Most large commercial jet engines include design features that ensure they can shut-down after "ingesting" a bird weighing up to . The engine does not have to survive the ingestion, just be safely shut down. This is a 'stand-alone' requirement, "i.e.", the engine, not the aircraft, must pass the test. Multiple strikes (from hitting a bird flock) on twin-engine jet aircraft are very serious events because they can disable multiple aircraft systems, requiring emergency action to land the aircraft, as in the January 15, 2009 forced ditching of US Airways Flight 1549.