Narrative:

Nearing the end of a hot; bumpy four-hour IFR flight to pvu that was largely conducted on oxygen at altitudes ranging from 8;000 to 14;000 feet MSL I was turned over to salt lake approach control north of the ymont intersection; given a vector and cleared from 14;000 ft to 13;000 ft MSL. The vector took me just north of pvu which was clearly visible. The location of the airport was clear and visibility was no factor. Winds were reported to be light and variable and the ATIS indicated runway 13 as the active. I requested the RNAV (GPS) runway 13 approach and was told to expect that. I loaded the approach into the GPS and verified that the waypoints were the same as were indicated on the paper chart; reviewed the approach and expected vectors to get me to zuxag (the IAF) without a need for a turn of greater than 90 degrees. As nearly as I can remember it; salt lake approach controller then said 'cleared direct ffu; cross ffu at or above 9000; cleared for the approach'. I must have been more tired and dehydrated than I thought I was; because I was having a struggle to figure out how to enter the approach from ffu without having to make a turn of greater than 90 degrees. Additionally; as I approached zuxag the GPS flight direction path indicated a need to make a left 270 degree turn rather than right turn I was expecting. I instinctively followed the indicated GPS flight path rather than doing what I earlier felt to be the right move; a turn to the right of more than 90 degrees. Shortly after entering the turn I disconnected the autopilot when roughly parallel and far offset from runway 13. When the controller then asked if I wanted the visual I said something to the effect of 'yes; thank you'. I was turned over to the tower and proceeded to position myself to enter the right downwind leg for runway 13 at a position just clear of a nearby rain cell. The tower indicated that the winds had changed dramatically from light and variable to 220 degrees at 16 knots gusting to 25 and asked if I wanted runway 18 instead of 13. I said yes and prepared to enter a right base for runway 18. While five hundred feet above pattern altitude about to make the turn to final the tower said that they had a low altitude alert. When I told them my current altitude they cleared me to land on runway 18. The last minute changes and altitude warning had the effect of concentrating my attention and I made an uneventful landing; back taxied per the instruction from the tower and taxied for fuel and overnight parking. I'm unsure what the source of the low altitude alert may have been; a false altitude report from my transponder or a glitch in the ATC computer but it certainly got my attention. The most disturbing part of this event filled; but ultimately safe arrival at pvu was the unanticipated clearance from ATC and the equally unanticipated steering directions provided by the GPS while converging on the IAF for the approach. I will spend some time on the garmin simulator trying to determine why the GPS reacted as it did. I will also caution myself in the future to not build-in false expectations regarding up-coming ATC instructions. The expectation that I would be vectored onto the leg from zuxag to dicot following ffu to avoid the greater than 90 degree turn led me to take actions that could have been dangerous had IMC prevailed. Flying into unfamiliar airports in complex airspace requires a degree of concentration that fatigue and dehydration make difficult.

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Original NASA ASRS Text

Title: GA pilot reported a GPS anomaly led to a poorly executed approach with an altitude alert.

Narrative: Nearing the end of a hot; bumpy four-hour IFR flight to PVU that was largely conducted on oxygen at altitudes ranging from 8;000 to 14;000 feet MSL I was turned over to Salt Lake Approach Control north of the YMONT intersection; given a vector and cleared from 14;000 ft to 13;000 ft MSL. The vector took me just north of PVU which was clearly visible. The location of the airport was clear and visibility was no factor. Winds were reported to be light and variable and the ATIS indicated RWY 13 as the active. I requested the RNAV (GPS) RWY 13 approach and was told to expect that. I loaded the approach into the GPS and verified that the waypoints were the same as were indicated on the paper chart; reviewed the approach and expected vectors to get me to ZUXAG (the IAF) without a need for a turn of greater than 90 degrees. As nearly as I can remember it; Salt Lake Approach Controller then said 'Cleared direct FFU; cross FFU at or above 9000; cleared for the approach'. I must have been more tired and dehydrated than I thought I was; because I was having a struggle to figure out how to enter the approach from FFU without having to make a turn of greater than 90 degrees. Additionally; as I approached ZUXAG the GPS flight direction path indicated a need to make a left 270 degree turn rather than right turn I was expecting. I instinctively followed the indicated GPS flight path rather than doing what I earlier felt to be the right move; a turn to the right of more than 90 degrees. Shortly after entering the turn I disconnected the autopilot when roughly parallel and far offset from RWY 13. When the controller then asked if I wanted the visual I said something to the effect of 'Yes; thank you'. I was turned over to the Tower and proceeded to position myself to enter the right downwind leg for RWY 13 at a position just clear of a nearby rain cell. The Tower indicated that the winds had changed dramatically from light and variable to 220 degrees at 16 knots gusting to 25 and asked if I wanted RWY 18 instead of 13. I said yes and prepared to enter a right base for RWY 18. While five hundred feet above pattern altitude about to make the turn to final the Tower said that they had a low altitude alert. When I told them my current altitude they cleared me to land on RWY 18. The last minute changes and altitude warning had the effect of concentrating my attention and I made an uneventful landing; back taxied per the instruction from the Tower and taxied for fuel and overnight parking. I'm unsure what the source of the low altitude alert may have been; a false altitude report from my transponder or a glitch in the ATC computer but it certainly got my attention. The most disturbing part of this event filled; but ultimately safe arrival at PVU was the unanticipated clearance from ATC and the equally unanticipated steering directions provided by the GPS while converging on the IAF for the approach. I will spend some time on the Garmin simulator trying to determine why the GPS reacted as it did. I will also caution myself in the future to not build-in false expectations regarding up-coming ATC instructions. The expectation that I would be vectored onto the leg from ZUXAG to DICOT following FFU to avoid the greater than 90 degree turn led me to take actions that could have been dangerous had IMC prevailed. Flying into unfamiliar airports in complex airspace requires a degree of concentration that fatigue and dehydration make difficult.

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.