Narrative:

Crossed bason OM with G/south needle centered and altimeter reading close to specified 1894' MSL. Altimeter had a 50' error at takeoff from san (altitude read 50' high with current altitude setting entered). At about 1 NM from MM with G/south needle centered and altimeter reading slightly above 400' MSL, instrument observed trees, power poles and bldgs only about 100' below aircraft (DH for ach is 295' MSL). At slightly below 400' MSL instrument directed student to level off and proceed visually for normal landing. Student was as surprised as instrument at closeness of obstacles. First observance of 4 degree VASI after level off was red over red. Toa ILS G/south is 3.4 degrees. After landing, PIC proceeded directly to tower to discuss what CFI considered a potentially hazardous incident. Tower personnel stated that ILS monitor lights indicated all was ok with their equipment and nobody else had commented on G/south problems. Tower personnel did not remember observing our aircraft low at 1.5 NM. Prior to takeoff from toa, altimeter again noted reading 50' high with current altitude setting. Aircraft proceed directly to nearby long beach for practice ILS approach where all appeared normal throughout approach. Cause of apparent low altitude at 1.5 NM from threshold not known. Misjudgment of visual altitude by instrument and student possible. Anomaly in G/south also a possibility. Recommend further investigation. Recommend also pilots add any adverse altimeter error to the approach DH or compensate for the error when entering the current altimeter setting in the barometer pressure window.

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Original NASA ASRS Text

Title: GA SMA ON ILS APCH IN VFR CONDITIONS ALARMED AT CLOSENESS OF OBSTRUCTIONS ON FINAL APCH.

Narrative: CROSSED BASON OM WITH G/S NEEDLE CENTERED AND ALTIMETER READING CLOSE TO SPECIFIED 1894' MSL. ALTIMETER HAD A 50' ERROR AT TKOF FROM SAN (ALT READ 50' HIGH WITH CURRENT ALT SETTING ENTERED). AT ABOUT 1 NM FROM MM WITH G/S NEEDLE CENTERED AND ALTIMETER READING SLIGHTLY ABOVE 400' MSL, INSTR OBSERVED TREES, PWR POLES AND BLDGS ONLY ABOUT 100' BELOW ACFT (DH FOR ACH IS 295' MSL). AT SLIGHTLY BELOW 400' MSL INSTR DIRECTED STUDENT TO LEVEL OFF AND PROCEED VISUALLY FOR NORMAL LNDG. STUDENT WAS AS SURPRISED AS INSTR AT CLOSENESS OF OBSTACLES. FIRST OBSERVANCE OF 4 DEG VASI AFTER LEVEL OFF WAS RED OVER RED. TOA ILS G/S IS 3.4 DEGS. AFTER LNDG, PIC PROCEEDED DIRECTLY TO TWR TO DISCUSS WHAT CFI CONSIDERED A POTENTIALLY HAZARDOUS INCIDENT. TWR PERSONNEL STATED THAT ILS MONITOR LIGHTS INDICATED ALL WAS OK WITH THEIR EQUIP AND NOBODY ELSE HAD COMMENTED ON G/S PROBS. TWR PERSONNEL DID NOT REMEMBER OBSERVING OUR ACFT LOW AT 1.5 NM. PRIOR TO TKOF FROM TOA, ALTIMETER AGAIN NOTED READING 50' HIGH WITH CURRENT ALT SETTING. ACFT PROCEED DIRECTLY TO NEARBY LONG BEACH FOR PRACTICE ILS APCH WHERE ALL APPEARED NORMAL THROUGHOUT APCH. CAUSE OF APPARENT LOW ALT AT 1.5 NM FROM THRESHOLD NOT KNOWN. MISJUDGMENT OF VISUAL ALT BY INSTR AND STUDENT POSSIBLE. ANOMALY IN G/S ALSO A POSSIBILITY. RECOMMEND FURTHER INVESTIGATION. RECOMMEND ALSO PLTS ADD ANY ADVERSE ALTIMETER ERROR TO THE APCH DH OR COMPENSATE FOR THE ERROR WHEN ENTERING THE CURRENT ALTIMETER SETTING IN THE BAROMETER PRESSURE WINDOW.

Data retrieved from NASA's ASRS site as of August 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.