C152 INSTRUCTOR AND STUDENT PLT DETECT ALTIMETER ERROR BY VISUAL REF TO TERRAIN.

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MY STUDENT AND I HAD JUST FLOWN N TO ZZZ VIA THE SPECIAL FLT RULES RTE. WE CALLED ZZZ TWR; WHILE AT AN ALT OF 4500 FT MSL; TO REQUEST LNDG. THE TWR HAD TOLD US TO MAKE A L DSNDING 270 DEG TURN TO LOSE ALT; PRIOR TO ENTERING THE TFC PATTERN. WE COMPLIED; AND STARTED DOWN. WE WERE DSNDING AT ABOUT 1000 FPM; TO GET DOWN TO PATTERN ALT EXPEDITIOUSLY. AFTER MAKING THE 270 DEG TURN; OUR ALTIMETER STILL INDICATED THAT WE WERE AT ONLY ABOUT 3000 FT; SO WE REQUESTED AND RECEIVED CLRNC TO CONTINUE WITH OUR DSNDING TURN. AS WE CONTINUED INTO THE SECOND TURN; I BEGAN TO SUSPECT THE ALTIMETER. THE VSI WAS SHOWING A STEADY DSCNT AT 800-1000 FPM; BUT AS BOTH MY STUDENT AND I COMPARED THE VIEW OUT THE WINDOW WITH THE INDICATED ALT; THE TWO DID NOT AGREE. WE APPEARED TO BE LOWER THAN THE ALTIMETER INDICATED. I POINTED THIS OUT TO MY STUDENT; AND ASKED WHAT HE SAW. WE BOTH AGREED IT DID NOT LOOK RIGHT. AT THAT POINT; AS IT LOOKED LIKE WE WERE IN FACT LOWER THAN THE 2600 FT THAT THE ALTIMETER INDICATED; I CALLED TWR TO ASK WHAT THEY SHOWED FOR US ON THEIR RADAR. THE TWR REPLIED THAT OUR MODE 'C' WAS INDICATING ONLY 600 FT MSL! I KNEW WE WERE LOW; BUT NOT THAT LOW! WE IMMEDIATELY APPLIED FULL PWR; AND CLBED BACK UP TO WHAT WE THOUGHT WAS AN APPROPRIATE PATTERN ALT. DURING THIS TIME; OUR VSI WAS SHOWING A CLB; WHILE THE ALTIMETER CONTINUED TO SHOW A DSCNT. THROUGHOUT THIS; THE AIRSPD APPEARED TO OPERATE NORMALLY. IN THE MEANTIME; WE TAPPED ON THE INST; SINCE IT SEEMED TO BE STUCK; BUT NOTHING HAPPENED. WE LEVELED OFF CLOSE TO PATTERN ALT; WITHOUT HAVING AN ALTIMETER TO REF. AGAIN; I CALLED TWR TO ASK FOR OUR MODE 'C' READOUT; AND THEY TOLD ME 900 FT. AT THAT POINT; WE WERE DOWNWIND ABEAM THE NUMBERS; AND TWR ASKED US IF WE COULD MAKE A 'SHORT APCH;' AS THEY HAD A HAWKER ON FINAL. I SAID YES; AND WE TURNED TOWARDS THE RWY TO COMPLY. THE ALTIMETER WAS STILL MALFUNCTIONING; BUT THE VSI APPEARED TO BE WORKING. AGAIN WHEN I WAS ON SHORT FINAL; I AGAIN TAPPED THE ALTIMETER; AND IT SUDDENLY BECAME 'UNSTUCK;' RAPIDLY UNWINDING; SPINNING; IN A BLUR; UNTIL IT STOPPED ABRUPTLY AT ABOUT 200 FT MSL. THEN; IT SUDDENLY APPEARED TO OPERATE NORMALLY; AS IF NOTHING HAD HAPPENED. THE RESULTANT LNDG WAS UNEVENTFUL. AFTER WE WERE ON THE GND; IT INDICATED THE PROPER FIELD ELEVATION FOR ZZZ. WHILE ON THE GND; IT OPERATED NORMALLY. AFTER LNDG; I ASKED THE TWR; BEFORE I LEFT THE FREQ; IF THERE WAS ANY PROB WITH OUR BEING SO LOW; WELL BELOW PATTERN ALT; AND KNOWING THE NOISE SENSITIVITY OF THE AREA. BUT; THE TWR DID NOT REPLY; OR THEY DID NOT HEAR US. DURING THE TIME THAT THIS HAPPENED; THEY DID NOT MAKE ANY MENTION OF OUR BEING LOW; WITH A LOW ALT ALERT; AS WE WERE WELL BELOW TFC PATTERN ALT; NOR DID THEY QUESTION OUR LOW ALT; EVEN WHEN I CALLED. THEY JUST CALMLY STATED OUR ALT AS 600 FT. WE DID HAVE THE CORRECT ALTIMETER SETTING IN THE KOLLSMAN WINDOW. THIS IS THE FIRST TIME IN MY FLYING CAREER THAT I HAVE EXPERIENCED A FAILURE OF THE ALTIMETER; LIKE THIS; WHILE THE REST OF THE PITOT-STATIC SYS SEEMED TO OPERATE NORMALLY. SINCE THE ALTIMETER SEEMED TO FIX ITSELF; I ELECTED TO FLY BACK TO MY DEP POINT SINCE THE WX WAS VMC. WE HAD NO MORE PROBS. I SPOKE TO MAINT UPON RETURNING.