737 NG optimum flight level reduction with reducing mach no.
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737 NG optimum flight level reduction with reducing mach no.
737 NG optimum flight level reduction with reducing mach no.
Can someone explain how my 737 optimum altitude is calculated.
Yesterday I completed an automatic FMC max rate climb to meet an ATC restriction ... the speed came back to 230 knots/0.68 mach and approaching FL350 ....ISA -10...at 69,000 tonnes the FMC optimum calculation was around 3,000' lower ( I would typically expect FL380 @ 0.78 mach in these conditions).
I understand part of the calculation is based on the fuel cost index (cost index zero giving the highest optimum)?
Thanks in advance.
Can someone explain how my 737 optimum altitude is calculated.
Yesterday I completed an automatic FMC max rate climb to meet an ATC restriction ... the speed came back to 230 knots/0.68 mach and approaching FL350 ....ISA -10...at 69,000 tonnes the FMC optimum calculation was around 3,000' lower ( I would typically expect FL380 @ 0.78 mach in these conditions).
I understand part of the calculation is based on the fuel cost index (cost index zero giving the highest optimum)?
Thanks in advance.
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By selecting a lower speed, you brought your 'coffin corner' down, lowering your optimum cruise at that alltitude. Your low speed buffet margin was degraded, so the FMS was telling you to cruise lower. Your optimum altitude, as well as your maximum altitude, changes according to selected cruise speed.
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Yesterday I completed an automatic FMC max rate climb to meet an ATC restriction ... the speed came back to 230 knots/0.68 mach and approaching FL350 ....ISA -10...at 69,000 tonnes the FMC optimum calculation was around 3,000' lower ( I would typically expect FL380 @ 0.78 mach in these conditions).
Those speeds look more like Max Angle.
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specimen figures
Good answers. I back engineered the data a little bit to try figure out the F390 part.
Let us suppose B737-800 OEM 41413kg, ISA+10, 69600kg take-off with 13500kg payload for 2000nm block distance is just eligible for F380 at M0.78 with a climb burn of 1500kg. Thus at ToD we are at 68100kg say. Instantaneous climb performance reserve at constant 247kcas is 630 feet per minute, L/D 17.4 for a lift coefft of 0.61, tsfc 0.65kg/kg-hr and a buffet margin of 1.39g. Breguet Range Factor is (V/c) * (L/D) so roughly the RF is 12050 to slide rule accuracy. Provided that this does not infringe any limitations, fine. But if it does and/or a better RF can be found at F360 this is what the FMC will suggest for optimum.
At the higher lift coeffts the aircraft tends to run out of buffet margin and climb performance. If L/D falls off too much the engines have to work harder and the tsfc starts to enter hot country.
Given that the computer chip is probably still a common or garden 386 I’d guess the sums are largely based on lookup tables.
Factor in what the other guys are saying, especially the coffin corner warning. But essentially don't poke the bear and don't thrash the engines. You can get away with the latter but if you attempt the former you may run out of sky.
I hope that helps.
Let us suppose B737-800 OEM 41413kg, ISA+10, 69600kg take-off with 13500kg payload for 2000nm block distance is just eligible for F380 at M0.78 with a climb burn of 1500kg. Thus at ToD we are at 68100kg say. Instantaneous climb performance reserve at constant 247kcas is 630 feet per minute, L/D 17.4 for a lift coefft of 0.61, tsfc 0.65kg/kg-hr and a buffet margin of 1.39g. Breguet Range Factor is (V/c) * (L/D) so roughly the RF is 12050 to slide rule accuracy. Provided that this does not infringe any limitations, fine. But if it does and/or a better RF can be found at F360 this is what the FMC will suggest for optimum.
At the higher lift coeffts the aircraft tends to run out of buffet margin and climb performance. If L/D falls off too much the engines have to work harder and the tsfc starts to enter hot country.
Given that the computer chip is probably still a common or garden 386 I’d guess the sums are largely based on lookup tables.
Factor in what the other guys are saying, especially the coffin corner warning. But essentially don't poke the bear and don't thrash the engines. You can get away with the latter but if you attempt the former you may run out of sky.
I hope that helps.
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I think running out of sky is what happened to the Tu154 that went into an unrecoverable flat spin when it tried to climb over weather.
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Mathy - you deserve that name! 
Rainbow is right, if you play around with speeds you can change the optimum CRZ ALT. For instance, put 320 Knots in the cruise page, and I imagine you'll get something around FL320? As that is where the low end of the high speed buffet boundary is.
Horgy
Rainbow is right, if you play around with speeds you can change the optimum CRZ ALT. For instance, put 320 Knots in the cruise page, and I imagine you'll get something around FL320? As that is where the low end of the high speed buffet boundary is.
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Props...my mistake it was max angle.
Mathy sounds like good data ...where is the source that allowed you to
back engineer.
Mathy sounds like good data ...where is the source that allowed you to
back engineer.
Last edited by Zoyberg; 7th Feb 2009 at 19:56.
Man I take my hat off to some of you smart chaps/chapesses!
I would have said that at the slower speeds you would reach low speed buffet margin earlier and therefore both your max height and your optimum height will be lower. If you selected a slightly higher speed it would give you that minimum margin at a slightly higher altitude, and so both your optimum and maximum heights would increase a bit. Do I need to get a personal tutor or is that about the guts of it?
Regards, Framer
I would have said that at the slower speeds you would reach low speed buffet margin earlier and therefore both your max height and your optimum height will be lower. If you selected a slightly higher speed it would give you that minimum margin at a slightly higher altitude, and so both your optimum and maximum heights would increase a bit. Do I need to get a personal tutor or is that about the guts of it?
Regards, Framer
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sources
My sources are simple. Company laptop, proprietary software and good old "Fundamentals of Flight". Wodges of manuals that fall open at the usual pages. Plus the odd snippet and gem picked up on two well-run Boeing courses under the tutelage of Lee Monson, then VP of B737s. Non disclosure agreements apply but given a simple request and time on my hands I could try and knock up a more illustrative example and pdf it to reside in the pprune notes if a moderator could advise how this is done. I can also point you at some pretty powerful software out there which is almost as good as the Boeing stuff and a darn sight cheaper although still the price of a small family saloon.
