Composites at the Tip of the Iceberg

By Lee Gaillard

The crash of American Airlines Flight 587 is a three-layered conundrum of which advanced composite structures form a significant part – but only a part.

Layer one: a catalyst. Accidents don’t occur in a vacuum; they require a triggering event. On November 12, 2001, that may have been trailing wingtip vortices from a departing aircraft. Layer two: context – background and underlying conditions that drive the unfolding of the event. Layer three: advanced composite load-bearing structures now approaching what may be their critical fatigue plateau in commercial aircraft. And composites are a relatively immature technology about which we are still learning.

Layer One: Catalyst

On that Monday morning, the weather was clear, with no reports of windshear or clear air turbulence. Japan Air Lines Flight 47, a Boeing 747-400, had taken off shortly before AA Flt. 587, an Airbus A300-600R with aircraft tail number N14053. Sixty-six seconds into its own flight climbing out of New York’s JFK International Airport, AA587 felt a bump as it, presumably, passed through JAL 47’s first dissipating wingtip vortex. A "rattling noise" was heard at this point on the Cockpit Voice Recorder (CVR); the captain commented, "wake encounter," with a relatively mild 0.1g registered on the Flight Data Recorder (FDR). Slightly more than 14 seconds later came a second bump and a second rattling noise. Radar showed the aircraft to be climbing at roughly 260 knots (about 300 mph).

Then all hell broke loose.

Over the next seven seconds, the plane’s rudder apparently slammed to the right, right again, then left, jarring the aircraft with unusual lateral stresses of 0.3 G, 0.4 G, and 0.3 G. FDR then indicates five or more rudder deflections between 5 and 11 degrees – if the FDR filtering system (apparently operating despite an earlier NTSB recommendation against their installation) has not averaged the range of these figures. Included was a final rudder reversal. With the aircraft entering a right sideslip as it yawed left at 10 deg./sec., the rudder kicked right. Under those conditions, the plane swung to the right, exerting an unusually high force of 0.8g against the left side. But this sudden leftward rudder pressure against the fin’s rear hinge line may have set up an aeroelastic divergence, causing the fin’s leading edge to rotate slightly to the right under the highest possible load conditions, turning the full area of the fin into a massive control surface that suddenly attempted to reverse the just-induced right swing but, overstressed, then rotated to starboard around its own mid-chord axis running from center attachment lugs to tip, cleanly shearing off both pairs of front and rear composite attachment lugs. As it twisted farther right, tension on the left center attachment fitting caused fin skin to fail above the lug attachment, leaving a small swatch of skin fibers still clinging to its rear; the right center fitting would have undergone compression first, then torsion and tension as the fin skin tore away from its massive composite attachment lug and the fin fluttered into Jamaica Bay.

More at:

http://www.aviationtoday.com/reports/iceberg.htm