Narrative:

During our descent into dfw on the byp 3 (little rock transition), we were cleared to FL240. The descent was made in vertical speed on the autoplt from FL350 to FL240. Both the copilot and I verbalized FL240 and pointed to FL240 in the window. Going through FL260 we discussed a southern deviation due to a thunderstorm just north of the arrival. ATC granted the left deviation and we made the turn in heading select and slowed our descent rate to 1000 FPM. During the descent I was having trouble clearing my ears because of a sinus infection. About the same time, the flight attendant in charge called to enter the cockpit to ask a question. After I opened the door and said hello, I turned to check our altitude. We were passing through FL235 when I first noticed the problem. I quickly rotated the thumbwheel to raise the nose, but by the time we started climbing we had descended to FL230. Right after we started climbing, ATC called about our altitude. I don't know if I bumped the altitude select button with a finger while changing our descent rate or if the copilot bumped it, but we are both sure FL240 was dialed in. (FL220 was set in the window.) with the distrs of the thunderstorms, the flight attendant, and my ears, we missed the leveloff at FL240. I never thought the automatic leveloff would not capture at FL240. I will be very sure to keep an eye on the altitude select window, just in case it ever gets bumped again.

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Original NASA ASRS Text

Title: CAPT OF AN MD88 OVERSHOT DSCNT ASSIGNED ALT DUE TO EITHER BUMPING THE ALT SELECT BUTTON, OR INADVERTENTLY SETTING THE WRONG ALT.

Narrative: DURING OUR DSCNT INTO DFW ON THE BYP 3 (LITTLE ROCK TRANSITION), WE WERE CLRED TO FL240. THE DSCNT WAS MADE IN VERT SPD ON THE AUTOPLT FROM FL350 TO FL240. BOTH THE COPLT AND I VERBALIZED FL240 AND POINTED TO FL240 IN THE WINDOW. GOING THROUGH FL260 WE DISCUSSED A SOUTHERN DEV DUE TO A TSTM JUST N OF THE ARR. ATC GRANTED THE L DEV AND WE MADE THE TURN IN HDG SELECT AND SLOWED OUR DSCNT RATE TO 1000 FPM. DURING THE DSCNT I WAS HAVING TROUBLE CLRING MY EARS BECAUSE OF A SINUS INFECTION. ABOUT THE SAME TIME, THE FLT ATTENDANT IN CHARGE CALLED TO ENTER THE COCKPIT TO ASK A QUESTION. AFTER I OPENED THE DOOR AND SAID HELLO, I TURNED TO CHK OUR ALT. WE WERE PASSING THROUGH FL235 WHEN I FIRST NOTICED THE PROB. I QUICKLY ROTATED THE THUMBWHEEL TO RAISE THE NOSE, BUT BY THE TIME WE STARTED CLBING WE HAD DSNDED TO FL230. RIGHT AFTER WE STARTED CLBING, ATC CALLED ABOUT OUR ALT. I DON'T KNOW IF I BUMPED THE ALT SELECT BUTTON WITH A FINGER WHILE CHANGING OUR DSCNT RATE OR IF THE COPLT BUMPED IT, BUT WE ARE BOTH SURE FL240 WAS DIALED IN. (FL220 WAS SET IN THE WINDOW.) WITH THE DISTRS OF THE TSTMS, THE FLT ATTENDANT, AND MY EARS, WE MISSED THE LEVELOFF AT FL240. I NEVER THOUGHT THE AUTO LEVELOFF WOULD NOT CAPTURE AT FL240. I WILL BE VERY SURE TO KEEP AN EYE ON THE ALT SELECT WINDOW, JUST IN CASE IT EVER GETS BUMPED AGAIN.

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.