Narrative:

ATC facility: I think it was ZBW. Location: 30 mi west cyper extol arrival lga. Cruising at FL290, given pilot's discretion FL210 expect cyper at FL180. Captain's leg. I set FL210 in altitude select window, did the point and shoot. Captain began descent and I called ATC to advise. Later, passing approximately FL230 or FL240, ATC gave us direct cyper, and we turned direct cyper. Short time later, ATC checked out altitude, we were 300 ft below FL210 and descending. ATC told us to return to FL210. Captain said he reset to FL180 because that was what he had understood. I never saw him change the altitude select window. Rest of flight continued normally. Interesting note is that we had both said we were both tired. It would appear that the altitude problem was a combination of misunderstood clearance and fatigue.

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Original NASA ASRS Text

Title: ALTDEV. DSCNT BELOW CLRED ALT. MISCOM AND MISUNDERSTANDING WITHIN FLC ABOUT ALT CLRNC. FATIGUE A FACTOR.

Narrative: ATC FACILITY: I THINK IT WAS ZBW. LOCATION: 30 MI W CYPER EXTOL ARRIVAL LGA. CRUISING AT FL290, GIVEN PLT'S DISCRETION FL210 EXPECT CYPER AT FL180. CAPT'S LEG. I SET FL210 IN ALT SELECT WINDOW, DID THE POINT AND SHOOT. CAPT BEGAN DSCNT AND I CALLED ATC TO ADVISE. LATER, PASSING APPROX FL230 OR FL240, ATC GAVE US DIRECT CYPER, AND WE TURNED DIRECT CYPER. SHORT TIME LATER, ATC CHKED OUT ALT, WE WERE 300 FT BELOW FL210 AND DSNDING. ATC TOLD US TO RETURN TO FL210. CAPT SAID HE RESET TO FL180 BECAUSE THAT WAS WHAT HE HAD UNDERSTOOD. I NEVER SAW HIM CHANGE THE ALT SELECT WINDOW. REST OF FLT CONTINUED NORMALLY. INTERESTING NOTE IS THAT WE HAD BOTH SAID WE WERE BOTH TIRED. IT WOULD APPEAR THAT THE ALT PROB WAS A COMBINATION OF MISUNDERSTOOD CLRNC AND FATIGUE.

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.