Narrative:

My student and I had just flown north to ZZZ via the special flight rules route. We called ZZZ tower, while at an altitude of 4500 ft MSL, to request landing. The tower had told us to make a left descending 270 degree turn to lose altitude, prior to entering the traffic pattern. We complied, and started down. We were descending at about 1000 FPM, to get down to pattern altitude expeditiously. After making the 270 degree turn, our altimeter still indicated that we were at only about 3000 ft, so we requested and received clearance to continue with our descending turn. As we continued into the second turn, I began to suspect the altimeter. The vsi was showing a steady descent at 800-1000 FPM, but as both my student and I compared the view out the window with the indicated altitude, 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 tower to ask what they showed for us on their radar. The tower replied that our mode 'C' was indicating only 600 ft MSL! I knew we were low, but not that low! We immediately applied full power, and climbed back up to what we thought was an appropriate pattern altitude. During this time, our vsi was showing a climb, while the altimeter continued to show a descent. Throughout this, the airspeed appeared to operate normally. In the meantime, we tapped on the instrument, since it seemed to be stuck, but nothing happened. We leveled off close to pattern altitude, without having an altimeter to reference. Again, I called tower to ask for our mode 'C' readout, and they told me 900 ft. At that point, we were downwind abeam the numbers, and tower asked us if we could make a 'short approach,' as they had a hawker on final. I said yes, and we turned towards the runway 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 landing was uneventful. After we were on the ground, it indicated the proper field elevation for ZZZ. While on the ground, it operated normally. After landing, I asked the tower, before I left the frequency, if there was any problem with our being so low, well below pattern altitude, and knowing the noise sensitivity of the area. But, the tower 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 altitude alert, as we were well below traffic pattern altitude, nor did they question our low altitude, even when I called. They just calmly stated our altitude 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 system seemed to operate normally. Since the altimeter seemed to fix itself, I elected to fly back to my departure point since the WX was VMC. We had no more problems. I spoke to maintenance upon returning.

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Original NASA ASRS Text

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

Narrative: 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.

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.