Air carrier medium large transport track deviation using IRS navigation system.

Attributes
ACN	142355
Time
Date	199004
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : las
State Reference	NV
Altitude	msl bound lower : 16000 msl bound upper : 16000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 18000 flight time type : 6000
ASRS Report	142355
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Aircraft was reported by ATC as 4 mi north of course. IRS navigation showed on course. When we switched to VOR's to verify course, we found ATC was correct. Shortly thereafter, we got FMC message, 'verify position.' somehow an error had entered the IRS navigation system and the message was not displayed on the FMC before ATC had noted the 4-5 mi error. VOR/ILS was used for the rest of the flight.

Original NASA ASRS Text

Title: ACR MLG TRACK DEVIATION USING IRS NAVIGATION SYSTEM.

Narrative: ACFT WAS RPTED BY ATC AS 4 MI N OF COURSE. IRS NAV SHOWED ON COURSE. WHEN WE SWITCHED TO VOR'S TO VERIFY COURSE, WE FOUND ATC WAS CORRECT. SHORTLY THEREAFTER, WE GOT FMC MESSAGE, 'VERIFY POS.' SOMEHOW AN ERROR HAD ENTERED THE IRS NAV SYS AND THE MESSAGE WAS NOT DISPLAYED ON THE FMC BEFORE ATC HAD NOTED THE 4-5 MI ERROR. VOR/ILS WAS USED FOR THE REST OF THE FLT.

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.