B737-300 departed from cleared route due to FMC malfunction or FMC operator error.

Attributes
ACN	574999
Time
Date	200302
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : oak.airport
State Reference	CA
Altitude	msl single value : 23000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
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 : 11200 flight time type : 5000
ASRS Report	574999
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : commercial
Experience	flight time last 90 days : 200 flight time total : 4000 flight time type : 2200
ASRS Report	575002
Events
Anomaly	aircraft equipment problem : critical maintenance problem : improper documentation non adherence : clearance non adherence : company policies non adherence : far non adherence : published procedure other spatial deviation
Independent Detector	atc equipment other atc equipment : radar aircraft equipment other aircraft equipment : fmc scratchpad alert other controllera
Resolutory Action	controller : issued alert controller : issued new clearance flight crew : became reoriented flight crew : overcame equipment problem flight crew : returned to intended or assigned course flight crew : overrode automation
Consequence	other other
Factors
Maintenance	performance deficiency : logbook entry performance deficiency : non compliance with legal requirements
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance Environmental Factor Company
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Navigation, fml, LNAV/off course. Flight was uneventful until we were cleared direct bty VOR tragr 2 arrival las. After clearance, we were given 2 10-degree turns, vectors for traffic. Shortly after ATC cleared us direct fuzzy TRAGR2. On the FMC scratch pad display, we were alerted 'verify position.' we checked LNAV. It was in use. FMC was showing direct fuzzy, rnp/actual showing 2.00/.26. Placed both msi switches to navigation. Re-entered direct fuzzy. ATC queried, 'company number, are you direct fuzzy?' we checked again and told ATC affirmative. ATC said 'you are drifting off course.' we checked FMC again. Re-entered direct fuzzy and told ATC. Several mins later ATC said 'company number, you are northwest of fuzzy not proceeding direct, off course by 30 degrees! Turn left 095 degrees direct fuzzy.' I checked FMC and no alert was shown. But rnp/actual was showing we were off course by 9 mi right of course. I told ATC we would need vectors to fuzzy and changed our equipment type to alpha. We proceeded to las. Uneventful approach and landing via fuzzy 2 arrival runway 25R using raw data. Informed outbound crew of situation, suggested full IRS alignment. Outbound captain agreed and said he would handle any discrepancies. Question: on RNAV SID/stars, how can we verify position if FMC IRS are only source of navigation? Would 2 FMC's in aircraft make a difference?

Original NASA ASRS Text

Title: B737-300 DEPARTED FROM CLRED RTE DUE TO FMC MALFUNCTION OR FMC OPERATOR ERROR.

Narrative: NAV, FML, LNAV/OFF COURSE. FLT WAS UNEVENTFUL UNTIL WE WERE CLRED DIRECT BTY VOR TRAGR 2 ARR LAS. AFTER CLRNC, WE WERE GIVEN 2 10-DEG TURNS, VECTORS FOR TFC. SHORTLY AFTER ATC CLRED US DIRECT FUZZY TRAGR2. ON THE FMC SCRATCH PAD DISPLAY, WE WERE ALERTED 'VERIFY POS.' WE CHKED LNAV. IT WAS IN USE. FMC WAS SHOWING DIRECT FUZZY, RNP/ACTUAL SHOWING 2.00/.26. PLACED BOTH MSI SWITCHES TO NAV. RE-ENTERED DIRECT FUZZY. ATC QUERIED, 'COMPANY NUMBER, ARE YOU DIRECT FUZZY?' WE CHKED AGAIN AND TOLD ATC AFFIRMATIVE. ATC SAID 'YOU ARE DRIFTING OFF COURSE.' WE CHKED FMC AGAIN. RE-ENTERED DIRECT FUZZY AND TOLD ATC. SEVERAL MINS LATER ATC SAID 'COMPANY NUMBER, YOU ARE NW OF FUZZY NOT PROCEEDING DIRECT, OFF COURSE BY 30 DEGS! TURN L 095 DEGS DIRECT FUZZY.' I CHKED FMC AND NO ALERT WAS SHOWN. BUT RNP/ACTUAL WAS SHOWING WE WERE OFF COURSE BY 9 MI R OF COURSE. I TOLD ATC WE WOULD NEED VECTORS TO FUZZY AND CHANGED OUR EQUIP TYPE TO ALPHA. WE PROCEEDED TO LAS. UNEVENTFUL APCH AND LNDG VIA FUZZY 2 ARR RWY 25R USING RAW DATA. INFORMED OUTBOUND CREW OF SIT, SUGGESTED FULL IRS ALIGNMENT. OUTBOUND CAPT AGREED AND SAID HE WOULD HANDLE ANY DISCREPANCIES. QUESTION: ON RNAV SID/STARS, HOW CAN WE VERIFY POS IF FMC IRS ARE ONLY SOURCE OF NAV? WOULD 2 FMC'S IN ACFT MAKE A DIFFERENCE?

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.