Air carrier medium large transport answered wrong clearance resulting in altitude excursion and potential conflict with unk traffic.

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Attributes
ACN	97711
Time
Date	198811
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : lgb
State Reference	CA
Altitude	msl bound lower : 8500 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sna tower : cmi
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 3200 flight time type : 580
ASRS Report	97711
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 0 vertical : 1000
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Airlines flight exy santa barbara to lgb. At approximately XA15 we had a near miss with another aircraft with approximately 1000' of vertical sep. Prior to this we were initially cleared from 17000 to 16000' and slowed from 305 KTS to 280 KTS. Then as we reached 16000' we were cleared to 14000' and slowed to 250 KTS. Upon reaching 14000' we were cleared to 11000'. While leveling at 11000' we understood that we were cleared to 5000' and expedite through 6000'. That was the clearance we read back. At 8700' we saw an oncoming aircraft which appeared to be at 7500' and we momentarily leveled at 8500' until he passed and then continued descent. Upon questioning ATC on the presence of the oncoming aircraft we were told our assigned altitude was 11000' and that we had read back the clearance for company bcxy. We did read back the clearance to descend to 5000' as company exy and if that was not our clearance it should have been corrected. We also do not understand how we could get nearly 3000' off altitude with someone at ATC noticing.

Original NASA ASRS Text

Title: ACR MLG ANSWERED WRONG CLRNC RESULTING IN ALT EXCURSION AND POTENTIAL CONFLICT WITH UNK TRAFFIC.

Narrative: AIRLINES FLT EXY SANTA BARBARA TO LGB. AT APPROX XA15 WE HAD A NEAR MISS WITH ANOTHER ACFT WITH APPROX 1000' OF VERT SEP. PRIOR TO THIS WE WERE INITIALLY CLRED FROM 17000 TO 16000' AND SLOWED FROM 305 KTS TO 280 KTS. THEN AS WE REACHED 16000' WE WERE CLRED TO 14000' AND SLOWED TO 250 KTS. UPON REACHING 14000' WE WERE CLRED TO 11000'. WHILE LEVELING AT 11000' WE UNDERSTOOD THAT WE WERE CLRED TO 5000' AND EXPEDITE THROUGH 6000'. THAT WAS THE CLRNC WE READ BACK. AT 8700' WE SAW AN ONCOMING ACFT WHICH APPEARED TO BE AT 7500' AND WE MOMENTARILY LEVELED AT 8500' UNTIL HE PASSED AND THEN CONTINUED DSCNT. UPON QUESTIONING ATC ON THE PRESENCE OF THE ONCOMING ACFT WE WERE TOLD OUR ASSIGNED ALT WAS 11000' AND THAT WE HAD READ BACK THE CLRNC FOR COMPANY BCXY. WE DID READ BACK THE CLRNC TO DSND TO 5000' AS COMPANY EXY AND IF THAT WAS NOT OUR CLRNC IT SHOULD HAVE BEEN CORRECTED. WE ALSO DO NOT UNDERSTAND HOW WE COULD GET NEARLY 3000' OFF ALT WITH SOMEONE AT ATC NOTICING.

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.