRC (unless we're getting low on fuel for arrival,
but to be honest it doesn't change much for the last hour of cruise).Not exactly what you were looking for but hope it helps !
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http://www.boeing.com/commercial/aer..._07/index.html
Try this, all good stuff and from the manufacturer. The previous edition contains the start of the explaination on cost index and the two articles relate cost index to LRC. Some of it is a bit generic but if you want max range go for CI = 0 and not LRC, try it, it does make a difference.
Also interesting is the fact that LRC equates to 35 in a 73NG and 25 in the classic. It also gives the speeds that the FMC will fly referenced to head and tailwinds.
Another good source on information regarding this is the Bulfer FMC guide.
Hope the link works! PM me if it does not, or just go to the Boeing website and the commercial section looking for the aero publication.
Try this, all good stuff and from the manufacturer. The previous edition contains the start of the explaination on cost index and the two articles relate cost index to LRC. Some of it is a bit generic but if you want max range go for CI = 0 and not LRC, try it, it does make a difference.
Also interesting is the fact that LRC equates to 35 in a 73NG and 25 in the classic. It also gives the speeds that the FMC will fly referenced to head and tailwinds.
Another good source on information regarding this is the Bulfer FMC guide.
Hope the link works! PM me if it does not, or just go to the Boeing website and the commercial section looking for the aero publication.
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mulleye,
i believe you mean ECON speed? LRC does not change with wind. it varies with weight and altitude but not wind. thats why before the 'fuel crisis', most airlines i know simply flys a CI which is close to LRC at OPT ALT (and caters for wind at the same time). however, in the current situation, most of them now flys a much lower than LRC speed. as for rule of thumb, i guess the QRH (boeing) would be a good start?
SR
i believe you mean ECON speed? LRC does not change with wind. it varies with weight and altitude but not wind. thats why before the 'fuel crisis', most airlines i know simply flys a CI which is close to LRC at OPT ALT (and caters for wind at the same time). however, in the current situation, most of them now flys a much lower than LRC speed. as for rule of thumb, i guess the QRH (boeing) would be a good start?
SR
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I agree with the first part od SRs post until this
. slower than LRC and you use more fuel.
If you are using cost index the slowest you should find yourself is LRC. At this speed the calculation is fully baised to saving fuel rather than time. Any slower and you will use more fule and time. Not a good idea.
however, in the current situation, most of them now flys a much lower than LRC speed.
If you are using cost index the slowest you should find yourself is LRC. At this speed the calculation is fully baised to saving fuel rather than time. Any slower and you will use more fule and time. Not a good idea.
In the manuals I'm looking at, Long Range Cruise is defined as giving 99% of best range. On that basis, I would infer that there are more economical speeds to fly at than LRC.
This seems to have some truth in it as on the 777 LRC is always > ECON @CI=0 (200kt headwinds excepted...)
This seems to have some truth in it as on the 777 LRC is always > ECON @CI=0 (200kt headwinds excepted...)
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Orp Tolip, You are confusing LRC with MRC (Maximum Range Cruise speed). MRC does what it says on the tin. LRC, as stated above by FullWings is a cruise speed that sacrifies 1% of range capability for a significant increase in speed.
On an FMC equiped aircraft ECON speed takes, amongst other parameters, head & tailwind into account. In still air, ECON speed is related to the Cost Index, CI, which compares fuel costs to fixed costs. Many pilots call the CI a 'fuel cost index', but it is simply a 'cost index'. For any aircraft there is a CI which would equate to LRC in still air.
In a headwind ECON is faster, and in a tailwind ECON is slower.
LRC is a rough and ready number to use as a trade off between fuel and range (range at LRC is 1% less than range at MRC). CI is a more scientific way, with the head/tail wind input being the icing on the cake. However, to many airlines it has become apparent that calculating the fixed costs is very difficult to quantify, so they use a single CI that seems to be 'about right' for their operation. Even if the CI is not calculated accurately, the benefit of the head/tail wind input means that using ECON may still be better than plain old LRC.
CI 0 = MRC
CI X=LRC (in still air)(most AFMs have a graph to show the CI equivalent to LRC)
CI 99 = free fuel and high speed (could be 999 or some other high number depending on the FMC)
I’m not aware of any rules of thumb though to answer TheMullEye’s original question.
On an FMC equiped aircraft ECON speed takes, amongst other parameters, head & tailwind into account. In still air, ECON speed is related to the Cost Index, CI, which compares fuel costs to fixed costs. Many pilots call the CI a 'fuel cost index', but it is simply a 'cost index'. For any aircraft there is a CI which would equate to LRC in still air.
In a headwind ECON is faster, and in a tailwind ECON is slower.
LRC is a rough and ready number to use as a trade off between fuel and range (range at LRC is 1% less than range at MRC). CI is a more scientific way, with the head/tail wind input being the icing on the cake. However, to many airlines it has become apparent that calculating the fixed costs is very difficult to quantify, so they use a single CI that seems to be 'about right' for their operation. Even if the CI is not calculated accurately, the benefit of the head/tail wind input means that using ECON may still be better than plain old LRC.
CI 0 = MRC
CI X=LRC (in still air)(most AFMs have a graph to show the CI equivalent to LRC)
CI 99 = free fuel and high speed (could be 999 or some other high number depending on the FMC)
I’m not aware of any rules of thumb though to answer TheMullEye’s original question.
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FE Hoppy,
just to give you a better illustration. on our b777, we used to fly CI 130 which approx equates LRC of M0.84 in still air. but now, we are flying in the range of CI 16-32 which only gives M0.828 at OPT ALT. on an average 12 hrs flight, we lose 10mins but 'gain' about 1,000kg fuel. as i've said earlier, at current times, 1,000kg multiplies the number of long-haul flights we have a day, it can work out to be quite substantial!
SR
just to give you a better illustration. on our b777, we used to fly CI 130 which approx equates LRC of M0.84 in still air. but now, we are flying in the range of CI 16-32 which only gives M0.828 at OPT ALT. on an average 12 hrs flight, we lose 10mins but 'gain' about 1,000kg fuel. as i've said earlier, at current times, 1,000kg multiplies the number of long-haul flights we have a day, it can work out to be quite substantial!
SR
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I don't think anyone can give you "hard numbers" or rules of thumb for that, unless of course you find an FMC geek, or an engineer from Boeing...
There are actually many rules of thumb that you can apply, but many of them apply to specific airplanes. Though they may work to some extent on your airplane, you'll have to experiment if you can't find any recommendations in your airplane's Flight handbook. Among those rules of thumb are:
Increase/decrease TAS by 40% of head/tail wind
Increase/decrease IMN by .001 for each 10 kt of head/tail wind
In any case, minimum speed will be max endurance (holding) speed. Maximum speed will vary with the airplane, but will be at a point where the drag curve starts rising more steeply with speed. In one airplane that normally cruised at .70-.72, max was .80; in another that cruised at .83-.85, max was .88 (in both cases, that "max" cruise was well below Mmo).
If you have an FMS, you don't need a rule of thumb; just fly ECON! Use CI=0 for most fuel savings, or as directed by your company to approximate lowest overall operating cost.
Last edited by Intruder; 22nd Nov 2007 at 17:09.
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SR,
I agree with you but care must be taken in terms used. There is no correction to LRC speed. ECON yes because this is cost based but LRC is only performance based.
If your AFM includes MRC then thats nice because it will save a bit more fuel than LRC but how many AFMs include MRC figures?
I agree with you but care must be taken in terms used. There is no correction to LRC speed. ECON yes because this is cost based but LRC is only performance based.
If your AFM includes MRC then thats nice because it will save a bit more fuel than LRC but how many AFMs include MRC figures?
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Intruder thanks for the info
The problem with my FMS is no ECON or CI (Lear35). I only have the QRH. Looking at it now............
LRC @ ISA - FL390 = M.731
from your post "Increase/decrease IMN by .001 for each 10 kt of head/tail wind"
So based on 100KT Head Wind my best bet is M.741
Thanks
MULLEYE
The problem with my FMS is no ECON or CI (Lear35). I only have the QRH. Looking at it now............
LRC @ ISA - FL390 = M.731
from your post "Increase/decrease IMN by .001 for each 10 kt of head/tail wind"
So based on 100KT Head Wind my best bet is M.741
Thanks
MULLEYE
Last edited by themulleye; 23rd Nov 2007 at 16:35.
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In truth, 'LRC' is only 99% of best RANGE in still air. I have always thought it should be renamed L T(ime) C
. I cannot see an awful lot of use in it unless the FMC is dead. Even then I'd be tempted to adjust for wind.
. I cannot see an awful lot of use in it unless the FMC is dead. Even then I'd be tempted to adjust for wind.
