ATC took clearance meant for another aircraft and began descent through an occupied altitude.

Attributes
ACN	130401
Time
Date	198912
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : blh
State Reference	CA
Altitude	msl bound lower : 28700 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zla tower : oak
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : commercial pilot : atp pilot : instrument
Experience	flight time last 90 days : 500 flight time total : 11000 flight time type : 3000
ASRS Report	130401
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

While at cruise FL370 was told to descend to FL290. After receiving FL290, ZLA advised us to descend to FL240. I thought this unusual since we were so far from our destination (phx). The copilot read back the clearance, reset the altitude reminder and we began to descend. I believe we then switched frequencys. While descending through FL290 center advised traffic at 12 O'clock, 15 mi, opp direction at FL280. Copilot advised we were descending to FL240. We were told to maintain FL290 and stopped descent at about 28700' and climbed back to FL290. We were back at FL290 when traffic passed below. I do know that both copilot and I responded to the call to descend to FL240. I also know that the clearance was read back to ATC. Where the confusion came in, I can't say!

Original NASA ASRS Text

Title: ATC TOOK CLRNC MEANT FOR ANOTHER ACFT AND BEGAN DESCENT THROUGH AN OCCUPIED ALT.

Narrative: WHILE AT CRUISE FL370 WAS TOLD TO DSND TO FL290. AFTER RECEIVING FL290, ZLA ADVISED US TO DSND TO FL240. I THOUGHT THIS UNUSUAL SINCE WE WERE SO FAR FROM OUR DEST (PHX). THE COPLT READ BACK THE CLRNC, RESET THE ALT REMINDER AND WE BEGAN TO DSND. I BELIEVE WE THEN SWITCHED FREQS. WHILE DSNDING THROUGH FL290 CENTER ADVISED TFC AT 12 O'CLOCK, 15 MI, OPP DIRECTION AT FL280. COPLT ADVISED WE WERE DSNDING TO FL240. WE WERE TOLD TO MAINTAIN FL290 AND STOPPED DSCNT AT ABOUT 28700' AND CLBED BACK TO FL290. WE WERE BACK AT FL290 WHEN TFC PASSED BELOW. I DO KNOW THAT BOTH COPLT AND I RESPONDED TO THE CALL TO DSND TO FL240. I ALSO KNOW THAT THE CLRNC WAS READ BACK TO ATC. WHERE THE CONFUSION CAME IN, I CAN'T SAY!

Data retrieved from NASA's ASRS site as of August 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.