Narrative:

During descent on final, inbound to mwc from atown (LOM) I entered strong rain showers. Altimeter and visibility suddenly became wildly erratic showing deviations of 500 ft and returning to normal. Cycling every (approximately) 5 seconds for a period of time of approximately 1 - 1 1/2 mins. Tower (mwc) radioed that milwaukee approach said I was below altitude for that section of the approach, 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) tower radioed again and, again, said that approach said I was below the altitude for the section of the flight. I reconfirmed that I was at the correct altitude for that phase of the approach, proceeded on the approach and landed without any difficulty from the minimum descent altitude, after calling mwc tower and announcing that I was descending to land via the VASI, 15L, mwc. Forward visibility was about 2 mi. On may/mon/93, I had the repair facility check the altimeter, vsi, static system, 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 situation, I will immediately switch to the alternate static source. I am arranging to have an mei give me a few hours training with partial panel approachs included. Not once during the flight did I experience a sudden loss of altitude such as the instruments showed.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.