A BEECH KING AIR 100 CAPT GOT A MASTER CAUTION LIGHT AND NOTED THE CABIN DOOR ANNUNCIATOR SHORTLY AFTER TKOF. HE TURNED BACK TOWARD DEP ARPT. HE THEN NOTED A MALFUNCTIONING AIRSPD INDICATOR; DECLARED AN EMER; AND RETURNED TO LAND AT DEP ARPT.

Narrative

THE COPLT WAS THE PF AND I WAS THE PNF. AFTER TKOF AND DURING OUR INITIAL TURN OUTBOUND; I NOTED A LOW AIRSPD. I ASKED HIM TO MAINTAIN 140 KTS. HE SAID 'OK.' AT THAT TIME WE RECEIVED THE MASTER WARNING AND NOTED THE CABIN DOOR ANNUNCIATOR HAD ILLUMINATED. WE TOLD ATC OF THE PROB AND THAT WE WERE RETURNING. AT THIS TIME I TOOK CTL OF THE AIRPLANE AND WAS NOW THE PF. WITHIN SECONDS; I WAS HAVING TROUBLE MAINTAINING ADEQUATE AIRSPD. AT 2000 FT AND A CLB PWR SETTING I COULD BARELY HOLD 80 KTS. I ASKED THE COPLT WHERE THE ARPT WAS. HE REPLIED '4 O'CLOCK POS.' JUST THEN; ATC GAVE US A VECTOR AND ALT TO FOLLOW A 'SLOW MOVING CESSNA.' IT HAD BEEN LESS THAN 1 MIN SINCE I TOOK THE AIRPLANE. I WAS TRYING TO FIGURE OUT THE AIRSPD PROB THINKING MAYBE THE DOOR HAD OPENED AND WAS CAUSING EXCESSIVE DRAG; BUT THAT DIDN'T FIT BECAUSE I BELIEVE WE WOULD HAVE FELT IT IN THE CABIN. I DECIDED TO DECLARE AN EMER. AT THIS POINT; I DID NOT WANT TO GET FURTHER FROM THE ARPT. ATC CLRED THE CESSNA OUT OF THE WAY AND I TURNED BASE TO FINAL. AS I ROLLED ONTO FINAL; I XCHKED THE COPLT'S AIRSPD. HIS AIRSPD SEEMED TO COINCIDE MORE CLOSELY WITH THE GIVEN PWR SETTINGS. I ASKED HIM TO PLUG IN ZZZ INTO THE GPS AND NOTED THE GND SPD WAS WITHIN 10 KTS OF HIS AIRSPD INDICATORS. AT ABOUT 1 MI FINAL; I WAS CONFIDENT THAT THE PROB WAS MY AIRSPD INDICATOR AND THUS WAS ABLE TO IGNORE THE INDICATIONS. THE LNDG WAS UNEVENTFUL. POSTFLT; THE PLT'S STATIC DRAIN VALVE WAS FOUND IN THE OPEN POS. THE COVER HAD FALLEN OFF AND IT IS POSSIBLE THAT IT WAS INADVERTENTLY BUMPED. IT IS LOCATED ON THE LOWER R SIDE OF THE COCKPIT. HOWEVER; IT IS ALSO POSSIBLE THAT I LEFT IT OPEN. IT IS NORMAL FOR ME TO REMOVE THE COVER AND DRAIN THE SYS. IN HINDSIGHT; I RECALL REACHING FOR THE COVER AND NOTING IT WAS OFF. I DID NOT EXERT THE EFFORT TO PICK UP THE COVER AS IT WAS JUST OUT OF REACH. SO I PROCEEDED AS NORMAL COMPLETING THE BEFORE STARTING ENGS CHK. USUALLY I WOULD PLACE THE COVER ON THE CONSOLE WHERE IT WOULD BE IN MY WAY AND THIS WAS MY PLACE MARKER THAT WOULD REMIND ME TO CLOSE THE DRAIN SYS AND REPLACE THE COVER. I DO NOT REMEMBER PUTTING THE COVER ON THE CONSOLE FOR THE REASON STATED ABOVE AND THIS IS WHY IT IS POSSIBLE THAT THE AIRSPD ISSUE MAY HAVE BEEN HUMAN ERROR. THE ACT OF DRAINING THE STATIC SYS DOES NOT EXIST IN THE CHKLIST. REPAIRING THE COVER AND MAKING A NOTE IN THE CHKLIST WOULD BE PRUDENT MEASURES TO ENSURE THAT THIS DOES NOT HAPPEN AGAIN. AS FAR AS THE CABIN DOOR ANNUNCIATOR IS CONCERNED MAINT COULD NOT GET IT TO FAIL ON THE GND. HOWEVER; IN PREPARING FOR A TEST FLT ON THE SAME DAY IT FAILED ON THE GND WITH MAINT STANDING OUTSIDE. WE COULD NOT GET IT TO FAIL AGAIN AT THAT TIME. WE CALLED IT A DAY AND THEY ARE LOOKING INTO THE PROB AS I WRITE THIS RPT.