AN ATX AMB ACFT TOOK OFF WITH ITS STATIC DRAIN LINE PARTIALLY OPEN.

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SOON AFTER TAKING OFF FROM HNL AS I PASSED THROUGH APPROX 250 FT AGL; I NOTICED MY AIRSPD BEGAN TO DECREASE. BOTH ENGS WERE DEVELOPING FULL PWR AND AT THIS WT THE ACFT WOULD NORMALLY BE CLBING AT ABOUT 1200 FPM. I ALSO NOTICED THAT MY RATE OF CLB WAS ALSO DECREASING. AS I PUSHED THE NOSE OVER TO GAIN AIRSPD; THE AIRSPD INDICATOR DISPLAYED A CONTINUING DECREASE FROM APPROX 120 KIAS TO APPROX 80 KIAS. AS THE AIRSPD DECREASED THROUGH ABOUT 90 KIAS; I BEGAN TO REALIZE THAT THERE WAS PROBABLY A PROB WITH THE AIRSPD INDICATOR. AT THIS POINT I ADVISED HNL TWR THAT I WOULD NEED TO RETURN FOR LNDG. I THEN MADE A R HAND TURN TO ENTER THE DOWNWIND LEG OF THE TFC PATTERN TO RWY 4R IN HNL. AT THIS POINT; I ALSO NOTICED THAT MY ALTIMETER DID NOT APPEAR TO BE DISPLAYING THE PROPER ALT; AS IT WAS ONLY SHOWING APPROX 300 FT AND I ESTIMATED THE ACFT'S ALT TO BE APPROX 800-900 FT. AS I RETURNED FOR LNDG; THE AIRSPD INDICATOR SHOWED AN INCREASE TO APPROX 120 KIAS AND STAYED THERE UNTIL I LANDED AND BEGAN MY TAXI OFF OF THE RWY. AFTER CLRING THE RWY; IT TOOK ABOUT 1 MIN FOR THE AIRSPD INDICATOR TO DECREASE TO '0' KIAS. THE APPARENT CAUSE OF THE PROB WAS THE ACFT'S STATIC PRESSURE SYS DRAIN VALVE WAS PARTIALLY OPEN. IN THE PRESSURIZED CABIN THIS PRODUCED ERRONEOUS INDICATIONS TO THE STATIC SYS INSTS (AIRSPD INDICATOR; ALTIMETER; RATE OF CLB INDICATOR). THIS VALVE IS LOCATED ON THE SIDE OF THE ACFT WITHIN ABOUT 3 INCHES OF THE PLT'S L KNEE WHILE SITTING. SINCE THE VALVE IS VERY EASY TO MOVE; I BELIEVE I MAY HAVE HIT IT WITH MY KNEE AS I SAT DOWN. TO PREVENT ANY FUTURE OCCURRENCE OF THIS; I WILL ALWAYS CHK THIS VALVE AFTER I AM IN MY SEAT TO ENSURE THAT IT IS COMPLETELY SHUT. I ALSO BELIEVE IT SHOULD BE HARDER TO TURN IT TO PREVENT IT FROM BEING MOVED INADVERTENTLY. ALSO POSSIBLE CONSIDERATION MAY NEED TO BE GIVEN TO MOVING THE DRAIN VALVE TO ANOTHER LOCATION ON FUTURE ACFT.