Narrative:

While descending on the modesto 2 arrival to sfo we were cleared to 'descend and maintain FL240 and expect to cross cedes at 11000 ft and 250 KTS.' we descended below FL240 to approximately FL233 before the error was noticed and we returned to FL240. We were not aware of any conflicts and ZOA never called us or commented on the altitude error. We were subsequently cleared lower and the rest of the flight was normal. The main contributing factors to this excursion were: 1) our airline has 2 different types of autoplts and altitude alerting system. One has an aural tone, a 1000 ft to go, and autolevels if selected, the other will not automatic level and has a light with no tone. Our aircraft this day had no automatic level or tone alert. 2) my attention was diverted approaching leveloff by a large error in the omega course and my effort to disconnect the omega and correct to course. 3) my descent calculation showed me high for the expected crossing restr and I was recalculating my descent. 4) I am a low time pilot in the B727 with less than 60 hours since qualifying, and my previous aircraft had the automatic level autoplt feature. I was still aware of these traps and let myself be distracted at the time I needed to be leveling off. Standardizing the autoplts and altitude alerting system would have helped avoiding this problem.

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Original NASA ASRS Text

Title: FLC OF A B727 OVERSHOT ASSIGNED ALT ON DSCNT DUE TO FAILING TO LEVEL AT ASSIGNED ALT.

Narrative: WHILE DSNDING ON THE MODESTO 2 ARR TO SFO WE WERE CLRED TO 'DSND AND MAINTAIN FL240 AND EXPECT TO CROSS CEDES AT 11000 FT AND 250 KTS.' WE DSNDED BELOW FL240 TO APPROX FL233 BEFORE THE ERROR WAS NOTICED AND WE RETURNED TO FL240. WE WERE NOT AWARE OF ANY CONFLICTS AND ZOA NEVER CALLED US OR COMMENTED ON THE ALT ERROR. WE WERE SUBSEQUENTLY CLRED LOWER AND THE REST OF THE FLT WAS NORMAL. THE MAIN CONTRIBUTING FACTORS TO THIS EXCURSION WERE: 1) OUR AIRLINE HAS 2 DIFFERENT TYPES OF AUTOPLTS AND ALT ALERTING SYS. ONE HAS AN AURAL TONE, A 1000 FT TO GO, AND AUTOLEVELS IF SELECTED, THE OTHER WILL NOT AUTO LEVEL AND HAS A LIGHT WITH NO TONE. OUR ACFT THIS DAY HAD NO AUTO LEVEL OR TONE ALERT. 2) MY ATTN WAS DIVERTED APCHING LEVELOFF BY A LARGE ERROR IN THE OMEGA COURSE AND MY EFFORT TO DISCONNECT THE OMEGA AND CORRECT TO COURSE. 3) MY DSCNT CALCULATION SHOWED ME HIGH FOR THE EXPECTED XING RESTR AND I WAS RECALCULATING MY DSCNT. 4) I AM A LOW TIME PLT IN THE B727 WITH LESS THAN 60 HRS SINCE QUALIFYING, AND MY PREVIOUS ACFT HAD THE AUTO LEVEL AUTOPLT FEATURE. I WAS STILL AWARE OF THESE TRAPS AND LET MYSELF BE DISTRACTED AT THE TIME I NEEDED TO BE LEVELING OFF. STANDARDIZING THE AUTOPLTS AND ALT ALERTING SYS WOULD HAVE HELPED AVOIDING THIS PROB.

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.