Narrative:

While descending into the slc area we were given a different SID than we were navigating on. We had just gotten the new SID out and reprogrammed the FMC with all the new crossing restrs when ATC also assigned a new runway for landing. Around this time we were given a descent from the low 20's to 17000 ft. The captain initiated the descent using vertical speed while still LNAV'ing in on the SID. At about FL190 I began accomplishing my descent flow. At that time we began experiencing moderate chop and it looked as if we would soon be descending into a cloud layer that looked to have some rough air in it. The captain made a call to the cabin crew to ask them to take their seats and remained on line with them for a short while telling them about the expected rides ahead. I looked at my altimeter verifying it was set at local altimeter of 29.37. It was at this time we were passing through about 17200 ft. I looked at his altimeter and it was still set at standard. I called 29.37 on the altimeter and he began dialing it down. Once he realized we were going to level off below our assigned altitude; he disengaged the autoplt and began correcting our flight path. We descended to around 16500 ft or so before reversing our trend. At this time ATC asked our altitude and I told them we were late getting the proper altimeter setting and were correcting to 17000 ft. We then leveled at 17000 ft and reengaged the autoplt. Loaded the new runway in the FMC; briefed the new approach; accomplished the descent checklist and landed uneventfully. When the captain began talking to the cabin crew; we should have accomplished a change in controls of the aircraft. A simple; 'you have the aircraft/I have the aircraft;' would probably have prevented this from happening.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN ACR CREW DESCENDING TO SLC WAS GIVEN A LATE SID CHANGE WITH A LOW ALTIMETER SETTING AND MOMENTARILY WENT BELOW THE ASSIGNED ALT.

Narrative: WHILE DSNDING INTO THE SLC AREA WE WERE GIVEN A DIFFERENT SID THAN WE WERE NAVING ON. WE HAD JUST GOTTEN THE NEW SID OUT AND REPROGRAMMED THE FMC WITH ALL THE NEW XING RESTRS WHEN ATC ALSO ASSIGNED A NEW RWY FOR LNDG. AROUND THIS TIME WE WERE GIVEN A DSCNT FROM THE LOW 20'S TO 17000 FT. THE CAPT INITIATED THE DSCNT USING VERT SPD WHILE STILL LNAV'ING IN ON THE SID. AT ABOUT FL190 I BEGAN ACCOMPLISHING MY DSCNT FLOW. AT THAT TIME WE BEGAN EXPERIENCING MODERATE CHOP AND IT LOOKED AS IF WE WOULD SOON BE DSNDING INTO A CLOUD LAYER THAT LOOKED TO HAVE SOME ROUGH AIR IN IT. THE CAPT MADE A CALL TO THE CABIN CREW TO ASK THEM TO TAKE THEIR SEATS AND REMAINED ON LINE WITH THEM FOR A SHORT WHILE TELLING THEM ABOUT THE EXPECTED RIDES AHEAD. I LOOKED AT MY ALTIMETER VERIFYING IT WAS SET AT LCL ALTIMETER OF 29.37. IT WAS AT THIS TIME WE WERE PASSING THROUGH ABOUT 17200 FT. I LOOKED AT HIS ALTIMETER AND IT WAS STILL SET AT STANDARD. I CALLED 29.37 ON THE ALTIMETER AND HE BEGAN DIALING IT DOWN. ONCE HE REALIZED WE WERE GOING TO LEVEL OFF BELOW OUR ASSIGNED ALT; HE DISENGAGED THE AUTOPLT AND BEGAN CORRECTING OUR FLT PATH. WE DSNDED TO AROUND 16500 FT OR SO BEFORE REVERSING OUR TREND. AT THIS TIME ATC ASKED OUR ALT AND I TOLD THEM WE WERE LATE GETTING THE PROPER ALTIMETER SETTING AND WERE CORRECTING TO 17000 FT. WE THEN LEVELED AT 17000 FT AND REENGAGED THE AUTOPLT. LOADED THE NEW RWY IN THE FMC; BRIEFED THE NEW APCH; ACCOMPLISHED THE DSCNT CHKLIST AND LANDED UNEVENTFULLY. WHEN THE CAPT BEGAN TALKING TO THE CABIN CREW; WE SHOULD HAVE ACCOMPLISHED A CHANGE IN CTLS OF THE ACFT. A SIMPLE; 'YOU HAVE THE ACFT/I HAVE THE ACFT;' WOULD PROBABLY HAVE PREVENTED THIS FROM HAPPENING.

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.