WHILE HOLDING ON AN ARR RTE; AN FK10 FLC EXPERIENCED TURB THAT DISPLACED THE ACFT BTWN 300 AND 400 FT FROM ASSIGNED ALT. THE AUTOPLT WAS DISCONNECTED AND THE FLC RETURNED TO THE ORIGINAL CLRNC ALT.

Narrative

WHILE ON THE FNKS6 ARR TO DCA WE WERE TOLD TO HOLD AT DOCCS INTXN AT 16000 FT. WE WERE EXPERIENCING MODERATE TURB AS WE ENTERED HOLDING. AUTOPLT WAS ON WITH +/-800 FT ON THE IVSI AND WITH ALT CHANGES OF +/-100 FT. SUDDENLY WE RECEIVED A NOSE UP OUT OF TRIM WARNING ON THE MULTI FUNCTION DISPLAY UNIT; BUT THE AUTOPLT CORRECTED BY LOWERING THE NOSE. THEN SUDDENLY WE ENTERED A SEVERE DOWNDRAFT. THE AUTOPLT COULD NOT HANDLE THE TURB AND THE ACFT KEPT DSNDING. AT 15710 FT I HAD THE AUTOPLT DISCONNECTED AND WE CLBED BACK TO 16000 FT. WE TOLD CTR ABOUT THE TURB AND HE SAID THAT THERE WAS NO CONFLICT. SEVERAL ACFT IN THE HOLDING PATTERN WERE RPTING THE TURB; BUT CTR HAD NOWHERE ELSE TO SEND US; SO WE HAD TO CONTINUE TO HOLD IN THE TURB. SUPPLEMENTAL INFO FROM ACN 331004: DURING OUR FIRST TURN IN HOLDING; WHILE IN 25 DEGS OF BANK; WE ENCOUNTERED A LARGE UP- AND DOWNDRAFT AND THE ALT ALERT WENT OFF. THE AUTOPLT DID NOT KICK ITSELF OFF AND WAS TRYING TO RETURN THE ACFT TO 16000 FT. AT THAT TIME ZDC CALLED AND ASKED US TO CHK OUR ALT. LATER WE ASKED IF THERE HAD BEEN A CONFLICT. CTR SAID THERE WAS NO CONFLICT; BUT HAD NOTED US 400 FT LOW. LESSONS LEARNED: EVEN WITH THE AUTOFLT SYS ENGAGED AND CLOSE MONITORING BY THE FLC; SEVERE CONDITIONS CAN ALTER ACFT POS SURPRISINGLY FAST.