Narrative:

In 6/88 at approximately XA05 UTC, I departed 23J in VFR conditions on a proposed IFR flight to sus. Previously, I had extended the proposed time of departure with st petersburg automated FSS. In one of my conversations with the FSS, I was advised by them that I should depart VFR and obtain my clearance from jacksonville approach control after becoming airborne. After I had climbed for several minutes, I was issued and acknowledged my clearance as follows: 'clrd to the sus airport as filed. Climb to and maintain 7000'.' I shortly thereafter began to suspect that my actual altitude was much higher than the altitude indicated by my altimeter, which at that point showed 4500'. I had no way to definitely ascertain my altitude, except from a visual estimate of my altitude by observing the ground and by my time climbing. Both estimates did lead me to believe that I was actually at a higher altitude than that shown by the altimeter. My first action was to check and reset my altimeter setting. This had very little effect. Soon, the indicated airspeed stopped climbing, whereas the aircraft had not. I then opened the alternate static air valve, but this also had no effect. It appeared to me that the altimeter had stuck in position, because I was observing normal airspeed and rate of climb indications. Because I then realized that I would be unable to determine my correct altitude, I called the controller to inform him that I was having difficulty with the altimeter, that it appeared stuck, and that I would appreciate him informing me when I was approaching 7000'. Which was my assigned altitude. I believe that the controller confirmed my report however, I am not certain at this time that he did. But, since I then assumed I would be advised before reaching 7000', I continued to climb while continuing to work out a solution, but became somewhat distracted by the problem that I was having with the altimeter. I continued to try to correct the problem but was unable to do so, until as a result of vigorously rotating the altimeter setting knob, the instrument suddenly appeared to become operative after a sudden winding up in the altitude. At the point it appeared to begin working again however, it indicated an altitude of 11500'. Concerned that it might be a correct reading, I asked the controller for confirmation of my altitude, which he did, but seemed rather surprised and somewhat upset that I had climbed to 11500'. I explained that I had asked for his help, that he had not advised me when I approached 7000' as I had requested, but was descending to 7000' according to my altimeter, which I still felt to be suspect of providing me with incorrect readings. Several times during this entire process, I requested and received from the controller a confirmation of my reported altitude. Each time the controller reported to me that I was at an altitude within 300' of that which was indicated and, for this reason, could not fully comprehend the problem I was having. Because I then realized that neither the controller nor I had a way to definitely confirm my correct altitude, I cancelled IFR and continued VFR in VFR conditions so I could investigate the problem further. I later confirmed my reported altitude with jacksonville ARTCC when it appeared to me that the actual altitude I observed and the altimeter altitude reading were reasonably the same and after observing the altimeter responsive to a climb or descent. Since then by opening the alternate static air valve, I also observed the correct effect, I concluded that my altimeter system was working again. I requested that my IFR flight plan be reinstated by jacksonville ARTCC, received and acknowledged my clearance, and proceeded as filed to my destination without further incident. On the ground at my destination home base with my altimeter set to the airport altimeter setting, it read the correct altitude, thus further confirming that the instrument was again working. Before the next flight, I had my radio shop check the entire system, and they were unable to find anything wrong with it. My last biennial static and transponder system check was done just 3 months prior to this incident. Given the flight conditions that I was operating in at the time, the incident did not appear to be a situation of extremely degraded safety. However, in other situations of terrain or WX, the problem could have been extremely hazardous. My reason for reporting this incident is that at all times during the altimeter malfunction, the controller confirmed that he saw me at the altitude that my altimeter showed, which was obviously incorrect. The lesson to be learned here by ATC is that, just because an aircraft is equipped with mode C and that the controller confirms altitude, does that necessarily mean that the aircraft is actually being flown at the altitude reported. It seems to me, whenever a pilot reports a suspected malfunctioning altimeter, that the controller should give some special attention to that aircraft and that the aircraft be provided horizontal separation from all aircraft and certainly not be required to cancel IFR because of equipment malfunctions solely for the purpose of 'staying legal'.

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Original NASA ASRS Text

Title: ALTIMETER MALFUNCTION CAUSED ALT DEVIATION ON IFR FLT PLAN.

Narrative: IN 6/88 AT APPROX XA05 UTC, I DEPARTED 23J IN VFR CONDITIONS ON A PROPOSED IFR FLT TO SUS. PREVIOUSLY, I HAD EXTENDED THE PROPOSED TIME OF DEP WITH ST PETERSBURG AUTOMATED FSS. IN ONE OF MY CONVERSATIONS WITH THE FSS, I WAS ADVISED BY THEM THAT I SHOULD DEPART VFR AND OBTAIN MY CLRNC FROM JACKSONVILLE APCH CTL AFTER BECOMING AIRBORNE. AFTER I HAD CLIMBED FOR SEVERAL MINUTES, I WAS ISSUED AND ACKNOWLEDGED MY CLRNC AS FOLLOWS: 'CLRD TO THE SUS ARPT AS FILED. CLIMB TO AND MAINTAIN 7000'.' I SHORTLY THEREAFTER BEGAN TO SUSPECT THAT MY ACTUAL ALT WAS MUCH HIGHER THAN THE ALT INDICATED BY MY ALTIMETER, WHICH AT THAT POINT SHOWED 4500'. I HAD NO WAY TO DEFINITELY ASCERTAIN MY ALT, EXCEPT FROM A VISUAL ESTIMATE OF MY ALT BY OBSERVING THE GND AND BY MY TIME CLIMBING. BOTH ESTIMATES DID LEAD ME TO BELIEVE THAT I WAS ACTUALLY AT A HIGHER ALT THAN THAT SHOWN BY THE ALTIMETER. MY FIRST ACTION WAS TO CHECK AND RESET MY ALTIMETER SETTING. THIS HAD VERY LITTLE EFFECT. SOON, THE INDICATED AIRSPEED STOPPED CLIMBING, WHEREAS THE ACFT HAD NOT. I THEN OPENED THE ALTERNATE STATIC AIR VALVE, BUT THIS ALSO HAD NO EFFECT. IT APPEARED TO ME THAT THE ALTIMETER HAD STUCK IN POSITION, BECAUSE I WAS OBSERVING NORMAL AIRSPEED AND RATE OF CLIMB INDICATIONS. BECAUSE I THEN REALIZED THAT I WOULD BE UNABLE TO DETERMINE MY CORRECT ALT, I CALLED THE CTLR TO INFORM HIM THAT I WAS HAVING DIFFICULTY WITH THE ALTIMETER, THAT IT APPEARED STUCK, AND THAT I WOULD APPRECIATE HIM INFORMING ME WHEN I WAS APCHING 7000'. WHICH WAS MY ASSIGNED ALT. I BELIEVE THAT THE CTLR CONFIRMED MY REPORT HOWEVER, I AM NOT CERTAIN AT THIS TIME THAT HE DID. BUT, SINCE I THEN ASSUMED I WOULD BE ADVISED BEFORE REACHING 7000', I CONTINUED TO CLIMB WHILE CONTINUING TO WORK OUT A SOLUTION, BUT BECAME SOMEWHAT DISTRACTED BY THE PROBLEM THAT I WAS HAVING WITH THE ALTIMETER. I CONTINUED TO TRY TO CORRECT THE PROBLEM BUT WAS UNABLE TO DO SO, UNTIL AS A RESULT OF VIGOROUSLY ROTATING THE ALTIMETER SETTING KNOB, THE INSTRUMENT SUDDENLY APPEARED TO BECOME OPERATIVE AFTER A SUDDEN WINDING UP IN THE ALT. AT THE POINT IT APPEARED TO BEGIN WORKING AGAIN HOWEVER, IT INDICATED AN ALT OF 11500'. CONCERNED THAT IT MIGHT BE A CORRECT READING, I ASKED THE CTLR FOR CONFIRMATION OF MY ALT, WHICH HE DID, BUT SEEMED RATHER SURPRISED AND SOMEWHAT UPSET THAT I HAD CLIMBED TO 11500'. I EXPLAINED THAT I HAD ASKED FOR HIS HELP, THAT HE HAD NOT ADVISED ME WHEN I APCHED 7000' AS I HAD REQUESTED, BUT WAS DSNDING TO 7000' ACCORDING TO MY ALTIMETER, WHICH I STILL FELT TO BE SUSPECT OF PROVIDING ME WITH INCORRECT READINGS. SEVERAL TIMES DURING THIS ENTIRE PROCESS, I REQUESTED AND RECEIVED FROM THE CTLR A CONFIRMATION OF MY REPORTED ALT. EACH TIME THE CTLR REPORTED TO ME THAT I WAS AT AN ALT WITHIN 300' OF THAT WHICH WAS INDICATED AND, FOR THIS REASON, COULD NOT FULLY COMPREHEND THE PROBLEM I WAS HAVING. BECAUSE I THEN REALIZED THAT NEITHER THE CTLR NOR I HAD A WAY TO DEFINITELY CONFIRM MY CORRECT ALT, I CANCELLED IFR AND CONTINUED VFR IN VFR CONDITIONS SO I COULD INVESTIGATE THE PROBLEM FURTHER. I LATER CONFIRMED MY REPORTED ALT WITH JACKSONVILLE ARTCC WHEN IT APPEARED TO ME THAT THE ACTUAL ALT I OBSERVED AND THE ALTIMETER ALT READING WERE REASONABLY THE SAME AND AFTER OBSERVING THE ALTIMETER RESPONSIVE TO A CLIMB OR DESCENT. SINCE THEN BY OPENING THE ALTERNATE STATIC AIR VALVE, I ALSO OBSERVED THE CORRECT EFFECT, I CONCLUDED THAT MY ALTIMETER SYSTEM WAS WORKING AGAIN. I REQUESTED THAT MY IFR FLT PLAN BE REINSTATED BY JACKSONVILLE ARTCC, RECEIVED AND ACKNOWLEDGED MY CLRNC, AND PROCEEDED AS FILED TO MY DEST WITHOUT FURTHER INCIDENT. ON THE GND AT MY DEST HOME BASE WITH MY ALTIMETER SET TO THE ARPT ALTIMETER SETTING, IT READ THE CORRECT ALT, THUS FURTHER CONFIRMING THAT THE INSTRUMENT WAS AGAIN WORKING. BEFORE THE NEXT FLT, I HAD MY RADIO SHOP CHECK THE ENTIRE SYSTEM, AND THEY WERE UNABLE TO FIND ANYTHING WRONG WITH IT. MY LAST BIENNIAL STATIC AND XPONDER SYSTEM CHECK WAS DONE JUST 3 MONTHS PRIOR TO THIS INCIDENT. GIVEN THE FLT CONDITIONS THAT I WAS OPERATING IN AT THE TIME, THE INCIDENT DID NOT APPEAR TO BE A SITUATION OF EXTREMELY DEGRADED SAFETY. HOWEVER, IN OTHER SITUATIONS OF TERRAIN OR WX, THE PROBLEM COULD HAVE BEEN EXTREMELY HAZARDOUS. MY REASON FOR REPORTING THIS INCIDENT IS THAT AT ALL TIMES DURING THE ALTIMETER MALFUNCTION, THE CTLR CONFIRMED THAT HE SAW ME AT THE ALT THAT MY ALTIMETER SHOWED, WHICH WAS OBVIOUSLY INCORRECT. THE LESSON TO BE LEARNED HERE BY ATC IS THAT, JUST BECAUSE AN ACFT IS EQUIPPED WITH MODE C AND THAT THE CTLR CONFIRMS ALT, DOES THAT NECESSARILY MEAN THAT THE ACFT IS ACTUALLY BEING FLOWN AT THE ALT REPORTED. IT SEEMS TO ME, WHENEVER A PLT REPORTS A SUSPECTED MALFUNCTIONING ALTIMETER, THAT THE CTLR SHOULD GIVE SOME SPECIAL ATTN TO THAT ACFT AND THAT THE ACFT BE PROVIDED HORIZONTAL SEPARATION FROM ALL ACFT AND CERTAINLY NOT BE REQUIRED TO CANCEL IFR BECAUSE OF EQUIPMENT MALFUNCTIONS SOLELY FOR THE PURPOSE OF 'STAYING LEGAL'.

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.