Narrative:

As a brand new pilot (just recently passing my check ride) flying on a cross country, except for just transitioning directly above the phx airport between north and south, I had never requested to fly through the phoenix class B airspace before. I could have flown underneath the airspace but it seemed to me that it would be safer to get radar following, flying through the class B airspace. So after checking the WX I filed a flight plan, for an altitude more than 500 ft below the bottoms of the clouds appropriate for the direction of flight. Shortly after takeoff, I opened my flight plan choosing an altitude suggested by flight center and then contacted ATC to request clearance to fly through their class B airspace and shortly thereafter received clearance, entered the airspace, ascending to my requested and assigned altitude. After flying for 15 mins or so, as I approached the southern end of the class B airspace, I realized some of the clouds ahead were at or below my altitude, and it became increasingly apparent that I would have to rapidly descend. So I told ATC that I wanted to change frequencys to modify my flight plan. They approved the frequency change and I called, whereupon the flight center accepted the change. I switched back to the ATC frequency and began descending, not realizing that I hadn't gotten ATC to officially approve my altitude change first. By this time I was probably outside the edge of the class B airspace. 5 mins or so flying went by when the ATC controller called asking if I was descending. I said yes. I was definitely outside his airspace by then and he seemed to realize it suddenly, so he terminated radar following without any fuss. No danger to other aircraft, no incident occurred, and though the flight controller released me immediately from radar, in the future, when making changes, I will always think -- ATC first, flight plan second. Also, I will allow a wider margin for WX ceiling differences from predictions.

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Original NASA ASRS Text

Title: NEW PLT EXPERIENCED CONFUSION REGARDING VFR FILED ALT AND ATC ASSIGNED ALT TRANSITIONING PHX CLASS B AIRSPACE.

Narrative: AS A BRAND NEW PLT (JUST RECENTLY PASSING MY CHK RIDE) FLYING ON A XCOUNTRY, EXCEPT FOR JUST TRANSITIONING DIRECTLY ABOVE THE PHX ARPT BTWN N AND S, I HAD NEVER REQUESTED TO FLY THROUGH THE PHOENIX CLASS B AIRSPACE BEFORE. I COULD HAVE FLOWN UNDERNEATH THE AIRSPACE BUT IT SEEMED TO ME THAT IT WOULD BE SAFER TO GET RADAR FOLLOWING, FLYING THROUGH THE CLASS B AIRSPACE. SO AFTER CHKING THE WX I FILED A FLT PLAN, FOR AN ALT MORE THAN 500 FT BELOW THE BOTTOMS OF THE CLOUDS APPROPRIATE FOR THE DIRECTION OF FLT. SHORTLY AFTER TKOF, I OPENED MY FLT PLAN CHOOSING AN ALT SUGGESTED BY FLT CTR AND THEN CONTACTED ATC TO REQUEST CLRNC TO FLY THROUGH THEIR CLASS B AIRSPACE AND SHORTLY THEREAFTER RECEIVED CLRNC, ENTERED THE AIRSPACE, ASCENDING TO MY REQUESTED AND ASSIGNED ALT. AFTER FLYING FOR 15 MINS OR SO, AS I APCHED THE SOUTHERN END OF THE CLASS B AIRSPACE, I REALIZED SOME OF THE CLOUDS AHEAD WERE AT OR BELOW MY ALT, AND IT BECAME INCREASINGLY APPARENT THAT I WOULD HAVE TO RAPIDLY DSND. SO I TOLD ATC THAT I WANTED TO CHANGE FREQS TO MODIFY MY FLT PLAN. THEY APPROVED THE FREQ CHANGE AND I CALLED, WHEREUPON THE FLT CTR ACCEPTED THE CHANGE. I SWITCHED BACK TO THE ATC FREQ AND BEGAN DSNDING, NOT REALIZING THAT I HADN'T GOTTEN ATC TO OFFICIALLY APPROVE MY ALT CHANGE FIRST. BY THIS TIME I WAS PROBABLY OUTSIDE THE EDGE OF THE CLASS B AIRSPACE. 5 MINS OR SO FLYING WENT BY WHEN THE ATC CTLR CALLED ASKING IF I WAS DSNDING. I SAID YES. I WAS DEFINITELY OUTSIDE HIS AIRSPACE BY THEN AND HE SEEMED TO REALIZE IT SUDDENLY, SO HE TERMINATED RADAR FOLLOWING WITHOUT ANY FUSS. NO DANGER TO OTHER ACFT, NO INCIDENT OCCURRED, AND THOUGH THE FLT CTLR RELEASED ME IMMEDIATELY FROM RADAR, IN THE FUTURE, WHEN MAKING CHANGES, I WILL ALWAYS THINK -- ATC FIRST, FLT PLAN SECOND. ALSO, I WILL ALLOW A WIDER MARGIN FOR WX CEILING DIFFERENCES FROM PREDICTIONS.

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.