Infrastructure in Africa is generally poor, and it's hard to find even a handful of decent roads, especially in South Sudan, where even a smooth dirt road is a rarity. Thus, small aircraft have become the most common mode of transportation.
In most African countries, if you're taking a plane, delays aren't a big deal, nor is the age or condition of the plane. As for expecting any sort of humanitarian service, you can forget about that altogether.
Regular flights can't be counted on for much, and as for those small planes that operate on short routes, it's even less of a concern. The fact that they can take off and land safely is already a successful flight.
Gao Yang was fortunate to have a chartered plane at his disposal, and he was the only passenger, so there was no need to fight for a seat when boarding. Moreover, Kolm swore up and down that the pilot he found was the best in Malakal, and the plane was the newest and best one in all of Malakal.