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Altitude And Fuel Efficiency  
User currently offlineVtdl From United States of America, joined Jul 2004, 81 posts, RR: 0
Posted (5 years 11 months 4 weeks 1 day 14 hours ago) and read 9181 times:

I have a feeling that someone must have asked this question before. If so, I apologize. Please point me to it.

I understand that planes fly at high altitude to save fuel. But it also burns more fuel to get to that altitude. Am I right? For a long distance fly, of course. How about for a short flight, let's say 30 to 40 minutes? Does the saving really enough to offset the extra fuel to fly high?

12 replies: All unread, jump to last
 
User currently offlineRwessel From United States of America, joined Jan 2007, 2238 posts, RR: 2
Reply 1, posted (5 years 11 months 4 weeks 1 day 14 hours ago) and read 9176 times:
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That's correct - for a short flight it's not worth it to climb to a high altitude. Exactly where the break-even is depends on a number of factors, but for a 30 minute flight you're basically never going to climb to 30+kft in an airliner.

User currently offlineRoseFlyer From United States of America, joined Feb 2004, 9372 posts, RR: 52
Reply 2, posted (5 years 11 months 4 weeks 1 day 14 hours ago) and read 9175 times:

There's a most efficient altitude for all flights. It does vary based on factors such as length of flight, destination airport arrival conditions and weight of the plane. The dispatchers on here can give you a better idea of when it is most economical to climb to a cruising altitude in the normal 290-410 range and when the flight is too short for that to be economical. It is somewhere around the 40-50 minute mark where planes will climb to a higher cruising altitude. Sometimes it is most efficient to climb to a cruising altitude and only stay there for about 5 minutes. Nowadays software makes those decisions.

To answer your question about a 30-40 minute flight, I would say most jets would climb to somewhere around 170 to 230. Turboprops are obviously different.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 22
Reply 3, posted (5 years 11 months 4 weeks 1 day 7 hours ago) and read 9109 times:

Hi VTDL, Buzz here. I prefer to fly with the windows open, lowly and slowly. So I'm in the thicker air down here among the grass airfield airplanes  Smile

The advantage of climbing into the thinner air is that in thin air for the same airspeed you get a higher ground speed. And there are fewer oxygen molecules available for the engine to breathe, so you should lean the fuel-air mixture (or have the fuel control automatically do it for you) So there's less thrust up there because of the thin air, but the fuel burn is less too.

So it depends on the airplane. how heavy it is, and how long you plan to cruise.

g'day


User currently onlinePihero From France, joined Jan 2005, 4130 posts, RR: 76
Reply 4, posted (5 years 11 months 4 weeks 1 day 7 hours ago) and read 9101 times:
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Quoting Vtdl (Thread starter):
How about for a short flight, let's say 30 to 40 minutes? Does the saving really enough to offset the extra fuel to fly high?



Quoting RoseFlyer (Reply 2):
To answer your question about a 30-40 minute flight, I would say most jets would climb to somewhere around 170 to 230.

For a short flight, the optimum is a *no cruise profile*, i.e climb / descent.
As a ball-park simplification, the higher your altitude,the longer your descent and both are equivalent to a cruise at that altitude in terms of fuel. That means, with a modern twin engined airliner, a 30 / 40 min sector could be planned as high as FL 350...even more if the plane is light.



Contrail designer
User currently offlineMusapapaya From United Kingdom, joined Apr 2004, 1053 posts, RR: 0
Reply 5, posted (5 years 11 months 4 weeks 1 day 4 hours ago) and read 9043 times:



Quoting Pihero (Reply 4):
For a short flight, the optimum is a *no cruise profile*, i.e climb / descent.
As a ball-park simplification, the higher your altitude,the longer your descent and both are equivalent to a cruise at that altitude in terms of fuel. That means, with a modern twin engined airliner, a 30 / 40 min sector could be planned as high as FL 350...even more if the plane is light.

Thats right and is routinely done here on Europe on A320 and B737 series.



Lufthansa Group of Airlines
User currently offlineQANTAS747-438 From United States of America, joined Jan 2001, 1886 posts, RR: 2
Reply 6, posted (5 years 11 months 4 weeks 17 hours ago) and read 8977 times:

Just flew on a 737-700 from OAK-LAX on a 55 min flt and we took it up to 41,000 with 90 paxs on board. Thought that was a little high for such a short flight, but like it's been said, perhaps a dispatcher can fill us in... OPNLguy???


My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
User currently offlinePilotpip From United States of America, joined Sep 2003, 3139 posts, RR: 11
Reply 7, posted (5 years 11 months 4 weeks 17 hours ago) and read 8972 times:

Ideally, even on a short flight you'd want to get up to whatever the optimal cruise altitude is and stay high as long as possible to do an idle descent. This would save the most fuel.

However in the real world this isn't possible espeically when going into a busy airport where they'd be slowing you and dropping you down low early.



DMI
User currently offlineAAR90 From United States of America, joined Jan 2000, 3451 posts, RR: 47
Reply 8, posted (5 years 11 months 3 weeks 6 days 23 hours ago) and read 8898 times:



Quoting QANTAS747-438 (Reply 6):
Just flew on a 737-700 from OAK-LAX on a 55 min flt and we took it up to 41,000 with 90 paxs on board. Thought that was a little high for such a short flight, but like it's been said, perhaps a dispatcher can fill us in... OPNLguy???

Depending upon weight, you're looking at 20-30 minutes cruising at FL410 and that's a whole lot of fuel savings.  Smile

Traffic is another factor to be considered. I've been doing a lot of DFW-TUL and DFW-AUS flights with the outbound legs (away from DFW) at high altitudes and the inbound legs (to DFW) at FL220-230... primarily due to inbound traffic.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineAjd1992 From , joined Dec 1969, posts, RR:
Reply 9, posted (5 years 11 months 3 weeks 6 days 22 hours ago) and read 8889 times:

When British Airways first introduced the B757 on the Manchester-Heathrow shuttle, they were getting up to FL410, and it's a 25 minute flight, or about 150 nautical miles. Now that's high Big grin

The B737's that Easyjet used to use on the Liverpool-Luton flight only got up to around FL200, so it depends on how heavy you are as well as all the other factors.


User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 10, posted (5 years 11 months 3 weeks 6 days 20 hours ago) and read 8875 times:



Quoting Vtdl (Thread starter):
Does the saving really enough to offset the extra fuel to fly high?

In many cases, yes. But you are right, there is a certain "break even" altitude above which it is not profitable to climb. However, if you loiter high as long as you can and use a continuous descent at idle power as much as possible, your average fuel burn can come down substantially. On long flights we do recalculate the ideal altitude as the weight and temp changes, and try to fly as close to that as possible.



Proud OOTSK member
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 11, posted (5 years 11 months 3 weeks 6 days 16 hours ago) and read 8854 times:

Looking at the B707-320B series airplanes, PanAmerican had a nice plastic laminated chart, called "Flight conduct, 300 fans" which enabled the pilots to determine the most economical cruising altitude, depending on aircraft weight and the sector length.
Used mostly for weather diversions.

The L1011-500 that I fly now has a similar chart.
Very handy.


User currently offlineHeinzmahrer From Switzerland, joined Feb 2008, 6 posts, RR: 0
Reply 12, posted (5 years 11 months 2 weeks 4 days 14 hours ago) and read 8684 times:

The optimum cruising flight level depends on actual aircraft weight (less weight = higher flight level), the actual air temperature at altitude and to a lesser degree the wind.
For a long distance flight that means as the aircraft gets lighter due to fuel burn the optimum cruising flight level increases and indeed this procedure is used and the crew will climb as soon as possible.
For short flights in high density traffic areas such as central Europe cruising flight levels are determined of what is made available by air traffic control (ATC). This of course is "optimum" for ATC but not for fuel burn. Between London and Zürich for instance the maximum flight level you can get from ATC is usually FL290 even though the aircraft would perform much better at FL 370 or even 390.


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