Narrative:

Eastbound, south of western most extent of phx class B airspace, descending to fly below 4000 ft floor of class B airspace, en route to chd airport. There is a mountain range you have to cross, then descend very rapidly to 4000 ft or less -- boundary of class B sector is defined by gila river, difficult to see distinctly even under half moon. Mountain pretty distinct, but to be conservative at night, I delayed starting my descent. Unsure of exactly where the river was, I turned south to try to stay out of class B airspace. Noticed probably the river to my right, which meant I was probably in class B airspace at about 4500 ft. I continued my descent to 3500 ft and turned toward chandler. Caused by defining class B boundary by river, not distinct at night, and my belief I could identify the river, which I couldn't. Contributing factor was making boundary so close to the mountain range. Solution is to pick a different route next time.

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Original NASA ASRS Text

Title: AN SMA PLT TRYING TO CIRCUMNAV CLASS B AIRSPACE, MISCALCULATED HIS RTE OF FLT VIOLATING THE AIRSPACE HE WAS TRYING TO AVOID.

Narrative: EBOUND, S OF WESTERN MOST EXTENT OF PHX CLASS B AIRSPACE, DSNDING TO FLY BELOW 4000 FT FLOOR OF CLASS B AIRSPACE, ENRTE TO CHD ARPT. THERE IS A MOUNTAIN RANGE YOU HAVE TO CROSS, THEN DSND VERY RAPIDLY TO 4000 FT OR LESS -- BOUNDARY OF CLASS B SECTOR IS DEFINED BY GILA RIVER, DIFFICULT TO SEE DISTINCTLY EVEN UNDER HALF MOON. MOUNTAIN PRETTY DISTINCT, BUT TO BE CONSERVATIVE AT NIGHT, I DELAYED STARTING MY DSCNT. UNSURE OF EXACTLY WHERE THE RIVER WAS, I TURNED S TO TRY TO STAY OUT OF CLASS B AIRSPACE. NOTICED PROBABLY THE RIVER TO MY R, WHICH MEANT I WAS PROBABLY IN CLASS B AIRSPACE AT ABOUT 4500 FT. I CONTINUED MY DSCNT TO 3500 FT AND TURNED TOWARD CHANDLER. CAUSED BY DEFINING CLASS B BOUNDARY BY RIVER, NOT DISTINCT AT NIGHT, AND MY BELIEF I COULD IDENT THE RIVER, WHICH I COULDN'T. CONTRIBUTING FACTOR WAS MAKING BOUNDARY SO CLOSE TO THE MOUNTAIN RANGE. SOLUTION IS TO PICK A DIFFERENT RTE NEXT TIME.

Data retrieved from NASA's ASRS site as of July 2007 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.