Hi,
I used to fly the route that they crashed on almost 5 days a week for the united nations. The weather on that route could very possibly be the worst weather you will experience in Africa(having flown west central south and north africa) The weather around Bukarvu is extremely bad(and world renowned for it), and scared the hell out of me!!!!down drafts over the top of those mountains are very common, and alot of them would be strong enough to put any aircraft flying fairly low over the top into the terrain below. I didnt know your son, but i new the other pilot, and know his flying history, and i have no doubt in my mind that they were experienced enough to be doing the type of flying they were doing, I know the contracts in that area quiet well, and the requirements to fly one of those aircraft are extremely steep......I really am sorry about what happened to your son, and hope you do get to the bottom of it....having seen where the a/c crashed and the position of the wreckage, i am willing to bet a severe down draft played a roll in it, and if as you said earlier they were having issues with one of the engines.......this could make a recoverable attitude, impossible to get out of! Good luck, the truth will surface eventually!