OWNER OF LJ35 FLIES THROUGH CLRED ALTITUDE WHILE FLYING IN THE LEFT SEAT UNDER THE TUTELAGE OF THE LISTED CAPTAIN IN THE RIGHT SEAT.

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I WAS THE LISTED CAPT ON THIS FLT AND MR A WAS TO BE MY FO. MR A IS THE OWNER OF THE AIRPLANE AND HE DECIDED THAT HE WANTED TO FLY IN THE L SEAT. SO WE LEFT THAT DAY WITH MR A IN THE L SEAT AND I TOOK THE R SEAT TO BE THE PNF. ON DEP; WE WERE ASSIGNED FL210 AND A L TURN TO HDG 090 DEGS. MR A WAS FLYING WITH THE AUTOPLT OFF. DURING OUR ASCENT WE ENTERED IMC AT APPROX FL195. MR A BECAME DISORIENTED AND STOPPED HIS TURN TO 090 DEGS AND ROLLED WINGS LEVEL. I REMINDED HIM WE WERE INSTRUCTED HDG 090 DEGS FOR TFC AND HE THEN RESUMED THE TURN. AT FL200 I MADE THE PNF CALL OUT OF 'FL200 FOR FL210' AND ADVISED HIM TO REDUCE HIS CLB RATE WHICH WAS STILL 3500 FPM. MR A REPLIED; 'I GOT IT.' AT FL205 I TOLD MR A HE WAS GOING TO BUST HIS ALT ASSIGNMENT. HE REPLIED; 'OH MAYBE BY A LITTLE.' KNOWING THAT MR A WAS STILL DISORIENTED; I TOOK THE CTLS BUT MR A WOULD NOT LET GO. (I SHOULD REMIND YOU THAT MR A IS THE ACFT OWNER AND MY BOSS.) HE FINALLY STOPPED HIS CLB AT FL214 (OVER THE ASSIGNED ALT BY 400 FT) AND WE PROMPTLY RETURNED TO OUR ASSIGNED ALT. HE THEN TURNED ON THE AUTOPLT AND WE CONTINUED THE FLT.