Air carrier flight crew misunderstood clearance.

Attributes
ACN	432910
Time
Date	199904
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iow.airport
State Reference	IA
Altitude	msl bound lower : 31000 msl bound upper : 35000
Environment
Flight Conditions	Mixed
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : flight engineer pilot : commercial pilot : atp pilot : multi engine
Experience	flight time last 90 days : 230 flight time total : 6000 flight time type : 2000
ASRS Report	432910
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : turbulence non adherence : clearance
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	controller : issued new clearance none taken : detected after the fact
Supplementary
Problem Areas	Weather ATC Human Performance Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While at FL310, we encountered turbulence. Asked controller if FL330 was available for a short period, and he said no. Then asked if FL350 was available and he said yes, but he would have to give us a radar vector for traffic separation. We conferred on this in the cockpit, and then said that would be fine and we were issued a turn to a 250 degree heading and we read back a clearance to FL350. At FL335 the controller queried us as to our altitude, and said we were never issued a climb clearance to FL350. He said there was no problem and no conflict and instructed us to continue climb to FL350. We (captain/first officer) had set and confirmed FL350 in the altitude alert when we thought we had a climb clearance. I think the similar heading and the semantics of the exchange between us and the controller, led us to perceive that if we took the heading, then we had a clearance to FL350. When the controller missed our readback of FL350 (he was busy with a lot of WX deviations), it only confirmed our belief we had clearance.

Original NASA ASRS Text

Title: ACR FLC MISUNDERSTOOD CLRNC.

Narrative: WHILE AT FL310, WE ENCOUNTERED TURB. ASKED CTLR IF FL330 WAS AVAILABLE FOR A SHORT PERIOD, AND HE SAID NO. THEN ASKED IF FL350 WAS AVAILABLE AND HE SAID YES, BUT HE WOULD HAVE TO GIVE US A RADAR VECTOR FOR TFC SEPARATION. WE CONFERRED ON THIS IN THE COCKPIT, AND THEN SAID THAT WOULD BE FINE AND WE WERE ISSUED A TURN TO A 250 DEG HDG AND WE READ BACK A CLRNC TO FL350. AT FL335 THE CTLR QUERIED US AS TO OUR ALT, AND SAID WE WERE NEVER ISSUED A CLB CLRNC TO FL350. HE SAID THERE WAS NO PROB AND NO CONFLICT AND INSTRUCTED US TO CONTINUE CLB TO FL350. WE (CAPT/FO) HAD SET AND CONFIRMED FL350 IN THE ALT ALERT WHEN WE THOUGHT WE HAD A CLB CLRNC. I THINK THE SIMILAR HDG AND THE SEMANTICS OF THE EXCHANGE BTWN US AND THE CTLR, LED US TO PERCEIVE THAT IF WE TOOK THE HDG, THEN WE HAD A CLRNC TO FL350. WHEN THE CTLR MISSED OUR READBACK OF FL350 (HE WAS BUSY WITH A LOT OF WX DEVS), IT ONLY CONFIRMED OUR BELIEF WE HAD CLRNC.

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.