Narrative:

On a flight to dpa I was receiving vectors and was told to expect the GPS 20R approach at dpa. I loaded the GPS 20R approach into the FMS with 'vectors' as the transition. I then received a clearance to go direct to dukbe. I had some difficulty changing the flight plan in the FMS for the new transition so I asked for a delay vector and shortly after that I had the FMS properly configured and advised ATC that I was ready for the approach. I was then cleared for the GPS 20R at dpa and told to maintain 'at or above 2;000 until established' on the approach course. Which I thought was odd because no segment of the approach was below 2;000 until passing fritz (final approach fix). I briefed the approach and proceeded as cleared.after passing dukbe I began a descent from 3;000 ft I was out of the clouds shortly after passing dukbe and was in descent and I was able to see everything (except the airport) clearly and I was in VMC. During the descent I remember seeing the ground and thinking 'this seems a little too low for this far out from the airport.' I then saw a low altitude annunciator come on and I quickly double checked altitudes and realized that I had descended below the minimum altitude for that part of the approach; and then immediately climbed back to the appropriate altitude (2;400 ft). I had incorrectly believed that I could descend to 1;380 ft which was the minimum altitude for the next segment of the approach. I was at approximately 2;000 ft at the lowest point.I believe the following to be contributing factors:1. Lack of recency of instrument experience even though I was legal within the fars and had recently completed an instrument proficiency check. 2. Lack of actual instrument time (only had about 1 hour of actual instrument experience). 3. Autopilot was inoperative which if was operative could have reduced pilot work load. 4. Moderate turbulence that made approach plates and instruments difficult to read; and made altitude more difficult to control.I believe the following to be appropriate corrective actions:1. Instrument flying skills degrade rapidly. Set up a regular routine to maintain proficiency either in a simulator or airplane.2. Take along a safety pilot or an instructor on flights in actual instrument conditions until more actual instrument experience is developed to help reduce work load and act as a second set of eyes.3. Become more familiar with approach plates before takeoff.

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Original NASA ASRS Text

Title: Confused by his ATC altitude assignment given with his GPS approach clearance; a C182 pilot with little IFR experience descended below the altitude appropriate for the approach segment prior to the FAF. When alerted by the Low Altitude warning on his GPS he returned to the appropriate altitude.

Narrative: On a flight to DPA I was receiving vectors and was told to expect the GPS 20R approach at DPA. I loaded the GPS 20R approach into the FMS with 'VECTORS' as the transition. I then received a clearance to go direct to DUKBE. I had some difficulty changing the flight plan in the FMS for the new transition so I asked for a delay vector and shortly after that I had the FMS properly configured and advised ATC that I was ready for the approach. I was then cleared for the GPS 20R at DPA and told to maintain 'at or above 2;000 until established' on the approach course. Which I thought was odd because no segment of the approach was below 2;000 until passing FRITZ (final approach fix). I briefed the approach and proceeded as cleared.After passing DUKBE I began a descent from 3;000 FT I was out of the clouds shortly after passing DUKBE and was in descent and I was able to see everything (except the airport) clearly and I was in VMC. During the descent I remember seeing the ground and thinking 'this seems a little too low for this far out from the airport.' I then saw a Low Altitude annunciator come on and I quickly double checked altitudes and realized that I had descended below the minimum altitude for that part of the approach; and then immediately climbed back to the appropriate altitude (2;400 FT). I had incorrectly believed that I could descend to 1;380 FT which was the minimum altitude for the next segment of the approach. I was at approximately 2;000 FT at the lowest point.I believe the following to be contributing factors:1. Lack of recency of instrument experience even though I was legal within the FARs and had recently completed an Instrument Proficiency Check. 2. Lack of actual instrument time (only had about 1 hour of actual instrument experience). 3. Autopilot was inoperative which if was operative could have reduced pilot work load. 4. Moderate turbulence that made approach plates and instruments difficult to read; and made altitude more difficult to control.I believe the following to be appropriate corrective actions:1. Instrument flying skills degrade rapidly. Set up a regular routine to maintain proficiency either in a simulator or airplane.2. Take along a safety pilot or an instructor on flights in actual instrument conditions until more actual instrument experience is developed to help reduce work load and act as a second set of eyes.3. Become more familiar with approach plates before takeoff.

Data retrieved from NASA's ASRS site as of April 2012 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.