It destroys the engines. Any pilot worth flying with would not fly into the ash cloud. The alaska volcano observatory is using helicopters to fly around Mt. Redoubt. That is how they are getting their video. Volcanic ash shows up on Doppler Radar, and this is one reason why the Alaska Volcano Observatory (AVO) works closely with the National Weather Service; it gives AVO the use of Doppler Radar to schedule their flights to repair the cameras, to record video, and for other scientific studies they do on or around Mt. Redoubt during the eruptions.