Narrative:

Scheduled service from san to mcc included flying the wimps 5 arrival. Flight proceeded normally on the STAR and was descending to cross twane at 11000 ft. During this phase of the descent, as the PNF, I left the center frequency to obtain an updated ATIS and to contact the company for a gate assignment upon arrival at mcc. I experienced some difficulty interpreting the revised ATIS since the instrument approach in use had been changed. Of the 2 approachs to the south, 1 was not transmitting due to scheduled maintenance while the other had inoperative DME. Once I understood the ATIS, I then had difficulty contacting the station. I had to call 3 times before I was able to communicate with them and receive the gate information. A normally 1-2 min absence from the frequency became 4-5 mins. Meanwhile, the captain was descending toward the 11000 ft leveloff and I noticed that he was speaking to ATC as he confirmed descending through 11000 ft. At this point, I thought that he had received clearance to descend lower. As we approached 10500 ft, I pointed to the altitude set in the altitude alerter to remind him to reset the new altitude clearance. He immediately added power and began to climb back to 11000 ft. At this time I returned to the ATC frequency to hear him discussing his climb back to 11000 ft with approach control. It turns out that his communication with ATC was simply a frequency change and not a new altitude assignment, and in the confusion (he was also apparently monitoring the company frequency) the PF descended toward 10000 ft rather than the assigned 11000 ft. Subsequent conversation with the TRACON controllers indicated no conflict resulted from our deviation. This type of event has been described many times by NASA. There is no substitute for increased vigilance when things get busy and, as always, the first rule is to aviate -- then the other duties follow.

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Original NASA ASRS Text

Title: B737-200 FLC DSNDS BELOW ASSIGNED ALT DUE TO DISTR OF ATC COM.

Narrative: SCHEDULED SVC FROM SAN TO MCC INCLUDED FLYING THE WIMPS 5 ARR. FLT PROCEEDED NORMALLY ON THE STAR AND WAS DSNDING TO CROSS TWANE AT 11000 FT. DURING THIS PHASE OF THE DSCNT, AS THE PNF, I LEFT THE CTR FREQ TO OBTAIN AN UPDATED ATIS AND TO CONTACT THE COMPANY FOR A GATE ASSIGNMENT UPON ARR AT MCC. I EXPERIENCED SOME DIFFICULTY INTERPRETING THE REVISED ATIS SINCE THE INST APCH IN USE HAD BEEN CHANGED. OF THE 2 APCHS TO THE S, 1 WAS NOT XMITTING DUE TO SCHEDULED MAINT WHILE THE OTHER HAD INOP DME. ONCE I UNDERSTOOD THE ATIS, I THEN HAD DIFFICULTY CONTACTING THE STATION. I HAD TO CALL 3 TIMES BEFORE I WAS ABLE TO COMMUNICATE WITH THEM AND RECEIVE THE GATE INFO. A NORMALLY 1-2 MIN ABSENCE FROM THE FREQ BECAME 4-5 MINS. MEANWHILE, THE CAPT WAS DSNDING TOWARD THE 11000 FT LEVELOFF AND I NOTICED THAT HE WAS SPEAKING TO ATC AS HE CONFIRMED DSNDING THROUGH 11000 FT. AT THIS POINT, I THOUGHT THAT HE HAD RECEIVED CLRNC TO DSND LOWER. AS WE APCHED 10500 FT, I POINTED TO THE ALT SET IN THE ALT ALERTER TO REMIND HIM TO RESET THE NEW ALT CLRNC. HE IMMEDIATELY ADDED PWR AND BEGAN TO CLB BACK TO 11000 FT. AT THIS TIME I RETURNED TO THE ATC FREQ TO HEAR HIM DISCUSSING HIS CLB BACK TO 11000 FT WITH APCH CTL. IT TURNS OUT THAT HIS COM WITH ATC WAS SIMPLY A FREQ CHANGE AND NOT A NEW ALT ASSIGNMENT, AND IN THE CONFUSION (HE WAS ALSO APPARENTLY MONITORING THE COMPANY FREQ) THE PF DSNDED TOWARD 10000 FT RATHER THAN THE ASSIGNED 11000 FT. SUBSEQUENT CONVERSATION WITH THE TRACON CTLRS INDICATED NO CONFLICT RESULTED FROM OUR DEV. THIS TYPE OF EVENT HAS BEEN DESCRIBED MANY TIMES BY NASA. THERE IS NO SUBSTITUTE FOR INCREASED VIGILANCE WHEN THINGS GET BUSY AND, AS ALWAYS, THE FIRST RULE IS TO AVIATE -- THEN THE OTHER DUTIES FOLLOW.

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.