h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The reason you are warned that littering could cause an air crash is obviously the risk of FOD - Foreign Object Damage, a term well known to anyone in the air business. Modern jet engines suck in a collossal amount of air which is then ejected from the other end, causing the thrust nescessary to fly. This suction can cause any small, loose objects, such as litter, to be sucked into the inner workings of the engines, which include very finely balanced turbine blades rotating at up to 45,000 RPM. Depending on the mass and nature of the foreign object, various levels of damage may result. Soft objects, such as small birds, normally will do little damage (except to the birds), whereas larger birds can be a problem. Any kind of metallic objects are the worst causes of FOD. FOD can extend from a minor bending of a single turbine blade to complete engine failure with separation of turbine blades. You don't want to be near a turbine blade that separates at 45,000 RPM. It will cut clear through the body of the airplane any anything in it. Such a complete engine failure may not cause disaster on landing, but on takeoff may well cause a non-surviable crash, depending of course on the number of engines on the aircraft and lots of other factors.

And yes - there are safety zones for ground personell around the engine inlets. As they say - don't become a foreign object ! It is claimed that this has actually happend - someone being sucked into a jet engine - but that may just be rumours. Confirmation, anyone?

So - don't litter (particularly nuts & bolts) and fly safely!