AN LJ25 FLC ON A POSITIONING FLT DISCOVER THAT AT LOW ALTS THE CAPT'S ALTIMETER COULD READ AS MUCH AS 140 FT HIGHER THAN ACTUAL.

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DURING VOR RWY 31 APCH IN IMC; WE DSNDED FROM 1500 FT MSL AT THE BERGR FAF TO 440 FT MSL (375 FT AGL) AS PER THE COMMERCIAL APCH PLATE. THIS FIX IS CO-LOCATED WITH THE 5.0 DME FIX OF ACY VOR. THE RWY THRESHOLD IS LOCATED AT .8 DME AS IS THE MISSED APCH POINT; WHILE THE DEPICTED VDP IS AT 1.8 DME. WE INITIATED A 900 FPM DSCNT TO MINIMUMS (440 FT MSL) AT 5.0 DME AND UPON PASSING 500 FT MSL; THE TWR TOLD US OUR MODE C INDICATED 300 FT MSL AND TO CONFIRM AN ALTIMETER SETTING OF 29.64; WHICH WE CONFIRMED. (THE WX WAS BORDERLINE SEVERE; WITH ALTIMETER UPDATES RANGING FROM 29.62 TO 29.78 WITHIN A 15 MIN PERIOD.) THE CAPT'S ALTIMETER AND THE COPLT'S ALTIMETER READ WITHIN 40 FT OF EACH OTHER. THE CTLR AGAIN STATED THAT MODE C SHOWED US AT 300 FT MSL; AT WHICH TIME I STARTED A CLB TO 500 FT MSL; WELL ABOVE MINIMUMS. JUST PRIOR TO REACHING THE VDP; THE RWY CAME INTO VIEW AND WE LANDED UNEVENTFULLY. INDICATED ALT AND FIELD ELEVATION WERE APPROX 50 FT DIFFERENT; WITH INDICATED ALT OF APPROX 120 FT AT A FIELD ELEVATION OF 76 FT -- WELL INDICATED ALT OF APPROX 120 FT AT A FIELD ELEVATION OF 76 FT; WELL WITHIN REQUIRED TOLERANCES. NEVERTHELESS; I CHOSE TO MONITOR OUR INDICATED ALT ON THE NEXT APCH (VISUAL BACKED UP BY ILS AT KFLL). OVER KNOWN LONGITUDINAL FIX/XING ALTS ON THE GS; IT APPEARED THE CAPT'S ALTIMETER WAS INDICATING APPROX 140 FT HIGHER THAN ACTUAL ALT. IN THE LNDG CONFIGN; ON THE GND; HOWEVER; THE DISAGREEMENT WAS ONLY 60 FT. NEVERTHELESS; I INFORMED MAINT OF THE PROB AND WROTE THE CONDITION UP IN THE LOGBOOK; WHICH GNDED THE ACFT FOR SEVERAL DAYS. MAINT LATER INFORMED ME THAT THE ELECTRICALLY SERVOED ALTIMETER WAS INDEED OUT OF TOLERANCE AT SEA LEVEL AND ACTUALLY BECAME MORE INACCURATE UP TO APPROX 300 FT MSL; WHERE THE ALTIMETER INDICATED 140 FT HIGHER THAN ACTUAL ALT. THIS WAS THE HIGHEST DISCREPANCY NOTED DURING THE BENCH TEST. THE 1969 LEARJET WAS NOT EQUIPPED WITH A GPWS/RADAR ALTIMETER; NOR WAS IT REQUIRED TO. ANOTHER FACTOR IS THE FACT THAT VINTAGE LEARJETS (ALL 25 SERIES AND SOME 30 SERIES) ARE NOT EQUIPPED WITH ANY AIR DATA COMPUTERS AT ALL; UNLIKE NEWER ACFT. INSTEAD; A 'STATIC DEFECT CORRECTION MODULE' IS INSTALLED IN THE CAPT'S ALTIMETER SYS; WHILE THE COPLT'S ALTIMETER SYS USES UNCORRECTED AIR DATA; WHICH REQUIRES THE PLTS TO CONSULT THE AFM TO APPLY PROPER ALT/AIRSPD CORRECTIONS. UNFORTUNATELY; THIS CAUSES THE TYPICAL LEARJET PLT TO FIXATE ON THE CAPT'S ALTIMETER; TO THE EXCLUSION OF THE FO'S ALTIMETER. THUS; WHEN THERE IS A PARTIAL; UNFLAGGED FAILURE OF THE CAPT'S ALTIMETER; IT IS DIFFICULT TO RECOGNIZE SINCE THERE IS NO EASY WAY TO ACCURATELY COMPARE THE DATA. AFTER 3 DAYS; MAINT IS ALSO LOOKING INTO THE POSSIBILITY OF A 'COCKED' STATIC PORT; WHICH COULD CAUSE VARYING INACCURACIES DEPENDING ON AIRSPD/ALT CONFIGNS.