Narrative:

Inbound to dbq on an instrument flight plan; we were at 8;500 msl ready to begin our descent for a visual approach to the airport. Chicago center advised that the ATIS had changed and I should listen to it again. The new ATIS revealed that the airport had just gone IFR and they were now using the RNAV 18 approach. When I reported that I had the updated ATIS; center assigned a heading around 010 or 020; a descent to maintain 3;000 and said to expect vectors to the RNAV 18 approach. I took that to mean vectors to final on the RNAV 18 and activated vectors to final RNAV 18 on our garmin 430. I turned to the assigned heading and began the descent.we entered the cloud layer at about 4;000 feet. I was hand flying the aircraft and almost immediately upon cloud entry began feeling disoriented with a distinct lean to the right. Trusting the instruments I held the aircraft wings level and leveled off at 3;000. We were in and out of the base of the cloud layer at this point.still struggling with the disorientation but stable according to instruments we flew on expecting a turn to the right to vector to the final for RNAV 18. Instead the controller asked me for direct witok; an initial approach fix that was still further north of my position. Unfortunately; my GPS was already set for vectors to final (zugni); and didn't offer witok.now a little stressed and still fighting the disorientation; I tried to reset the GPS to show the witok waypoint but as I was working with the GPS; the aircraft began an actual turn to the right and a descent of about 500'; allowing a view of the ground. I recovered; stabilized and began climbing back to the assigned altitude but was already on a heading toward the final approach fix inside of witok. I decided that manipulating the GPS was a bad idea and asked the controller for a vector to zugni. Instead of a vector he questioned why I couldn't get to witok.before I could answer; he warned me that my altitude was 500' low asked if I could see the ground. I said I could and that we were inbound to zugni. He replied continue to zugni and contact tower. From there the rest of the approach was uneventful; we landed without incident and taxied to the ramp.I had used garmin 430s frequently in other aircraft and felt familiar with their operation.the feeling of disorientation was a surprise and a great distraction. Coupled with the unexpected instructions; I was overwhelmed. I continued to fly the aircraft and we got safely to landing but the experience was not ideal.going forward; I will fly more actual IMC approaches with an instructor in this aircraft to become solidly proficient. I also need to re-familiarize myself with editing GPS flight plans in the avionics. But; most importantly; I will now specify the initial approach fix I will use on an approach and ask the controller for that fix. In the case of the dbq RNAV 18 approach; there were four possible initial fixes I could have used; any of which; when planned for in the GPS would have prevented the confusing instructions at a time when my workload was heightened due to the disorientation.

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Original NASA ASRS Text

Title: General aviation pilot on an IFR flight plan reported getting vertigo while on vectors for an RNAV approach. While attempting to make GPS entries 500 feet of altitude is lost and the heading drifts to the right. ATC issues instructions that do not require GPS inputs and the approach is successfully flown.

Narrative: Inbound to DBQ on an instrument flight plan; we were at 8;500 msl ready to begin our descent for a visual approach to the airport. Chicago Center advised that the ATIS had changed and I should listen to it again. The new ATIS revealed that the airport had just gone IFR and they were now using the RNAV 18 approach. When I reported that I had the updated ATIS; Center assigned a heading around 010 or 020; a descent to maintain 3;000 and said to expect vectors to the RNAV 18 approach. I took that to mean vectors to final on the RNAV 18 and activated vectors to final RNAV 18 on our Garmin 430. I turned to the assigned heading and began the descent.We entered the cloud layer at about 4;000 feet. I was hand flying the aircraft and almost immediately upon cloud entry began feeling disoriented with a distinct lean to the right. Trusting the instruments I held the aircraft wings level and leveled off at 3;000. We were in and out of the base of the cloud layer at this point.Still struggling with the disorientation but stable according to instruments we flew on expecting a turn to the right to vector to the final for RNAV 18. Instead the controller asked me for direct WITOK; an initial approach fix that was still further north of my position. Unfortunately; my GPS was already set for vectors to final (ZUGNI); and didn't offer WITOK.Now a little stressed and still fighting the disorientation; I tried to reset the GPS to show the WITOK waypoint but as I was working with the GPS; the aircraft began an actual turn to the right and a descent of about 500'; allowing a view of the ground. I recovered; stabilized and began climbing back to the assigned altitude but was already on a heading toward the final approach fix inside of WITOK. I decided that manipulating the GPS was a bad idea and asked the controller for a vector to ZUGNI. Instead of a vector he questioned why I couldn't get to WITOK.Before I could answer; he warned me that my altitude was 500' low asked if I could see the ground. I said I could and that we were inbound to ZUGNI. He replied continue to ZUGNI and contact tower. From there the rest of the approach was uneventful; we landed without incident and taxied to the ramp.I had used Garmin 430s frequently in other aircraft and felt familiar with their operation.The feeling of disorientation was a surprise and a great distraction. Coupled with the unexpected instructions; I was overwhelmed. I continued to fly the aircraft and we got safely to landing but the experience was not ideal.Going forward; I will fly more actual IMC approaches with an instructor in this aircraft to become solidly proficient. I also need to re-familiarize myself with editing GPS flight plans in the avionics. But; most importantly; I will now specify the initial approach fix I will use on an approach and ask the controller for that fix. In the case of the DBQ RNAV 18 approach; there were four possible initial fixes I could have used; any of which; when planned for in the GPS would have prevented the confusing instructions at a time when my workload was heightened due to the disorientation.

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.