According to the Rule Compendium, "A winged creature can be tripped, and if it is, it falls as if it didn’t maintain its minimum forward speed." (P. 145) But if the target has good or perfect maneuverability it has no minimum forward speed. I'm inclined to interpret it as falling anyway, since the wording doesn't seem to imply you actually need to fall below the MFS, just you fall "as if" you had. Has this ever been clarified?
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