Narrative:

Aircraft was cleared to climb and maintain 10000 ft. At 9500 ft, a leveloff was initiated. During the leveloff, the captain's altimeter continued to show a climb. During the time the altimeter was passing through 10500 ft, the aircraft was actually in a descent with a 10 degree nose down pitch and a 2500-3000 FPM descent showing on the ivsi. Departure control was reading what the captain's altimeter was showing, which was 10700 ft, before that altimeter began to show a descent. We immediately asked for a descent and vectors back to ZZZ. We were cleared to 3000 ft with a turn to the airport. Believing the copilot's altimeter was correct, we used that information. During a reasonably constant descent, we noticed that this altimeter too, was possibly incorrect. As we leveled at 3000 ft (using the copilot's altimeter) we broke out of the clouds and visually verified that we, in fact, were not at 3000 ft. We estimated our altitude to be approximately 2200-2500 ft MSL (sea level elevation is approximately 750 ft in this area). We climbed to approximately 3000 ft and intercepted the ILS and landed without incident. Upon inspection, it was found that the captain's altimeter was improperly installed the previous night while in for maintenance. When the altimeter was put in the panel, the static lines had not been routed properly, causing them to be pinched. This had obviously rendered the altimeters inaccurate and unreliable.

Google
 

Original NASA ASRS Text

Title: A DA20 ON LEVELOFF AT 10000 FT DIVERTED AND RETURNED TO THE FIELD DUE TO CAPT'S AND FO'S ALTIMETERS AND IVSI INDICATOR BECAME ERRATIC CAUSED BY PINCHED STATIC PRESSURE HOSES.

Narrative: ACFT WAS CLRED TO CLB AND MAINTAIN 10000 FT. AT 9500 FT, A LEVELOFF WAS INITIATED. DURING THE LEVELOFF, THE CAPT'S ALTIMETER CONTINUED TO SHOW A CLB. DURING THE TIME THE ALTIMETER WAS PASSING THROUGH 10500 FT, THE ACFT WAS ACTUALLY IN A DSCNT WITH A 10 DEG NOSE DOWN PITCH AND A 2500-3000 FPM DSCNT SHOWING ON THE IVSI. DEP CTL WAS READING WHAT THE CAPT'S ALTIMETER WAS SHOWING, WHICH WAS 10700 FT, BEFORE THAT ALTIMETER BEGAN TO SHOW A DSCNT. WE IMMEDIATELY ASKED FOR A DSCNT AND VECTORS BACK TO ZZZ. WE WERE CLRED TO 3000 FT WITH A TURN TO THE ARPT. BELIEVING THE COPLT'S ALTIMETER WAS CORRECT, WE USED THAT INFO. DURING A REASONABLY CONSTANT DSCNT, WE NOTICED THAT THIS ALTIMETER TOO, WAS POSSIBLY INCORRECT. AS WE LEVELED AT 3000 FT (USING THE COPLT'S ALTIMETER) WE BROKE OUT OF THE CLOUDS AND VISUALLY VERIFIED THAT WE, IN FACT, WERE NOT AT 3000 FT. WE ESTIMATED OUR ALT TO BE APPROX 2200-2500 FT MSL (SEA LEVEL ELEVATION IS APPROX 750 FT IN THIS AREA). WE CLBED TO APPROX 3000 FT AND INTERCEPTED THE ILS AND LANDED WITHOUT INCIDENT. UPON INSPECTION, IT WAS FOUND THAT THE CAPT'S ALTIMETER WAS IMPROPERLY INSTALLED THE PREVIOUS NIGHT WHILE IN FOR MAINT. WHEN THE ALTIMETER WAS PUT IN THE PANEL, THE STATIC LINES HAD NOT BEEN ROUTED PROPERLY, CAUSING THEM TO BE PINCHED. THIS HAD OBVIOUSLY RENDERED THE ALTIMETERS INACCURATE AND UNRELIABLE.

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.