a very quick question from ATC
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Oceanic eastboud, only 100 miles from destination. One of six a/c that required streaming for the same popular london airport, I asked to fly at 0.82 and the lady jockey came back to say 0.80 MAX!!
Seemed a little slow to me and didnt help with my traffic situation, resulted in the a/c behind being swung 45 degress to the left of his track with a 65 degree turn back 2 mins later to make the required space.
Seemed a little slow to me and didnt help with my traffic situation, resulted in the a/c behind being swung 45 degress to the left of his track with a 65 degree turn back 2 mins later to make the required space.
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Hmm, I'm not second guessing either ATC or the crew...
BUT...100 miles from destination at FL330...
It *seems* they should be ready to start down and an increase of 0.02 ain't gonna kill anybody.
I would have given it to you in a descent but I'm just that kinda guy.
BUT...100 miles from destination at FL330...
It *seems* they should be ready to start down and an increase of 0.02 ain't gonna kill anybody.
I would have given it to you in a descent but I'm just that kinda guy.
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quick answer
If you mean common everyday expectation and not MMO then I’d guess M0.80 as you say. On my last acquaintance, when flat out economy was the driver then M0.77 and a lift coefficient of 0.55 would sit nicely at FL330. Sacrificing a little but with an eye on good long range performance over ca. 3900nm then M0.785 perhaps with a lift coefft of 0.53 would do nicely. High speed? Then M0.795 and a lift coefft of 0.52 seems in order. Fuel load would be based around ideas like these.
In general the B767 does not like to go above M0.80 in cruise because the L/D starts to reduce rather sharply because compressibility drag for one thing starts to increase very rapidly. Whereas induced drag follows a square law, compressibility has terms that can be modelled in cube and quad laws. I say "modelled" because the most commonly used interpolation formulae go to third and fourth powers. What the aerodynamics is doing is another thing but the mathematical fit of these interpolation equations is good.
I take it as read that if you ask for more than M0.80 you mean short duration for pressing ATC reasons otherwise the route fuel calculations are liable to go out the window. But you know that.
In general the B767 does not like to go above M0.80 in cruise because the L/D starts to reduce rather sharply because compressibility drag for one thing starts to increase very rapidly. Whereas induced drag follows a square law, compressibility has terms that can be modelled in cube and quad laws. I say "modelled" because the most commonly used interpolation formulae go to third and fourth powers. What the aerodynamics is doing is another thing but the mathematical fit of these interpolation equations is good.
I take it as read that if you ask for more than M0.80 you mean short duration for pressing ATC reasons otherwise the route fuel calculations are liable to go out the window. But you know that.
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Question from the great unwashed.
Mathy's post was, for me, both very interesting and worryingly complex. Right on the limit of what I - a hi-tech kit engineer, but entirely without aeronautical training or teaching - could understand.
I can see from his and the other answers that the 767 has the power reserve to punch it along faster, but it's intrinsic aerodynamics don't want to go that fast. Now, I have always understood that the B747, in most of its forms, was and, so far as I know, still is the fastest subsonic airliner in the business?
The paradox for me is; the 767 was designed after the 747; the 747 is much bigger and has more or less all the same features, ostensibly, as a 767 and yet, it seems, its aerodynamics are happy to troll along at M0.9X and harrumpty-bump thousand feet while running on fumes. Why is this please?
Roger
PS: Mathy - lovely answer mate, but try not to live up to your name so much - I can't keep up!
I can see from his and the other answers that the 767 has the power reserve to punch it along faster, but it's intrinsic aerodynamics don't want to go that fast. Now, I have always understood that the B747, in most of its forms, was and, so far as I know, still is the fastest subsonic airliner in the business?
The paradox for me is; the 767 was designed after the 747; the 747 is much bigger and has more or less all the same features, ostensibly, as a 767 and yet, it seems, its aerodynamics are happy to troll along at M0.9X and harrumpty-bump thousand feet while running on fumes. Why is this please?
Roger
PS: Mathy - lovely answer mate, but try not to live up to your name so much - I can't keep up!
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Hey, Landroger, I love Mathy's stuff. Its different for sure, but adds a much needed empirical explanation to those atypical perfomance situations. He and Enicalythe seem to be the two main practitioners
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BwatchGRUNT
Given the variables of fuel price, holding into LHR, turbulence, reserves, company fuel policies, I am not surprised at her reply.
I am sure it does not apply to you, but some ATCOs seem to think speed / RoC / RoD are just things we dial into a computer and go back to the crossword... rather than burning 10% more fuel than we need, making it uncomfortable for all on board, and/or increasing the risk of an overspeed.
I appreciate there are also pilots who just treat fuel burnt as a number on the screen, and not £££ out of the compnay's bottom line.
If they were @ FL330, and 100NM out, they are already "high" (33x3=99 + 10 for speed or so=109NM +/-wind), and wanting to lose energy, not pour precious fuel/energy into the motors that they later need to throw away with the speedbrakes.
Do appreciate you have your needs as well, and we all need to work together, but you did ask the qu
NoD
Given the variables of fuel price, holding into LHR, turbulence, reserves, company fuel policies, I am not surprised at her reply.
I am sure it does not apply to you, but some ATCOs seem to think speed / RoC / RoD are just things we dial into a computer and go back to the crossword... rather than burning 10% more fuel than we need, making it uncomfortable for all on board, and/or increasing the risk of an overspeed.
I appreciate there are also pilots who just treat fuel burnt as a number on the screen, and not £££ out of the compnay's bottom line.
If they were @ FL330, and 100NM out, they are already "high" (33x3=99 + 10 for speed or so=109NM +/-wind), and wanting to lose energy, not pour precious fuel/energy into the motors that they later need to throw away with the speedbrakes.
Do appreciate you have your needs as well, and we all need to work together, but you did ask the qu
NoD
Originally Posted by mathy
with an eye on good long range performance over ca. 3900nm then M0.785 perhaps with a lift coefft of 0.53 would do nicely
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landroger:
No, that title goes to the Convair 990.
The 707s I flew had a planned cruise of Mach 0.86 and we could easily do Mach 0.89. This was when jet fuel was 21 to 27 cents per gallon.
I can see from his and the other answers that the 767 has the power reserve to punch it along faster, but it's intrinsic aerodynamics don't want to go that fast. Now, I have always understood that the B747, in most of its forms, was and, so far as I know, still is the fastest subsonic airliner in the business?
The 707s I flew had a planned cruise of Mach 0.86 and we could easily do Mach 0.89. This was when jet fuel was 21 to 27 cents per gallon.
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M0.785 perhaps with a lift coefft of 0.53
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Seemed a little slow to me and didnt help with my traffic situation, resulted in the a/c behind being swung 45 degress to the left of his track with a 65 degree turn back 2 mins later to make the required space.
The 767 is slow and so is the 330 at .82 I understand the 330 comes back to .78 in turbulence. The 744 can give you .85 in turbulence and 310 kts on descent. If you ask nicely I have no problem with .87 and 350 kts on descent in a 744. Why not drop a faster aircraft in an early descent underneath slower traffic ahead? I know it's rude to push in etc but some of the vectors/speed restrictions to sit behind a 330 are agony at times. Why not use the speedy ones to advantage. The North Atlantic is filling up with slow traffic these days and it seems to cause more flow problems than the faster ones. I must admit I'd rather have a 330 if I was paying for the fuel.
