Narrative:

I was flying the aircraft. Captain was on the radios. The aircraft, an large transport, was on autoplt with LNAV and VNAV modes engaged. During descent into slc, the controller gave us a crossing restriction to cross 10 mi east of ffu at 17000 ft. As I was setting a waypoint 11 mi east of ffu (I usually use an extra mile as a pad), the captain asked for clarification of the clearance, to confirm 17 mi east at 10000 ft. The controller replied in the affirmative, so I changed the waypoint to 18 mi east of ffu at 10000 ft. At the VNAV computed top of descent point, the aircraft started down. We were subsequently given a vector of 220 degrees for spacing. During the descent, I began to become concerned the low altitude east of the wasatch mountains. Just as I was about to speak up, the controller asked about our altitude (we were just passing 15500 ft). The captain replied that we understood we were cleared to 10000 ft. The controller told us to level at 15000 ft, which was no problem since I had taken the aircraft off autoplt and began leveling off as soon as he spoke. We were then cleared direct to ffu, and the rest of the flight went smoothly. So, yet another case of miscom resulting in an altitude deviation (I read callback). Someone unfamiliar with the terrain and a cloudy night and this scenario could have had a tragic conclusion. Also, another good reason to accelerate development of data link communication. The skies are too crowded for busy terminal areas that provide altitude and airspeed restrictions in print at given pints to help these overworked controllers cope with the ever-increasing volume of traffic converging on the larger terminal areas. Ask any pilot (or controller for that matter) about a well-proceduralized approach and departure system, like atlanta, for instance. The system works well when everyone knows what to expect, and everyone is being predictable. A mountainous area is no place for an old fashioned ATC system. Supplemental information from acn 222299: I read back the clearance and got it wrong. The center gave us the clearance again. I readback the clearance and got it wrong. The center gave us the clearance again. I read back the clearance. The center said 'roger.' I watched the copilot enter my first readback that was wrong into the FMS and mode control panel. It didn't register that it was the wrong restriction.

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Original NASA ASRS Text

Title: AN ACR LGT CREW WAS DSNDING TO 10000 FT AT 17 MI. THE CLRNC WAS 17000 FT AT 10 MI.

Narrative: I WAS FLYING THE ACFT. CAPT WAS ON THE RADIOS. THE ACFT, AN LGT, WAS ON AUTOPLT WITH LNAV AND VNAV MODES ENGAGED. DURING DSCNT INTO SLC, THE CTLR GAVE US A XING RESTRICTION TO CROSS 10 MI E OF FFU AT 17000 FT. AS I WAS SETTING A WAYPOINT 11 MI E OF FFU (I USUALLY USE AN EXTRA MILE AS A PAD), THE CAPT ASKED FOR CLARIFICATION OF THE CLRNC, TO CONFIRM 17 MI E AT 10000 FT. THE CTLR REPLIED IN THE AFFIRMATIVE, SO I CHANGED THE WAYPOINT TO 18 MI EAST OF FFU AT 10000 FT. AT THE VNAV COMPUTED TOP OF DSCNT POINT, THE ACFT STARTED DOWN. WE WERE SUBSEQUENTLY GIVEN A VECTOR OF 220 DEGS FOR SPACING. DURING THE DSCNT, I BEGAN TO BECOME CONCERNED THE LOW ALT E OF THE WASATCH MOUNTAINS. JUST AS I WAS ABOUT TO SPEAK UP, THE CTLR ASKED ABOUT OUR ALT (WE WERE JUST PASSING 15500 FT). THE CAPT REPLIED THAT WE UNDERSTOOD WE WERE CLRED TO 10000 FT. THE CTLR TOLD US TO LEVEL AT 15000 FT, WHICH WAS NO PROBLEM SINCE I HAD TAKEN THE ACFT OFF AUTOPLT AND BEGAN LEVELING OFF AS SOON AS HE SPOKE. WE WERE THEN CLRED DIRECT TO FFU, AND THE REST OF THE FLT WENT SMOOTHLY. SO, YET ANOTHER CASE OF MISCOM RESULTING IN AN ALTDEV (I READ CALLBACK). SOMEONE UNFAMILIAR WITH THE TERRAIN AND A CLOUDY NIGHT AND THIS SCENARIO COULD HAVE HAD A TRAGIC CONCLUSION. ALSO, ANOTHER GOOD REASON TO ACCELERATE DEVELOPMENT OF DATA LINK COM. THE SKIES ARE TOO CROWDED FOR BUSY TERMINAL AREAS THAT PROVIDE ALT AND AIRSPD RESTRICTIONS IN PRINT AT GIVEN PINTS TO HELP THESE OVERWORKED CTLRS COPE WITH THE EVER-INCREASING VOLUME OF TFC CONVERGING ON THE LARGER TERMINAL AREAS. ASK ANY PLT (OR CTLR FOR THAT MATTER) ABOUT A WELL-PROCEDURALIZED APCH AND DEP SYS, LIKE ATLANTA, FOR INSTANCE. THE SYS WORKS WELL WHEN EVERYONE KNOWS WHAT TO EXPECT, AND EVERYONE IS BEING PREDICTABLE. A MOUNTAINOUS AREA IS NO PLACE FOR AN OLD FASHIONED ATC SYS. SUPPLEMENTAL INFO FROM ACN 222299: I READ BACK THE CLRNC AND GOT IT WRONG. THE CTR GAVE US THE CLRNC AGAIN. I READBACK THE CLRNC AND GOT IT WRONG. THE CTR GAVE US THE CLRNC AGAIN. I READ BACK THE CLRNC. THE CTR SAID 'ROGER.' I WATCHED THE COPLT ENTER MY FIRST READBACK THAT WAS WRONG INTO THE FMS AND MODE CTL PANEL. IT DIDN'T REGISTER THAT IT WAS THE WRONG RESTRICTION.

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.