Sometimes one event leads to unexpected consequences and for 9/11 it was the end of major U.S. airline crashes*. With the crash of American Airlines flight 587 in November of 2001 in New York there has been only one airline crash with 50 or more fatalities in the last 22 years**
Prior to 9/11 these were the major airline crashes
2000 - Alaskan Airlines in the Pacific Ocean - 88 Deaths
1999 - Egypt airlines just off of Nantucket - 217 Deaths
1997 -Korean Air Flight near Gaum - 229 Deaths
1996 - July TWA Flight 800 just off the coast of New York - 230 Deaths
1996 - May - Value Jet the Florida Everglades - 110 Deaths
1994 - October - American Eagle flight Indiana - 68 Deaths
1994 - September -USA Flight 427 Pittsburgh - 132 Deaths
1990 - Aviance Flight New York - 73 Deaths
1989 - UA Flight Iowa - 112 Deaths
1987 - Northwest Flight Detroit - 156 Death
1986 - AeroMexico Flight California - 82 Deaths
1985 - August Delta Airlines Irving TX -135 Deaths
1985 - January Galaxy Airlines - Reno - 70 Deaths
1982 - July - Pan Am Flight Louisiana - 153 Deaths
1982 - January - Air Florida Potamic River DC - 78 Deaths
5/25/79 - America Airlines Chicago (the famous photo of the plane inverted) - 273 Deaths
1978 - PSA Flight - San Diego - 144 Deaths
In the 22 years since 9/11 there have been 2 major airline crashes with 50 or more fatalities (Colgan Air Buffalo - 50 Deaths). In the 22 years prior to 9/11 there were 17 major airline crashes in the U.S. Unexpected Consequences.
*Major airline crashes is defined by 50 or more fatalities from the crash.
**Not that 50 is an arbitrary number but there was a plane crash in Lexington KY in 2006 where 49 people lost their lives.