Originally Posted by Brian Carlton

As an example, the fan on the engine that powers the B-757 is about 80" diameter and spins at about 4500 rpm on takeoff. The engine develops 40K lb. thrust, 80% of it coming from the fan. Each blade weighs about 13 lb. and the centrifugal force on the blade is over 100K lb. at 4500 rpm. The fan disc, one of the most critical pieces in the engine, must withstand the force of all the blades trying to tear it to pieces. This force is on the order of 3.5 million lb. The force occurs every time the engine is run near maximum speed (every takeoff).

Any anomaly in the disc, the slightest crack or discontinuity in the forging, will eventually cause the disc to explode. There are several examples including the United DC-10 that crashed in Sioux City and the Delta DC-9-80 that killed a woman sitting right next to the fan inside the passenger cabin.

It's a very complex business whereby there is no margin for any mistakes.