PLT DEVIATED FROM ASSIGNED ALT DUE TO LOSING CTL OF THE ACFT'S ALT IN HVY TURB.

Narrative

I WAS CRUISING IN A MOONEY TLS AT 165 KIAS AT 6000 IN IMC ON AN IFR FLT PLAN FROM GAI TO MRB V143 GSO. THERE WAS A FRONTAL BOUNDARY. ABOUT 50 MI S OF MRB; I BEGAN EXPERIENCING MODERATE TURB. I ASKED FOR AND WAS CLRED TO 8000. SHORTLY AFTER REACHING 8000 I HIT AN AREA OF INCREASING TURB -- UP 300 FT; DOWN TO 8000 FT; THEN UP 300 AND THEN VIOLENTLY DOWN 800 FT TO 7200 FT. I EXTENDED THE SPD BRAKES AND REDUCED PWR TO SLOW TOWARDS THE MANEUVERING SPD OF ABOUT 120 KIAS. THEN I CALLED CTR; RPTED 'MORE THAN MODERATE TURB;' AND ANNOUNCED I WAS UNABLE TO HOLD ALT PLUS OR MINUS 300 FT. THE CTLR RESPONDED: 'YES; YOU'LL HAVE THAT ALL THE WAY DOWN TO LYNCHBURG.' SO I ASKED FOR AND WAS CLRED TO 14000. I DONNED OXYGEN AND CLBED TO 14000; WHERE THERE WAS NO FURTHER TURB. I AM UNAWARE OF MY COLLISION HAZARD CREATED BY THE ALTDEV. I'VE GOT A STORMSCOPE; BUT NOT DOPPLER RADAR. THE LESSON I LEARNED IS TO SLOW DOWN VIA SPD BRAKES AND PWR REDUCTION WHEN ENCOUNTERING MODERATE TURB NEAR A FRONTAL ZONE. IF IT SUDDENLY GETS WORSE; FAR BETTER TO BE AT; OR AT LEAST NEAR; VA.