I saw the chart below in A320's airport planning manual, and there is one thing I am curious about. The green curve can be divided into 3 segments. The first one is horizontal, which makes sense because there is a max zero fuel weight limit. But why the second and third segments have different slopes?

 

My guess is that with payload less than 16 tons the aircraft's optimum cruising altitude would hit its limit (41000 ft). Therefore the aircraft can no longer cruise at its optimum altitude so fuel savings are less significant. The same chart for other aircraft types are very similar as well. I am wondering what's the correct explanation?

(Source: pdf page 150)

I saw the chart below in A320's airport planning manual, and there is one thing I am curious about. The green curve can be divided into 3 segments. The first one is horizontal, which makes sense because there is a max zero fuel weight limit. But why the second and third segments have different slopes?

 

My guess is that with payload less than 16 tons the aircraft's optimum cruising altitude would hit its limit (41000 ft). Therefore the aircraft can no longer cruise at its optimum altitude so fuel savings are less significant. The same chart for other aircraft types are very similar as well. I am wondering what's the correct explanation?

 

(Source: pdf page 150)

 

Apparently it's because in the second segment, the limiting factor becomes the Maximum Landing Weight:

 

Apparently it's because in the second segment, the limiting factor becomes the Maximum Landing Weight:

 

Thank you for the diagram. But maybe it's max takeoff weight? Because with 20 tons of payload, A320 cannot takeoff with full fuel tank, but it can land with 7+ tons of reserve fuel, which should not be a limiting factor. 

Payloand and range is all about achieving the best compromise. If you have max payload, you are not gonna be able to take as much fuel as you can (limited by the max take off weight), therefore your range will be limited. Depending how far you fly, you need to consider the landing weight, because on a short flight, you will have too much fuel left.

And vice versa, if you take maximum fuel, you can't take max payload....and you need to watch the landing weight.

Payloand and range is all about achieving the best compromise. If you have max payload, you are not gonna be able to take as much fuel as you can (limited by the max take off weight), therefore your range will be limited. Depending how far you fly, you need to consider the landing weight, because on a short flight, you will have too much fuel left.

And vice versa, if you take maximum fuel, you can't take max payload....and you need to watch the landing weight.

Yep I now understand the chart. Thank you.