The UN Humanitarian Air Service (UNHAS) isn't free. Have been an aid worker for some time and these flights are generally operated by third party operators, wet or dry leased. At the moment Somalia flights are operated out of Wilson or JKIA in Nairobi, using Caravans, one or two CRJs, B1900s and various other types including an RJ85 at one point. The agencies pay for the tickets on these flights, whether UN or NGO. For the UN, the World Food Programme (WFP) is the organisation that coordinates the air services on behalf of NGOs and partner UN agencies. Congo is another story, I am not sure who selects the air operators there or how it's done, but I lost a friend there when an ATR 42 went down a number years back, carrying UN workers. so in terms of the internal UN oversight from country to country, I'm uncertain as to how standardised it is, if it is at all. In Liberia we used to go around on UNMIL Mil-8 helicopters, which was fine but not that safe. A couple of low-level non-fatal crashes led to most agencies not allowing their staff to use those things. The usual stereotype of the vodka-fuelled Russian pilots prevailed, how true it was I'm not sure.