B757 crews dealing with an automatic speed brk EICAS after takeoff discovered an unacceptable IRS na...

Narrative:

I was the non-flying pilot in may 2009. While climbing out I was completing checklists and speaking to maintenance and dispatch about an automatic speed brk EICAS we got on strut extension so I was completing my ETOPS checklist a little later than usual. We were 100 NM from our departure airport when I began the gross navigation error check. The FMC showed a 4.5NM difference from the raw data DME readout; the bearing checked right on; using departure station VOR. My first officer then checked the same thing on his side and got the same result. We then tried to use our destination VOR as it was closer and got the same results. We then pulled up the IRS position page on the FMC and went to the bearing/distance prompt and saw the IRU's were showing a 4.2 NM to 5.6 NM discrepancy. This discrepancy was increasing with our distance from shore. After speaking with dispatch and maintenance again we decided to turn back for our departure airport. We made an uneventful overweight landing with equipment standing by.

Original NASA ASRS Text

Title: B757 crews dealing with an AUTO SPD BRK EICAS after takeoff discovered an unacceptable IRS navigation deviation during delayed navigation accuracy check. The flight returned to land overweight.

Narrative: I was the non-flying pilot in May 2009. While climbing out I was completing checklists and speaking to Maintenance and Dispatch about an AUTO SPD BRK EICAS we got on strut extension so I was completing my ETOPS checklist a little later than usual. We were 100 NM from our departure airport when I began the gross navigation error check. The FMC showed a 4.5NM difference from the raw data DME readout; the bearing checked right on; using departure station VOR. My First Officer then checked the same thing on his side and got the same result. We then tried to use our destination VOR as it was closer and got the same results. We then pulled up the IRS POS page on the FMC and went to the bearing/distance prompt and saw the IRU's were showing a 4.2 NM to 5.6 NM discrepancy. This discrepancy was increasing with our distance from shore. After speaking with dispatch and maintenance again we decided to turn back for our departure airport. We made an uneventful overweight landing with equipment standing by.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	835472
Time
Date	200905
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B757-200
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Antiskid System
Person 1
Function	Captain Pilot Not Flying
Experience	Flight Crew Last 90 Days 155 Flight Crew Total 19000 Flight Crew Type 5025
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Clearance Deviation - Track / Heading All Types

B757 crews dealing with an automatic speed brk EICAS after takeoff discovered an unacceptable IRS navigation deviation during delayed navigation accuracy check. The flight returned to land overweight.