A B737 flight crew deviated from cleared course when their IRU's failed to position update.

Attributes
ACN	809788
Time
Date	200810
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zhu.artcc
State Reference	TX
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zhu.artcc
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
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 : 70 flight time total : 4800 flight time type : 850
ASRS Report	809788
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

Approaching kelpp intersection overwater south of houston; tx; reported kelpp; a mandatory reporting point. ZHU reported radar contact and advised we were 12 mi west of kelpp. IRU's didn't update while over central america; right IRU drifted 8.8 mi from FMC position; left IRU drifted 9.4 mi from FMC position. We received DME's when approaching land in texas but IRU's did not correct themselves. ATC sent us direct stros intersection; but we missed that as well. Vectored to destination; without incident.

Original NASA ASRS Text

Title: A B737 FLT CREW DEVIATED FROM CLRED COURSE WHEN THEIR IRU'S FAILED TO POSITION UPDATE.

Narrative: APCHING KELPP INTXN OVERWATER S OF HOUSTON; TX; RPTED KELPP; A MANDATORY RPTING POINT. ZHU RPTED RADAR CONTACT AND ADVISED WE WERE 12 MI W OF KELPP. IRU'S DIDN'T UPDATE WHILE OVER CENTRAL AMERICA; R IRU DRIFTED 8.8 MI FROM FMC POS; L IRU DRIFTED 9.4 MI FROM FMC POS. WE RECEIVED DME'S WHEN APCHING LAND IN TEXAS BUT IRU'S DID NOT CORRECT THEMSELVES. ATC SENT US DIRECT STROS INTXN; BUT WE MISSED THAT AS WELL. VECTORED TO DEST; WITHOUT INCIDENT.

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.