Narrative:

After departure from gso airport runway 23 climbed to 6000', runway heading, as cleared. Several other small private aircraft also on frequency being controled by departure control. At least 2 of which had weak radios and covered up control xmissions. The first transmission to us by departure control was covered by one such operator. The second transmission was to turn left to 040, climb to, what we thought was 14000'. The clearance was read back, left turn to 040, climb to 14000'. 14000' was set into altitude alert. Approaching 10000', we were instructed to switch to ZDC and checked in at 10000' climbing to 14000'. Center informed us that we had only been cleared to 10000'. At that point the actual altitude was 10300'. FMC was accelerating to normal climb speed and climbing at approximately 500 FPM. The 10000' cleared altitude was recaptured immediately. Altitude cleared to was misunderstood because of radio interference by other aircraft on frequency. The wrong readback was not caught by controller, possibly because that transmission was also covered. A gross error was averted by a timely frequency change and a FMC that reduces climb rates to a minimum in order to accelerate to the best climb speed at 10000'. I'm not sure what to recommend to prevent a recurrence. I believe though that requiring operators of small aircraft to wear headsets instead of using the overhead speakers would eliminate some of the covered xmissions by allowing them thus to hear more clearly when the frequency is clear before they transmit.

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Original NASA ASRS Text

Title: ALT EXCURSION OCCURS DURING A PERIOD OF FREQ CONGESTION.

Narrative: AFTER DEP FROM GSO ARPT RWY 23 CLIMBED TO 6000', RWY HDG, AS CLRED. SEVERAL OTHER SMALL PRIVATE ACFT ALSO ON FREQ BEING CTLED BY DEP CTL. AT LEAST 2 OF WHICH HAD WEAK RADIOS AND COVERED UP CTL XMISSIONS. THE FIRST XMISSION TO US BY DEP CTL WAS COVERED BY ONE SUCH OPERATOR. THE SECOND XMISSION WAS TO TURN LEFT TO 040, CLIMB TO, WHAT WE THOUGHT WAS 14000'. THE CLRNC WAS READ BACK, LEFT TURN TO 040, CLIMB TO 14000'. 14000' WAS SET INTO ALT ALERT. APCHING 10000', WE WERE INSTRUCTED TO SWITCH TO ZDC AND CHECKED IN AT 10000' CLIMBING TO 14000'. CENTER INFORMED US THAT WE HAD ONLY BEEN CLRED TO 10000'. AT THAT POINT THE ACTUAL ALT WAS 10300'. FMC WAS ACCELERATING TO NORMAL CLIMB SPEED AND CLIMBING AT APPROX 500 FPM. THE 10000' CLRED ALT WAS RECAPTURED IMMEDIATELY. ALT CLRED TO WAS MISUNDERSTOOD BECAUSE OF RADIO INTERFERENCE BY OTHER ACFT ON FREQ. THE WRONG READBACK WAS NOT CAUGHT BY CTLR, POSSIBLY BECAUSE THAT XMISSION WAS ALSO COVERED. A GROSS ERROR WAS AVERTED BY A TIMELY FREQ CHANGE AND A FMC THAT REDUCES CLIMB RATES TO A MINIMUM IN ORDER TO ACCELERATE TO THE BEST CLIMB SPEED AT 10000'. I'M NOT SURE WHAT TO RECOMMEND TO PREVENT A RECURRENCE. I BELIEVE THOUGH THAT REQUIRING OPERATORS OF SMALL ACFT TO WEAR HEADSETS INSTEAD OF USING THE OVERHEAD SPEAKERS WOULD ELIMINATE SOME OF THE COVERED XMISSIONS BY ALLOWING THEM THUS TO HEAR MORE CLEARLY WHEN THE FREQ IS CLEAR BEFORE THEY XMIT.

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.