Narrative:

I recently checked out and have less than 40 hours in the aircraft. I departed under IMC as a single pilot for a short flight in high workload IMC to pdx. The weather was low overcast and the temperature was near freezing. I departed with instructions to level at 4000 ft MSL. Shortly after departure I was switched to pdx approach control who gave me a vector to intercept the localizer for runway 10L. I had all the correct frequencies tuned and identified; I had the correct weather and had briefed myself on the approach into pdx. The weather was about 800 ft MSL overcast and 2 miles visibility. I remained on the vector until I flew through the localizer and received a query from ATC as to my intentions. My HSI showed a full deflection yet the GPS screen showed me flying through it. I realized too late that I had my garmin GNS 530 west set on GPS mode instead of vloc mode and therefore I was not receiving correct navigation information for the ILS approach. Rather than fly a missed approach and risk being late to pickup my passengers; I attempted to follow the GPS course indicated on the screen rather than switch from GPS mode to vloc mode and recapture. I descended below the minimum altitude at the marker for runway 10L had got an altitude warning from ATC telling me to pull up immediately. I broke out around 700 ft MSL and was within a few hundred feet of the interstate 5 bridge towers. The bottom line is that was really stupid and I should have gone missed or asked for vectors immediately as soon as I realized I flown through the localizer for runway 10L. I could have had ample time to reset the approach in the correct navigation mode. Since that incident I have amended my checklist to include a 'GPS mode' verification check on the before takeoff check as well as the before landing checklist. I believe this will help fix any future problems as long as I'm diligent with my checklist usage.

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Original NASA ASRS Text

Title: The pilot of a light turbine twin reports attempting to fly the ILS to Runway 10L at PDX with the course display set to GPS instead of VOR/LOC. Reporter descends to VMC below 700 feet MSL just west of the Interstate 5 bridge towers resulting in a low altitude call from ATC.

Narrative: I recently checked out and have less than 40 hours in the aircraft. I departed under IMC as a single pilot for a short flight in high workload IMC to PDX. The weather was low overcast and the temperature was near freezing. I departed with instructions to level at 4000 FT MSL. Shortly after departure I was switched to PDX Approach Control who gave me a vector to intercept the localizer for Runway 10L. I had all the correct frequencies tuned and identified; I had the correct weather and had briefed myself on the approach into PDX. The weather was about 800 FT MSL overcast and 2 miles visibility. I remained on the vector until I flew through the localizer and received a query from ATC as to my intentions. My HSI showed a full deflection yet the GPS screen showed me flying through it. I realized too late that I had my Garmin GNS 530 W set on GPS mode instead of VLOC mode and therefore I was not receiving correct navigation information for the ILS approach. Rather than fly a missed approach and risk being late to pickup my passengers; I attempted to follow the GPS course indicated on the screen rather than switch from GPS mode to VLOC mode and recapture. I descended below the minimum altitude at the marker for Runway 10L had got an altitude warning from ATC telling me to pull up immediately. I broke out around 700 FT MSL and was within a few hundred feet of the interstate 5 bridge towers. The bottom line is that was really stupid and I should have gone missed or asked for vectors immediately as soon as I realized I flown through the localizer for Runway 10L. I could have had ample time to reset the approach in the correct navigation mode. Since that incident I have amended my checklist to include a 'GPS MODE' verification check on the before takeoff check as well as the before landing checklist. I believe this will help fix any future problems as long as I'm diligent with my checklist usage.

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.