Narrative:

During VOR runway 31 approach in IMC, we descended from 1500 ft MSL at the bergr FAF to 440 ft MSL (375 ft AGL) as per the commercial approach plate. This fix is co-located with the 5.0 DME fix of acy VOR. The runway threshold is located at .8 DME as is the missed approach point, while the depicted vdp is at 1.8 DME. We initiated a 900 FPM descent to minimums (440 ft MSL) at 5.0 DME and upon passing 500 ft MSL, the tower 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 captain's altimeter and the copilot's altimeter read within 40 ft of each other. The controller again stated that mode C showed us at 300 ft MSL, at which time I started a climb to 500 ft MSL, well above minimums. Just prior to reaching the vdp, the runway came into view and we landed uneventfully. Indicated altitude and field elevation were approximately 50 ft different, with indicated altitude of approximately 120 ft at a field elevation of 76 ft -- well indicated altitude of approximately 120 ft at a field elevation of 76 ft, well within required tolerances. Nevertheless, I chose to monitor our indicated altitude on the next approach (visual backed up by ILS at kfll). Over known longitudinal fix/crossing altitudes on the GS, it appeared the captain's altimeter was indicating approximately 140 ft higher than actual altitude. In the landing confign, on the ground, however, the disagreement was only 60 ft. Nevertheless, I informed maintenance of the problem and wrote the condition up in the logbook, which grounded the aircraft for several days. Maintenance later informed me that the electrically servoed altimeter was indeed out of tolerance at sea level and actually became more inaccurate up to approximately 300 ft MSL, where the altimeter indicated 140 ft higher than actual altitude. 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 aircraft. Instead, a 'static defect correction module' is installed in the captain's altimeter system, while the copilot's altimeter system uses uncorrected air data, which requires the pilots to consult the afm to apply proper altitude/airspeed corrections. Unfortunately, this causes the typical learjet pilot to fixate on the captain's altimeter, to the exclusion of the first officer's altimeter. Thus, when there is a partial, unflagged failure of the captain's altimeter, it is difficult to recognize since there is no easy way to accurately compare the data. After 3 days, maintenance is also looking into the possibility of a 'cocked' static port, which could cause varying inaccuracies depending on airspeed/altitude configns.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.