A B737-300 ENCOUNTERS SEVERE CLR AIR TURB IN ITS CLB TO FL270. THE AUTOPLT REVERTS TO CTL WHEEL STEERING AND THE ACFT OVERSHOT ASSIGNED ALT.

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ON CLB OUT OF BWI; ON THE SWANN TWO DEP; WE ENCOUNTERED MODERATE TO SEVERE TURB AT APPROX 26500-27000 FT. OUR LEVELOFF ALT WAS FL270. THE AUTOPLT WAS ENGAGED. INITIALLY THE AUTOPLT ATTEMPTED TO LEVEL AT FL270 BUT THE TURB THRUST US INTO A VERT CLB AND ROLLED THE ACFT FROM L TO R AND BACK. THE AUTOPLT DISCONNECTED AND REVERTED TO CTL WHEEL STEERING. OUR BANK ANGLE REACHED 30-40 DEGS EACH SIDE OF LEVEL. THE TURB LASTED APPROX 15 SECONDS. WE IMMEDIATELY NOTIFIED ATC OF THE TURB AND DSNDED TO FL270. NO INJURIES WERE SUSTAINED IN THE CABIN. THE TURB WASN'T VIOLENT BUT JUST ROLLED THE ACFT FROM SIDE TO SIDE EVEN THOUGH WE HAD FULL AILERON DEFLECTION IN THE OPPOSITE DIRECTION. NO CONFLICT WAS NOTED WITH ATC TFC AND THE FLT PROCEEDED TO PVD WITHOUT FURTHER PROBS. NORMAL APCH AND LNDG AT PVD. THERE WAS NO INDICATION OF WX ON OUR RADAR AND NO RPTS OF TURB FROM ATC. SUPPLEMENTAL INFO FROM ACN 391148: THE AUTOPLT WAS UNABLE TO MAINTAIN HEADING AND ALT AND REVERTED TO CTL WHEEL STEERING AT ABOUT 26500 FT. THE SUDDEN UPSURGE WAS TOO GREAT FOR THE AUTOPLT TO CAPTURE FL270. WE REGAINED CTL OF ACFT AT 27900 FT AND DSNDED BACK TO FL270.