i worked for a south african company that had contracts with the UN in the congo...thats basically how it works- all the UN and Red cross / aid flying is contracted out to companies, and because south africa can provide a 2nd world standard at 3 rd world prices then the UN awards south african operators these contracts.
thats who you will need to work for...
be warned, it is not as easy as it sounds, but the only way to get such a job is to turn up there and speak to the people face to face...no point emailing or phoning.