# What factors cause a turbine aircraft to have decreasing fuel burn over time?

Consider a fictional aircraft with a fuel schedule similar to the following:

1st hour - 10,000 (lb or kg, doesn't matter)
2nd hour  - 8,000
3rd hour  - 6,000
4th* hour - 5,000
* and subsequent


As we can see the fourth hour of flight burns only half the fuel of the first hour, so fuel consumption is a function of time. Is the decreasing weight of the aircraft as fuel is burned the only reason for this increase in fuel efficiency, or are there other influences? For example if a jet launches with half tanks versus full tanks, would you use the same 1st hour fuel schedule?

I may be way off base here, so if there are fundamental errors in my question, please correct me.

• – ymb1
May 7 at 22:24
• @ymb1 - Very relevant - I'll spend some time reading your answer there (and possibly mark this question as dupe later). Thanks! May 7 at 22:28
• Those numbers seem a bit exaggerated to me, but in any case, it's mostly due to the power reduction to keep a constant cruise speed as fuel is burned off, plus the extra burn of the takeoff and climb at the start. My own little plane is about 7 mph faster solo with minimal fuel compared to at gross, and that would represent a noticeable reduction on power to maintain the same speed. May 8 at 0:48
• The reduction in fuel consumption over time also applies to piston aircraft. Piston aircraft are generally more fuel efficient, so the effect is less prominent. May 9 at 13:29
• @John K: While the fuel consumption/speed increase isn't all that noticable (at least to me) in small piston aircraft, where you really see an effect is in rate of climb. (And takeoff distance.) May 11 at 18:19