Narrative:

During climb en route to dca from opf, both the pilot's and copilot's vertical speeds were observed to vary several hundred ft. Approximately 20 mins after level off the autoplt began to follow excursions of the pilot's altimeter and vertical speeds during light turbulence. As the turbulence increased to moderate the autoplt was disconnected. Shortly thereafter both the pilot's and copilot's altimeters began to vary 500-800' with 1500-2000 FPM variance of the vertical speeds. Frequently these variances were abrupt and in opp directions. ATC was immediately advised that we were having altitude indication problems on both system. We were given a vector east of AR7 to avoid all traffic. The aircraft was flown manually at a fixed attitude and as the turbulence decreased it was determined that the pilot's indication was more stable than the copilot's. A block altitude was requested and FL370 through FL390 was assigned. The aircraft was flown at 38000' and was stable within 100' as long as no pitch changes. Approaching ric a descent was requested to evaluate performance at lower altitudes. Block 290-310 was assigned and flight at FL300 proved stable within 100'. Descent and landing were made with minimum attitude changes and altimeter indications remained stable. The aircraft had been on the ground 2 days during heavy rains with the pitot covers installed. Suspect that the design of these covers can increase the possibility of water in the static lines, as was found. Alternate static source was not selected because on a previous flight with minor vertical speed variances, selection of the pilot's alternate static source caused altitude readings significantly (1000') off the flight manual data. We were check this in flight with the known good systems.

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Original NASA ASRS Text

Title: WATER IN ACFT STATIC SYSTEM CAUSED ERRATIC INDICATIONS FROM INSTRUMENTS AND SYSTEMS USING STATIC INPUTS.

Narrative: DURING CLB ENRTE TO DCA FROM OPF, BOTH THE PLT'S AND COPLT'S VERT SPDS WERE OBSERVED TO VARY SEVERAL HUNDRED FT. APPROX 20 MINS AFTER LEVEL OFF THE AUTOPLT BEGAN TO FOLLOW EXCURSIONS OF THE PLT'S ALTIMETER AND VERT SPDS DURING LIGHT TURB. AS THE TURB INCREASED TO MODERATE THE AUTOPLT WAS DISCONNECTED. SHORTLY THEREAFTER BOTH THE PLT'S AND COPLT'S ALTIMETERS BEGAN TO VARY 500-800' WITH 1500-2000 FPM VARIANCE OF THE VERT SPDS. FREQUENTLY THESE VARIANCES WERE ABRUPT AND IN OPP DIRECTIONS. ATC WAS IMMEDIATELY ADVISED THAT WE WERE HAVING ALT INDICATION PROBS ON BOTH SYS. WE WERE GIVEN A VECTOR E OF AR7 TO AVOID ALL TFC. THE ACFT WAS FLOWN MANUALLY AT A FIXED ATTITUDE AND AS THE TURB DECREASED IT WAS DETERMINED THAT THE PLT'S INDICATION WAS MORE STABLE THAN THE COPLT'S. A BLOCK ALT WAS REQUESTED AND FL370 THROUGH FL390 WAS ASSIGNED. THE ACFT WAS FLOWN AT 38000' AND WAS STABLE WITHIN 100' AS LONG AS NO PITCH CHANGES. APCHING RIC A DSCNT WAS REQUESTED TO EVALUATE PERFORMANCE AT LOWER ALTS. BLOCK 290-310 WAS ASSIGNED AND FLT AT FL300 PROVED STABLE WITHIN 100'. DSCNT AND LNDG WERE MADE WITH MINIMUM ATTITUDE CHANGES AND ALTIMETER INDICATIONS REMAINED STABLE. THE ACFT HAD BEEN ON THE GND 2 DAYS DURING HEAVY RAINS WITH THE PITOT COVERS INSTALLED. SUSPECT THAT THE DESIGN OF THESE COVERS CAN INCREASE THE POSSIBILITY OF WATER IN THE STATIC LINES, AS WAS FOUND. ALTERNATE STATIC SOURCE WAS NOT SELECTED BECAUSE ON A PREVIOUS FLT WITH MINOR VERT SPD VARIANCES, SELECTION OF THE PLT'S ALTERNATE STATIC SOURCE CAUSED ALT READINGS SIGNIFICANTLY (1000') OFF THE FLT MANUAL DATA. WE WERE CHK THIS IN FLT WITH THE KNOWN GOOD SYSTEMS.

Data retrieved from NASA's ASRS site as of August 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.