B757-200 flight crew suffers track deviation when IRS fail to provide accurate navigation data.

Attributes
ACN	810858
Time
Date	200811
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sna.airport
State Reference	CA
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	climbout : initial
Route In Use	departure sid : musel
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
ASRS Report	810858
Person 2
Affiliation	government : faa
Function	controller : departure
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory flight crew : returned to intended or assigned course
Supplementary
Problem Areas	ATC Human Performance Chart Or Publication Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Socal departure had given us a '...heading to intercept the sxc 061 degree radial to teyki; resume the MUSEL6 departure.' when approaching the 061 degree radial according to our FMC; departure asked us if we were intercepting the radial; upon which we replied that we were approaching the radial and would intercept. The controller indicated we had passed the radial and assigned us a heading to fly. We did as instructed; meanwhile checking raw data to confirm that our FMC was correct. The raw data radial did not overlay our FMC picture; but we could not figure out why not or which was correct. The PNF did a navigation accuracy check and found our vors to be accurate. We could not figure out the discrepancy in the navigation error.

Original NASA ASRS Text

Title: B757-200 FLT CREW SUFFERS TRACK DEVIATION WHEN IRS FAIL TO PROVIDE ACCURATE NAV DATA.

Narrative: SOCAL DEP HAD GIVEN US A '...HDG TO INTERCEPT THE SXC 061 DEG RADIAL TO TEYKI; RESUME THE MUSEL6 DEP.' WHEN APCHING THE 061 DEG RADIAL ACCORDING TO OUR FMC; DEP ASKED US IF WE WERE INTERCEPTING THE RADIAL; UPON WHICH WE REPLIED THAT WE WERE APCHING THE RADIAL AND WOULD INTERCEPT. THE CTLR INDICATED WE HAD PASSED THE RADIAL AND ASSIGNED US A HDG TO FLY. WE DID AS INSTRUCTED; MEANWHILE CHKING RAW DATA TO CONFIRM THAT OUR FMC WAS CORRECT. THE RAW DATA RADIAL DID NOT OVERLAY OUR FMC PICTURE; BUT WE COULD NOT FIGURE OUT WHY NOT OR WHICH WAS CORRECT. THE PNF DID A NAV ACCURACY CHK AND FOUND OUR VORS TO BE ACCURATE. WE COULD NOT FIGURE OUT THE DISCREPANCY IN THE NAV ERROR.

Data retrieved from NASA's ASRS site as of May 2009 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.