Air carrier questions controller about altitude clearance that they have read back and find to be too low for adequate terrain clearance. Controller had made no comment about the improper readback.

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Attributes
ACN	452465
Time
Date	199910
Day	Tue
Local Time Of Day	0001 To 0600
Place
Locale Reference	airport : hib.airport
State Reference	MN
Altitude	msl single value : 4000
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	tracon : dlh.tracon
Operator	common carrier : air carrier
Make Model Name	Saab-Scania Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	approach : instrument precision arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : atp pilot : cfi
Experience	flight time last 90 days : 120 flight time total : 3300 flight time type : 1700
ASRS Report	452465
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	other
Supplementary
Problem Areas	FAA Flight Crew Human Performance ATC Human Performance
Primary Problem	FAA

Narrative:

While on ATC vectors for the ILS runway 31 at hibbing, mn (hib) the controller issued us a descent clearance. I read back 'descend to 2600 ft,' which is what I thought the controller said. The first officer questioned the altitude as we started down. We checked the MSA on the chart and realized it is 3400 ft. He said that it sounded too low for a vector and I agreed. I asked the controller to verify the altitude and he corrected us and said it was 3600 ft. We only descended to 4000 ft MSL when we clarified the altitude. The controller missed our readback and we could have descended dangerously close to the terrain. I didn't notice at first as I had only slept 3 hours that night. We were on a continuos duty overnight and had been on for 11 hours 50 mins. Had I been better rested I may have caught the error sooner. Still, the FAA decision to not have to correct mistakes by controllers is wrong. Controllers are not required to verify readbacks, and as in this case can have the potential for disaster. This could have been avoided if we were rested and the controller corrected our wrong readback.

Original NASA ASRS Text

Title: ACR QUESTIONS CTLR ABOUT ALT CLRNC THAT THEY HAVE READ BACK AND FIND TO BE TOO LOW FOR ADEQUATE TERRAIN CLRNC. CTLR HAD MADE NO COMMENT ABOUT THE IMPROPER READBACK.

Narrative: WHILE ON ATC VECTORS FOR THE ILS RWY 31 AT HIBBING, MN (HIB) THE CTLR ISSUED US A DSCNT CLRNC. I READ BACK 'DSND TO 2600 FT,' WHICH IS WHAT I THOUGHT THE CTLR SAID. THE FO QUESTIONED THE ALT AS WE STARTED DOWN. WE CHKED THE MSA ON THE CHART AND REALIZED IT IS 3400 FT. HE SAID THAT IT SOUNDED TOO LOW FOR A VECTOR AND I AGREED. I ASKED THE CTLR TO VERIFY THE ALT AND HE CORRECTED US AND SAID IT WAS 3600 FT. WE ONLY DSNDED TO 4000 FT MSL WHEN WE CLARIFIED THE ALT. THE CTLR MISSED OUR READBACK AND WE COULD HAVE DSNDED DANGEROUSLY CLOSE TO THE TERRAIN. I DIDN'T NOTICE AT FIRST AS I HAD ONLY SLEPT 3 HRS THAT NIGHT. WE WERE ON A CONTINUOS DUTY OVERNIGHT AND HAD BEEN ON FOR 11 HRS 50 MINS. HAD I BEEN BETTER RESTED I MAY HAVE CAUGHT THE ERROR SOONER. STILL, THE FAA DECISION TO NOT HAVE TO CORRECT MISTAKES BY CTLRS IS WRONG. CTLRS ARE NOT REQUIRED TO VERIFY READBACKS, AND AS IN THIS CASE CAN HAVE THE POTENTIAL FOR DISASTER. THIS COULD HAVE BEEN AVOIDED IF WE WERE RESTED AND THE CTLR CORRECTED OUR WRONG READBACK.

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.