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Change In Altitude During Long Haul Flights

Posted: Mon Feb 01, 2010 12:55 am
by AT
Just flew JFK DXB and back recently (on an Emirates 77-300ER) and for both flights, the pilot announced during the preflight information that we would be flying at a certain altitude (i think it was 29000 ft on the outbound) and that as the flight progressed and we burned fuel, we would climb to higher altitudes.

Can someone explain the physics of this? If it is more efficient to fly at higher altitudes where the air is thinner shouldn't that be true regardless of the weight of the plane? Why would one want to burn fuel before climbing to full altitude?

It was interesting though hearing the sounds as we changed altitude- the engines would suddenly go from a whine to a roar for a couple minutes and then die down again.

RE: Change In Altitude During Long Haul Flights

Posted: Mon Feb 01, 2010 1:02 am
by Moose135
It usually is more efficient to fly at higher altitudes, however not always. When the aircraft is heavy, it may have a tough time reaching higher altitudes, if it is able to do so at all. It may burn more fuel to reach and maintain a higher altitude, so an intermediate cruise level is used until weight is reduced. A step climb tries to keep the aircraft at its most efficient altitude as fuel burns off and weight is reduced.

RE: Change In Altitude During Long Haul Flights

Posted: Mon Feb 01, 2010 1:05 am
by tdscanuck


Quoting AT (Thread starter):
Can someone explain the physics of this?

As you climb, air density drops. This means the wing needs more Cl (more angle of attack) to provide the needed lift; since Cd goes up with Cl^2, you end up with a lower L/D and burn more fuel per mile. For any particular weight and speed, there is an optimum altitude for minimum fuel burn.

Quoting AT (Thread starter):
If it is more efficient to fly at higher altitudes where the air is thinner shouldn't that be true regardless of the weight of the plane?

No. The optimum altitude is a function of weight.

Quoting AT (Thread starter):
Why would one want to burn fuel before climbing to full altitude?

Because you need to get lighter before the higher altitude becomes optimum.

Quoting AT (Thread starter):
It was interesting though hearing the sounds as we changed altitude- the engines would suddenly go from a whine to a roar for a couple minutes and then die down again.

That's a step climb...it's typically done at some moderate rate of climb (not nearly as hard on the engines as the initial climb-out from takeoff).

Tom.m

RE: Change In Altitude During Long Haul Flights

Posted: Thu Feb 04, 2010 1:42 am
by jambrain


Quoting Tdscanuck (Reply 2):
As you climb, air density drops. This means the wing needs more Cl (more angle of attack) to provide the needed lift; since Cd goes up with Cl^2, you end up with a lower L/D and burn more fuel per mile. For any particular weight and speed, there is an optimum altitude for minimum fuel burn.

As T could add the engines TSFC curve needs to be factored in as well, to get the engines closer to peak thermodynamic efficiency you need to match the thrust to the drag as close to peak efficiency as possible. Drag and weight drops as fuel is burnt, so you climb in order that the air density drops so engines are still running at 85-90% peak speed but produce less thrust (low density air less mass flow less thrust)

a GE90 will only be producing 13 or 14 klb thrust at cruise altitude towards the end of a flight not 115 klb

RE: Change In Altitude During Long Haul Flights

Posted: Sat Feb 13, 2010 11:14 pm
by kimberlyRJ
Quoting Tdscanuck (Reply 2):
That's a step climb...it's typically done at some moderate rate of climb (not nearly as hard on the engines as the initial climb-out from takeoff).

I find you get this quite a bit when you have completed a westbound pond NATrack crossing.

After passing your last waypoint ATC will often ask you to climb (if the pilots have not already asked ATC) if the aircraft can climb to a higher altitude – thinking about it this is done on most flights I have served on but as Tdscanuck said the climb is quite shallow and you only really notice the engines sounding a little louder, or the pitch of the sound changing – I find this especially on our Boeing 777 aircraft.

My last flew our NATrack at FL320 then once cleared we climbed to FL400.

Kimberly

RE: Change In Altitude During Long Haul Flights

Posted: Sat Feb 13, 2010 11:36 pm
by A342
Quoting Tdscanuck (Reply 2):
No. The optimum altitude is a function of weight.

And airspeed, correct? The higher the mach number, the more lift you get at equal angle of attack.

RE: Change In Altitude During Long Haul Flights

Posted: Sat Feb 13, 2010 11:54 pm
by CosmicCruiser
Quoting kimberlyrj (Reply 4):
the climb is quite shallow and you only really notice the engines sounding a little louder

The cruise power will be very close to climb power therefore the difference is not as noticeable. Optimum is always better for fuel burn.

RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 2:48 am
by Starlionblue
Quoting A342 (Reply 5):
And airspeed, correct? The higher the mach number, the more lift you get at equal angle of attack.

I don't claim to understand how it affects lift, but there is a "speed limit" for subsonic airliners, depending on the onset of mach buffet on that particular wing.

RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 4:57 am
by tdscanuck
Quoting A342 (Reply 5):
Quoting Tdscanuck (Reply 2):
No. The optimum altitude is a function of weight.

And airspeed, correct? The higher the mach number, the more lift you get at equal angle of attack.

Correct; good catch, I should have included that. For a given weight and airspeed, there's an optimum altitude. If you change either factor, the optimum altitude will change. How you figure out the cruise speed is a relatively complicated calculation inside the FMC based on how the airline weights time against fuel burn.

Quoting Starlionblue (Reply 7):
I don't claim to understand how it affects lift, but there is a "speed limit" for subsonic airliners, depending on the onset of mach buffet on that particular wing.

As you start to go "too fast", the shock wave on the top of the wing gets stronger. This causes a rise in drag, and can cause separation of the flow over the top of the wing aft of the shock. The separation also causes a drag rise, and can force you to a higher angle of attack, which also causes drag rise. The separation is what causes the buffet.

Tom.

RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 8:57 am
by Starlionblue
Thanks Tom I knew the essence of that. What I was wondering (and should have asked) is how it affects cruise altitude. I mean given that angle of attack changes with weight. Thanks for clearing that out in the first part.

RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 10:35 am
by Fabo
Quoting tdscanuck (Reply 8):
How you figure out the cruise speed is a relatively complicated calculation inside the FMC based on how the airline weights time against fuel burn.

Should you want to play with it a bit, for ex. in Flightsim, this is the "Cost Index" entry. In FMCs, you could also nicely observe the change in OPT FL and MAX FL as you burn fuel and lower mass, and for ex. PMDG 747 FMC will also calculate optimum step-climb process for you (at least by means how long&far you are from when step climb should be made for effectivity). You can try the freeware iFly, but I have no idea how well (if ever) is this modelled.

RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 2:45 pm
by 747classic
In the "good old times" (Without FMS, FMC or PMS ) we used graphs and tables, to calculate the optimum flight level versus weight. Here two examples from the 747/CF6-50E2 QRH.
As discussed by others in this thread the optimum altitude is depended from aircraft weight at a fixed speed (M.84 in this example).



The actual climb to a higher level could be earlier or later, if the nose or tail wind on the requested flight level was known.
(Asking another flight already flying on your requested level.).


RE: Change In Altitude During Long Haul Flights

Posted: Sun Feb 14, 2010 6:11 pm
by tdscanuck
Quoting Starlionblue (Reply 9):
What I was wondering (and should have asked) is how it affects cruise altitude. I mean given that angle of attack changes with weight.

I tried to think through this but couldn't get to an answer based on first principles...I think I'd need to play with an FMC. Higher cruise Mach is going to cause more wave drag but less induced drag (because you don't need as much AoA for the same lift). The exact balance point would depend on the drag polars, I think.

Tom.