FALSE WARNING ON LOW ALT ALERT DUE TO TERMINAL AREA WX; RAIN SHOWERS.

Narrative

DURING DSCNT ON FINAL; INBOUND TO MWC FROM ATOWN (LOM) I ENTERED STRONG RAIN SHOWERS. ALTIMETER AND VISIBILITY SUDDENLY BECAME WILDLY ERRATIC SHOWING DEVS OF 500 FT AND RETURNING TO NORMAL. CYCLING EVERY (APPROX) 5 SECONDS FOR A PERIOD OF TIME OF APPROX 1 - 1 1/2 MINS. TWR (MWC) RADIOED THAT MILWAUKEE APCH SAID I WAS BELOW ALT FOR THAT SECTION OF THE APCH; AND GAVE ME THE ALTIMETER SETTING OF 29.62 INCHES. I CONFIRMED AND RECHKED ALTIMETER. ALTIMETER WAS STEADY AT THIS PHASE AND SHOWED (INDICATED) 2700 FT MSL. A SHORT TIME LATER (MAYBE 1 MIN) TWR RADIOED AGAIN AND; AGAIN; SAID THAT APCH SAID I WAS BELOW THE ALT FOR THE SECTION OF THE FLT. I RECONFIRMED THAT I WAS AT THE CORRECT ALT FOR THAT PHASE OF THE APCH; PROCEEDED ON THE APCH AND LANDED WITHOUT ANY DIFFICULTY FROM THE MINIMUM DSCNT ALT; AFTER CALLING MWC TWR AND ANNOUNCING THAT I WAS DSNDING TO LAND VIA THE VASI; 15L; MWC. FORWARD VISIBILITY WAS ABOUT 2 MI. ON MAY/MON/93; I HAD THE REPAIR FACILITY CHK THE ALTIMETER; VSI; STATIC SYS; AFTER TELLING THEM THE PREVIOUSLY STATED CHAIN OF EVENTS. THEY FOUND A 'LARGE' QUANTITY OF WATER IN THE STATIC DRAIN BOTTLES. REMOVED THE WATER; 'BLEW-OUT' THE LINES AND TOLD ME THAT THE STRONG WIND/RAIN CAUSED THE VISIBILITY/ALTIMETER AND ENCODING ALTIMETER TO ACT ERRATICALLY. THIS WAS PROBABLY THE REASON THAT THE MODE C INDICATED RAPID OR EXCESSIVE DSCNTS; MUCH LIKE I SAW ON THE INDICATORS THEMSELVES. WHILE THE INDICATORS WERE ERRATIC FOR ONLY A SHORT TIME PERIOD; IT MAY HAVE BEEN THAT THE ENCODER WAS STILL CYCLING LONGER. IN THE FUTURE; IF I SHOULD EXPERIENCE A SIMILAR SIT; I WILL IMMEDIATELY SWITCH TO THE ALTERNATE STATIC SOURCE. I AM ARRANGING TO HAVE AN MEI GIVE ME A FEW HRS TRAINING WITH PARTIAL PANEL APCHS INCLUDED. NOT ONCE DURING THE FLT DID I EXPERIENCE A SUDDEN LOSS OF ALT SUCH AS THE INSTS SHOWED.