B737 in descent for sna is given confusing ATC instructions during vectors.

Attributes
ACN	602120
Time
Date	200312
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : lax.vortac
State Reference	CA
Altitude	msl single value : 14000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival : on vectors arrival star : tandy
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 250 flight time total : 3600 flight time type : 650
ASRS Report	602120
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 180 flight time total : 21000 flight time type : 15000
ASRS Report	602123
Events
Anomaly	other anomaly other other spatial deviation
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to original clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	ATC Human Performance

Narrative:

On descent to sna, we were on the arrival to the airport. At an altitude of approximately FL200, we were given a heading of 130 degrees to fly. A few mins later, approach or center said 'cross merma at 14000 ft.' approximately 15 mi to merma, still on the 130 degree heading, I inquired the captain if ATC still wanted us on the 130 degree heading or direct merma. The captain asked ATC. The response was that we were to 'cross merma at 14000 ft and should have been direct merma.' our 130 degree heading would put us 5 mi away from merma. I immediately initiated a turn to merma. We were level at 14000 ft at this point. The controller said to fly heading 160 degrees. Approximately 15 seconds later, the controller said to fly heading 170 degrees. We complied with both vectors. ATC handed us off to next controller. What we did wrong: 1) we should have clarified the 'cross merma at 14000 ft' by asking if they wanted us 'direct merma' also when we initially received clearance. 2) we should have queried ATC earlier in the descent regarding the 130 degree heading. What we did right: 1) we did notice the discrepancy, albeit not early enough, and queried the controller. 2) remained professional at all times. Not the time to change barbs over the radio. What I learned: double and triple check all clrncs. If they don't add up, ask!

Original NASA ASRS Text

Title: B737 IN DSCNT FOR SNA IS GIVEN CONFUSING ATC INSTRUCTIONS DURING VECTORS.

Narrative: ON DSCNT TO SNA, WE WERE ON THE ARR TO THE ARPT. AT AN ALT OF APPROX FL200, WE WERE GIVEN A HEADING OF 130 DEGS TO FLY. A FEW MINS LATER, APCH OR CTR SAID 'CROSS MERMA AT 14000 FT.' APPROX 15 MI TO MERMA, STILL ON THE 130 DEG HDG, I INQUIRED THE CAPT IF ATC STILL WANTED US ON THE 130 DEG HDG OR DIRECT MERMA. THE CAPT ASKED ATC. THE RESPONSE WAS THAT WE WERE TO 'CROSS MERMA AT 14000 FT AND SHOULD HAVE BEEN DIRECT MERMA.' OUR 130 DEG HDG WOULD PUT US 5 MI AWAY FROM MERMA. I IMMEDIATELY INITIATED A TURN TO MERMA. WE WERE LEVEL AT 14000 FT AT THIS POINT. THE CTLR SAID TO FLY HEADING 160 DEGS. APPROX 15 SECONDS LATER, THE CTLR SAID TO FLY HEADING 170 DEGS. WE COMPLIED WITH BOTH VECTORS. ATC HANDED US OFF TO NEXT CTLR. WHAT WE DID WRONG: 1) WE SHOULD HAVE CLARIFIED THE 'CROSS MERMA AT 14000 FT' BY ASKING IF THEY WANTED US 'DIRECT MERMA' ALSO WHEN WE INITIALLY RECEIVED CLRNC. 2) WE SHOULD HAVE QUERIED ATC EARLIER IN THE DSCNT REGARDING THE 130 DEG HDG. WHAT WE DID RIGHT: 1) WE DID NOTICE THE DISCREPANCY, ALBEIT NOT EARLY ENOUGH, AND QUERIED THE CTLR. 2) REMAINED PROFESSIONAL AT ALL TIMES. NOT THE TIME TO CHANGE BARBS OVER THE RADIO. WHAT I LEARNED: DOUBLE AND TRIPLE CHK ALL CLRNCS. IF THEY DON'T ADD UP, ASK!

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.