PLT FAILS TO MAINTAIN ASSIGNED ALT WHEN ENCOUNTERING DOWNDRAFT.

Narrative

WHILE BEING VECTORED FOR AN APCH AT OAJ THE ACFT LOST APPROX 400-500 FT. I WAS ENTERING AN AREA OF HVY RAIN THAT WAS PART OF A TROPICAL AIR MASS IN THE WESTERN PART OF THE STATE. WHILE BEING VECTORED TOWARD THE LOM BY ILM DEP CTL; I NOTICED THE ACFT BEGINNING TO LOSE ALT. I BEGAN RAISING THE NOSE OF THE ACFT TO COMPENSATE. NOTICING THAT I WAS NOT GETTING THE RESPONSE THAT I REQUIRED; I INCREASED THE PWR. THE COMBINATION OF PITCH AND PWR WAS ENOUGH TO STOP THE DSCNT AND BEGIN REGAINING THE LOST ALT. DURING THE ENCOUNTER; ILM DEP CTL REMINDED ME OF THE 1700 FT ASSIGNED ALT. I RESPONDED BY STATING THAT I WAS IN SEVERE TURB AND REQUIRED A VECTOR OUT OF THE STORM CELL. I WAS GIVEN THE VECTOR THAT I REQUESTED AND WAS IN VMC CONDITIONS IN APPROX 2-3 MI. THE VECTOR PLACED ME IN AN AREA OF VMC BTWN THE LOM AND THE ARPT. THIS POS MADE IT POSSIBLE TO MAKE A VISUAL APCH TO THE RWY. SEVERAL FACTORS I BELIEVE CAME INTO PLAY ARE THE LACK OF RADAR IN THE ACFT AND THE POS OF THE CELL OVER THE LOM. I BELIEVE THAT I WAS TOLD OF THE WX IN QUESTION BY ILM CTLRS AND THAT ITS INTENSITY WAS THAT OF LEVEL 1 AND/OR 2. IN RETROSPECT; MY DESIRE TO COMPLETE THE APCH SHOULD NOT HAVE PLAYED A FACTOR. THE DECISION SHOULD HAVE BEEN MADE TO HOLD UNTIL THE APCH CORRIDOR WAS CLR. MY DECISION TO CONTINUE WAS BASED UPON THE INFO GIVEN TO ME BY THE CTLR. AREAS OF LEVEL 1 AND 2 WX ARE ROUTINELY PENETRATED UNEVENTFULLY. IT IS MY OPINION THAT I ENCOUNTERED A SUBSTANTIAL WINDSHEAR. I WOULD ALSO LIKE TO NOTE THAT I HAVE BEEN TRAINED BY MY COMPANY IN THE RECOGNITION AND RECOVERY FROM WINDSHEAR ENCOUNTERS.