Fuel Burn at Different Altitudes

Bernoulli Fan

Controller
Sometimes we have to hold you down longer than you'd like, or bring you down earlier than you'd like; I'm interested to know how large the fuel burn difference is between some altitudes, all other factors being equal. Good approximations welcome.

757 FL340 vs FL240
A320 FL340 vs FL240
E120 17,000 vs 11,000
CRJ FL340 vs FL280
 

Autothrust Blue

Commander Air Group, BSG-75
I'll take notes for you next time as I'm still really new to the airplane...

But for our intermediate level-offs on the Brasilia, we don't wind it up to max forward speed. SOP has us maintain 200 knots, which saves us a lot of gas compared to cruise max torque down low. (We can do 215 flat-out, but it takes us a hell of a lot more gas.)
 

dasleben

That's just, like, your opinion, man
I'm pulling this out of the 757 manuals I have (I simply don't remember typical cruise fuel burns for it), but between FL230 and FL330 at 240,000#, the total difference in fuel burn is about 500 lbs/hr. Not a whole lot of difference. That's at 310 KIAS at FL230 (~M0.71), and M0.80 at FL330. The issue is that holding the jet down at significantly lower altitudes for an extended period reduces the TAS, and combined with the increase in fuel burn at low altitude, results in higher fuel burns along the entire route of flight. Winds can also be a major factor.

Some guys like to make a big deal out of not being able to get the planned altitude and/or mach number, but unless you really hold us down, it's not a huge deal.
 

Minuteman

“Dongola”
I'm pulling this out of the 757 manuals I have (I simply don't remember typical cruise fuel burns for it), but between FL230 and FL330 at 240,000#, the total difference in fuel burn is about 500 lbs/hr. Not a whole lot of difference. That's at 310 KIAS at FL230 (~M0.71), and M0.80 at FL330. The issue is that holding the jet down at significantly lower altitudes for an extended period reduces the TAS, and combined with the increase in fuel burn at low altitude, results in higher fuel burns along the entire route of flight. Winds can also be a major factor.
This is pretty much my thinking as well. Fuel Flow at the Green Dot speed in an "typical A320" is around 4,400 pph at FL300 vs 4,500 pph at 10,000 ft.

Traditionally, the some of best things for fuel economy is to let the jet get to altitude with the most power possible (DCLB is for engine life), direct routing, and idle descents. In a perfect world. 1% changes are significant for the fuel desk.
 

dasleben

That's just, like, your opinion, man
This is pretty much my thinking as well. Fuel Flow at the Green Dot speed in an "typical A320" is around 4,400 pph at FL300 vs 4,500 pph at 10,000 ft.
Sounds about right. I just looked at some of our paperwork for a flight a little over 5 hours long in the 767-200ER. The difference between FL320 and FL380 in destination fuel burn for the entire route was a total of 1600# (55,400# vs. 53,800#). FL380 took 11 minutes longer. Not much difference. I recall getting held down at FL310 for the oceanic segment on one of the legs (planned at FL370), and the difference shown in the FMC was about 2,000# burn.

Traditionally, the some of best things for fuel economy is to let the jet get to altitude with the most power possible (DCLB is for engine life), direct routing, and idle descents. In a perfect world. 1% changes are significant for the fuel desk.
We try to do that as much as possible, too. Usually full climb thrust beginning at 12,000' (the jet removes derate automatically in the FADEC aircraft), with idle descents planned by the VNAV.
 

greg1016

Trustworthy Source
Sounds about right. I just looked at some of our paperwork for a flight a little over 5 hours long in the 767-200ER. The difference between FL320 and FL380 in destination fuel burn for the entire route was a total of 1600# (55,400# vs. 53,800#). FL380 took 11 minutes longer. Not much difference. I recall getting held down at FL310 for the oceanic segment on one of the legs (planned at FL370), and the difference shown in the FMC was about 2,000# burn.



We try to do that as much as possible, too. Usually full climb thrust beginning at 12,000' (the jet removes derate automatically in the FADEC aircraft), with idle descents planned by the VNAV.
So what you're saying is when a certain European carrier's dispatch is on the phone with my supervisor because a flight is stuck at F310 with a planned altitude of F380, it is not nearly as big a deal as the dispatcher and my supervisor are trying to turn it in to?

Not that I don't try my hardest to get everyone at an altitude they are happy with, because I do, but sometimes that is just the way the cards fall. Cleaning up conflicts, issuing weather deviations and coordination with other centers has to take precedent when you are working 70 aircraft and they all are screaming for something.
 

dasleben

That's just, like, your opinion, man
So what you're saying is when a certain European carrier's dispatch is on the phone with my supervisor because a flight is stuck at F310 with a planned altitude of F380, it is not nearly as big a deal as the dispatcher and my supervisor are trying to turn it in to?

Not that I don't try my hardest to get everyone at an altitude they are happy with, because I do, but sometimes that is just the way the cards fall. Cleaning up conflicts, issuing weather deviations and coordination with other centers has to take precedent when you are working 70 aircraft and they all are screaming for something.
Well, I don't want to say that it's never a big deal; for their operation and aircraft, being held down for a significant period may turn into an issue down the road. From a pilot's perspective, we realize that it's perfectly possible not to get our requested altitude on an ocean crossing, and that the majority of the time, it doesn't present a problem for fuel burn as long as we can eventually work in a climb. Significant wind variations at different altitudes can create problems, however.

I'd like to add, the benefit of having an FMC is that we can have the computer calculate burn for various altitudes, with and without step climbs.
 

greg1016

Trustworthy Source
Well, I don't want to say that it's never a big deal; for their operation and aircraft, being held down for a significant period may turn into an issue down the road. From a pilot's perspective, we realize that it's perfectly possible not to get our requested altitude on an ocean crossing, and that the majority of the time, it doesn't present a problem for fuel burn as long as we can eventually work in a climb. Significant wind variations at different altitudes can create problems, however.

I'd like to add, the benefit of having an FMC is that we can have the computer calculate burn for various altitudes, with and without step climbs.
Well in this particular instance, it was an adjacent center, not me, that would not accept the aircraft at a higher altitude (which is usually the case), the aircraft eventually agreed to take a significant speed decrease in order to get a higher altitude, something like M.84 to M.78 or something like that. How does that trade-off work? IIRC it was a B744, but it could have been an Airbus, the carrier operates both. A reroute for a higher altitude was not an option in this case.
 

dasleben

That's just, like, your opinion, man
Well in this particular instance, it was an adjacent center, not me, that would not accept the aircraft at a higher altitude (which is usually the case), the aircraft eventually agreed to take a significant speed decrease in order to get a higher altitude, something like M.84 to M.78 or something like that. How does that trade-off work? IIRC it was a B744, but it could have been an Airbus, the carrier operates both. A reroute for a higher altitude was not an option in this case.
Without knowing their typical fuel burns and what their flightplan looked like, there's not much I can guess on that one. Longer flights will naturally see the most impact for sustained lower altitudes. What we'd do in my aircraft is start plugging in different scenarios to see what'd work best. A slower mach speed at a higher altitude may work out better depending on a wide range of factors, though often the lower altitude works best while maintaining the mach. This scenario is why I tend to take the time to input accurate wind data into the FMC, sometimes to the chagrin of my coworkers. :) It can be beneficial in this situation. Dispatch is a great resource, too.

Digging into the manuals a bit further, for the 767-300ER, a flight of 5,000 air miles at FL290 requires ~145,000# burn (assuming just under MTOW), while the same flight at FL370 requires ~133,000#. That's about a 12 hour leg, and while a 12,000# difference in burn would be significant at the destination, there's very little chance of being stuck down at FL290 for a 12+ hour leg.
 

greg1016

Trustworthy Source
dasleben
Well thanks for that, when the aircraft and my system don't have accurate wind data it really increases my workload. Especially on a busy night when I am running minimum separation and my computer isn't tracking the aircraft profiles accurately.
 

Bernoulli Fan

Controller
Thanks for all the numbers so far. I'm happy to discover that not canceling 'til the gate, running oversquare, and raising the flaps in the flare are all worse than a few minutes at FL240.
 

dasleben

That's just, like, your opinion, man
Thanks for all the numbers so far. I'm happy to discover that not canceling 'til the gate, running oversquare, and raising the flaps in the flare are all worse than a few minutes at FL240.
Yeah, a few minutes is not a big deal. The problems begin when you start talking about hours.
 

Chief Captain

Well-Known Member
I had a Captain accept a dispatch with just about the absolute minimum fuel required for a 3+ hour flight. After raising my concerns, I'm told "it'll be fine". 20 minutes after takeoff, he's arguing with ATC because we're stuck at FL240.

I couldn't help but side with the controller on that one. Apart from the obvious separation issue, it's kind of silly to accept a fuel load so low that a few minutes below the planned level causes significant anxiety.
 

Baronman

Well-Known Member
On the ERJ we'll be at approximately the same TAS at FL280 and FL340 (450Kts).

Off the top of my head it's about a 600 lb/hr difference total. For a longer flight of ours it could be significant 2+hrs depending on how much extra gas we have.
 

Baronman

Well-Known Member
On the ERJ we'll be at approximately the same TAS at FL280 and FL340 (450Kts).

Off the top of my head it's about a 600 lb/hr difference total. For a longer flight of ours it could be significant 2+hrs depending on how much extra gas we have.
 

Vector4Food

This job would be easier without all the airplanes
Geez, most crews start pissing and moaning if we descend them 1000 feet.

"Can't you move the other guy??"

"Actually no, and look I have more traffic, descend 3000 feet"
 

greg1016

Trustworthy Source
Geez, most crews start pissing and moaning if we descend them 1000 feet.

"Can't you move the other guy??"

"Actually no, and look I have more traffic, descend 3000 feet"
I think I already went off on that little diatribe: aircraft at 360. climb and maintain f380 due to traffic. unable. descend and maintain f330 due to traffic. request climb to f380. f380 no longer available descend and maintain f330.

long story short, tell us the truth the first time, a clearance is not an arbitrary request.
 

dasleben

That's just, like, your opinion, man
On the ERJ we'll be at approximately the same TAS at FL280 and FL340 (450Kts).

Off the top of my head it's about a 600 lb/hr difference total. For a longer flight of ours it could be significant 2+hrs depending on how much extra gas we have.
That's even more of a difference than a 757. I recall those things being inefficient, but dayum. :)
 

dasleben

That's just, like, your opinion, man
I think I already went off on that little diatribe: aircraft at 360. climb and maintain f380 due to traffic. unable. descend and maintain f330 due to traffic. request climb to f380. f380 no longer available descend and maintain f330.

long story short, tell us the truth the first time, a clearance is not an arbitrary request.
Well, it's not that they're lying to you; they're just trying to figure out the best way to do it. Climbing when heavy may put the aircraft high enough above the optimum altitude that fuel burn actually increases, and stall margins are reduced. Descending can put the airplane back into the weather, and increase fuel burns as well. Usually we'll take the descent first, then bother you for higher again once fuel is burned off. :)
 

Vector4Food

This job would be easier without all the airplanes
Post Edited out because it came across more with angst than was intended.

Short story:

We issue/offer higher for traffic because 99.5% of the time this is the crews first choice.

Saying you're "Unable" then saying "able" after being issued a descent is unprofessional, and thus those crews can expect to be treated likewise.

No qualms with the following exchange:

"ABC123 are you able FL380 for traffic?"
"Negative"
"ABC123 roger, descend FL 340"
"In this case we are able FL380"
"ABC123 descend FL320"
 

dasleben

That's just, like, your opinion, man
No qualms with the following exchange:

"ABC123 are you able FL380 for traffic?"
"Negative"
"ABC123 roger, descend FL 340"
"In this case we are able FL380"
"ABC123 descend FL320"
I do actually have qualms with that. Sometimes if FL380 is right on the edge (but safe), it's better to try to see if it's possible to maintain FL360 and see if the controller has another plan. If the plan is to descend, it's sometimes better to take the climb. Remember, we're the ones with the limited fuel supply, and don't have access to "The Plan." Descending the aircraft further just out of spite... Well... I think it'd be wise not to respond to that.

The point is, nobody's trying to lie to you, screw with you, or trick you. We're just trying to do what's best for the flight.
 
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