A B737-200 flying in ZAU airspace, reads back the wrong altitude they were cleared to. After a 1500 ft deviation, the controller notices the deviation and reclears the aircraft back to a lower altitude. Controller allegedly states he did not hear the erroneous readback.

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Attributes
ACN	375064
Time
Date	199707
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : jot
State Reference	IL
Altitude	msl bound lower : 31000 msl bound upper : 32500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : atp pilot : commercial
Experience	flight time last 90 days : 163 flight time total : 14081 flight time type : 4000
ASRS Report	375064
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time total : 5900 flight time type : 210
ASRS Report	375500
Events
Anomaly	altitude deviation : overshoot other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

While climbing from an intermediate altitude of FL290 to FL330, the center controller asked what altitude we were cleared to. When we responded 'FL330,' he asked us to level off at our present altitude and we leveled off at FL325. After about 30 seconds, he stated that he had cleared us to FL310 and may have missed our readback of 'FL330.' he reclred us to FL310, and we descended to FL310. Both of us on the flight deck had understood our clearance to be FL330 and I had set FL330 in the altitude alert panel, since I was flying, using the autoflt system. We do not know if we misheard the clearance or the controller mistakenly cleared us to the wrong altitude. There was no apparent traffic conflict. Supplemental information from acn 375500: ZAU cleared us to FL330. This seemed odd, first officer read back FL330, captain set FL330 in MCP after verbalizing clearance. At approximately FL321 ZAU asked what altitude we were cleared to, we said FL330. Center said stop climb at FL320, then reclred us back to FL310. Center said he missed the readback.

Original NASA ASRS Text

Title: A B737-200 FLYING IN ZAU AIRSPACE, READS BACK THE WRONG ALT THEY WERE CLRED TO. AFTER A 1500 FT DEV, THE CTLR NOTICES THE DEV AND RECLEARS THE ACFT BACK TO A LOWER ALT. CTLR ALLEGEDLY STATES HE DID NOT HEAR THE ERRONEOUS READBACK.

Narrative: WHILE CLBING FROM AN INTERMEDIATE ALT OF FL290 TO FL330, THE CTR CTLR ASKED WHAT ALT WE WERE CLRED TO. WHEN WE RESPONDED 'FL330,' HE ASKED US TO LEVEL OFF AT OUR PRESENT ALT AND WE LEVELED OFF AT FL325. AFTER ABOUT 30 SECONDS, HE STATED THAT HE HAD CLRED US TO FL310 AND MAY HAVE MISSED OUR READBACK OF 'FL330.' HE RECLRED US TO FL310, AND WE DSNDED TO FL310. BOTH OF US ON THE FLT DECK HAD UNDERSTOOD OUR CLRNC TO BE FL330 AND I HAD SET FL330 IN THE ALT ALERT PANEL, SINCE I WAS FLYING, USING THE AUTOFLT SYS. WE DO NOT KNOW IF WE MISHEARD THE CLRNC OR THE CTLR MISTAKENLY CLRED US TO THE WRONG ALT. THERE WAS NO APPARENT TFC CONFLICT. SUPPLEMENTAL INFO FROM ACN 375500: ZAU CLRED US TO FL330. THIS SEEMED ODD, FO READ BACK FL330, CAPT SET FL330 IN MCP AFTER VERBALIZING CLRNC. AT APPROX FL321 ZAU ASKED WHAT ALT WE WERE CLRED TO, WE SAID FL330. CTR SAID STOP CLB AT FL320, THEN RECLRED US BACK TO FL310. CTR SAID HE MISSED THE 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.