Narrative:

Approximately 40 mi south of ZZZ we were given a clearance to cross ZZZ at FL280; then descend pilot's discretion to FL240. Put crossing restr in FMC and set FL240 in MCP window. Just before top of descent we got an altitude alert horn and we noticed the MCP altitude was showing FL500. I reset the MCP altitude to FL240 and elected to start down early using vertical speed to make crossing restr. At approximately FL320 the altitude alert horn went off again and the MCP window reset to FL500 again. We again reset FL240 in MCP window. This sequence of events happened again at approximately FL310 and again we reset the MCP altitude to FL240. Each time this happened I verified MCP altitudes with PNF. Anyway; I got distraction enough that we crossed ZZZ at FL284 instead of assigned altitude FL280. Distrs were: 1) working with ZZZ1 maintenance on another problem via ACARS. 2) 3 times the altitude alerter went off and the MCP altitude reset to FL500. This happened in a short time span. 3) ATC asked us to slow to 250 KTS about 2 mi before ZZZ which I did after I got below FL280. Bottom-line: I got distraction enough with the MCP altitude alert horn and MCP altitude window resetting to FL500 3 times in a short time frame and close to ZZZ. I had good intentions when I started down earlier via vertical speed rather than waiting for VNAV to start the descent; but I didn't have enough vertical speed selected for what was needed to make the crossing restr. Maybe I should have let VNAV do the flying; or totally disregard the problem and work on the restr first; or maybe I should have told ATC that we could not make the restr that we were working on a problem.

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Original NASA ASRS Text

Title: A B737 PILOT REPORTS MISSING A CROSSING ALT RESTRICTION CAUSED BY THE DISTRACTION OF THE MCP ALT SET RESETTING TO FL500 SEVERAL TIMES DURING DSCNT.

Narrative: APPROX 40 MI S OF ZZZ WE WERE GIVEN A CLRNC TO CROSS ZZZ AT FL280; THEN DSND PLT'S DISCRETION TO FL240. PUT XING RESTR IN FMC AND SET FL240 IN MCP WINDOW. JUST BEFORE TOP OF DSCNT WE GOT AN ALT ALERT HORN AND WE NOTICED THE MCP ALT WAS SHOWING FL500. I RESET THE MCP ALT TO FL240 AND ELECTED TO START DOWN EARLY USING VERT SPD TO MAKE XING RESTR. AT APPROX FL320 THE ALT ALERT HORN WENT OFF AGAIN AND THE MCP WINDOW RESET TO FL500 AGAIN. WE AGAIN RESET FL240 IN MCP WINDOW. THIS SEQUENCE OF EVENTS HAPPENED AGAIN AT APPROX FL310 AND AGAIN WE RESET THE MCP ALT TO FL240. EACH TIME THIS HAPPENED I VERIFIED MCP ALTS WITH PNF. ANYWAY; I GOT DISTR ENOUGH THAT WE CROSSED ZZZ AT FL284 INSTEAD OF ASSIGNED ALT FL280. DISTRS WERE: 1) WORKING WITH ZZZ1 MAINT ON ANOTHER PROB VIA ACARS. 2) 3 TIMES THE ALT ALERTER WENT OFF AND THE MCP ALT RESET TO FL500. THIS HAPPENED IN A SHORT TIME SPAN. 3) ATC ASKED US TO SLOW TO 250 KTS ABOUT 2 MI BEFORE ZZZ WHICH I DID AFTER I GOT BELOW FL280. BOTTOM-LINE: I GOT DISTR ENOUGH WITH THE MCP ALT ALERT HORN AND MCP ALT WINDOW RESETTING TO FL500 3 TIMES IN A SHORT TIME FRAME AND CLOSE TO ZZZ. I HAD GOOD INTENTIONS WHEN I STARTED DOWN EARLIER VIA VERT SPD RATHER THAN WAITING FOR VNAV TO START THE DSCNT; BUT I DIDN'T HAVE ENOUGH VERT SPD SELECTED FOR WHAT WAS NEEDED TO MAKE THE XING RESTR. MAYBE I SHOULD HAVE LET VNAV DO THE FLYING; OR TOTALLY DISREGARD THE PROB AND WORK ON THE RESTR FIRST; OR MAYBE I SHOULD HAVE TOLD ATC THAT WE COULD NOT MAKE THE RESTR THAT WE WERE WORKING ON A PROB.

Data retrieved from NASA's ASRS site as of May 2009 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.