Narrative:

On initial descent we had a clearance to cross a fix somewhere in the vicinity of abc at FL270. As the autoplt descended in 'prof' we got instrument comparison errors followed immediately by the autoplt kicking off and numerous annunciations on the 'ead.' I began hand flying and trying to determine what was happening. The airspeed on the captain's instruments was in the range of 260 KTS; first officer's was 310 to 320 approaching redline. Initially I tried to slow to keep the first officer's side from overspeeding as I evaluated. In doing so the captain side airspeed approached the low side footer. Based on other cues -- ground speed; pitch attitude; power setting; and feel of the aircraft; I determined that the captain side instruments were most likely correct; and instructed the first officer to put his air data instruments on my side to get rid of what I thought were erroneous and highly distracting overspeed and clacker warnings. About this time we advised ATC that we were having airspeed problems and noticed we had overshot our descent by approximately 800 ft. ATC didn't seem too concerned and issued us a new clearance to a lower altitude. As we were reporting our issue; the whole problem corrected itself and the instruments worked perfectly for the uneventful remainder of the flight. The first officer returned his air data source to normal operations. The whole incident took place in approximately 2 mins. This is my guess: winds were strong out of the west giving us a headwind with a strong crosswind component from the right. It seemed the air was extra moist and a little colder than usual at altitude FL340. I think some ice may have gotten into the right static system; and as we descended it lodged in a way to cause a malfunction. Only a theory.

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Original NASA ASRS Text

Title: B717 ON DESCENT EXPERIENCES APPARENT PITOT/STATIC SYSTEM ERRORS ON THE FO SIDE INSTRUMENTS. DISTRACTION WHICH RESULTS CONTRIBUTES TO AN ALT DEVIATION. INCIDENT LASTS ABOUT TWO MINUTES AND THEN ABATES COMPLETELY.

Narrative: ON INITIAL DSCNT WE HAD A CLRNC TO CROSS A FIX SOMEWHERE IN THE VICINITY OF ABC AT FL270. AS THE AUTOPLT DSNDED IN 'PROF' WE GOT INST COMPARISON ERRORS FOLLOWED IMMEDIATELY BY THE AUTOPLT KICKING OFF AND NUMEROUS ANNUNCIATIONS ON THE 'EAD.' I BEGAN HAND FLYING AND TRYING TO DETERMINE WHAT WAS HAPPENING. THE AIRSPD ON THE CAPT'S INSTS WAS IN THE RANGE OF 260 KTS; FO'S WAS 310 TO 320 APPROACHING REDLINE. INITIALLY I TRIED TO SLOW TO KEEP THE FO'S SIDE FROM OVERSPEEDING AS I EVALUATED. IN DOING SO THE CAPT SIDE AIRSPD APCHED THE LOW SIDE FOOTER. BASED ON OTHER CUES -- GND SPD; PITCH ATTITUDE; PWR SETTING; AND FEEL OF THE ACFT; I DETERMINED THAT THE CAPT SIDE INSTS WERE MOST LIKELY CORRECT; AND INSTRUCTED THE FO TO PUT HIS AIR DATA INSTS ON MY SIDE TO GET RID OF WHAT I THOUGHT WERE ERRONEOUS AND HIGHLY DISTRACTING OVERSPEED AND CLACKER WARNINGS. ABOUT THIS TIME WE ADVISED ATC THAT WE WERE HAVING AIRSPD PROBS AND NOTICED WE HAD OVERSHOT OUR DSCNT BY APPROX 800 FT. ATC DIDN'T SEEM TOO CONCERNED AND ISSUED US A NEW CLRNC TO A LOWER ALT. AS WE WERE RPTING OUR ISSUE; THE WHOLE PROB CORRECTED ITSELF AND THE INSTS WORKED PERFECTLY FOR THE UNEVENTFUL REMAINDER OF THE FLT. THE FO RETURNED HIS AIR DATA SOURCE TO NORMAL OPS. THE WHOLE INCIDENT TOOK PLACE IN APPROX 2 MINS. THIS IS MY GUESS: WINDS WERE STRONG OUT OF THE WEST GIVING US A HEADWIND WITH A STRONG XWIND COMPONENT FROM THE R. IT SEEMED THE AIR WAS EXTRA MOIST AND A LITTLE COLDER THAN USUAL AT ALT FL340. I THINK SOME ICE MAY HAVE GOTTEN INTO THE R STATIC SYS; AND AS WE DSNDED IT LODGED IN A WAY TO CAUSE A MALFUNCTION. ONLY A THEORY.

Data retrieved from NASA's ASRS site as of May 2009 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.