It's proven to be NP-Hard, but with a well known "good enough" approximation ("the knapsack problem has a fully polynomial time approximation scheme (FPTAS)")
I guess it's only part of the problem, solving for one plane.
In practice you would want to solve for varying demand, for multiple customers, for planes that eventually are delayed, crews that get sick/delayed, for different planes, for routes that pick/drop different cargo at different places.
And of course, you want to price all of that in a way that makes you money.