330lover
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Difference In Costs Betw Flying High/low Altitude

Tue Aug 07, 2012 8:47 am

We were wondering what the difference in costs is between flying at high and low altitude.
Specifically, we are talking about small regional planes in Africa. Climbing to altitude or staying at lower altitude (sightseeing-like altitude).
What factors have an influence on the costs of such quite short flights?

Thanks for any info on this.

To the mods: if this is better suited for another forum, please move accordingly. Thanks.
Britten Norman Islander VP-FBR on Falkland Islands. THAT'S FLYING!
 
tdscanuck
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RE: Difference In Costs Betw Flying High/low Altitude

Tue Aug 07, 2012 11:57 am

Quoting 330lover (Thread starter):
We were wondering what the difference in costs is between flying at high and low altitude

It can be very large (factor of 2 or more) for large airliners. The differential probably isn't as large for smaller regional aircraft.

Quoting 330lover (Thread starter):
Specifically, we are talking about small regional planes in Africa. Climbing to altitude or staying at lower altitude (sightseeing-like altitude).
What factors have an influence on the costs of such quite short flights?

The big trade is climb performance vs. stage length vs. weight. Almost all aircraft burn less fuel at altitude but they spend more to get to altitude and they can't go as high when they're heavy. The FMC will compute the optimal profile for any chosen balance of fuel cost, time cost, and speed.

Piston prop planes typically have terrible climb performance, relative to jets, so their optimal altitude will tend to be lower over the same stage length. With turboprops they tend to be slower so, even though they're jets from a thermodynamic point of view, they also tend to want to be lower than jets.

Tom.
 
330lover
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RE: Difference In Costs Betw Flying High/low Altitude

Tue Aug 07, 2012 3:48 pm

Thanks Tom.

So the difference is mainly in the climb performance?
I suppose on higher altitude, the air resistance is lower than flying on low, warmer flight levels?
Britten Norman Islander VP-FBR on Falkland Islands. THAT'S FLYING!
 
TW
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RE: Difference In Costs Betw Flying High/low Altitude

Tue Aug 07, 2012 7:38 pm

There's a PDF document from ATR that shows how the flight level affects fuel consumption. Here the link: http://www.atraircraft.com/media/downloads/fuelsaving2011_1.pdf
According to this document, flying higher saves fuel, but takes a bit longer. You have to take into account what the increased flight time costs you in terms of personnel costs, engine maintenance (under a PBH contract), and maybe fewer flights per day.

Airlines who operate in sandy regions have more wear and tear on their jet engines. Maybe this would be a factor as well if you fly at lower altitudes in such regions than higher in cleaner air.
 
flight152
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RE: Difference In Costs Betw Flying High/low Altitude

Tue Aug 07, 2012 8:21 pm

Quoting tdscanuck (Reply 1):
The differential probably isn't as large for smaller regional aircraft.

Not sure what you identitify as small, but the E145 burns up to 50% more at FL250 versus FL370 (approx 2400 pph vs 3500 pph)
 
tdscanuck
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 3:39 am

Quoting 330lover (Reply 2):
So the difference is mainly in the climb performance?
I suppose on higher altitude, the air resistance is lower than flying on low, warmer flight levels?

Yes, air resistance goes down (lower density). This also takes power from the engine (lower density again) but it's also colder (helps offset) and the drop in drag is bigger than the drop in engine power so you end up ahead.

Quoting Flight152 (Reply 4):
Quoting tdscanuck (Reply 1):
The differential probably isn't as large for smaller regional aircraft.

Not sure what you identitify as small, but the E145 burns up to 50% more at FL250 versus FL370 (approx 2400 pph vs 3500 pph)

I meant more stuff like Dash-8's and ATR's...I'd expect an E145 to behave the same way as any other twin jet airliner with respect to fuel vs. altitude.

Tom.
 
330lover
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 7:33 am

Thanks for the answers.
The reason I asked is because one of our clients had a charter flight booked in Africa on a small prop airliner.
When asking the flight crew if they could lower their flight level to be able to make some nice film shots of the landscape, they had to pay extra because it is more expensive to fly at 1000 ft than higher. At least, that's what the flight crew explained.

Does this makes sense, or is it an example of using tourists as money source?
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Starlionblue
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 9:29 am

Quoting 330lover (Reply 6):
The reason I asked is because one of our clients had a charter flight booked in Africa on a small prop airliner.
When asking the flight crew if they could lower their flight level to be able to make some nice film shots of the landscape, they had to pay extra because it is more expensive to fly at 1000 ft than higher. At least, that's what the flight crew explained.

Does this makes sense, or is it an example of using tourists as money source?

It makes perfect sense. Heck, you can look at a Cessna 172 and 2000ft will use more fuel than 4000ft. Well.. Barely...

[Edited 2012-08-08 02:32:00]
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tdscanuck
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 1:30 pm

Quoting 330lover (Reply 6):
When asking the flight crew if they could lower their flight level to be able to make some nice film shots of the landscape, they had to pay extra because it is more expensive to fly at 1000 ft than higher. At least, that's what the flight crew explained.

Does this makes sense, or is it an example of using tourists as money source?

That makes perfect sense. Even for an underpowered prop plane, 1000' is going to drive fuel consumption way up (either because you're trying to go fast in thick air or because you slow down and take way longer). I don't think the optimum altitude for fuel burn on any aircraft except maybe an ultralight is down as low as 1000'.

Tom.
 
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bikerthai
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 2:16 pm

Quoting tdscanuck (Reply 5):
but it's also colder (helps offset) and the drop in drag is bigger than the drop in engine power so you end up ahead.

If I remember right, colder air will give you better compression ratio for your engines, so it will make your engine more efficient. Although in the grand scheme of thing, the efficiency difference may be over shadowed by all the other stuff everyone else is mentioning.

bt
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MD11Engineer
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RE: Difference In Costs Betw Flying High/low Altitude

Wed Aug 08, 2012 2:19 pm

On the Morane Saulnier 760 Paris jet (4 seater, two Maboré IV turbojets) it makes quite a bit of difference:

At an N1 cruise setting of 19500 RPM at standard atmospheric conditions under similar climbing and descend conditions (climb: N1 at 21500 RPM, IAS 200 kts up to 20,000 ft, then - 10 kts each 3000 ft / descend: N1 at 17000 RPM air brakes out , IAS 180 kts) I´ll get the following values:

Cruise altitude___________Range___Time to TOC______Time from TOD to GRD_______Total flight time

10,000 ft________________560 mn_______5´_________________6´______________________1h57´
20,000 ft________________720 nm______11´_________________9´______________________2h30´
25,000 ft________________810 nm______16´________________11´______________________2h50´
30,000 ft________________860 nm______21´________________12´______________________3h05´

To account for taxi and circuit fuel subtract 100 - 150 nm from each range figure

Also from the manual:

at 19000 RPM both engines together use 400 l/h at 30.000 ft while they use 640 l/h at 10.000 ft (the manual and aircraft are from the 1950s, they gave fuel consumption in litres per hour)
at 21500 RPM (highest power) fuel consumption is 625 l/h at 30.000 ft and 1075 l/h at 10.000 ft.


The manual advises to climb as high as possible as fast as possible.

Jan

[Edited 2012-08-08 07:28:27]
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