Air carrier widebody transport track heading deviation when IRS units malfunctioned.

Attributes
ACN	197620
Time
Date	199112
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : sju
State Reference	PR
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : tjzs
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	enroute : other oceanic enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 90 flight time total : 16200
ASRS Report	197620
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument
Experience	flight time last 90 days : 230 flight time total : 6900 flight time type : 1125
ASRS Report	197553
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Departed san juan, destination barbados. Shortly after takeoff, we were cleared direct to iluri intersection, about 150 mi away. Shortly before or after reaching cruising altitude, san juan center controller informed us that we appeared to be drifting left of a direct course to iluri. At this time the FMC/autoplt was engaged in 'navigation' and track deviation was annunciated as 0.1 mi left of course. An immediate correction was initiated into the wind (to the right). A crosswind existed from 90 degrees to the right at about 75 KTS. By manually tuning in coy and ppr and fof vors, navigation to iluri and thence to destination was accomplished as these navaids became available, without incident. ETA at iluri was met within 1 min. It quickly became apparent that FMC navigation system was producing substantial position errors even after manual updates of position were accomplished using VOR/DME information. After landing, IRU performance check indicated drift rates of 99.9 for all 3 IRU units, which is radial position error and for this approximately 1 1/2 hour flight a reading of '3' would be maximum allowed. Appropriate E-6 logbook entries were made.

Original NASA ASRS Text

Title: ACR WDB TRACK HDG DEV WHEN IRS UNITS MALFUNCTIONED.

Narrative: DEPARTED SAN JUAN, DEST BARBADOS. SHORTLY AFTER TKOF, WE WERE CLRED DIRECT TO ILURI INTXN, ABOUT 150 MI AWAY. SHORTLY BEFORE OR AFTER REACHING CRUISING ALT, SAN JUAN CENTER CTLR INFORMED US THAT WE APPEARED TO BE DRIFTING L OF A DIRECT COURSE TO ILURI. AT THIS TIME THE FMC/AUTOPLT WAS ENGAGED IN 'NAV' AND TRACK DEV WAS ANNUNCIATED AS 0.1 MI L OF COURSE. AN IMMEDIATE CORRECTION WAS INITIATED INTO THE WIND (TO THE R). A XWIND EXISTED FROM 90 DEGS TO THE R AT ABOUT 75 KTS. BY MANUALLY TUNING IN COY AND PPR AND FOF VORS, NAV TO ILURI AND THENCE TO DEST WAS ACCOMPLISHED AS THESE NAVAIDS BECAME AVAILABLE, WITHOUT INCIDENT. ETA AT ILURI WAS MET WITHIN 1 MIN. IT QUICKLY BECAME APPARENT THAT FMC NAV SYS WAS PRODUCING SUBSTANTIAL POS ERRORS EVEN AFTER MANUAL UPDATES OF POS WERE ACCOMPLISHED USING VOR/DME INFO. AFTER LNDG, IRU PERFORMANCE CHK INDICATED DRIFT RATES OF 99.9 FOR ALL 3 IRU UNITS, WHICH IS RADIAL POS ERROR AND FOR THIS APPROX 1 1/2 HR FLT A READING OF '3' WOULD BE MAX ALLOWED. APPROPRIATE E-6 LOGBOOK ENTRIES WERE MADE.

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.