Cruise speeds and how close do jets go to "optimal"
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Cruise speeds and how close do jets go to "optimal"
Hi all,
Question on cruise speeds, hope this is an appropriate place to ask.
Take for example on Wikipedia or Janes etc it might say the cruise speed of a 747 is .85M. I guess this is an idealized approximation and it's actually quite rare to cruise @ .8500M, because of temperature, weighting, altitude etc. My question is not about a 747 in particular, but passenger jets in general, how much variance on the "ideal" value would you typically see, first of all during a "typical" flight. So from the start of cruise to end of cruise, through stepped cruise etc, would you be looking at eg .85M +/- .01, .03, .05M? More than that?
Second of all, suppose you follow the same route from time to time over so many months or years. Is there much variance due to occasional / unforeseen issues that might cause a flight to occasionally have to adopt a bit slower or faster cruise due to unexpected events? How much variance from an intended cruise speed might a flight encounter on an occasional basis.
I'm sure there is no special "ideal cruise speed" for a given plane but I'm just wondering how much variance there is due to regular or unexpected operational factors from what might be deemed "ideal" for a given flight on a given day.
Hope it makes sense!
Question on cruise speeds, hope this is an appropriate place to ask.
Take for example on Wikipedia or Janes etc it might say the cruise speed of a 747 is .85M. I guess this is an idealized approximation and it's actually quite rare to cruise @ .8500M, because of temperature, weighting, altitude etc. My question is not about a 747 in particular, but passenger jets in general, how much variance on the "ideal" value would you typically see, first of all during a "typical" flight. So from the start of cruise to end of cruise, through stepped cruise etc, would you be looking at eg .85M +/- .01, .03, .05M? More than that?
Second of all, suppose you follow the same route from time to time over so many months or years. Is there much variance due to occasional / unforeseen issues that might cause a flight to occasionally have to adopt a bit slower or faster cruise due to unexpected events? How much variance from an intended cruise speed might a flight encounter on an occasional basis.
I'm sure there is no special "ideal cruise speed" for a given plane but I'm just wondering how much variance there is due to regular or unexpected operational factors from what might be deemed "ideal" for a given flight on a given day.
Hope it makes sense!
Moderator
Most aircraft have a bit of a drag bucket which leads to whatever speed being the preferred nil-wind situation for a given set of conditions. Unless there is a good reason not to do so, an airline will seek to operate at the preferred speed for dollar reasons. Slow down or speed up and the drag goes up and fuel costs as well. Target Mach will be set as accurately as reasonably possible - certainly to better than ± 0.005 (for want of a number) and, generally, considerably better depending on the equipment. At the risk of hanging myself out to dry, you probably could expect accuracy in the vicinity of 0.001-0.002.
Some considerations which will lead to a varying of the target speed -
(a) individual aircraft differences which are monitored by inflight data recording for ops engineering backroom number crunching
(b) strong tailwind - slow down a bit
(c) strong headwind - speed up a bit
(d) curfew/schedule - speed up/down as appropriate
(e) while fuel costs are a significant driver, the airline should be looking to minimise overall cost. Sector length, capital costs, maintenance costs then start to be a noticeable component in the equations.
Generally, jets are operated within a narrow altitude band unless other constraints dictate (eg ATC, weather) so it all gets a tad rigid.
Back 20-30 years ago and earlier we had a reasonable discretion regarding speed and, on occasion, it was clacker all the way - several fond recollections of having to get an aircraft to another port ASAP for whatever reason with nil pax/cabin folk
. These days, with the mighty dollar reigning, fuel burn control (mainly speed driven) dictates a far more rigid approach to the conduct of the operation.
Some considerations which will lead to a varying of the target speed -
(a) individual aircraft differences which are monitored by inflight data recording for ops engineering backroom number crunching
(b) strong tailwind - slow down a bit
(c) strong headwind - speed up a bit
(d) curfew/schedule - speed up/down as appropriate
(e) while fuel costs are a significant driver, the airline should be looking to minimise overall cost. Sector length, capital costs, maintenance costs then start to be a noticeable component in the equations.
Generally, jets are operated within a narrow altitude band unless other constraints dictate (eg ATC, weather) so it all gets a tad rigid.
Back 20-30 years ago and earlier we had a reasonable discretion regarding speed and, on occasion, it was clacker all the way - several fond recollections of having to get an aircraft to another port ASAP for whatever reason with nil pax/cabin folk
. These days, with the mighty dollar reigning, fuel burn control (mainly speed driven) dictates a far more rigid approach to the conduct of the operation.
To add to the eminent J_T's comments, most airliners use a "cost index" which balances fuel cost, crew cost and maintenance cost. Maintenance is usually accounted for by the hour, so added flying time might reduce burn, increase maintenance and, for that matter, crew costs. The index is a formula set up by the airline based on its particulars. Each sector will have a cost index for the crew to enter into the Flight Management Computer (FMC) and an optimal profile will be flown including climb, cruise and descent speeds, flight level (if ATC can accommodate) for that sector.
As John said, there are fairly limited range of levels and speeds that can be flown, basically FL 280 to FL 410, with levels above FL 370 only at the end of a long sector or a light weight take-off. Speeds are restricted by level, turbulence and range required, but typically for a B747 between M.82 and M.86, for the most part, a 744 pilot could chime in.
For a rule of thumb, each change of .01 Mach will shorten or lengthen the flight 1 minute for each 600 nautical miles flown.
GF
As John said, there are fairly limited range of levels and speeds that can be flown, basically FL 280 to FL 410, with levels above FL 370 only at the end of a long sector or a light weight take-off. Speeds are restricted by level, turbulence and range required, but typically for a B747 between M.82 and M.86, for the most part, a 744 pilot could chime in.
For a rule of thumb, each change of .01 Mach will shorten or lengthen the flight 1 minute for each 600 nautical miles flown.
GF
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daveonhols:
The Cost Index mentioned is generally for newer generation aircraft with flight management computers. The computer flight plan fashioned by flight control/management is prioritized by the company for their benefit, balanced by the operating costs and the need to keep the airplane on schedule.
When I was last at the airline I flew the classic 747's (100/200). The airplane had both .84M, .86M and long range cruise charts. After 16 years on the airplane, I found that the 747 seemed to be most comfortable at .85M to .855M, and my fuel burns vs the CFP data burns confirm that. Anything slower than .84M caused a higher body attitude and more drag and all the negatives that result. The Whale didn't like slow. .86M and higher sucked down fuel at a prodigious rate, so those speeds were reserved for making time and service on mostly rare occasions.
Over the North Pacific between Alaska and Japan, where I commonly flew, the common Mach was .84-.86M for the large widebodies. The 767 flew at .80-.82M, resulting in separation issues.
The other issue in cruise is optimum flight level for a particular weight, taking into account ISA deviation (temperature deviation from standard). Flying at this flight level resulted in the best efficiency. The -100 @ 660,000# had an optimum cruise flight level of 330 and the -200 optimum was 40,000# above that weight at FL330. Optimum cruising flight level increased 1,000' for every 25,000# of fuel burned (about an hour). As a result, my rule was to maintain .85M and go to the optimum flight level for the weight if ATC allowed the profile.
There a few other variables, but constant Mach and optimum altitude were generally the targets for long overwater flights.
The Cost Index mentioned is generally for newer generation aircraft with flight management computers. The computer flight plan fashioned by flight control/management is prioritized by the company for their benefit, balanced by the operating costs and the need to keep the airplane on schedule.
When I was last at the airline I flew the classic 747's (100/200). The airplane had both .84M, .86M and long range cruise charts. After 16 years on the airplane, I found that the 747 seemed to be most comfortable at .85M to .855M, and my fuel burns vs the CFP data burns confirm that. Anything slower than .84M caused a higher body attitude and more drag and all the negatives that result. The Whale didn't like slow. .86M and higher sucked down fuel at a prodigious rate, so those speeds were reserved for making time and service on mostly rare occasions.
Over the North Pacific between Alaska and Japan, where I commonly flew, the common Mach was .84-.86M for the large widebodies. The 767 flew at .80-.82M, resulting in separation issues.
The other issue in cruise is optimum flight level for a particular weight, taking into account ISA deviation (temperature deviation from standard). Flying at this flight level resulted in the best efficiency. The -100 @ 660,000# had an optimum cruise flight level of 330 and the -200 optimum was 40,000# above that weight at FL330. Optimum cruising flight level increased 1,000' for every 25,000# of fuel burned (about an hour). As a result, my rule was to maintain .85M and go to the optimum flight level for the weight if ATC allowed the profile.
There a few other variables, but constant Mach and optimum altitude were generally the targets for long overwater flights.
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Hi All,
Thanks for the great responses. Judging by the quotes of ".84-.86M for the large widebodies. The 767 flew at .80-.82M" I guess I can look at something around optimal +/- 0.01M, as a reasonable estimate.
Related to this, I was thinking about stepped cruise. What would be the cruise speed implications of not being allowed to do stepped cruise? If I remember correctly, stepped cruise is all about flying at the "right" speed, but a lighter plane after burning fuel, needs to do that at higher altitude in less dense air, generating less lift? Have I remember wrong or is that a gross over simplification or am I along the right lines?
Picking some random numbers that don't seem too unreasonable based on my long-haul experience ... suppose you started out at 36k feet and planned to step up to 40k feet (if anyone has a more real world example I would love to hear it). I was thinking about the steady state equation of flight, L=0.5pSV^2CL and trying to calculate how much slower you would have to go at 36k to generate the same amount of lift that you would at 40k, for example if ATC denied you permissions to climb for stepped cruise. How much of an impact would this have? I figured something like you would have to fly 10% slower in this case (it comes down to the ratios of the air densities vs the ratios of the squared velocities in my calcs). Somehow it seems too much, especially in light of the +/-.01M I figured from the above. Maybe I can infer that ATC would never (or rarely) deny a pilot the chance to climb for stepped cruise, or are there situations where taking a ~10% hit on your intended cruise speed would be required? Or have I got this completely wrong?
Thanks for any insights on this!
Thanks for the great responses. Judging by the quotes of ".84-.86M for the large widebodies. The 767 flew at .80-.82M" I guess I can look at something around optimal +/- 0.01M, as a reasonable estimate.
Related to this, I was thinking about stepped cruise. What would be the cruise speed implications of not being allowed to do stepped cruise? If I remember correctly, stepped cruise is all about flying at the "right" speed, but a lighter plane after burning fuel, needs to do that at higher altitude in less dense air, generating less lift? Have I remember wrong or is that a gross over simplification or am I along the right lines?
Picking some random numbers that don't seem too unreasonable based on my long-haul experience ... suppose you started out at 36k feet and planned to step up to 40k feet (if anyone has a more real world example I would love to hear it). I was thinking about the steady state equation of flight, L=0.5pSV^2CL and trying to calculate how much slower you would have to go at 36k to generate the same amount of lift that you would at 40k, for example if ATC denied you permissions to climb for stepped cruise. How much of an impact would this have? I figured something like you would have to fly 10% slower in this case (it comes down to the ratios of the air densities vs the ratios of the squared velocities in my calcs). Somehow it seems too much, especially in light of the +/-.01M I figured from the above. Maybe I can infer that ATC would never (or rarely) deny a pilot the chance to climb for stepped cruise, or are there situations where taking a ~10% hit on your intended cruise speed would be required? Or have I got this completely wrong?
Thanks for any insights on this!
