The Assault on the S.H.I.E.L.D Helicarrier was an in-flight attack aboard the S.H.I.E.L.D. Helicarrier that was launched by a team led by Clint Barton, who was, at that point, under the mental control of Loki.
The assault resulted in the (impermanent) death of S.H.I.E.L.D. agent Phil Coulson and the escape of Loki, precipitating the Battle of New York. The death of Coulson prompted Nick Fury to go to extreme measures and eventually resurrect the fallen agent.