parapente
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Optimum cruising altitude (long haul)

Fri Nov 03, 2017 10:38 am

Since the beginning of jet flight I believe that the 'standard' optimal cruising altitude (after initial step climbs) is circa 38-40,000 ft.
This has never changed (improved) . I was wondering why.
Clearly the wings have become more efficient with higher aspect ratios and greater laminar flow.
A carbon fuse is far mor able to take the pressure differentials at altitude and less prone to the dreaded fatigue.
Engines have a far higher bypass ratio (but I am not sure that this is necessarily a good thing at altitude-I simply don't know)
But geared fans on large engines are surely coming one day.Perhaps that's a good thing (again I don't know).

I assume that if it were possible it would be desirable for an aircraft to operate at say a Fl of 50,000 feet rather than 40k due to the far lower drag.But (Concorde apart) this has never happened in 60 years or indeed even planned.
 
mmo
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 11:16 am

I don't know where you are getting your information from but it is incorrect!
Optimum altitude is a function of several things. But, basically, it comes down to gross weight. The lighter you get; the higher you can go.

A fully loaded 747-400 will struggle to get to FL310, but 10 hours later the optimum altitude will be around FL370-390 depending on weight.

There is not a direct correlation between altitude and efficiency of climbing to optimum FL. For example, you need to realize the once you are in the tropopause the OAT remains constant so the efficiency with altitude is lowered, you need to consider the time remaining vs. the climb fuel you will burn. On the 747/777 I found it was often more fuel efficient to remain at FL 370-390 than it would be to climb to 410.

Finally, aircraft are limited for max altitude based how fast they can descend in the event of a rapid decompression. Much higher than 430-451 the aircraft can not make the certification requirement.
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Flow2706
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 2:18 pm

Yep, higher altitude does not necessarily equate to better fuel economy. You usually get better shortcuts if you go higher (due to less traffic), but if everybody else is doing the same thing the benefit is gone...on some flights we stay at FL320/340 instead of going to FL360/380 even if performance permits as it may give you a saving (f.e. if you are flying against a Jetstream with its core at FL370 - you are probably better of trying to stay beneath it). Obviously I can also be the other way around. Some pilots actually prefer to stay lower to avoid exposure to higher doses of cosmic radiation - I heard the radiation dose doubles roughly every 2000m/6000ft (not sure if the radiation really causes measurable effects, but thats how some people argue). Some people are also complaining that they are more fatigued due to the higher cabin altitude at higher levels...
 
RetiredWeasel
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 2:28 pm

Flow2706 wrote:
Some pilots actually prefer to stay lower to avoid exposure to higher doses of cosmic radiation - I heard the radiation dose doubles roughly every 2000m/6000ft (not sure if the radiation really causes measurable effects, but thats how some people argue). ...


That would be a tough sell to a Chief Pilot when you have to see him because you're not flying the profile on the flight plan: "I'm worried about the cosmic radiation.." Huh?
 
Flow2706
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 2:31 pm

RetiredWeasel wrote:
That would be a tough sell to a Chief Pilot when you have to see him because you're not flying the profile on the flight plan: "I'm worried about the cosmic radiation.." Huh?

Nobody actually checks the reasons for a different level - it would be hard to do anyway, because often you don't fly your flight planned level for other reasons (ATC constraints, turbulence, higher weights then planned, technical defects etc etc.). Also some times the flight plan is not planed at the optimum level anyway (there may be level capping etc or slots generated at certain levels, so it may be better to plan at a "non optimum" level and try to get the optimum level in flight anyway).
 
Flow2706
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 2:36 pm

It's depending on the company I guess - I do short haul flying on the small bus (A320) and we don't even have an ACARS system installed...
 
RetiredWeasel
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 2:37 pm

Flow2706 wrote:
RetiredWeasel wrote:
That would be a tough sell to a Chief Pilot when you have to see him because you're not flying the profile on the flight plan: "I'm worried about the cosmic radiation.." Huh?

Nobody actually checks it - it would be hard to do anyway, because often you don't fly your flight planned level for other reasons (ATC constraints, turbulence, higher weights then planned, technical defects etc etc.)


Actually most airlines do check FOAs and fuel burns as part of their data collection. If you are a pilot that consistently has a fuel overburn (long haul) then sooner or later you will get noticed and your flight profiles will get scrutiny. Edit: Not to mention the FO's who would probably ask the CPT why aren't we climbing according to the flight plan and he replies due to cosmic radiation. That would more than likely cause the FO to report him to the union and things could snowball from there. As a former FO on the 400, I would be skeptical of a CPT who gets to NRT 3-4000 lbs short on fuel because of his fear of cosmic particles.
Last edited by RetiredWeasel on Fri Nov 03, 2017 2:58 pm, edited 1 time in total.
 
mmo
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 3:39 pm

Flow2706 wrote:
Nobody actually checks the reasons for a different level - it would be hard to do anyway, because often you don't fly your flight planned level for other reasons


As RetiredWeasel pointed out it is very to check and it is checked. On long haul flights, the dispatcher will monitor the flight by way of position reports and ACARS updates. Generally speaking, it is noted on the flight plan why the flight isn't flown in accordance with the flight plan.

I can guarantee you if you are habitually not flying the flight plan and arriving short of fuel, you will be invited for a chat with the Chief Pilot. Those kind of chats you are best to avoid!!
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BravoOne
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 6:27 pm

You must recall that the flight plan is just that, a "plan" usually based om optimized conditions. Once you have pushed the power up for takeoff it becomes very dynamic and for that reason most crews will maintain a "Howgozit: so as to monitor the difference between the what the plan predicted and actual performance. Most FMCs are programed to calculate the "ICAO" step climb. (google it), and thus will forecast performance based on a prefect wind.weight.temp algorithm, which is seldom seen in real workd global flight ops.
 
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SheikhDjibouti
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Re: Optimum cruising altitude (long haul)

Fri Nov 03, 2017 11:02 pm

mmo wrote:
Optimum altitude is a function of several things. But, basically, it comes down to gross weight. The lighter you get; the higher you can go.

A fully loaded 747-400 will struggle to get to FL310, but 10 hours later the optimum altitude will be around FL370-390 depending on weight.

Ok, I understand that, but what I am guessing the OP is asking (and what I want to know regardless) is whether fuel burn continues to improve the higher you go?
Is there a trade off between [less dense air = lower drag/friction losses] vs [less dense air = jet engines unable to function economically] ? Mostly, the only aircraft that regularly reached higher levels are powered by old technology turbojets; I'm thinking specifically of SR-71, U-2 and of course Concorde. Do modern high bypass engines thrive at extremely high altitudes ?

Put it another way; your 747-400 struggles to get to FL310 early in the flight, but later on when it is able to climb, why bother climbing - unless there is an efficiency gain to be made. I'm guessing that there is, but at what point does it cease?

For example, you need to realize the once you are in the tropopause the OAT remains constant so the efficiency with altitude is lowered,

Is it possible you have already answered some of my questions here? I cannot be sure because I don't fully understand what you are saying, or the physics behind it. If you can somehow give a technical explanation, and yet also keep it in layman's terms, I would be grateful. :D
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Starlionblue
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Re: Optimum cruising altitude (long haul)

Sat Nov 04, 2017 1:16 am

SheikhDjibouti wrote:
mmo wrote:
Optimum altitude is a function of several things. But, basically, it comes down to gross weight. The lighter you get; the higher you can go.

A fully loaded 747-400 will struggle to get to FL310, but 10 hours later the optimum altitude will be around FL370-390 depending on weight.

Ok, I understand that, but what I am guessing the OP is asking (and what I want to know regardless) is whether fuel burn continues to improve the higher you go?
Is there a trade off between [less dense air = lower drag/friction losses] vs [less dense air = jet engines unable to function economically] ? Mostly, the only aircraft that regularly reached higher levels are powered by old technology turbojets; I'm thinking specifically of SR-71, U-2 and of course Concorde. Do modern high bypass engines thrive at extremely high altitudes ?

Put it another way; your 747-400 struggles to get to FL310 early in the flight, but later on when it is able to climb, why bother climbing - unless there is an efficiency gain to be made. I'm guessing that there is, but at what point does it cease?


Efficiency gains from climbing cease when you reach the service ceiling, or when temperature stops decreasing with altitude, or when the gains from climbing are not sufficient given the remaining flight time. At some point the FM will say "No optimal" if you ask it to calculate the time for a step climb.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mmo
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Re: Optimum cruising altitude (long haul)

Sat Nov 04, 2017 7:13 am

SheikhDjibouti wrote:

Put it another way; your 747-400 struggles to get to FL310 early in the flight, but later on when it is able to climb, why bother climbing - unless there is an efficiency gain to be made. I'm guessing that there is, but at what point does it cease?

For example, you need to realize the once you are in the tropopause the OAT remains constant so the efficiency with altitude is lowered,

Is it possible you have already answered some of my questions here? I cannot be sure because I don't fully understand what you are saying, or the physics behind it. If you can somehow give a technical explanation, and yet also keep it in layman's terms, I would be grateful. :D


You reference the SR-71, the engine when it was at it's operational altitude was not a turbojet engine, but more of a ram jet, while the turbine part still functioned the majority of the thrust was from the ram effect. That was not "old technology".

Perhaps a review of basics might be helpful. The standard lapse rate is 2degreedC/1000' decrease in temp until you reach the tropopause, at which htime the temp remains constant. The trop generally is it's highest at the equator and it decreases altitude as you move towards the poles. In the northern hemisphere, the trop is higher in the summer than it is in the winter (southern hemisphere is the opposite). Turbojet/Turbofan engines love cold temps (air is more dense) but the altitude increase makes the air less dense. So, the higher you climb, GW permitting, the engines become more efficient. Unless you have a significant amount of time remaining, once you hit the trop you will not gain efficiency from temperature reductions. Therefore, it does not make sense to continue climbing as your weight reduces.

With that said, there are other factors to consider. For example, it might be worth it to climb to catch a more favorable tail wind. (HF position reports were great at determining the winds at other altitudes), the converse is true. It might be worth it to descend to catch a more favorable tail wind. You might have to climb or descent to get a smoother ride. As a rule of thumb you want to avoid the trop altitude -/+ 2000 feet as you will "generally" get a better ride avoiding those altitudes.

Hope that helps, if not, ask away!
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Balerit
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Re: Optimum cruising altitude (long haul)

Sat Nov 04, 2017 9:05 am

With the increase in altitude, the density of air decreases. The reduced density means a lower number of air molecules entering the core engine. For the fuel/air mixture to maintain its stoichiometric ratio, the required number of fuel molecules must decrease to cater for the decreased number of air molecules. This results in a lower amount of fuel required at higher altitudes.

Moreover, lower air density at high altitudes also increases flight efficiency by decreasing the skin-friction drag produced by the interaction of air molecules with the surface of the aircraft.
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kalvado
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Re: Optimum cruising altitude (long haul)

Sat Nov 04, 2017 7:24 pm

and a very hypothetical question along same lines:
If we would have a different atmospheric pressure - say 45 or 15 in mercury (1100 or 400 mm Hg) at sea level - would plane still be designed for optimum flight at pressure of roughly FL350-FL400, or it would be different? And is current FL350-400 range a matter of design choice or aerodynamics driven?
Two factors come to mind - descent time for pressurization loss - which is sort of a separate story; and ability to use same wing for takeoff and landing (might be interesting).
Or in other words - how would design parameters change if atmospheric pressure would be way different than what it is?
 
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Balerit
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Re: Optimum cruising altitude (long haul)

Sat Nov 04, 2017 10:03 pm

Just remember that the design parameter used to determine wing size is the stall speed for the gross weight, i.e when landing.
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26point2
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Re: Optimum cruising altitude (long haul)

Sun Nov 05, 2017 6:04 pm

parapente wrote:

I assume that if it were possible it would be desirable for an aircraft to operate at say a Fl of 50,000 feet rather than 40k due to the far lower drag.But (Concorde apart) this has never happened in 60 years or indeed even planned.


I have flown a plane certified to FL510 for the past 9 yrs and have never been up there. Never been above FL470 as a matter of fact. A few good reasons for this are....

Aircraft performance is marginal, TAS is lower, plane is nearer to stall speed, radiation levels are higher, cabin altitude is higher, time to descend in case of emergency is longer. There is no reason to fly that high in a modern passenger plane.
 
parapente
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Re: Optimum cruising altitude (long haul)

Mon Nov 06, 2017 9:38 am

Thanks above for the answers,particularly 26point2 - from the horses mouth so to speak!
 
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Pellegrine
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Re: Optimum cruising altitude (long haul)

Wed Nov 08, 2017 9:45 am

Given a light enough gross weight, I was under the assumption that the optimum cruise level (i.e optimum fuel burn/range) is usually better at a higher FL. For example, AFAIR a B744 at ~550,000 lbs. GW can fly at FL410. So why would you want to fly it at FL350/370 at that GW, unless there were worse winds at FL410?

I read the responses, but I'm talking strictly in terms of fuel burn per hour and therefore range.
oh boy, here we go!!!
 
mmo
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Re: Optimum cruising altitude (long haul)

Wed Nov 08, 2017 12:57 pm

Pellegrine wrote:
Given a light enough gross weight, I was under the assumption that the optimum cruise level (i.e optimum fuel burn/range) is usually better at a higher FL. For example, AFAIR a B744 at ~550,000 lbs. GW can fly at FL410. So why would you want to fly it at FL350/370 at that GW, unless there were worse winds at FL410?

I read the responses, but I'm talking strictly in terms of fuel burn per hour and therefore range.


Depending where the trop is has an impact on how high you really want to go. Once you hit the trop the OAT reamains constant. So, if you climb the fuel burn stays just about constant. But you now have to factor in the fuel burn for the climb and most times it isn't worth it as you are far enough in the flight where you will not spend enough time to recover the fuel burn used in the climb. For instance, on the 744, you would be light enough for the last 2 hours to go to FL 410/430 but there is no overall savings due to the fuel burn used in the climb. The FMC would reflect that too.
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gloom
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Re: Optimum cruising altitude (long haul)

Wed Nov 08, 2017 8:54 pm

I don't know if we're talking the same thing there, but you guys seem to attribute lower fuel comsumption to an effect rather, than cause.

Whole airframe is optimized for specific AoA (angle of attack). At this specific AoA, it reaches highest L2D coefficient (lift-to-drag). So, basically the plane (and FMC, and all ops) try to calculate CoG vs MAC and currently best flight level to fly as close to the point as possible. It simply gives the least drag at given weight/lift possible. So, basically heavy plane starts at lower (as lower means higher density, and that increases lift at this specific AoA we have). As the weight drops down, less lift is required and lift-induced drag becomes lower. Plane gets higher to again achieve AoA as close to optimum, and total drag is lower from both effects: reduced lift-induced drag as described above, and lower parasitic drag as effect of lower pressure.

I have thought this could be valuable for getting an overall picture. The plane at specified weight could cruise higher (which is indicated by FMC/FMGS for example, and sometimes the difference between optimum and max could be as high as 4000ft for example). However, higher is not better in this case, as gains from lower parasite drag will be way lower than raise of lift drag, resulting in higher overall drag and higher fuel burn.

Of course this is simplified a bit, but hopefully useful to make a picture.

Cheers,
Adam
 
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Balerit
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Re: Optimum cruising altitude (long haul)

Wed Nov 08, 2017 9:01 pm

Adam, I guess you will reach an altitude where your angle of attack exceeds the airfoil lift and as you say, drag then becomes excessive.
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OldAeroGuy
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Re: Optimum cruising altitude (long haul)

Wed Nov 08, 2017 9:59 pm

gloom wrote:
I don't know if we're talking the same thing there, but you guys seem to attribute lower fuel comsumption to an effect rather, than cause.

Whole airframe is optimized for specific AoA (angle of attack). At this specific AoA, it reaches highest L2D coefficient (lift-to-drag). So, basically the plane (and FMC, and all ops) try to calculate CoG vs MAC and currently best flight level to fly as close to the point as possible. It simply gives the least drag at given weight/lift possible. So, basically heavy plane starts at lower (as lower means higher density, and that increases lift at this specific AoA we have). As the weight drops down, less lift is required and lift-induced drag becomes lower. Plane gets higher to again achieve AoA as close to optimum, and total drag is lower from both effects: reduced lift-induced drag as described above, and lower parasitic drag as effect of lower pressure.

I have thought this could be valuable for getting an overall picture. The plane at specified weight could cruise higher (which is indicated by FMC/FMGS for example, and sometimes the difference between optimum and max could be as high as 4000ft for example). However, higher is not better in this case, as gains from lower parasite drag will be way lower than raise of lift drag, resulting in higher overall drag and higher fuel burn.

Of course this is simplified a bit, but hopefully useful to make a picture.



Cheers,
Adam


Great job gloom. The is the best and most correct answer given so far in this discussion. The only thing I would add is that quantity that should be maximized for CI=0 flying is M(L/D). Straight out of the jet airplane Breguet range equation.
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Pellegrine
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Re: Optimum cruising altitude (long haul)

Thu Nov 09, 2017 1:05 am

mmo wrote:
Pellegrine wrote:
Given a light enough gross weight, I was under the assumption that the optimum cruise level (i.e optimum fuel burn/range) is usually better at a higher FL. For example, AFAIR a B744 at ~550,000 lbs. GW can fly at FL410. So why would you want to fly it at FL350/370 at that GW, unless there were worse winds at FL410?

I read the responses, but I'm talking strictly in terms of fuel burn per hour and therefore range.


Depending where the trop is has an impact on how high you really want to go. Once you hit the trop the OAT reamains constant. So, if you climb the fuel burn stays just about constant. But you now have to factor in the fuel burn for the climb and most times it isn't worth it as you are far enough in the flight where you will not spend enough time to recover the fuel burn used in the climb. For instance, on the 744, you would be light enough for the last 2 hours to go to FL 410/430 but there is no overall savings due to the fuel burn used in the climb. The FMC would reflect that too.


Well that makes sense as I can't recall flying at FL400 or FL410 in either a 744 or 777. Is there a good website to look up the height of the tropopause per location daily?
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Andre3K
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Re: Optimum cruising altitude (long haul)

Thu Nov 09, 2017 2:08 am

Starlionblue wrote:
Mostly, the only aircraft that regularly reached higher levels are powered by old technology turbojets; I'm thinking specifically of SR-71, U-2 and of course Concorde. Do modern high bypass engines thrive at extremely high altitudes ?



High bypass engines actually have a much greater thrust loss with speed and altitude increase. From what I have been able to find a GE90-84 (84370lb takeoff thrust) at altittude will have a maximum thrust of less than 20,000lb (cruise thrust 15556lb). The cruise thrust of a turbojet is like maybe half the takeoff thrust of the engine. I'm pretty sure it's because the higher and faster you go with high bypass, the closer the bypass air gets to the speed of the aircraft which means the thrust will drop. With a turbojet you will NEVER reach the exhaust speed.
 
LH707330
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Re: Optimum cruising altitude (long haul)

Thu Nov 09, 2017 8:49 pm

Andre3K wrote:
Starlionblue wrote:
Mostly, the only aircraft that regularly reached higher levels are powered by old technology turbojets; I'm thinking specifically of SR-71, U-2 and of course Concorde. Do modern high bypass engines thrive at extremely high altitudes ?



High bypass engines actually have a much greater thrust loss with speed and altitude increase. From what I have been able to find a GE90-84 (84370lb takeoff thrust) at altittude will have a maximum thrust of less than 20,000lb (cruise thrust 15556lb). The cruise thrust of a turbojet is like maybe half the takeoff thrust of the engine. I'm pretty sure it's because the higher and faster you go with high bypass, the closer the bypass air gets to the speed of the aircraft which means the thrust will drop. With a turbojet you will NEVER reach the exhaust speed.

Indeed. What you're describing is the thrust lapse rate of the engine. Some 707 skippers have reported that the JT3Ds were better for takeoff, but the JT4As provided more thrust at altitude.

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