Narrative:

Cleared by ZTL to descend from FL290 to what I (first officer) thought was FL240. I observed captain set what I thought was FL240 in altitude alert window and he began descent on autoplt. I then contacted company on non ATC radio for arrival questions, etc. When that was complete I again looked at our flight INS and observed us descending through approximately FL233 for what was in the altitude alert window (230). I questioned the captain about 'weren't we just cleared to 240?' he thought FL230 was our clearance, but said to confirm altitude cleared to with ZTL (128.75 MHZ). Center said it had been FL240. I advised center we were correcting. Altitude hold was pushed on flight control panel and I set FL240 back in altitude window. Aircraft descend (on autoplt) bottomed out at FL228 as captain corrected back toward FL240. At that time ZTL told us to maintain FL230, which we did. Captain spoke to controller about misunderstanding as captain stated he thought we were cleared to FL230. Controller said, 'no problem.' no conflict observed by crew and none reported by ZTL. Supplemental information from acn 145559: as a final note, we suspect FL240 was dialed into our MCP, but the selector switch was not seated in its detent and reverted to FL230 during our descent.

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Original NASA ASRS Text

Title: ALT DEVIATION.

Narrative: CLRED BY ZTL TO DSND FROM FL290 TO WHAT I (F/O) THOUGHT WAS FL240. I OBSERVED CAPT SET WHAT I THOUGHT WAS FL240 IN ALT ALERT WINDOW AND HE BEGAN DSNT ON AUTOPLT. I THEN CONTACTED COMPANY ON NON ATC RADIO FOR ARR QUESTIONS, ETC. WHEN THAT WAS COMPLETE I AGAIN LOOKED AT OUR FLT INS AND OBSERVED US DSNDING THROUGH APPROX FL233 FOR WHAT WAS IN THE ALT ALERT WINDOW (230). I QUESTIONED THE CAPT ABOUT 'WEREN'T WE JUST CLRED TO 240?' HE THOUGHT FL230 WAS OUR CLRNC, BUT SAID TO CONFIRM ALT CLRED TO WITH ZTL (128.75 MHZ). CENTER SAID IT HAD BEEN FL240. I ADVISED CENTER WE WERE CORRECTING. ALT HOLD WAS PUSHED ON FLT CTL PANEL AND I SET FL240 BACK IN ALT WINDOW. ACFT DSND (ON AUTOPLT) BOTTOMED OUT AT FL228 AS CAPT CORRECTED BACK TOWARD FL240. AT THAT TIME ZTL TOLD US TO MAINTAIN FL230, WHICH WE DID. CAPT SPOKE TO CTLR ABOUT MISUNDERSTANDING AS CAPT STATED HE THOUGHT WE WERE CLRED TO FL230. CTLR SAID, 'NO PROB.' NO CONFLICT OBSERVED BY CREW AND NONE RPTED BY ZTL. SUPPLEMENTAL INFO FROM ACN 145559: AS A FINAL NOTE, WE SUSPECT FL240 WAS DIALED INTO OUR MCP, BUT THE SELECTOR SWITCH WAS NOT SEATED IN ITS DETENT AND REVERTED TO FL230 DURING OUR DSNT.

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.