ZLC controller mistakenly cleared air carrier B767 to FL240, thinking FL250. Flight crew observed traffic 4 mi behind when scanning TCASII screen after hearing ATC clear another aircraft out of FL240. Captain asked to call ZLC. ATC investigation determined controller error.

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Attributes
ACN	387892
Time
Date	199712
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : ffu airport : slc
State Reference	UT
Altitude	msl bound lower : 24000 msl bound upper : 31000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zlc tower : ont
Operator	common carrier : air carrier
Make Model Name	B767-300 and 300 ER
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	arrival star : star
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 220 flight time total : 5000 flight time type : 2000
ASRS Report	387892
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 20000 flight time type : 150
ASRS Report	387889
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 24000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

First officer received a clearance from ZLC to descend to FL240 and expect spane intersection at 16000 ft. First officer read back 'leaving FL310 for FL240' and proceeded to descend to FL240. Upon reaching FL245 captain and first officer heard the ATC controller tell an aircraft to start their descent from FL240. At this point the captain looked down at the TCASII screen and noticed an aircraft 4 mi behind us that was just starting a descent out of FL240. After landing we were instructed to call slc and talk to the area controller. The controller seemed to think we were cleared to FL250.

Original NASA ASRS Text

Title: ZLC CTLR MISTAKENLY CLRED ACR B767 TO FL240, THINKING FL250. FLC OBSERVED TFC 4 MI BEHIND WHEN SCANNING TCASII SCREEN AFTER HEARING ATC CLR ANOTHER ACFT OUT OF FL240. CAPT ASKED TO CALL ZLC. ATC INVESTIGATION DETERMINED CTLR ERROR.

Narrative: FO RECEIVED A CLRNC FROM ZLC TO DSND TO FL240 AND EXPECT SPANE INTXN AT 16000 FT. FO READ BACK 'LEAVING FL310 FOR FL240' AND PROCEEDED TO DSND TO FL240. UPON REACHING FL245 CAPT AND FO HEARD THE ATC CTLR TELL AN ACFT TO START THEIR DSCNT FROM FL240. AT THIS POINT THE CAPT LOOKED DOWN AT THE TCASII SCREEN AND NOTICED AN ACFT 4 MI BEHIND US THAT WAS JUST STARTING A DSCNT OUT OF FL240. AFTER LNDG WE WERE INSTRUCTED TO CALL SLC AND TALK TO THE AREA CTLR. THE CTLR SEEMED TO THINK WE WERE CLRED TO FL250.

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.