Narrative:

While level at 5000 ft MSL during the en route portion of an IFR flight, I received instructions from the approach controller directing me to the OM for an ILS approach. From my standpoint, there was some confusion regarding the instructions at this point as I heard the controller clear me to 'descend at my discretion' and turn to a new heading. I acknowledged the instructions 'roger, skyhawk XXX is out of 5000 ft and turning 150 degrees' and began my descent and turn to the new heading. The controller, busy with other airplanes on the frequency, didn't acknowledge my transmission. After descending on the new heading, the controller came on frequency and reprimanded me for descending saying I was instructed to remain at 5000 ft. It was at this point that I realized I may have misunderstood the clearance to descend, if there in fact was one, and I immediately stopped the descent. The controller said he could give me a lower altitude in a few mins if that is what I wanted and there were no further xmissions regarding the misunderstanding after this. The rest of the flight progressed normally after this and I was soon switched to another frequency and controller. The obvious cause of this whole situation is that I misunderstood the clearance to descend. What further aggravated the problem was that the controller didn't catch my readback, after which I started and continued the descent. At no point did I descend with the willful intention of violating my clearance contrary to the controller instructions -- I descended based on the instructions I thought I heard the controller issue. Another factor that contributed to the situation was a busy frequency that may have caused me to misunderstand the instructions and which distraction the controller from hearing my readback. The corrective actions from my perspective are pretty straightforward. Obviously, in the future I'm going to make sure I get an acknowledgement from a controller before I execute instructions or a clearance (busy frequency or not). I'm also going to make sure I understand the instructions or clearance well enough so that I can at least write them down for future reference -- whether that be a few seconds or a week later.

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Original NASA ASRS Text

Title: A COMMERCIAL INST RATED PLT FLYING A C172 AT MVW FAILED TO MAINTAIN THE ALT ASSIGNED BY ATC.

Narrative: WHILE LEVEL AT 5000 FT MSL DURING THE ENRTE PORTION OF AN IFR FLT, I RECEIVED INSTRUCTIONS FROM THE APCH CTLR DIRECTING ME TO THE OM FOR AN ILS APCH. FROM MY STANDPOINT, THERE WAS SOME CONFUSION REGARDING THE INSTRUCTIONS AT THIS POINT AS I HEARD THE CTLR CLR ME TO 'DSND AT MY DISCRETION' AND TURN TO A NEW HDG. I ACKNOWLEDGED THE INSTRUCTIONS 'ROGER, SKYHAWK XXX IS OUT OF 5000 FT AND TURNING 150 DEGS' AND BEGAN MY DSCNT AND TURN TO THE NEW HDG. THE CTLR, BUSY WITH OTHER AIRPLANES ON THE FREQ, DIDN'T ACKNOWLEDGE MY XMISSION. AFTER DSNDING ON THE NEW HDG, THE CTLR CAME ON FREQ AND REPRIMANDED ME FOR DSNDING SAYING I WAS INSTRUCTED TO REMAIN AT 5000 FT. IT WAS AT THIS POINT THAT I REALIZED I MAY HAVE MISUNDERSTOOD THE CLRNC TO DSND, IF THERE IN FACT WAS ONE, AND I IMMEDIATELY STOPPED THE DSCNT. THE CTLR SAID HE COULD GIVE ME A LOWER ALT IN A FEW MINS IF THAT IS WHAT I WANTED AND THERE WERE NO FURTHER XMISSIONS REGARDING THE MISUNDERSTANDING AFTER THIS. THE REST OF THE FLT PROGRESSED NORMALLY AFTER THIS AND I WAS SOON SWITCHED TO ANOTHER FREQ AND CTLR. THE OBVIOUS CAUSE OF THIS WHOLE SIT IS THAT I MISUNDERSTOOD THE CLRNC TO DSND. WHAT FURTHER AGGRAVATED THE PROB WAS THAT THE CTLR DIDN'T CATCH MY READBACK, AFTER WHICH I STARTED AND CONTINUED THE DSCNT. AT NO POINT DID I DSND WITH THE WILLFUL INTENTION OF VIOLATING MY CLRNC CONTRARY TO THE CTLR INSTRUCTIONS -- I DSNDED BASED ON THE INSTRUCTIONS I THOUGHT I HEARD THE CTLR ISSUE. ANOTHER FACTOR THAT CONTRIBUTED TO THE SIT WAS A BUSY FREQ THAT MAY HAVE CAUSED ME TO MISUNDERSTAND THE INSTRUCTIONS AND WHICH DISTR THE CTLR FROM HEARING MY READBACK. THE CORRECTIVE ACTIONS FROM MY PERSPECTIVE ARE PRETTY STRAIGHTFORWARD. OBVIOUSLY, IN THE FUTURE I'M GOING TO MAKE SURE I GET AN ACKNOWLEDGEMENT FROM A CTLR BEFORE I EXECUTE INSTRUCTIONS OR A CLRNC (BUSY FREQ OR NOT). I'M ALSO GOING TO MAKE SURE I UNDERSTAND THE INSTRUCTIONS OR CLRNC WELL ENOUGH SO THAT I CAN AT LEAST WRITE THEM DOWN FOR FUTURE REF -- WHETHER THAT BE A FEW SECONDS OR A WK LATER.

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.