Narrative:

During climb out from phx, ATC cleared us to several intermediate altitudes. The last clearance received from ZAB was to climb from FL190 to FL330. While climbing through FL250, center advised us to maintain FL250. After asking the controller if she intended for us to descend back down to the altitude, she stated the previous clearance issued was to FL230. We then stated that FL330 was received and read back. She reissued FL250 and no apparent traffic conflict existed. Both pilots in the cockpit distinctly heard FL330. An acm in the jump seat also heard FL330. The problem arose from either: 1) a misassigned altitude, or 2) an altitude that was not distinctly transmitted but sounded distinctly like a different altitude. This flight was the first of the day in early afternoon. Fatigue was not a factor, nor were other outside performance factors involved. Contributing factors include: 1) unclear transmission of #'south which are heard clearly as something different, 2) also, certain voice modulations in a particular frequency range are less discernable, and 3) controllers not listening or responding to a readback. Solution: problem could be eased by identing those controllers and making them aware of the need to improve the quality of their transmission by 1) grater amplitude with certain #'south, or 2) slight pause between certain #'south; i.e. 'Fl tree...tree zero, fl two...tree zero.'

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING CLIMBOUT FROM PHX.

Narrative: DURING CLBOUT FROM PHX, ATC CLRED US TO SEVERAL INTERMEDIATE ALTS. THE LAST CLRNC RECEIVED FROM ZAB WAS TO CLB FROM FL190 TO FL330. WHILE CLBING THROUGH FL250, CENTER ADVISED US TO MAINTAIN FL250. AFTER ASKING THE CTLR IF SHE INTENDED FOR US TO DSND BACK DOWN TO THE ALT, SHE STATED THE PREVIOUS CLRNC ISSUED WAS TO FL230. WE THEN STATED THAT FL330 WAS RECEIVED AND READ BACK. SHE REISSUED FL250 AND NO APPARENT TFC CONFLICT EXISTED. BOTH PLTS IN THE COCKPIT DISTINCTLY HEARD FL330. AN ACM IN THE JUMP SEAT ALSO HEARD FL330. THE PROB AROSE FROM EITHER: 1) A MISASSIGNED ALT, OR 2) AN ALT THAT WAS NOT DISTINCTLY XMITTED BUT SOUNDED DISTINCTLY LIKE A DIFFERENT ALT. THIS FLT WAS THE FIRST OF THE DAY IN EARLY AFTERNOON. FATIGUE WAS NOT A FACTOR, NOR WERE OTHER OUTSIDE PERFORMANCE FACTORS INVOLVED. CONTRIBUTING FACTORS INCLUDE: 1) UNCLEAR XMISSION OF #'S WHICH ARE HEARD CLEARLY AS SOMETHING DIFFERENT, 2) ALSO, CERTAIN VOICE MODULATIONS IN A PARTICULAR FREQ RANGE ARE LESS DISCERNABLE, AND 3) CTLRS NOT LISTENING OR RESPONDING TO A READBACK. SOLUTION: PROB COULD BE EASED BY IDENTING THOSE CTLRS AND MAKING THEM AWARE OF THE NEED TO IMPROVE THE QUALITY OF THEIR XMISSION BY 1) GRATER AMPLITUDE WITH CERTAIN #'S, OR 2) SLIGHT PAUSE BTWN CERTAIN #'S; I.E. 'FL TREE...TREE ZERO, FL TWO...TREE ZERO.'

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.