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namida

Best altitude for 0.82 cruise?

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Hello, I usually fly close to optimum altitude but seems there is another altitude for max speed, not fuel saving.As I googled, the altitude is between FL240-FL300. I found that as aircraft passes FL260, GS and TAS started falling.Someone says the best speed is at the point when GS and TAS became same.. I learned about density altitude recently, so i can caculate this if someone let me know the formular.. :)Any solutions or links would be appreciated, thank you!! Sorry not GS TAS, MVO, MMO :(

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Just use a cost index of 500. Where did you read that thing about GS and TAS becoming the same????GS = TAS +- wind remember this mnemotecnic. ICETM (ice tea mint). That will tell you how speeds vary with altitude. I: IndicatedC: CalibratedE: EquivalentT: TrueM: Mach If you climb at a constant IAS, all the other speeds will increase. If you climb at constant TAS, I C and E will decrease with altitude but Mach will increase. If you climb at constant MACH, all the other speeds will decrease with altitude. A normal climb schedule consists of climbing at constant IAS up to 20.000ft, and then maintaining constant MACH (which will actually be a deceleration). So, up to 20000 you accelerate (increasing TAS), and from 20000 upwards, you decelerate (decreasing TAS).

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The GS generally raises with the altitude as you fly into less dense air (up to certain FL). You should consider also the high altitude winds (jetstream) which might severely reduce or increase your ground speed. I've personally seen tailwinds of 100+ knots in the RL resulting in 600+ knots groundspeed.

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Omar, could you provide some explanation on the first graph.. I-C-E-T-M? Cheers
It is easy. First graph. If you climb at constant IAS, and monitor all the other speeds, you would know that they all increase as you climb. TAS being your "real speed", to maintain a constant IAS means that the airplane has to accelerate constantly as you climb (provided the wind doesn't change). Second graph. If you maintain a constant mach during climb and monitor TAS, you'll notice that as you climb, you deceletate. *I believe that what you see in the 737 speed tape is actually EAS, and not just IAS.

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Thank you for explanation and plus ICETM :) So, is it correct the altitude TAS stops increasing is the VMO = MMO = Maximum Speed?And I found that the Crossover altitude is almost fixed to FL240 because 737is small plane.I think FL200 is little low for 737. (in FSX.. of course) I saw NGX's TAS goes up to 500 when passing FL260, and drops as climb continues..So.. the speed would be FL260 for NGX i flew.. right? laugh.png

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Thank you for explanation and plus ICETM :) So, is it correct the altitude TAS stops increasing is the VMO = MMO = Maximum Speed?And I found that the Crossover altitude is almost fixed to FL240 because 737is small plane.I think FL200 is little low for 737. (in FSX.. of course) I saw NGX's TAS goes up to 500 when passing FL260, and drops as climb continues..So.. the speed would be FL260 for NGX i flew.. right? laugh.png
The first part is not correct. The IAS/M climb schedule does not correspond to anything else than just chosen speeds. You can climb 280/0.78 or you can do 300/.80. Cost Index will define that. The crossover altitude depends purely on the selection of speeds you made. Once you have chosen your MACH cruise, you'll climb at constant IAS (accelerated climb) until your your TAS is the same as the TAS you want to carry along the flight (defined in MACH). At that point you'll switch to constant MACH climb (decelerated climb), and your TAS will decrease by the effects of temperature drop. The speeds you select for the schedule affects fuel consumption and time to TOC because it has a big impact on how the rate of climb progresses along the climb. PS: yes, 28000 makes more sense.unled1uuk.jpg

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Oh I see what you mean.. http://forums.xplane...sover-altitude/Still can't fully understand but I got a word roughly.. will study more with this graph xD Thank you for clearly let me know how to get Max Speed. Idea.gif
Be warned, performance is a complex topic. I had NO CLUE about it during my first 10 years of hardcore simulation and I only learned the real thing once i did my ATPL. FS often ignores these things because you can just fly a 747 without having studied the important "boring stuff" on the textbook. Here's another one for you.unled1udy.jpg

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Thanks for advice. I'm playing FS for 10 years and feels more fun to theories than flying. :)I have learned many new flight techniques for simulation and NGX made it even more better. Oh, and yes, I FELT that great rate of climb at cross over altitude.. it was because of that.. now I get it..Anyway I should level off at that altitude for MAX cruise.. *cough*(I usually fly in normal status. believe me xD)

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The crossover altitude depends purely on the selection of speeds you made.
Sorry man, isn't it a little more complex than that? After all speed of sound, hence TAS at given mach number varies... As does TAS - IAS. Higher temp, higher TAS for given IAS. Higher temp, higher TAS for given mach and vice versa. Well, not sure if the two correlate linear. Just a thought I had immediately reading all this. But maybe I'm on the wrong track here. sig.gif

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Sorry man, isn't it a little more complex than that? After all speed of sound, hence TAS at given mach number varies... As does TAS - IAS. Higher temp, higher TAS for given IAS. Higher temp, higher TAS for given mach and vice versa. Well, not sure if the two correlate linear. Just a thought I had immediately reading all this. But maybe I'm on the wrong track here. sig.gif
OF COURSE it is more complex than that. But for a given day, the cross over altitude will be fixed for the chosen speeds. What you want to do, is to change to a Mach number that you've already reached as TAS went decreasing throughout your climb.

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Higher temp, higher TAS for given mach
I've seen this, too. (somewhere over googling..)This is the reason I talk about 'density altitude'.. blah blah..It would be good to know how to calculate crossover altitude in certain circumstances, but will save it for now.Weather there will be a formula exists or not, I'm in good mood now. : ) Thanks Josef.

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That will be set by your cost index. I'm failing at explaining this apparently. Say you choose 280IAS for climb and .78 for cruise. The cross-over altitude will be that altitude at which 280KIAS results in the same TAS as M.78. TAS = EAS / sqr(ρ/ρ0) ρ stands for density.

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Man, you guys complicate this stuff. Say your climb is set to 280/.78. The plane will climb at 280IAS until the Mach increases to .78 and then it climbs at that.The rest is for your ATPL written.

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Man, you guys complicate this stuff. Say your climb is set to 280/.78. The plane will climb at 280IAS until the Mach increases to .78 and then it climbs at that. The rest is for your ATPL written.
While I agree with your first and last statement (I should say especially the last), your second one still doesn't help the OP's issue, as he was asking for exactly that altitude LOL LMAO.gif This might be theoretical, but why not just use a TAS formula depending on altitude and use a Mach number depending on altitude (thinking about it, this one might be easier), insert your speeds and solve for altitude? This might be a good start (like so often). In fact, I'm gonna give it a try, maybe it's possible to come up with something that way. sig.gif

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While I agree with your first and last statement (I should say especially the last), your second one still doesn't help the OP's issue, as he was asking for exactly that altitude LOL LMAO.gif This might be theoretical, but why not just use a TAS formula depending on altitude and use a Mach number depending on altitude (thinking about it, this one might be easier), insert your speeds and solve for altitude? This might be a good start (like so often). In fact, I'm gonna give it a try, maybe it's possible to come up with something that way. sig.gif
You know the airplane figures that out all by itself right?

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The relationship between true airspeed and mach number varies with temperature. Similarly the relationship between indicated airspeed and mach number varies with pressure. This all means that everytime you climb, the transition from airspeed to mach on the VNAV climb will be at a different altitude.Regarding the aircraft FMC being programmed with this knowledge, so the pilot is not required to understand basic aerodynamics, I beg to differ. If one is just playing a game, fine, just why play with an accurate replica of a fully complex aircraft. Isn't it like going to a Chinese theater play without understanding the language?

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You know the airplane figures that out all by itself right?
Well yes, so? It still doesn't help the OP, because I'm sure he knows the FMS is capable of doing so, but he was asking for a way to get that altitude. The FMS does nowhere output that altitude except for when you just do the climb and watch when it switches over, but I doubt that's the way the OP was looking for. sig.gif

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