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Is Mach 0.85 Becoming The Industry Standard?  
User currently offlineA380900 From France, joined Dec 2003, 1116 posts, RR: 1
Posted (1 year 4 months 3 weeks 2 days 5 hours ago) and read 11068 times:

757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

All new airliners seem to be shooting for 0.85. Is there a technical reason for that or is it just a consensus that appeared at this level. Can we expect to see the A320 and 737 replacements to go that fast?

By the way, I made a round trip across the Atlantic not long ago on a 757 and it seemed definitely slower than a 747.

48 replies: All unread, showing first 25:
 
User currently offlineatct From United States of America, joined Mar 2001, 2307 posts, RR: 38
Reply 1, posted (1 year 4 months 3 weeks 2 days 3 hours ago) and read 11032 times:

Just because the airliner can go that fast doesn't mean it will. I frequently see 757/767's at .76 etc.


"The way to get started is to quit talking and begin doing." - Walt Disney
User currently offlineA380900 From France, joined Dec 2003, 1116 posts, RR: 1
Reply 2, posted (1 year 4 months 3 weeks 1 day 21 hours ago) and read 10885 times:

Fair enough. They do seem to optimize the planes for one cruise speed and every single last airplane produced seems to be Mach 0.85.

User currently offline26point2 From United States of America, joined Mar 2010, 836 posts, RR: 0
Reply 3, posted (1 year 4 months 3 weeks 1 day 21 hours ago) and read 10886 times:

Most carriers, at least in the US, have been operating at slower speeds the past few years in order to save fuel/cost. We fly the Global Express at M0.85 all day long and I have never seen an airliner match our speed except perhaps 747. 737, 757, A320 are considerably slower. Quite often when 100 miles from destination ATC slows us down to M0.78 for sequencing to follow Southwest (737) or some such thing.

User currently offlinewilco737 From Greenland, joined Jun 2004, 9077 posts, RR: 76
Reply 4, posted (1 year 4 months 3 weeks 1 day 21 hours ago) and read 10886 times:
AIRLINERS.NET CREW
HEAD MODERATOR

An airplane has a design speed where the lift to drag ratio is the best. For the 744 it is M0.855. But due to the high fuel costs we do not cruise that fast anymore. Usually we are at M0.84 or if not heavy even slower. Of course we can still go faster, but we try to avoid it due to the fuel costs.

Problem is still in high demand traffic areas like the NAT OTS system. Many many airplanes on the same route with different speeds. So the 767 and 340s fly slower than M0.80 and we are in a hurry at M0.85... So you don't get your level or your speed...

It would be great to see if all airplanes cruise at the same speed.

wilco737
  



It it's not Boeing, I am not going.
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 5, posted (1 year 4 months 3 weeks 1 day 15 hours ago) and read 10721 times:

Technically, doesn't the wing shape effects the efficient speed at which you cruise?
So, if the 737 is being revised to the MAX with a new wing, will they be able to increase that cruise speed?

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlinehivue From United States of America, joined Feb 2013, 1096 posts, RR: 0
Reply 6, posted (1 year 4 months 3 weeks 1 day 9 hours ago) and read 10548 times:

Quoting A380900 (Thread starter):
All new airliners seem to be shooting for 0.85.

The 747 was 0.855 at EIS back in about 1970, right?

Quoting A380900 (Thread starter):
757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

I think the whole thing is about fuel.The 757/767 were 0.80 for fuel savings. They've been slowly working their way back up to the 747 number as aircraft/engines have become more fuel efficient.


User currently offlineonebadlt123 From United States of America, joined Oct 2004, 51 posts, RR: 0
Reply 7, posted (1 year 4 months 3 weeks 8 hours ago) and read 10096 times:

At my carrier we plan the following speeds. Note, these are default speeds and we can adjust faster or slower as needed . If there is no operational need to go faster we will leave the flight at default or even slow it down .01. Most of the time in the domestic market, or below FL 300 we will use LRC speeds


A319 - M.78
A320 - M.78

B737-500 - M.76
B737-700 - M.77
B738 - M.78
B739 - M.78
B739ER- M.78/79

B747-400 - M.84

757-200 - M.78
757-300 - M.79

767-200 - M.80
767-300 -M.78
767-400 - M.78

777-200 - M.83

787-8 - M.84

[Edited 2013-06-01 18:22:09]

User currently offline1337Delta764 From United States of America, joined Oct 2005, 6571 posts, RR: 2
Reply 8, posted (1 year 4 months 3 weeks 4 hours ago) and read 10018 times:

Quoting hivue (Reply 6):
I think the whole thing is about fuel.The 757/767 were 0.80 for fuel savings. They've been slowly working their way back up to the 747 number as aircraft/engines have become more fuel efficient.
Quoting A380900 (Thread starter):
757 and 767: Mach 0.8
A330 and A340: Mach 0.82
777: Mach 0.84
747, A380, A350, 787: Mach 0.85

Don't forget the A300, the world's slowest widebody with a design cruise speed of mach 0.78.

[Edited 2013-06-01 22:43:02]


The Pink Delta 767-400ER - The most beautiful aircraft in the sky
User currently offlineMax Q From United States of America, joined May 2001, 4648 posts, RR: 19
Reply 9, posted (1 year 4 months 3 weeks 3 hours ago) and read 10002 times:

Quoting Reply 7):
At my carrier we plan the following speeds. Note, these are default speeds and we can adjust faster or slower as needed . If there is no operational need to go faster we will leave the flight at default or even slow it down .01. Most of the time in the domestic market, or below FL 300 we will use LRC speeds


A319 - M.78
A320 - M.78

B737-500 - M.76
B737-700 - M.77
B738 - M.78
B739 - M.78
B739ER- M.78/79

B747-400 - M.84

757-200 - M.78
757-300 - M.79

767-200 - M.80
767-300 -M.78
767-400 - M.78

777-200 - M.83

787-8 - M.84

Don't know where you get these numbers from. I work for the same Airline and there are no fixed mach numbers.



Econ is used for planning with a variable cost index depending on the priorities of that particular flight, even if fixed mach numbers were used yours are incorrect (too slow)



The best contribution to safety is a competent Pilot.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 10, posted (1 year 4 months 2 weeks 6 days 12 hours ago) and read 9767 times:

As I understand it, designing an aircraft for high subsonic cruise is almost as difficult a feat as designing it for supersonic cruise...and M0.85 or thereabouts is about the limits for "conventional" subsonic jet design. That said, the 747 has a really high Mmo compared to the others, M0.90 IIRC...a product of a different time   Boeing thought the 2707 had a better future as a passenger aircraft..  


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25626 posts, RR: 22
Reply 11, posted (1 year 4 months 2 weeks 6 days 10 hours ago) and read 9708 times:

Quoting 1337Delta764 (Reply 8):
Don't forget the A300, the world's slowest widebody with a design cruise speed of mach 0.78.

Makes little difference on typical routes that were operated by the A300. If you find old schedules involving routes operated by the A300 and other types, I doubt the published block times are noticeably different.


User currently offlineMax Q From United States of America, joined May 2001, 4648 posts, RR: 19
Reply 12, posted (1 year 4 months 2 weeks 6 days 5 hours ago) and read 9585 times:

Quoting KELPkid (Reply 10):
That said, the 747 has a really high Mmo compared to the others, M0.90

B747 MMO is ,92



The best contribution to safety is a competent Pilot.
User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 13, posted (1 year 4 months 2 weeks 5 days 20 hours ago) and read 9461 times:

Here's a short homework assignment if anyone is willing to do it.

Please chart the comparison between cruise speed and wing sweep of the different airplane models.
While you are at it, please identified which model have a super-critical wing.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineFlighty From United States of America, joined Apr 2007, 8631 posts, RR: 2
Reply 14, posted (1 year 4 months 2 weeks 5 days 17 hours ago) and read 9375 times:

Quoting Reply 7):
787-8 - M.84

It will be interesting to know if the 787 has a "sweet spot" cruise that fast.

Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.


User currently offlineKlaus From Germany, joined Jul 2001, 21485 posts, RR: 53
Reply 15, posted (1 year 4 months 2 weeks 5 days 12 hours ago) and read 9279 times:

Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

If the fuel consumption penalty in going from (for instance) 0.8 to 0.84 mach should be significantly lower now than it was with the older airfoils (and corresponding engines), the revenue opportunity of faster flights would at some point be worth it even with the higher fuel prices nowadays.

If I understand this correctly, the older airfoils should have suffered more from transonic drag than the newer ones do. So with the older designs you have to pay more to get to a higher speed than you do with newer designs.

Other aerodynamic improvements due to newer simulation capabilities should have contributed to this as well (re-shaping fuselage, fairings, stabilizers).


User currently offlineBMI727 From United States of America, joined Feb 2009, 15780 posts, RR: 27
Reply 16, posted (1 year 4 months 2 weeks 5 days 4 hours ago) and read 9135 times:

Quoting Klaus (Reply 15):
Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

In practical terms, not so much. In practice, the supercritical airfoil is more commonly used to get the same cruise speeds with less sweep which makes the wing less tricky from a structural perspective. Older 747s had 37.5 degrees of sweep, while the 747-400 and -8 make do with 35. The A380 uses 33.5 while the G650 uses 36 degrees but flies faster.

You can use supercritical airfoils to fly that much faster for the same amount of drag, but in practice the speed is usually not worth the cost. Look at the Citation XLS+: it has a straight wing with a supercritical cross section and can do M 0.75 with a ceiling of FL 450 and fly off of a 3,560 ft. runway.



Why do Aerospace Engineering students have to turn things in on time?
User currently offlineKELPkid From United States of America, joined Nov 2005, 6407 posts, RR: 3
Reply 17, posted (1 year 4 months 2 weeks 5 days 4 hours ago) and read 9150 times:

Quoting Klaus (Reply 15):
Isn't a relevant part of this the advances in aerodynamics (such as supercritical airfoils) which have reduced the cost penalty of these increments in near-sonic speeds?

The big trick in moving around near the speed of sound is in controlling shock waves...airfoils and lifting surfaces, by their very nature, accellerate the air moving on the low pressure side of the airfoil. Once that air goes supersonic, a shock wave forms, and the rules governing subsonic flows are out the door in that localized area. This is why Mach 0.92 is tricky (besides the extra drag of moving faster...). And it all has to work well when you're at climb and approach speeds, too  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineKlaus From Germany, joined Jul 2001, 21485 posts, RR: 53
Reply 18, posted (1 year 4 months 2 weeks 4 days 23 hours ago) and read 9081 times:

Quoting BMI727 (Reply 16):
You can use supercritical airfoils to fly that much faster for the same amount of drag, but in practice the speed is usually not worth the cost.

It still looks like a plausible contributing reason why the design speeds have been going up again even despite rising fuel prices.

Does anybody have the relative fuel consumption numbers at hand for the different kinds of airfoils at different but comparable airspeeds, preferably with comparable engines?


User currently offlinefrancoflier From France, joined Oct 2001, 3790 posts, RR: 11
Reply 19, posted (1 year 4 months 2 weeks 4 days 2 hours ago) and read 8813 times:

Quoting Max Q (Reply 12):
Quoting KELPkid (Reply 10):
That said, the 747 has a really high Mmo compared to the others, M0.90

B747 MMO is ,92

You're both right.

M 0.90 for JAR certification and 0.92 for FAA certification.
Don't know why the difference though...



Looks like I picked the wrong week to quit posting...
User currently offlineRara From Germany, joined Jan 2007, 2111 posts, RR: 2
Reply 20, posted (1 year 4 months 2 weeks 3 days 11 hours ago) and read 8633 times:

Quoting wilco737 (Reply 4):
An airplane has a design speed where the lift to drag ratio is the best. For the 744 it is M0.855. But due to the high fuel costs we do not cruise that fast anymore.

What's the benefit of a better lift-to-drag ratio if the fuel burn is higher?

In other words, if you burn less fuel at mach 0.84, but the lift-to-drag ratio is better at mach 0.855, then what's the practical benefit of lift-to-drag?

Quoting Flighty (Reply 14):
Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.

Indeed... theoretically that should be reflected in the cost index, which is a function of fuel cost versus operating cost. In practice, most airlines use the same cost index for all flights because they find it too tedious to calculate.



Samson was a biblical tough guy, but his dad Samsonite was even more of a hard case.
User currently offlineLH707330 From United States of America, joined Jun 2012, 799 posts, RR: 0
Reply 21, posted (1 year 4 months 2 weeks 3 days 5 hours ago) and read 8554 times:

Quoting BMI727 (Reply 16):
Older 747s had 37.5 degrees of sweep, while the 747-400 and -8 make do with 35.

They all have the same basic planform, with the -400 having a small extension and winglet, and the -8 having the same extension and a raked tip (along with a changed cross-section and flap).


User currently offlinelebb757 From United States of America, joined Apr 2013, 9 posts, RR: 0
Reply 22, posted (1 year 4 months 2 weeks 2 days 18 hours ago) and read 8458 times:

Quoting Flighty (Reply 14):
Just wanted to add, speed can be important if it permits certain routings or schedules that another aircraft can't do. This isn't only about fuel burn. It can also be about pilot hours (expensive), daily revenue and even aircraft hours. An aircraft moving 5% faster will have 5% fewer hours on it. If speed allows one extra leg per day for a WN 737... wow. You can do the same network with fewer aircraft and pilots.

5% faster does not equal exactly 5% percent fewer hours. However, you are right that speed allows increased profitability in the cases you just mentioned.

Quoting Rara (Reply 20):
Indeed... theoretically that should be reflected in the cost index, which is a function of fuel cost versus operating cost. In practice, most airlines use the same cost index for all flights because they find it too tedious to calculate.

What variables does the FMC manage?? I'm pretty sure about fuel cost, maintenance and crew salaries. It would be interesting to know if it also takes into account aircraft utilisation and as Flighty mentioned the possibility of an extra leg.


User currently offlinebikerthai From United States of America, joined Apr 2010, 2155 posts, RR: 4
Reply 23, posted (1 year 4 months 2 weeks 2 days 14 hours ago) and read 8393 times:

Quoting Rara (Reply 20):
then what's the practical benefit of lift-to-drag?

From a simplistic stand point, at a given payload, the higher the lift-drag ratio, the less thrust you need to maintain speed, which directly impacts the efficiency of you're engines.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineonebadlt123 From United States of America, joined Oct 2004, 51 posts, RR: 0
Reply 24, posted (1 year 4 months 1 week 4 days 16 hours ago) and read 7969 times:

Quoting Max Q (Reply 9):
Don't know where you get these numbers from. I work for the same Airline and there are no fixed mach numbers.



Econ is used for planning with a variable cost index depending on the priorities of that particular flight, even if fixed mach numbers were used yours are incorrect (too slow)

I never said these were fixed in stone mach numbers. Now I may not be correct on the Sabre system, but old school wise it generally picked one of the above mentioned mach numbers 9 times out of 10 as it was the most efficient speed it liked to chose based on our preset variables. I cant tell you how many flight plans I run daily and see those numbers along with LRC, but LRC is different

They aren't too slow to be chosen, they are chosen because they are cheaper speeds to run given that time-frame. South side planning does not use CI planning.


25 Rara : So you're saying that the better the lift-drag ratio is, the more efficient the engines work - meaning you burn less fuel? Why then will you burn les
26 prebennorholm : You are really splitting hairs here. No plane has best L/D ratio at one single Mach number. It varies greatly with actual weight and altitude. A heav
27 bikerthai : When we talk about L/D ratio, most of us are just talking about the wing L/D. There is also parasite drag (includes the fuselage) that increases with
28 Post contains images Pihero : We are talking *Cost Index* here. It's just the ratio of hourly costs /fuelcosts . These include crew salaries, hourly maintenace costs / amortisatio
29 hivue : How much (if any) effect does this fact have on airlines being able to actually fly the best specific range? Could routes/airways be arranged so that
30 Pihero : The block altitude clearance now belongs to military traffic - especially tankers refuelling other jets in a hippodrome. There are still some die-har
31 bikerthai : From a hardware engineer standpoint . . . what you just explained was somewhat complex bt
32 SandroZRH : The A340 is optimally flown at M.81, the A330 at M.80-M.81.
33 Post contains images wilco737 : Optimally yes, but many friends of me are 330/ 340 Pilots and they fly slower than M0.80 at times. I know you fly them as well, but at LH the cost in
34 planewasted : I remember reading that Emirates says the A380 is more economical at the higher cruise speeds.
35 Starlionblue : This is unfortunately rather a vague statement. Higher relative to what?[Edited 2013-07-01 08:04:47]
36 seabosdca : I doubt it. Widebodies' speed has crept back up as their range has grown. There's less benefit to optimizing for higher speed if you're not flying as
37 Rara : What exactly is "optimal" at M0.80 if fuel burn is lower at lower speeds? Or why else do they fly slower?
38 Starlionblue : Lower fuel burn can mean lower endurance, but not necessarily longer range, especially in a headwind. Highest endurance speed is lower than highest r
39 Pihero : Higher, surely ? The associated costs of a given flight are of two orders : 1/- the fixed costs : mainly related to overflight and landing fees, flig
40 SmittyOne : Am I right in saying that all of this can and does go right out the window if/when ATC has you following a slower aircraft?!
41 Pihero : It would be a very rare occurrence to be blocked for very long by a slower airplane ahead. The simplest way is to accept a lower level, at your optim
42 Post contains images SmittyOne : Good to know! On a lighter note, I can almost picture ATC waving this:
43 Post contains links airmagnac : Total cost Made of energy cost and time-related cost The ultimate purpose of transport aircraft is to create movement from point A to point B. As the
44 Starlionblue : Yes! Oops damnit...
45 Post contains images SandroZRH : I hear you, they are starting to introduce CI0 standard at my company aswell. It's not going to work though as connections at ZRH are so tight that w
46 Rara : Thanks for the explanation. I take it, then, that the B744's L/D ration isn't best at M0.855, but at some speed lower than that, but that M0.855 is t
47 Post contains links airmagnac : We're dealing here with a what could be called a "multi-layered" optimization problem. Ultimately, the goal is to optimize the airline operating cost
48 B747400ERF : Optimum econ cruise speed for the 744 is around .84
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