a very quick question from ATC

[BwatchGRUNT]

Boeing 767 at FL330 - is your maximum speed 0.80 mach??

Thanks

[sleeve of wizard]

depending on the weight of the aircraft at the time it very well could be. generally speaking 767's don't cruise above M.82

[flyingchanges]

MMO is .86

[EW73]

Hell...in the '74 Classics, we used to cruise at 0.86M!

[sleeve of wizard]

under FAA certification MMO is M0.86, under UKCAA certification MMO is M0.84, LRC is M0.80, fuel burn increases markedly above M0.80

[underread east]

JAR-Ops follows FAA with MMO at 0.86M (767-3ERF)

[sleeve of wizard]

these were the limits when I flew the 767, would not surprise me that the UKCAA have come in line with JAROPS. Haven't flown the 767 in a few years now!

[BwatchGRUNT]

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.

[zerozero]

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.

:cool:

[boxmover]

Depends if it’s 100 straight line or track. All bets are of if holds are in the plan.

[mathy]

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.

[Landroger]

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!:eek:

[Mariner]

Perhaps they were flying through some rough air. If I remember correctly, the turbulent air penetration speed on the 76 is 290 KIAS/.78 Mach.

[hawk37]

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

[NigelOnDraft]

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

[Capn Bloggs]

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
Sorry for the thread creep but what is this lift coefficient you speak of ie is it CL, and more importantly, how do you adjust it from the flight deck?

[aterpster]

landroger:

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?

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.

[mutt]

M0.785 perhaps with a lift coefft of 0.53 I know that this information is available in certain Boeing manuals and software, but generally these aren't available to flight crew. So how do you actually use this information? Is it supplied to your crews?

Mutt

[Mercenary Pilot]

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.

Next time, punish the slow aircraft! :E

[By George]

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.