Flutter: Fast and Fatal

Yesterday the National Transportation Safety Board released a synopsis, subject to further review and editing, of its findings on the September 2011 crash of a modified P-51 at the Reno air races that killed 11 people.  “ probable cause…was the reduced stiffness of the elevator trim tab system that allowed aerodynamic flutter to occur at racing speeds.”

Peter Garrison dissected the phenomenon of flutter in his 2001 article, “The Hammer.”

Seldom reported and little understood, occupies one of those dimly lit and unsafe places that decent people prefer not to visit. The idea that an airplane could shatter—disintegrate—for no reason other than its own motion through the air—better to let sleeping horrors lie.

Garrison went on to explain that flutter is a form of resonance, or sympathetic vibration, as seen in an out-of-balance tire. However, “Out-of-balance tires seldom lead to structural failure of the car because automobile suspensions are vastly overbuilt for the loads they normally encounter. But airplanes, which must be kept as light as possible, are not superfluously stout. They are capable of failing with sudden explosiveness when flutter sets in.”

In our April 1987 article on the Dash 80 (the prototype Boeing 707), test pilot Dix Loesch recalled that the Dash 80′s tail also was prone to flutter, and that “flutter was a black science then . When the flutter guys started talking to their bosses, everybody else just sort of looked at the ceiling.”

The vibration frequency of an aircraft section in the throes of flutter is so fast that it can’t be detected by eye. It’s visible only in slow-motion videos like these: