B757 encounters track deviation on RNAV SID from dfw when FMS DME updating fails.

Attributes
ACN	674888
Time
Date	200510
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw.airport
State Reference	TX
Altitude	msl single value : 5000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : d10.tracon
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	other Other
Flight Phase	climbout : intermediate altitude
Route In Use	departure sid : ferra
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	674888
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	674889
Events
Anomaly	aircraft equipment problem : critical non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : eicas other controllera
Resolutory Action	controller : issued advisory
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

After takeoff; runway 18L; dfw; ferra 2 RNAV departure. ATC commented that we were outside the turn at bpark intersection. My tracking of the magenta line for the departure was not as aggressive as was needed and we were indeed outside of the turn; correcting. I don't believe any loss of traffic separation occurred. The controller did not indicate the amount of deviation that she noted. Approximately 26 mins after departure; IRS navigation only message appeared on the CDU and we determined that #1 DME had indeed failed. The IRS navigation only message continued to appear all throughout the flight. Inaccuracy within the aircraft systems combined with my inexact tracking of the departure may have made the deviation greater than was apparent to us in the cockpit. Before takeoff; the nd FMS position (aircraft) was touching the runway symbol. The #1 DME system was written up upon arrival at sea.

Original NASA ASRS Text

Title: B757 ENCOUNTERS TRACK DEVIATION ON RNAV SID FROM DFW WHEN FMS DME UPDATING FAILS.

Narrative: AFTER TAKEOFF; RWY 18L; DFW; FERRA 2 RNAV DEP. ATC COMMENTED THAT WE WERE OUTSIDE THE TURN AT BPARK INTERSECTION. MY TRACKING OF THE MAGENTA LINE FOR THE DEP WAS NOT AS AGGRESSIVE AS WAS NEEDED AND WE WERE INDEED OUTSIDE OF THE TURN; CORRECTING. I DON'T BELIEVE ANY LOSS OF TFC SEPARATION OCCURRED. THE CTLR DID NOT INDICATE THE AMOUNT OF DEV THAT SHE NOTED. APPROX 26 MINS AFTER DEP; IRS NAV ONLY MSG APPEARED ON THE CDU AND WE DETERMINED THAT #1 DME HAD INDEED FAILED. THE IRS NAV ONLY MSG CONTINUED TO APPEAR ALL THROUGHOUT THE FLT. INACCURACY WITHIN THE ACFT SYSTEMS COMBINED WITH MY INEXACT TRACKING OF THE DEP MAY HAVE MADE THE DEVIATION GREATER THAN WAS APPARENT TO US IN THE COCKPIT. BEFORE TAKEOFF; THE ND FMS POS (ACFT) WAS TOUCHING THE RWY SYMBOL. THE #1 DME SYSTEM WAS WRITTEN UP UPON ARR AT SEA.

Data retrieved from NASA's ASRS site as of January 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.