I once heard an answer to that very question from a jet engine mechanic. The answer was that the bird gets chopped up in the inlet pretty well leaving small particles that go into the next set of fans that are smaller then another set even smaller this is called the 'compressor' part of the engine. Usually, the blades are undamaged because they are built to withstand birds but when large enough birds (or multiple birds at once) are ingested it puts enough debris into the compressor to stall it momentarily and it 'back-fires' so to speak, causing a flame out. Restarting a jet engine takes a lot of time (time you do not have at low altitude, they were at 3200 feet and dropping fast). The idea that a grid could be put over the intake has been thought of before the problem is that it would constantly be plugging up. The force caused by a jet engine is tremendous, so things could easily be pulled through a grid or screen (like sucking a basketball through a garden hose.. LOL ) But keep thinking! I agree it is a problem that needs to be solved.