Accident Summary - continued

The investigation centered on vibratory characteristics of the fan blades and fan case. There was a debate on the role of the inlet on the blade/case relationship that asked: "Did the inlet failure cause tip rub?" or "Did tip rub cause the inlet failure?"

The initial triggering event was never completely agreed upon by all parties. However, the failure mechanism was sufficiently understood so that corrective actions to prevent additional occurrences could be defined.

The identified failure mechanism involved a fan case "6-wave" mode traveling at 1/2 fan speed in the direction of the fan rotation. Click on the graphic to the right to view an animation of this effect.

The fan blade assembly also exhibited a fore-and-aft rocking vibratory mode, which has a traveling wave shape at 1/2 fan speed in an opposite direction to the fan.

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