Loss of Tailfin the Focus of Crash Probe

Air Safety Week, Nov 19, 2001

Source: FAA/NASA Aviation Safety Reporting System (ASRS)

Violent Vibes

Uncontrolled flutter can shake an aircraft apart in seconds

Martin Aubury

"Flutter is a violent vibration caused by the interaction of structural flexibility, mass and aerodynamic forces....We are talking here of wing flexure interacting with aileron rotation, horizontal tail with elevator or fin with rudder....The term 'flutter' sounds innocuous, but it is not. It is violently destructive. Avoidance depends on meticulous design, maintenance and strict compliance with speed limits."

Mr. Aubury lectures on aircraft design and flutter at the Australian Defence Force Academy

Aubury, e-mail ozaubury@ozemail.com.au

Thriving Theories
Scenario: Bomb near center wing box or in wheel well, concealed before boarding
Comments: No evidence. FBI conducting parallel investigation.

Scenario: Suicide bomber in passenger cabin or bomb in checked bag
Comments: No evidence. Ditto FBI probe.

Scenario: Hijacking with intent to crash into Empire State Building
Comments: Not enough time, difficult to burst into cockpit and seize control
during climb.

Scenario: Command detonated small incendiary bomb in tailcone
Comments: No evidence. Investigators would see telltale charring and
distinctive
'petaling' of the metal immediately. They have not said so.

Scenario: Uncontained engine failure (one or both engines)
Comments: No evidence. Airworthiness directive history of the CF6-80C2A5
engines
on the accident aircraft shows "nothing really big" according to Ed Hurst of
Denver, Colorado-based Aviation DataSource Inc.

Scenario: Bird strike(s)
Comments: No evidence of "feathered FOD" (foreign object damage)

Scenario: Inadvertent thrust reverser deployment (full or partial)
Comments: No evidence

Scenario: Structural failure of engine pylons
Comments: No evidence. A June 2000 Airworthiness directive (AD 2000-10-18)
required inspection and repair of cracks to prevent "possible separation of the

engine from the airplane." Engines found still attached to their pylons.

Scenario: Sabotage of tail structure
Comments: No evidence. Accident airplane underwent maintenance "A" check two
days before crash. Loosened screws/bolts or drilled-out rivets would be
immediately apparent to investigators.

Scenario: Wake vortex encounter
Comments: Crew reported encountering wake vortex from JAL B747 ahead of them.
Possible flutter/fatigue failure of tailfin triggered by wake encounter.
Opinions differ significantly on this issue.

Scenario: Delamination in base of fin
Comments: Post-crash damage not attributed to any pre-existing condition.

Scenario: Crew response to wake turbulence encounter
Comments: Possible excessive rudder deflection associated with possible
malfunction of rudder travel limiting devices. Too early to tell.

>TK American Airlines [AMR]: Delta Air Lines [DAL]: United Airlines [UAL]: Continental Airlines [CAL]: