FLC OF ATX SMT OVERSHOT CLB DEP ALT DUE TO DISTR OF TURB AND 'EQUIP LOOSE' NOISE IN COCKPIT.

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WE DEPARTED RWY 19R ON A VFR DEP PROFILE (ALT 3500 FT; HDG 100 DEGS TILL STATE RTE 146; THEN BLD VORTAC VIA 260 DEG RADIAL; CLB TO / MAINTAIN 5500 FT). TFC PERMITTING; DPC WILL MOST OFTEN CLR US TO 5500 FT UPON 'RADAR CONTACT.' DUE TO A LARGE JET HAVING JUST DEPARTED RWY 25 PRIOR TO OUR 19R TKOF; WE REQUESTED AN 'EARLY L TURN' AND WERE ASSIGNED 'HDG 150 DEGS TILL CLR OF RWYS 25...THEN CONTINUE THE CHARLIE DEP.' XING RWYS 25; WE WERE APCHING 3500 FT: WE CONTACTED DPC AND DPC SAID 'ROGER.' SHORTLY AFTER THAT; WE BEGAN TO ENCOUNTER A VERY STRONG UPDRAFT WHICH IS QUITE COMMON IN OUR HIGH DESERT AREA IN MID- AFTERNOON. THE FO THOUGHT HE HEARD (OR FELT SOMETHING) HIT THE FLOOR BELOW HIS SEAT; AND WAS DISTRACTED BY LOOKING DOWN AT THE FLOOR MOMENTARILY. AS THIS UPDRAFT WAS AFFECTING OUR ACFT DPC SAID; '(ACR)...WHAT IS YOUR ALT?' THE CAPT REPLIED; '4000 FT.' DPC STATED; 'WHY ARE YOU AT 4000 FT WITH VFR TFC AT 1 O'CLOCK; 1.5 MI...TURN R 30 DEGS.' THE CAPT REPLIED; 'TURNING R 30 DEGS AND DSNDING TO 3500 FT.' XING TFC WAS SPOTTED BY THE CREW; AND DPC ADVISED BY THE FO. SHORTLY AFTER THAT DPC SAID; 'CLRED TO BLD VORTAC; CLB TO AND MAINTAIN 5500 FT.' UPON CLRING THE CLASS BRAVO AIRSPACE DPC SAID; 'CLRING CLASS B AIRSPACE'; SQUAWK 1200; GOOD DAY.' THE CAPT REPLIED; 'WE APOLOGIZE FOR THE PROB ON DEP.' DPC STATED; 'IT WAS NO PROB.' COUNTERING THE TYPE OF UPDRAFTS WE EXPERIENCE IN THE HIGH DESERT AREA IN SMALLER ACFT; THE PF IS NOT ONLY CONCERNED WITH PHYSICALLY HANDLING THE ACFT; BUT HE DOES NOT WANT TO HAVE PAX 'BOUNCING OFF THE OVERHEAD' EITHER. CREW MOMENTARY DISTRS ARE CAUSED BY THINGS SUCH AS FIRE EXTINGUISHER HOLD DOWN LATCHES WHICH COME OPEN TOO EASILY IN FLT. ESPECIALLY WHEN THE EXTINGUISHERS ARE MOUNTED UNDER PLT/CREW SEATS. IF BOTH THE PF AND PNF CAN MAINTAIN VISUAL AND SITUATIONAL AWARENESS DURING CRITICAL PHASES OF FLT; ALT 'BUSTS' LIKE OURS WILL BE MINIMIZED.