As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DCA-SDF - FL 201, THE CAPTAINS ALTIMETER WAS ERRATIC WITH A 500 FT DROP IN CORRECTED MODE. THE DIFFERENTIAL DECREASED WITH A DECREASE IN ALTITUDE. THE AIRCRAFT WAS DIVERTED TO PIT AND LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED THE MACH AIRSPEED INDICATOR AND A LEAKING STATIC LINE AT THE MACH SPEED INDICATOR. BOTH ALTIMETERS WERE ALSO REPLACED AS A PRECAUTIONARY MEASURE. (M)