Narrative:

Flying air carrier X on IFR flight to slc at FL370. On ZLC frequency, 128.55, cleared to cross 30 mi west of slc VOR at 15000'. We were approximately 80 mi west at the time, so initiated a descent with speed brakes deployed and descended at approximately 4000 FPM in order to comply with crossing restriction. Thunderstorms in the area caused a need for deviations from vector headings. At around 17600' ATC told us to level at 17000'. I set the altitude in the master control panel altitude window, but due to the excessive descent rate, the autoplt failed to capture 17000' and the descent continued until I disconnected the autoplt and leveled the aircraft at around 16600'. I then climbed back to 17000' and reconnected the autoplt. ATC did not mention the loss of altitude and the flight/approach to slc continued west/O incident. Approaching slc from the west, ATC is often slow to give descent clearance, which then causes higher than normal descent rates, usually compounded by the fact that there is often a strong tailwind in that area. Also, there is no standard crossing point, so flight planning is not possible until the moment the clearance is received. In this incident, even if the autoplt had captured 17000', I don't believe that the aircraft could have leveled at that altitude west/O causing discomfort and even harm to the passenger and F/a's. Supplemental information from acn 92312: we had a 65 KT tailwind at FL370, and we began our descent immediately with full speed brakes to make the restriction. Controller issued descent clearance with restriction too late with existing conditions (i.e., tailwind). Controller issued a new altitude that was nearly impossible to make with existing conditions (i.e., 4000 FPM rate of descent). Pilots should have informed controllers of problem in leveling off in that short of time.

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Original NASA ASRS Text

Title: ACR UNABLE TO COMPLY WITH ATC ALT ASSIGNMENT. COMPLAINT REFERENCE ATC PROC INTO SLC FROM THE WEST.

Narrative: FLYING ACR X ON IFR FLT TO SLC AT FL370. ON ZLC FREQ, 128.55, CLRED TO CROSS 30 MI W OF SLC VOR AT 15000'. WE WERE APPROX 80 MI W AT THE TIME, SO INITIATED A DSCNT WITH SPD BRAKES DEPLOYED AND DSNDED AT APPROX 4000 FPM IN ORDER TO COMPLY WITH XING RESTRICTION. TSTMS IN THE AREA CAUSED A NEED FOR DEVIATIONS FROM VECTOR HDGS. AT AROUND 17600' ATC TOLD US TO LEVEL AT 17000'. I SET THE ALT IN THE MASTER CTL PANEL ALT WINDOW, BUT DUE TO THE EXCESSIVE DSCNT RATE, THE AUTOPLT FAILED TO CAPTURE 17000' AND THE DSCNT CONTINUED UNTIL I DISCONNECTED THE AUTOPLT AND LEVELED THE ACFT AT AROUND 16600'. I THEN CLBED BACK TO 17000' AND RECONNECTED THE AUTOPLT. ATC DID NOT MENTION THE LOSS OF ALT AND THE FLT/APCH TO SLC CONTINUED W/O INCIDENT. APCHING SLC FROM THE W, ATC IS OFTEN SLOW TO GIVE DSCNT CLRNC, WHICH THEN CAUSES HIGHER THAN NORMAL DSCNT RATES, USUALLY COMPOUNDED BY THE FACT THAT THERE IS OFTEN A STRONG TAILWIND IN THAT AREA. ALSO, THERE IS NO STANDARD XING POINT, SO FLT PLANNING IS NOT POSSIBLE UNTIL THE MOMENT THE CLRNC IS RECEIVED. IN THIS INCIDENT, EVEN IF THE AUTOPLT HAD CAPTURED 17000', I DON'T BELIEVE THAT THE ACFT COULD HAVE LEVELED AT THAT ALT W/O CAUSING DISCOMFORT AND EVEN HARM TO THE PAX AND F/A'S. SUPPLEMENTAL INFO FROM ACN 92312: WE HAD A 65 KT TAILWIND AT FL370, AND WE BEGAN OUR DSCNT IMMEDIATELY WITH FULL SPD BRAKES TO MAKE THE RESTRICTION. CTLR ISSUED DSCNT CLRNC WITH RESTRICTION TOO LATE WITH EXISTING CONDITIONS (I.E., TAILWIND). CTLR ISSUED A NEW ALT THAT WAS NEARLY IMPOSSIBLE TO MAKE WITH EXISTING CONDITIONS (I.E., 4000 FPM RATE OF DSCNT). PLTS SHOULD HAVE INFORMED CTLRS OF PROB IN LEVELING OFF IN THAT SHORT OF TIME.

Data retrieved from NASA's ASRS site as of August 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.