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Higher/lower Altitudes And Fuel Burn  
User currently offlinePHX787 From Japan, joined Mar 2012, 7474 posts, RR: 18
Posted (2 years 2 months 5 days ago) and read 2953 times:

Simple question: when it comes to fuel efficiency, is it preferred to be at a higher altitude (FL37+) or a lower altitude (FL32-) to have better fuel efficiency, or in between?

Does anybody have any stats for ULH planes (747, 777, A340, A380) and their fuel burn in comparison to altitude?


Thanks, Z


次は、渋谷、渋谷。出口は、右側です。電車とホームの間は広く開いておりますので、足元に注意下さい。
3 replies: All unread, jump to last
 
User currently offlineAF1624 From France, joined Jul 2006, 658 posts, RR: 0
Reply 1, posted (2 years 2 months 5 days ago) and read 2936 times:

It strictly depends on gross weight, CG, temperature and winds.

The FMC (on aircraft equipped with it) will work out an optimum altitude depending on these factors + some others.

As a rule of thumb, higher altitude = thinner air + lower temperature = lower fuel burn

BUT

If the aircraft is too heavy, higher altitude = thinner air = higher angle of attack to keep level flight = higher fuel burn

AND of course winds play a big part in it.

That's pretty much how it works.

Usually, an airliner will do step climbs. i.e. start cruising at FL350, and then as fuel levels diminish weight decreases so 370, then 390, then 410.



Cheers
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 2, posted (2 years 2 months 4 days 18 hours ago) and read 2832 times:

Our dispatchers also plot say 4 routes and altitudes and choose route/altitude based on the best for time and or fuel burn depending on what's most important for that flight.

User currently offlineMir From United States of America, joined Jan 2004, 21571 posts, RR: 55
Reply 3, posted (2 years 2 months 4 days 7 hours ago) and read 2671 times:

It is, in general, better to be higher. However, if you're too heavy for a certain altitude, then you'll burn more fuel than you would if you stayed lower down until you became light enough to climb up. Aircraft will have an optimum altitude for a particular weight, and and significant deviation from that will result in higher fuel burn. The optimum altitude will increase over the course of the flight as the aircraft burns off fuel and becomes lighter.

-Mir



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