Narrative:

While on a direct clearance to slc airport at FL310, I (PNF) received a descent clearance to 17000 ft. I acknowledged a descent to 10000 ft and set the altitude alerter. My first officer (PF) acknowledged the clearance to 10000 ft. During the descent we were vectored 15 degrees right for about 10 mi and then back to direct slc. Passing 15000 ft, I stated to the first officer that I wasn't comfortable with the 10000 ft descent clearance with a 9410 ft obstruction on the planned approach. He concurred and stated that he was going to level at 12000 ft to slow the aircraft for confign. This was approximately 15 mi west of slc airport. Passing 13000 ft, center called and asked altitude we were cleared to. I stated 10000 ft, but we were leveling at 12000 ft. He then stated that we were cleared to 17000 ft to maintain 12000 ft and contact approach control. We contacted approach control and completed the flight. Upon deplaning, I contacted ZLC operational duty supervisor and requested this incident be looked into. Their quality assurance division determined that 1) we were cleared to 17000 ft. 2) I acknowledged and read back 10000 ft. 3) the controller did not catch this error. 4) a LOA between slc approach control and ZLC requires a handoff at 17000 ft when aircraft are cleared direct to slc and not on an arrival. 5) slc approach contacted ZLC due to our altitude being below 17000 ft and our position in relation to the airport without a handoff. 6) there was no loss of separation. 7) there were no altitude alerts. Both flight crew and controller were operating at the end of their duty periods on the back side of the clock. We were tired. Although uncomfortable, we/I did not question the controller about the altitude assignment. We were aware of the terrain but failed to question a clearance that we knew to be abnormal. Supplemental information from acn 489117: during the approach briefing, I briefed both MSA and the highest obstacle depicted on the plan view, which in this case is a spot elevation of 9410 ft MSL, northeast of the field. In retrospect, we should have immediately questioned the clearance to 10000 ft since we were aware of the terrain. 2 days prior, we were in slc and cleared for a visual to runway 17 from 15000 ft and 10 mi from the field with traffic at our 1 O'clock and 10 mi executing a visual to runway 16R. During this approach we were required to execute a 360 degree turn in order to descend while avoiding traffic on runway 16R. This was the reason I planned to be level at 12000 ft 15 mi east of the field. I was lulled into a false sense of security since everything seemed to be going according to plan or nearly so. Supplemental information from acn 489118: air carrier X landing slc was given pilot's discretion descent to 17000 ft. Pilot read back 10000 ft and the readback error was not caught. Aircraft went into slc approach descending to 10000 ft instead of descending to 17000 ft which constituted the pilot deviation.

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Original NASA ASRS Text

Title: DC8 FLC OVERSHOT ASSIGNED ALT DUE TO INCORRECT READBACK MISSED BY ZLC CTLR.

Narrative: WHILE ON A DIRECT CLRNC TO SLC ARPT AT FL310, I (PNF) RECEIVED A DSCNT CLRNC TO 17000 FT. I ACKNOWLEDGED A DSCNT TO 10000 FT AND SET THE ALT ALERTER. MY FO (PF) ACKNOWLEDGED THE CLRNC TO 10000 FT. DURING THE DSCNT WE WERE VECTORED 15 DEGS R FOR ABOUT 10 MI AND THEN BACK TO DIRECT SLC. PASSING 15000 FT, I STATED TO THE FO THAT I WASN'T COMFORTABLE WITH THE 10000 FT DSCNT CLRNC WITH A 9410 FT OBSTRUCTION ON THE PLANNED APCH. HE CONCURRED AND STATED THAT HE WAS GOING TO LEVEL AT 12000 FT TO SLOW THE ACFT FOR CONFIGN. THIS WAS APPROX 15 MI W OF SLC ARPT. PASSING 13000 FT, CTR CALLED AND ASKED ALT WE WERE CLRED TO. I STATED 10000 FT, BUT WE WERE LEVELING AT 12000 FT. HE THEN STATED THAT WE WERE CLRED TO 17000 FT TO MAINTAIN 12000 FT AND CONTACT APCH CTL. WE CONTACTED APCH CTL AND COMPLETED THE FLT. UPON DEPLANING, I CONTACTED ZLC OPERATIONAL DUTY SUPVR AND REQUESTED THIS INCIDENT BE LOOKED INTO. THEIR QUALITY ASSURANCE DIVISION DETERMINED THAT 1) WE WERE CLRED TO 17000 FT. 2) I ACKNOWLEDGED AND READ BACK 10000 FT. 3) THE CTLR DID NOT CATCH THIS ERROR. 4) A LOA BTWN SLC APCH CTL AND ZLC REQUIRES A HDOF AT 17000 FT WHEN ACFT ARE CLRED DIRECT TO SLC AND NOT ON AN ARR. 5) SLC APCH CONTACTED ZLC DUE TO OUR ALT BEING BELOW 17000 FT AND OUR POS IN RELATION TO THE ARPT WITHOUT A HDOF. 6) THERE WAS NO LOSS OF SEPARATION. 7) THERE WERE NO ALT ALERTS. BOTH FLC AND CTLR WERE OPERATING AT THE END OF THEIR DUTY PERIODS ON THE BACK SIDE OF THE CLOCK. WE WERE TIRED. ALTHOUGH UNCOMFORTABLE, WE/I DID NOT QUESTION THE CTLR ABOUT THE ALT ASSIGNMENT. WE WERE AWARE OF THE TERRAIN BUT FAILED TO QUESTION A CLRNC THAT WE KNEW TO BE ABNORMAL. SUPPLEMENTAL INFO FROM ACN 489117: DURING THE APCH BRIEFING, I BRIEFED BOTH MSA AND THE HIGHEST OBSTACLE DEPICTED ON THE PLAN VIEW, WHICH IN THIS CASE IS A SPOT ELEVATION OF 9410 FT MSL, NE OF THE FIELD. IN RETROSPECT, WE SHOULD HAVE IMMEDIATELY QUESTIONED THE CLRNC TO 10000 FT SINCE WE WERE AWARE OF THE TERRAIN. 2 DAYS PRIOR, WE WERE IN SLC AND CLRED FOR A VISUAL TO RWY 17 FROM 15000 FT AND 10 MI FROM THE FIELD WITH TFC AT OUR 1 O'CLOCK AND 10 MI EXECUTING A VISUAL TO RWY 16R. DURING THIS APCH WE WERE REQUIRED TO EXECUTE A 360 DEG TURN IN ORDER TO DSND WHILE AVOIDING TFC ON RWY 16R. THIS WAS THE REASON I PLANNED TO BE LEVEL AT 12000 FT 15 MI E OF THE FIELD. I WAS LULLED INTO A FALSE SENSE OF SECURITY SINCE EVERYTHING SEEMED TO BE GOING ACCORDING TO PLAN OR NEARLY SO. SUPPLEMENTAL INFO FROM ACN 489118: ACR X LNDG SLC WAS GIVEN PLT'S DISCRETION DSCNT TO 17000 FT. PLT READ BACK 10000 FT AND THE READBACK ERROR WAS NOT CAUGHT. ACFT WENT INTO SLC APCH DSNDING TO 10000 FT INSTEAD OF DSNDING TO 17000 FT WHICH CONSTITUTED THE PLTDEV.

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.