Narrative:

While flying at 10000' I, as PNF, received a clearance to climb to what I thought was 29000'. I set it in the altitude window and started climb to and leveled at 29000'. After about 3 mins level at 29000' I was asked by ZMA what altitude we were (29000'), and was told to immediately descend to 28000' (which we did). ZMA said we were cleared to 28000', not 29000'. Flight continued uneventfully. I think 1 of 2 things happened. If I read back 29000', then no error was made by me. If I read back 28000', then I either set 29000' and misread callback, or the altitude setting knob was inadvertently hit at a later time prior to level off. This is a possibility as the first officer said he re-engaged the profile mode and armed it by pulling out the altitude setting switch. It's possible while doing this, the altitude was moved one click to 29000' from 28000. Which scenario occurred, I do not know for sure, but perhaps another form of altitude setting west/O a very sensitive knob would be helpful. If I set 29000' and read back 28000', then I cannot offer any suggestions to improve that, except that the first officer was on another frequency talking to our company when I received and acknowledged the climb. A possible answer would be to not talk to company. The ACARS system was designed for this purpose, but it does not currently work. Supplemental information from acn 108978: we had been cleared to FL280 and somehow FL290 was set in our altitude select. I was on the radio talking to company when the clearance was received from ATC and did not hear it. The altitude select on this aircraft serves 2 functions, it selects altitude and arms profile mode of the FMS. We don't know if the captain selected FL280 and it jumped 1 notch when he released the knob, or if he selected FL280 and it jumped a notch when I selected profile mode and armed it for climb.

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Original NASA ASRS Text

Title: FLT CREW OF WDB CLEARED FL280, CLIMBED TO FL290. ARTCC CTLR INTERVENED AND FLT DESCENDED TO FL280.

Narrative: WHILE FLYING AT 10000' I, AS PNF, RECEIVED A CLRNC TO CLB TO WHAT I THOUGHT WAS 29000'. I SET IT IN THE ALT WINDOW AND STARTED CLB TO AND LEVELED AT 29000'. AFTER ABOUT 3 MINS LEVEL AT 29000' I WAS ASKED BY ZMA WHAT ALT WE WERE (29000'), AND WAS TOLD TO IMMEDIATELY DSND TO 28000' (WHICH WE DID). ZMA SAID WE WERE CLRED TO 28000', NOT 29000'. FLT CONTINUED UNEVENTFULLY. I THINK 1 OF 2 THINGS HAPPENED. IF I READ BACK 29000', THEN NO ERROR WAS MADE BY ME. IF I READ BACK 28000', THEN I EITHER SET 29000' AND MISREAD CALLBACK, OR THE ALT SETTING KNOB WAS INADVERTENTLY HIT AT A LATER TIME PRIOR TO LEVEL OFF. THIS IS A POSSIBILITY AS THE F/O SAID HE RE-ENGAGED THE PROFILE MODE AND ARMED IT BY PULLING OUT THE ALT SETTING SWITCH. IT'S POSSIBLE WHILE DOING THIS, THE ALT WAS MOVED ONE CLICK TO 29000' FROM 28000. WHICH SCENARIO OCCURRED, I DO NOT KNOW FOR SURE, BUT PERHAPS ANOTHER FORM OF ALT SETTING W/O A VERY SENSITIVE KNOB WOULD BE HELPFUL. IF I SET 29000' AND READ BACK 28000', THEN I CANNOT OFFER ANY SUGGESTIONS TO IMPROVE THAT, EXCEPT THAT THE F/O WAS ON ANOTHER FREQ TALKING TO OUR COMPANY WHEN I RECEIVED AND ACKNOWLEDGED THE CLB. A POSSIBLE ANSWER WOULD BE TO NOT TALK TO COMPANY. THE ACARS SYS WAS DESIGNED FOR THIS PURPOSE, BUT IT DOES NOT CURRENTLY WORK. SUPPLEMENTAL INFO FROM ACN 108978: WE HAD BEEN CLRED TO FL280 AND SOMEHOW FL290 WAS SET IN OUR ALT SELECT. I WAS ON THE RADIO TALKING TO COMPANY WHEN THE CLRNC WAS RECEIVED FROM ATC AND DID NOT HEAR IT. THE ALT SELECT ON THIS ACFT SERVES 2 FUNCTIONS, IT SELECTS ALT AND ARMS PROFILE MODE OF THE FMS. WE DON'T KNOW IF THE CAPT SELECTED FL280 AND IT JUMPED 1 NOTCH WHEN HE RELEASED THE KNOB, OR IF HE SELECTED FL280 AND IT JUMPED A NOTCH WHEN I SELECTED PROFILE MODE AND ARMED IT FOR CLB.

Data retrieved from NASA's ASRS site as of August 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.