UCSD Jacobs School of Engineering University of California San Diego
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LESSONS LEARNED: Flight 587 Tragedy

SINCE THE CRASH OF AMERICAN AIRLINES FLIGHT 587 ON NOVEMBER 12, THERE HAS BEEN SPECULATION AS TO ITS CAUSE. PROFESSORS JOHN KOSMATKA AND MIROSLAV KRSTIC OFFER THEIR OBSERVATIONS BASED ON EXPERIENCE AND NEWS REPORTS.

Impact of Turbulence

An expert in controls, Mechanical & Aerospace Engineering Professor Miroslav Krstic explains, "The American Airlines jet took off too soon after a Japanese airliner and became caught in its turbulent wake. This wake did not have enough time to adequately dissipate and could have acted like a strong vortex, ripping-off the tail and rolling the jet." The structure of this wake resembles a horizontal tornado that is generated by the wing of the preceding aircraft as it pierces through the air.

While in the case of flight 587, turbulence led to tragic results, Krstic says that the turbulent wake can actually be used to improve the efficiency of aircraft. Krstic has recently developed a dynamic, autopilot control system to allow airplanes to use turbulent wakes to conserve fuel by as much as 50 percent. Krstic's computer models show that by flying in a delta or "V" pattern like birds, planes can get a lift from the lead planes' wake. Because the wakes dissipate gradually, the planes can be safely spaced apart, coming together in formation and breaking apart as needed. "The extremum seeking control system reads the preexisting sensors and automatically navigates the plane to the optimal formation necessary to find the sweet spot," explains Krstic.

Structural Integrity of Composites

According to Structural Engineering Professor John Kosmatka, an expert on new composites for aerospace and military structures, "The vertical stabilizer failed at the root where it connects to the fuselage. This design has been proven to be reliable, and the turbulence alone was not enough to cause this; there had to be preexisting structural damage."

Stabilizers are constructed of thin layers of composite materials that sandwich a core of light, corrugated- like material. This allows the piece to be very light, and extremely stiff and resilient.

"I expect that the inner core may have been damaged from a prior accident or by rain water, cleaning fluids, or a de-icer," theorizes Kosmatka. Once the vertical stabilizer was damaged, its stiffness was reduced. Aerodynamic loads then probably caused flutter or vibrations in the craft, which resulted in failure. This, combined with the turbulent conditions, would have made operating the plane extremely difficult."

Kosmatka says that although composites have been proven to be safe, this incident points to the need for better maintenance procedures. "The composite sections of aircraft have very high inspection criteria and require precise ultrasonic testing to detect damage that is not visible to the naked eye."

Kosmatka has been analyzing the General Atomics unmanned Predator aircraft, which is made entirely of composite materials. By performing experimental vibration tests and computational flutter analyses, he studies the effects of aging on an airplane's structural integrity.