Step climb vs Opt FL A320
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Step climb vs Opt FL A320
Hi everyody.
When I insert a step climb with all the wind data at different levels
sometimes it differs so much comparing to your Opt FL in the progress
page so, how does the system compute each one?
Regards!
When I insert a step climb with all the wind data at different levels
sometimes it differs so much comparing to your Opt FL in the progress
page so, how does the system compute each one?
Regards!
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This made me wonder as well, and so I did alot of digging in our company efficiency literature and the FCOM.
First, the FMGC dilligently budgets processor time, and doesn't recalculate Opt FL very often. Reenter the cruise level, make the box think, and you may very well see Opt FL change before your eyes.
Secondly the PROG page Opt FL is rounded to nearest 500 foot, so we can't "see" when our optimum level may indeed be above our present level, if only marginally. i.e., presently at FL360, displayed Opt FL 360, when in reality our "true" optimum might be FL362, with even slightly favorable winds up high the FMGC may want to step to FL380, confounding the well-intentioned aviator.
Thirdly, assuming other requirements are met, it's generally better to fly much higher than optimum than even slightly below it; i.e., presently at FL360, Opt FL 365, the FMGC will recomend a step to FL380. The box consistently prefers 1500 above Opt rather than 500 below, and performance data backs that up.
Lastly, we can't in practice fly the extremely slow climb that would be theoretically optimal. The vertical profile of our climb does indeed look like a stairstep, when transposed over the smoothly rising optimal. Since being below optimal is so prohibitive, the stair step doesn't average above and below the optimum slope, but instead biases to climb early.
Further, and you've no doubt already seen, how the step pays no heed to to the Rec Max FL (other than staying below it), in search of a strong tailwind up high and to a lesser degree ducking headwinds.
First, the FMGC dilligently budgets processor time, and doesn't recalculate Opt FL very often. Reenter the cruise level, make the box think, and you may very well see Opt FL change before your eyes.
Secondly the PROG page Opt FL is rounded to nearest 500 foot, so we can't "see" when our optimum level may indeed be above our present level, if only marginally. i.e., presently at FL360, displayed Opt FL 360, when in reality our "true" optimum might be FL362, with even slightly favorable winds up high the FMGC may want to step to FL380, confounding the well-intentioned aviator.
Thirdly, assuming other requirements are met, it's generally better to fly much higher than optimum than even slightly below it; i.e., presently at FL360, Opt FL 365, the FMGC will recomend a step to FL380. The box consistently prefers 1500 above Opt rather than 500 below, and performance data backs that up.
Lastly, we can't in practice fly the extremely slow climb that would be theoretically optimal. The vertical profile of our climb does indeed look like a stairstep, when transposed over the smoothly rising optimal. Since being below optimal is so prohibitive, the stair step doesn't average above and below the optimum slope, but instead biases to climb early.
Further, and you've no doubt already seen, how the step pays no heed to to the Rec Max FL (other than staying below it), in search of a strong tailwind up high and to a lesser degree ducking headwinds.
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Cardinal, would you kindly give some more detailed explanation as to why FMGC prefers being at 1500ft above optimum altitude then 500ft below? I was always taught that a climb to a higher level should only be attempted ( for fuel economy ) when you are at least 1000ft below your optimum level
Thanks
Thanks
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On the Airbus the Optimum is the most economical flight level, taking into account Cost Index/Wind/OAT
Only 500 ft steps have an economical impact (as far as I know flight levels have 1000 ft steps), and unless you are given a block altitude, 500 steps are not feasible.
So in order to avoid "noising" the FMS i.e. making it computing for a long period of time a "meaningless data" the FMS uses 500 ft increments.
This applies for Honeywell Pegasus, as well as the Thales/Smith FMS2 since they are both developped from Airbus/Aespatial specification.
The 1500 ft is something I am not aware off, however all the above will work only if you are all the time in managed mode, because to the optimum flight level corresponds an optimum path that can only be flown if you are at the optimum speed.
Only 500 ft steps have an economical impact (as far as I know flight levels have 1000 ft steps), and unless you are given a block altitude, 500 steps are not feasible.
So in order to avoid "noising" the FMS i.e. making it computing for a long period of time a "meaningless data" the FMS uses 500 ft increments.
This applies for Honeywell Pegasus, as well as the Thales/Smith FMS2 since they are both developped from Airbus/Aespatial specification.
The 1500 ft is something I am not aware off, however all the above will work only if you are all the time in managed mode, because to the optimum flight level corresponds an optimum path that can only be flown if you are at the optimum speed.
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Step climb is a pilot imput, and it will be performed at a waypoint (geographical step) however the FMS2 has the capability to compute an optimized point (lat and long coordinate) from which it would be better to climb to the specified step climb flight level.
It takes into consideration the aircraft weight (power available, power required stuff) in order to minimize fuel burn during the climb (at max cruise thrust) to the desired step flight level along with the fuel flow(step TOC weight)/forcast wind.
All this has nothing to do with the optimum flight level, it is the optimization of the climb to a step altitude.
Step climb/descent can be an operational requirement (ATC Notams etc...) and will all know that there is no savings in that case, so the FMS will try through optimization of the step save some nickels, weithereas optimum cruise flight level will save more dollars
It takes into consideration the aircraft weight (power available, power required stuff
) in order to minimize fuel burn during the climb (at max cruise thrust) to the desired step flight level along with the fuel flow(step TOC weight)/forcast wind.All this has nothing to do with the optimum flight level, it is the optimization of the climb to a step altitude.
Step climb/descent can be an operational requirement (ATC Notams etc...) and will all know that there is no savings in that case, so the FMS will try through optimization of the step save some nickels, weithereas optimum cruise flight level will save more dollars
Last edited by kijangnim; 27th Jul 2008 at 09:09. Reason: to add more info
