ACR WDB IN THE HOLD AT WILLOW WITH HVY CUMULUS IN THE AREA; DSNDING TO 8000 FT; CAPT; PF; DISCONNECTS AUTOPLT ACCOUNT IN STRONG DOWNDRAFT; UNABLE TO STOP DSCNT. DSNDED TO 7800 FT BEFORE HE COULD REGAIN ALT. ADVISED APCH CTL TO CHANGE THE HOLD AT WILLOW.

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DURING DSCNT INTO GATWICK GIVEN HOLD AT WILLOW AS PUBLISHED ON ARR AT 9000 FT (FL90) HAD TO DEVIATE AROUND SMALL BUILDUPS ON ARR. REACHING WILLOW NOTICED BUILDUPS IN AREA BUILDING WITH HVY RAIN IN AREA. ANOTHER ACFT POINTED THIS OUT TO APCH WHO REPLIED IT WOULD ONLY BE A FEW MINS. WE WERE IMC WITH OCCASIONAL BREAKS OUT OF CLOUDS. ONLY PAINTED RAIN ON RADAR; NO DEFINED CELLS. AFTER ONE TURN IN PATTERN WITH ONLY RAIN AND OCCASIONAL TURB WE BRIEFLY BROKE OUT OF CLOUDS AND SAW A DARK CELL TO OUR L. I PICKED UP MIKE TO TELL APCH WE WOULD DEVIATE WHEN WE WENT IMC AGAIN. ABOUT THIS SAME TIME WE HIT SOME MORE TURB AND I HEARD AUTOPLT DISCONNECT. CAPT STATED IT WAS NOT HOLDING ALT. HE THEN TURNED R (N) AND TOLD ME TO INSTRUCT APCH OF SAME. WE WERE IN A DSCNT WHICH I CALLED OUT TO CAPT. HE HAD ALREADY APPLIED PWR AND TRIED TO MAINTAIN ALT. GOING THROUGH FL80 I CALLED ALT OUT TO CAPT. DSCNT STOPPED AT ABOUT FL78. CAPT STATED WE HIT A DOWNDRAFT AND THAT HE COULD NOT HOLD ALT. APCH VECTORED TO LOC WITHOUT FURTHER INCIDENT. I TOLD APCH HOLDING PATTERN NEEDED TO BE MOVED DUE TO WX.