AN EMB135 EXPERIENCED A 15 SECOND LONGITUDINAL FLUTTER AT FL190 AND 315 KTS. WITH THE AUTOPLT DISCONNECTED AND ACFT SLOWED; THE FLUTTER CEASED.

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I EXPERIENCED A SEVERE FLUTTER IN THE PITCH AXIS WHILE IN CRUISE FLT AT FL190 AND 315 KIAS. WE WERE IMC; BUT THERE WAS NO ICING WHATSOEVER. TURB WAS OCCASIONALLY MODERATE IN MAGNITUDE; AND AT FIRST I THOUGHT THE FLUTTER WAS JUST MORE TURB; BUT IT INCREASED GRADUALLY IN MAGNITUDE OVER A PERIOD OF A FEW SECONDS; AND WAS OF A VERY RHYTHMIC QUALITY THAT IT COULD NOT HAVE BEEN THE RESULT OF TURB. I REDUCED SPD FROM 315 KTS TO 290 KTS IMMEDIATELY; AND AS I DID SO; THE AUTOPLT DISCONNECTED BY ITSELF; WITH 'AUTOPLT FAILURE EICAS' MESSAGE. THE ENTIRE EPISODE LASTED 10-15 SECONDS. AFTER ABOUT 1 MIN; I RE-ENGAGED THE AUTOPLT; AND CONTINUED THE FLT NORMALLY AT A SLIGHTLY REDUCED SPD. THE FLUTTER NEVER RECURRED; AND THERE WERE NO OTHER PROBS ENCOUNTERED. I DID NOT DECLARE AN EMER; AND THE FLT PROCEEDED WITHOUT INCIDENT TO BOS. THERE WAS NO INDICATION OF ANY DAMAGE TO THE ACFT; INCLUDING THE FLT CTLS. THE FLUTTER WAS NOTICEABLE TO THE FLT ATTENDANT. I RPTED THE EVENT TO MAINT; AND WROTE IT UP IN THE MAINT LOG. THE ACFT WAS REMOVED FROM SVC FOR 3 DAYS. I WAS INTERVIEWED ABOUT THE INCIDENT BY THE BOS MAINT SUPVR; AND BY DIRECTOR OF FLT OPS (LINE). I OPERATED THE ACFT AT 315 KTS BECAUSE 1) WE WERE ASSIGNED '300 KTS OR BETTER' BY ATC; AND TOLD WE WERE 'LEADING THE PACK' INTO BOS. 2) OUR AOM SAYS THERE IS 'LITTLE USE' TO FLYING AT A SLOWER SPD IN OTHER THAN SEVERE TURB. AT NO TIME DID THE TURB APPROACH 'SEVERE' MAGNITUDE. DETECTION: THE FLUTTER WAS OBVIOUS: RHYTHMIC VIBRATION IN THE PITCH AXIS OF ABOUT 4 HERTZ; ROUGHLY. REACTION: REDUCED SPD; CHKED CTL EFFECTIVENESS; RE-ENGAGED AUTOPLT; HAND FLEW THE APCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PLT'S ACR MAINT COULD FIND NO DISCREPANCY CAUSING THIS PROB. RPTR STATED THAT WHILE DISCUSSING HIS EXPERIENCE; OTHER PLTS HAVE RPTED THE BRIEF HIGH SPD FLUTTER WITH THE AUTOPLT ENGAGED. THE EVENT WAS BRIEF AND THE CREW NOT FRIGHTENED; BUT CONCERNED.