Optimum cruise altitude for efficiency

C150J

Well-Known Member
Hi guys --

This is more of a theoretical question, so I'm definitely not looking for formulas or hard numbers. What is the most efficient TRAJECTORY of an aircraft on any given route absent of any mitigating factors (wind, weather, STARS, etc.). Obviously, for most aircraft, the higher you go, the less fuel is burned. Does this premise dictate that we climb to the highest altitude possible for just moments before needing to descend on a 3-4 degree descent to the destination (a curved path)?

I'm making the assumption that climb thrust doesn't affect fuel flow tremendously.


Again, purely hypothetical with the intent to furnish better understanding of fuel efficiency. A good example would be a flight we conducted last week from DCA-PVD. Company had filed us extremely low for a 300+nm leg (around FL210), so we asked for FL290. We were able to comfortably climb to that altitude (we were really light) and stay there for about 15 minutes before getting a crossing restriction. We saved 210lbs of fuel by doing this.


Forgive me if I'm missing something huge and this turns out to be a stupid question!
J.
 

SpiraMirabilis

Possible Subversive
Depends on the length of the flight cruise segments at your filed altitude. If it is a longer flight then you might benefit from a more steep climb path to reach cruise altitude quicker (although I would stay in a fast climb until 180 or so around the terminal so you dont become a roadblock from faster mainline jets.) However if you have a very short amount of time in the cruise segment it may not be as fuel efficient if you were to climb just to descend. Even at the most fuel efficient power-idle descent the fuel savings may be less than the fuel expended getting to that altitude.

A slower descent is generally more fuel efficient than a fast descent. I don't know enough about CRJs to give you speeds but descending a faster speed (say .76) would be less efficient than .74/300kias because youre descending far more rapidly which reduces the time you're able to be at flight idle.

We could create a graph showing the intersection between the curves when you become more efficient and when you become less efficient.
 

meritflyer

Well-Known Member
FWIW, a jet's most efficient TAS is found around FL250-290 but due to fuel conservation efforts, higher altitudes are filed.

Optimum route efficiency has long been something the books have debated. If you're familar with the "bracket method", you'll be able to determine whether over X amount of miles, a higher altitude for fuel conservation etc., is optimum. Most CRJ/ERJ AFM's I've seen have "brackets" which assist pilots in making informed decisions.
 

C150J

Well-Known Member
FWIW, the -200 doesn't get above FL290 99% of the time:). Those XR's have about 200lbs more thrust than the CF-34-3B1's, and I think that must make all the difference in the world.
 

Polar742

All the responsibility none of the authority
It's interesting. As the type of flying I've been involved with short-haul domestic, to short-haul int'l on heavies, and dhing (alot) on long-haul, I've seen some interesting things.

1) In the 747s, the Optimum altitude is 2000' below the ceiling for the weight. What all the parameters are, I'm not sure, as I'm still in the FNG phase of aircraft knowledge and that's grad-level info

2) Short-haul wise, you're right. I'm not sure of what percent your fuel burn the 210 lbs was, but the company might have flight planned at a lower altitude since ATC will cap regional (as in remaining in the NE corridor) at lower altitudes and thus ensured you had enough gas to complete the flight. Last month, I flew from ICN to HKG, and we ended up 4000' above flight plan, and saved 5 tonnes of fuel (or 11,000 pounds) our burn was about 37 tonnes, so that was a 10% savings.

3) Leaving on a long flight (ICN-LAX) the other day, and at MGTOW, and fuel critical, as the payload was maxed out for the flight, our intial cruise altitude was FL290. As the gas burned, they got up to FL370 within an hour or so of TOD. So on that flight, it was necessary for the crew to fly the plan, and continue climb as it was fuel critical.

Another thing to look at is specific range, and see what altitude that is most efficeint. (alert tgrayson)
 

tgrayson

New Member
What is the most efficient TRAJECTORY of an aircraft on any given route absent
According to the aerodynamics books, for a jet, it's generally considered to be a steady climb in order to maintain optimal L/D ratio (not max L/D). As fuel is burned off, the altitude must increase or else you'd have to reduce the AOA to maintain altitude. Stair-stepping is a practical alternative to a steady climb.
 

Hacker15e

Dunning–Kruger Observer
Does this premise dictate that we climb to the highest altitude possible for just moments before needing to descend on a 3-4 degree descent to the destination (a curved path)?
The emergency fuel divert profile for a military jet looks exactly like this.
Full power to speed up, then climbing to altitude in full power using a specified climb schedule. Then immediately leveling off, slowing down, and pitching over to perform a L/Dmax descent into the divert airfield.

The rule of thumb we use for the optimum altitude is 1/3 the distance to the divert airport.
 

JoelT

Well-Known Member
Generally, a parabola is what we are looking for on short-haul flights. Climb to your max altitude even if it is only for a few minutes. Then, an idle decent. Of course, ATC has their own plans.
 

ppragman

FLIPY FLAPS!
These things get skewed by the addition of wind. When you're climbing into a headwind, a shallower faster (kts not fpm) climb is better because you spend less time at the maximum headwind. Plus, when you get up to altitude (provided you're not torque or temping out) you should run at a higher power setting than normal. It saves fuel because you spend less time in the headwind. The same is true in reverse for saving fuel on a tailwind. Climb like a bat outa hell up to the flight levels, then throttle back a little, then wait until you're in real close then drop out of the sky like a stone. Saves fuel, I swear.
 

germb747

Well-Known Member
I think I remember a question on the ATP written test saying that the "optimum" altitude was FL360; anyone else remember this question? I'm not sure why it's there, because there are so many variables, but for what it's worth.

My understanding is that climbing higher is better up to a point (that point being around the tropopause), because the temperature above that point no longer decreases with altitude and you're just hanging on the fans. Does anyone buy that?
 
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