Answer appears to be Flight 93 - one of the 9/11 hijacked aircraft in America

4 planes flight 11 , 175, 77. and 93 both united an American airlines

United Airlines Flight 93.

Flight 93 was one of the four hijacked planes on 9/11. This plane crashed in the fields of Pennsylvania near Shanksville, after the passengers mounted a revolt. It possibly saved more lives by preventing the plane from getting to it's hijacker's destination, which was speculated to be the White House or the US Capitol.

The fourth plane that was hijacked on 9-11 was United Airlines flight 93. Some of the passengers upon hearing the news of the other flights and suspecting their fate decided to fight back. In the struggle to retake the plane it crashed into a field near Shanksville, Pennsylvania where everyone on board died.