Narrative:

While on the GPS 25R approach ATC alerted us that we were below altitude for the leg of the approach we were on and to stop descending. The analog altimeter was reading 300-400 higher than what ATC radar was receiving from our mode C transponder. The altimeter was set correctly; and we confirmed this setting with the controller. We stopped descent and checked alternate air in the case of static port blockage; but it had no effective change in the indicated altimeter. I alerted the tower we were having 'altimeter trouble' and requested that he standby as we were troubleshooting the situation.the weather was improving and the field was in sight and I was reasonably sure I could return to [home airport] for final landing; so I requested low approach only with a return to approach frequency as soon as possible. The filed flight plan said training/multiple approaches in the remarks section and I was hopeful for a seamless and constant communication with approach for the return leg. Tower ATC asked if we were canceling our IFR flight plan and continuing VFR. I said 'no' and that we could maintain VFR through the low approach and climb out; but to transfer us back to approach as soon as possible. ATC stated that he was in a 'quandary' as to whether we were canceling IFR and going VFR. I said no; so he then told us to go missed approach; which we did; and we were transferred to approach soon after.approach stated that our intention to return to [home airport] threw them off guard and that they were there to help. I stated how our flight plan said training flight and multiple approaches and apologized. He vectored us back without any more incident. While researching the potential problems with the altimeter; I learned that since there had been a significant rainstorm for the previous 2-3 days with heavy winds and downpours that there was a chance water had somehow entered into the static port.I am reporting this incident because after recollecting the series of transmissions and events during the critical phase of approach flight and the controller's curt and pressing demands for answers while we were in the midst of troubleshooting; that this event could have led to this situation becoming more serious. If I wasn't being pressed for hard decisions during this time and instead supported through the tower by more informational data regarding our current received transponder altitude and comparing it to what our analog altimeter was reading; I feel I could have made my transmission in a more concise and effective format. For instance; when he asked if we were canceling IFR and returning VFR and I said I could maintain VFR through the low approach; I should have said VMC instead of VFR; but my focus at that point was maintaining safe flight with this student and keeping the aircraft and cockpit managed. During the return leg to [home airport]; I kept an eye on both the transponder's indicated altitude as well as the analog altimeter. Minor discrepancies continued; but with the weather becoming VMC; it wasn't as critical to resuming a safe flight. The aircraft was squawked upon return.

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Original NASA ASRS Text

Title: A C172 pilot reported altimeter accuracy issues during a practice instrument approach. Pilot elected to break off approach in VMC conditions and return to base airport.

Narrative: While on the GPS 25R approach ATC alerted us that we were below altitude for the leg of the approach we were on and to stop descending. The analog altimeter was reading 300-400 higher than what ATC radar was receiving from our mode C transponder. The altimeter was set correctly; and we confirmed this setting with the controller. We stopped descent and checked alternate air in the case of static port blockage; but it had no effective change in the indicated altimeter. I alerted the tower we were having 'altimeter trouble' and requested that he standby as we were troubleshooting the situation.The weather was improving and the field was in sight and I was reasonably sure I could return to [home airport] for final landing; so I requested low approach only with a return to Approach frequency ASAP. The filed flight plan said training/multiple approaches in the remarks section and I was hopeful for a seamless and constant communication with Approach for the return leg. Tower ATC asked if we were canceling our IFR flight plan and continuing VFR. I said 'no' and that we could maintain VFR through the low approach and climb out; but to transfer us back to Approach ASAP. ATC stated that he was in a 'quandary' as to whether we were canceling IFR and going VFR. I said no; so he then told us to go missed approach; which we did; and we were transferred to Approach soon after.Approach stated that our intention to return to [home airport] threw them off guard and that they were there to help. I stated how our flight plan said training flight and multiple approaches and apologized. He vectored us back without any more incident. While researching the potential problems with the altimeter; I learned that since there had been a significant rainstorm for the previous 2-3 days with heavy winds and downpours that there was a chance water had somehow entered into the static port.I am reporting this incident because after recollecting the series of transmissions and events during the critical phase of approach flight and the controller's curt and pressing demands for answers while we were in the midst of troubleshooting; that this event could have led to this situation becoming more serious. If I wasn't being pressed for hard decisions during this time and instead supported through the tower by more informational data regarding our current received transponder altitude and comparing it to what our analog altimeter was reading; I feel I could have made my transmission in a more concise and effective format. For instance; when he asked if we were canceling IFR and returning VFR and I said I could maintain VFR through the low approach; I should have said VMC instead of VFR; but my focus at that point was maintaining safe flight with this student and keeping the aircraft and cockpit managed. During the return leg to [home airport]; I kept an eye on both the transponder's indicated altitude as well as the analog altimeter. Minor discrepancies continued; but with the weather becoming VMC; it wasn't as critical to resuming a safe flight. The aircraft was squawked upon return.

Data retrieved from NASA's ASRS site 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.