PLT ON TEST FLT DSNDS BELOW HIS ASSIGNED ALT. CITES PLT DISTR AND PLANNING FOR LOW LEVEL TESTS TO COME. HE AND CTLR NOTE THE ALTDEV AT THE SAME TIME.

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FOLLOWING A ROUTINE PERFORMANCE TEST FLT AT FL310; I WAS CLRED TO DSND TO FL240 IN PREPARATION FOR LEAVING CTLED AIRSPACE. I INADVERTENTLY DSNDED BELOW FL240 TO FL230; THEN IMMEDIATELY RETURNED TO FL240. CTLR ASKED FOR MY ASSIGNED ALT AND I RPTED FL240; AS I WAS RETURNING/CORRECTING BACK TO FL240. HE RPTED SEEING ALT AS FL230. A CONTRIBUTING FACTOR WAS MY ATTN DIVERTED TO A SERIES OF LOW ALT TESTS TO BE PERFORMED AT 10000 FT. I DISCOVERED MY ERROR THROUGH NORMAL SCAN ABOUT THE TIME THE CTLR CALLED ME. MY CORRECTIVE ACTION WAS IMMEDIATE UPON DISCOVERING MY ERROR. OBVIOUSLY; MY ATTN WAS DIVERTED FROM FLYING THE AIRPLANE AS I WAS ARRANGING CARDS AND THOUGHTS FOR THE LOW ALT TESTS. IN RETROSPECT; I SHOULD HAVE CONCENTRATED ALL ATTN ON FLYING THE AIRPLANE IN THE CTLED ENVIRONMENT; REVIEWING THE UPCOMING TESTS ONLY AFTER DSNDING BELOW CTLED AIRSPACE AND CANCELING MY INST FLT PLAN. THIS INCIDENT UNDERSCORES THE NEED FOR CONTINUAL VIGILANCE; PARTICULARLY DURING FLT TESTS WITH ONLY 1 PLT IN THE ACFT. WITHOUT A COPLT TO XCHK; THE PLT FLYING THE AIRPLANE CANNOT AFFORD TO RELAX OR ALLOW HIS ATTN TO BE DIVERTED FROM FLYING THE AIRPLANE FIRST.