Narrative:

I was safety pilot for an IFR rated pilot doing an instrument approach in IMC. Aircraft was equipped with a KLN94 IFR approved GPS. The data base was current at the time of the deviation. We were cleared to the approach and crossed the FAF at 3000 MSL. The GPS did not have the step down fixes identified on the database. It only showed the map. Looking at the chart the minimum altitudes have an asterisk indicating LNAV only. I interpreted this as minimum altitudes to abide by if you had the fixes in the database. Since our plan was a circling approach; I thought we were not required to stay above those altitudes. I recommended to the pilot in command (PIC) that we descend down to the MDA. The PIC agreed and began a descent at approximately 500 feet per minute.upon descent below 2000 we received a call from the controller. He relayed an altitude alert and asked us to verify altitude. The PIC leveled off and said our indicated altitude was 1900 MSL. The controller acknowledged and did not comment further. The PIC decided to stay at 1900 and attempt to identify the step down fixes using fore flight with geo referenced instrument approach plates.we finished the approach at the map and identified the runway environment. We went missed and continued our flight to our destination without further incident.there was a training deficiency on both our parts. We are both IFR rated and current; but we owe some additional training on some of the quirks of older RNAV systems such as the KLN94. Since many aircraft in our fleet are older airframes with legacy avionics; the solution is to increase training to ensure we know the ins and outs of how each GPS system gives information.

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Original NASA ASRS Text

Title: C182 pilot deviated from published altitudes during RNAV approach.

Narrative: I was safety pilot for an IFR rated pilot doing an instrument approach in IMC. Aircraft was equipped with a KLN94 IFR approved GPS. The data base was current at the time of the deviation. We were cleared to the approach and crossed the FAF at 3000 MSL. The GPS did not have the step down fixes identified on the database. It only showed the MAP. Looking at the chart the minimum altitudes have an asterisk indicating LNAV only. I interpreted this as minimum altitudes to abide by if you had the fixes in the database. Since our plan was a circling approach; I thought we were not required to stay above those altitudes. I recommended to the Pilot In Command (PIC) that we descend down to the MDA. The PIC agreed and began a descent at approximately 500 feet per minute.Upon descent below 2000 we received a call from the controller. He relayed an altitude alert and asked us to verify altitude. The PIC leveled off and said our indicated altitude was 1900 MSL. The controller acknowledged and did not comment further. The PIC decided to stay at 1900 and attempt to identify the step down fixes using fore flight with Geo referenced instrument approach plates.We finished the approach at the MAP and identified the runway environment. We went missed and continued our flight to our destination without further incident.There was a training deficiency on both our parts. We are both IFR rated and current; but we owe some additional training on some of the quirks of older RNAV systems such as the KLN94. Since many aircraft in our fleet are older airframes with Legacy avionics; the solution is to increase training to ensure we know the ins and outs of how each GPS system gives information.

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.