FAILED TO COMPLY WITH PUBLISHED PROC; LEFT ALT EARLY.

Narrative

THE FLT WAS AN INSTRUMENT COMPETENCY CHK WITH A CFII IN THE RIGHT SEAT. FLT WAS RADAR VECTORED BY WASHINGTON CENTER TO INTERCEPT THE RWY 27 LOC A FEW MILES W OF THE ARPT; FLY THE LOC BACKCOURSE TO HAIGS INTXN; AND THEN COMPLETE THE APCH. APCH CLRNC INCLUDED; 'MAINTAIN 4000 UNTIL ESTABLISHED.' I XED HAIGS OUTBND AT 4000' AND INITIATED A TEARDROP ENTRY TO THE HOLDING PATTERN FOR COURSE REVERSAL. AT THE TIME OF INTERCEPTING THE LOC INBND ABOUT 3 MI E OF HAIGS; I MADE A SERIOUS ERROR AND COMMENCED DSCNT TO 2900'. WASHINGTON CENTER CAUGHT THE ERROR VERY QUICKLY AND TELEPHONED THE TWR CTLR WHO WAS NOW HANDLING THE FLT. THE TWR CTLR RADIOED INSTRUCTIONS TO CLB BACK TO 4000' UNTIL W OF HAIGS. I COMPLIED IMMEDIATELY AND REALIZED MY ERROR AT THAT POINT. LOWEST ALT REACHED WHILE STILL E OF HAIGS WAS ABOUT 3200'. THE REMAINDER OF THE APCH AND LNDG WERE NORMAL. THIS ERROR WAS TOTALLY MINE. I INTERPRETED THE PROFILE VIEW LIKE A PROC TURN COURSE REVERSAL WITH STEP DOWN TO LOWER ALT ONCE ESTABLISHED ON THE LOC INBND. I IGNORED THE PROFILE VIEW'S CLR DEPICTION OF HAIGS AS A SPECIFIC STEP DOWN FIX. THE APCH CHART IS CLR; AND MY ERROR IS EQUALLY CLR. WASHINGTON CENTER'S ALERTNESS MAY HAVE PREVENTED AN ACCIDENT; AND THAT CTLR DID A FIRST CLASS JOB. I BELIEVE THIS MISTAKE TO BE THE SAME ONE WHICH RESULTED IN THE ACR CRASH ON APCH TO IAD SOME YRS AGO. I UNDERSTAND THAT THE PHRASE 'MAINTAIN 4000 UNTIL ESTABLISHED' WAS INTENDED TO PREVENT EXACTLY THIS MISTAKE. NONETHELESS; I STILL MADE THE MISTAKE. VIGILANCE AND CONCENTRATION WERE INADEQUATE! IT SHOULD BE NOTED THAT THE CFII QUESTIONED MY PREMATURE DSCNT AS SOON AS I STARTED IT. BELIEVING FIRMLY (BUT WRONGLY) THAT THE DSCNT WAS PROPER; I TALKED HIM OUT OF HIS QUESTION. HE LEARNED SOMETHING TOO. SUPPLEMENTAL INFO FROM ACN 82201: THE PLT RECEIVING THE INSTRUMENT COMPETENCY CHK IS ONE OF THE BEST IN TERMS OF ATTITUDE INSTRUMENT FLYING AND KNOWLEDGE OF IFR PROC THAT I'VE FLOWN WITH. THUS; I ALLOWED THIS ASSESSMENT OF HIS SKILLS TO UNDULY INFLUENCE MY ROLE AS PIC DURING THIS PROC.