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Change In Altitude During Long Haul Flights  
User currently offlineAT From United States of America, joined Jul 2000, 1042 posts, RR: 0
Posted (4 years 7 months 2 weeks 6 days 3 hours ago) and read 6253 times:

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.

12 replies: All unread, jump to last
 
User currently offlineMoose135 From United States of America, joined Oct 2004, 2322 posts, RR: 10
Reply 1, posted (4 years 7 months 2 weeks 6 days 2 hours ago) and read 6245 times:

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.


KC-135 - Passing gas and taking names!
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 2, posted (4 years 7 months 2 weeks 6 days 2 hours ago) and read 6244 times:



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


User currently offlineJambrain From United Kingdom, joined Sep 2008, 251 posts, RR: 0
Reply 3, posted (4 years 7 months 2 weeks 3 days 2 hours ago) and read 5802 times:



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



Jambrain
User currently offlinekimberlyrj From United Kingdom, joined Mar 2008, 385 posts, RR: 1
Reply 4, posted (4 years 7 months 1 week 4 hours ago) and read 5429 times:

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


User currently offlineA342 From Germany, joined Jul 2005, 4682 posts, RR: 3
Reply 5, posted (4 years 7 months 1 week 4 hours ago) and read 5421 times:

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.



Exceptions confirm the rule.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 6, posted (4 years 7 months 1 week 4 hours ago) and read 5409 times:

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.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17039 posts, RR: 66
Reply 7, posted (4 years 7 months 1 week 1 hour ago) and read 5365 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 8, posted (4 years 7 months 6 days 22 hours ago) and read 5342 times:

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.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17039 posts, RR: 66
Reply 9, posted (4 years 7 months 6 days 18 hours ago) and read 5291 times:

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.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 10, posted (4 years 7 months 6 days 17 hours ago) and read 5258 times:

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.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offline747classic From Netherlands, joined Aug 2009, 2141 posts, RR: 14
Reply 11, posted (4 years 7 months 6 days 13 hours ago) and read 5221 times:

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.).




Operating a twin over the ocean, you're always one engine failure from a total emergency.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 12, posted (4 years 7 months 6 days 9 hours ago) and read 5193 times:

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.


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