Flight crew of B737-300 descended without clearance in ZLA airspace.

Attributes
ACN	592439
Time
Date	200309
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : lax.vortac
State Reference	CA
Altitude	msl bound lower : 36000 msl bound upper : 37000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 12000 flight time type : 9000
ASRS Report	592439
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Experience	flight time last 90 days : 240 flight time total : 11000 flight time type : 4100
ASRS Report	592215
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : company policies non adherence : clearance non adherence : far
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While cruising at FL370, we had just encountered light to moderate chop. I turned on the seatbelt sign and went off the frequency to make a PA. When I came back on, I was relieved to see we had received descent clearance to FL240 and began a cruise descent hoping to find smoother air. I recall being about 30 mi from top of descent point so descent clearance sounded reasonable. The controller told us to stop descent as we were passing through FL360 and asked us when we received descent clearance. My first officer answered, 'about a min ago.' the controller stated that descent clearance was for another aircraft. My first officer told me he read back the clearance. Maybe the readback was blocked or readback for mistaken identify was not caught, or there was no mistaken identify and the controller made an honest mistake. The remainder of flight continued uneventfully and the controller did not sound frustrated with us. In the future, I will always verify clearance with controller if only one of us hears the clearance.

Original NASA ASRS Text

Title: FLT CREW OF B737-300 DSNDED WITHOUT CLRNC IN ZLA AIRSPACE.

Narrative: WHILE CRUISING AT FL370, WE HAD JUST ENCOUNTERED LIGHT TO MODERATE CHOP. I TURNED ON THE SEATBELT SIGN AND WENT OFF THE FREQ TO MAKE A PA. WHEN I CAME BACK ON, I WAS RELIEVED TO SEE WE HAD RECEIVED DSCNT CLRNC TO FL240 AND BEGAN A CRUISE DSCNT HOPING TO FIND SMOOTHER AIR. I RECALL BEING ABOUT 30 MI FROM TOP OF DSCNT POINT SO DSCNT CLRNC SOUNDED REASONABLE. THE CTLR TOLD US TO STOP DSCNT AS WE WERE PASSING THROUGH FL360 AND ASKED US WHEN WE RECEIVED DSCNT CLRNC. MY FO ANSWERED, 'ABOUT A MIN AGO.' THE CTLR STATED THAT DSCNT CLRNC WAS FOR ANOTHER ACFT. MY FO TOLD ME HE READ BACK THE CLRNC. MAYBE THE READBACK WAS BLOCKED OR READBACK FOR MISTAKEN IDENT WAS NOT CAUGHT, OR THERE WAS NO MISTAKEN IDENT AND THE CTLR MADE AN HONEST MISTAKE. THE REMAINDER OF FLT CONTINUED UNEVENTFULLY AND THE CTLR DID NOT SOUND FRUSTRATED WITH US. IN THE FUTURE, I WILL ALWAYS VERIFY CLRNC WITH CTLR IF ONLY ONE OF US HEARS THE 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.