777 vanished more than three months ago with 239 people on board. joining me now is david susi, cnn safety analyst and former safety inspector for the faa. good to see you again, david. one of the big assumptions reflects a belief it s likely the pilots suffered oxygen loss. what combination of factors has to happen before that happens? there s two different possibilities here, fredericka. one is lost prez yourization in the cabin and not able to reduce the air flight speed and lower it to an area where they could breathe so that is a possibility number one, just that there was something that incapacitated them such as an explosion, decompression to cause a problem with the aircraft. secondly, could have been that the oxygen bottle itself was simply empty in the event and assumed they had oxygen and continued to fly.