A BA4100 TURBOPROP FLC LOSES CTL OF THEIR ACFT WHEN IN PROX OF A LARGE TSTM NEAR RICHMOND; VA. THE ACFT ASCENDS 500 FT BEFORE IT IS BROUGHT UNDER CTL.

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I WAS THE CAPT ON FLT FROM JFK TO RIC. THE FLT WAS UNEVENTFUL UNTIL OUR APCH INTO RIC. A SQUALL LINE ARRIVED AT THE RIC ARPT SHORTLY BEFORE WE ARRIVED TO THE RIC AREA. WE WERE BEING VECTORED FOR AN ILS RWY 34. (THE FO WAS THE PF.) FROM OUR POS; IT DID NOT LOOK LIKE WE WERE GOING TO BE ABLE TO SHOOT THE ILS TO RWY 34 DUE TO TSTMS BTWN US AND THE ARPT. RIC APCH ADVISED US OF LEVEL 4 ACTIVITY ON THE FINAL APCH COURSE FOR RWY 34. I CONCURRED THAT WE SHOWED LEVEL 4 ACTIVITY IN FRONT OF US AND WE ELECTED TO BREAK OFF THE APCH AND FLY SBOUND UNTIL WE COULD FIND A HOLE IN THE LINE AND HEAD WBOUND IN ORDER TO GET INTO RIC BEHIND THE SQUALL LINE. AS WE CONTINUED SBOUND; MY FO WAS HAND FLYING THE AIRPLANE. WE WERE NAVING AROUND ANOTHER LARGE CELL WHEN WE ENCOUNTERED A SEVERE UPDRAFT AND WERE UNABLE TO MAINTAIN OUR ASSIGNED ALT OF 2000 FT. AT APPROX 2500 FT; WE WERE ABLE TO REGAIN CTL OF THE ACFT. DIRECTLY ABOVE US WAS A JET CLBING OUT OF RIC. I HAD VISUAL CONTACT WITH HIM; BUT I DON'T KNOW HIS ALT. WE DID NOT GET A TCASII TA OR RA. THE APCH CTLR ADVISED US OF THE AIRPLANE AND I TOLD HIM I HAD IT IN SIGHT; BUT WE COULD NOT MAINTAIN 2000 FT. HE SAID 'ROGER' AND THAT WAS THE END OF ANY POTENTIAL CONFLICT. WE EVENTUALLY WERE ABLE TO PICK OUR WAY THROUGH THE LINE AND GET INTO RIC.