Air carrier descends on expected clearance thinking he had heard it given.

Attributes
ACN	468860
Time
Date	200004
Day	Thu
Local Time Of Day	1201 To 1800
Place
State Reference	WY
Altitude	msl bound lower : 34000 msl bound upper : 35000
Environment
Flight Conditions	IMC
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	A320
Operating Under FAR Part	Part 121
Navigation In Use	other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 13000 flight time type : 1500
ASRS Report	468860
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The flight was experiencing light turbulence at FL350 with reports of moderate ahead. I requested FL280. Just as the controller began transmitting, the lead flight attendant came onto the flight deck and began talking. I thought I heard the controller say that the flight was cleared to FL330 now and to expect FL280. I read back 'air carrier X out of FL350 for FL330.' the descent was initiated. At about FL340, the controller said that he had not actually given us a clearance to FL330, but that we were now cleared to do so. I should have asked for the controller to repeat his initial statement, but I thought I heard everything correctly. As the controller made no reply or correction to my call out of FL350, I had no idea that we had departed the altitude without a clearance.

Original NASA ASRS Text

Title: ACR DSNDS ON EXPECTED CLRNC THINKING HE HAD HEARD IT GIVEN.

Narrative: THE FLT WAS EXPERIENCING LIGHT TURB AT FL350 WITH RPTS OF MODERATE AHEAD. I REQUESTED FL280. JUST AS THE CTLR BEGAN XMITTING, THE LEAD FLT ATTENDANT CAME ONTO THE FLT DECK AND BEGAN TALKING. I THOUGHT I HEARD THE CTLR SAY THAT THE FLT WAS CLRED TO FL330 NOW AND TO EXPECT FL280. I READ BACK 'ACR X OUT OF FL350 FOR FL330.' THE DSCNT WAS INITIATED. AT ABOUT FL340, THE CTLR SAID THAT HE HAD NOT ACTUALLY GIVEN US A CLRNC TO FL330, BUT THAT WE WERE NOW CLRED TO DO SO. I SHOULD HAVE ASKED FOR THE CTLR TO REPEAT HIS INITIAL STATEMENT, BUT I THOUGHT I HEARD EVERYTHING CORRECTLY. AS THE CTLR MADE NO REPLY OR CORRECTION TO MY CALL OUT OF FL350, I HAD NO IDEA THAT WE HAD DEPARTED THE ALT WITHOUT A 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.