On departure, using LNAV off of IRS flight crew told they are 1 1/2 mi off course.

Attributes
ACN	278322
Time
Date	199407
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : phx
State Reference	AZ
Altitude	msl bound lower : 3000 msl bound upper : 5000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : phx tower : mvy
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude
Route In Use	departure sid : sid
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 12000 flight time type : 150
ASRS Report	278322
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : became reoriented other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

After departure out of phx on the st johns 1 departure, we were advised by departure that we were about 1 1/2 mi off course, and over a noise sensitive area. At the time we were navigating using LNAV off of the IRS. An accuracy check on the IRS showed 2 mi error. We had a mechanical problem shortly before takeoff that required a return to the gate and about a 1 1/2 hour delay. I had checked the accuracy of the IRS on initial taxi out and they were ok. Apparently, they had drifted during our ground delay. On glass cockpit aircraft we really become dependent on the FMC 'magic' and it is usually dependable. In the case of this SID, a heading is called for rather than track but the difference is normally non -existent. With the error in the IRS today, that was not the case. This stresses the importance of backing up IRS generated information with radio navigation and doing an accuracy check just before takeoff.

Original NASA ASRS Text

Title: ON DEP, USING LNAV OFF OF IRS FLC TOLD THEY ARE 1 1/2 MI OFF COURSE.

Narrative: AFTER DEP OUT OF PHX ON THE ST JOHNS 1 DEP, WE WERE ADVISED BY DEP THAT WE WERE ABOUT 1 1/2 MI OFF COURSE, AND OVER A NOISE SENSITIVE AREA. AT THE TIME WE WERE NAVING USING LNAV OFF OF THE IRS. AN ACCURACY CHK ON THE IRS SHOWED 2 MI ERROR. WE HAD A MECHANICAL PROB SHORTLY BEFORE TKOF THAT REQUIRED A RETURN TO THE GATE AND ABOUT A 1 1/2 HR DELAY. I HAD CHKED THE ACCURACY OF THE IRS ON INITIAL TAXI OUT AND THEY WERE OK. APPARENTLY, THEY HAD DRIFTED DURING OUR GND DELAY. ON GLASS COCKPIT ACFT WE REALLY BECOME DEPENDENT ON THE FMC 'MAGIC' AND IT IS USUALLY DEPENDABLE. IN THE CASE OF THIS SID, A HDG IS CALLED FOR RATHER THAN TRACK BUT THE DIFFERENCE IS NORMALLY NON -EXISTENT. WITH THE ERROR IN THE IRS TODAY, THAT WAS NOT THE CASE. THIS STRESSES THE IMPORTANCE OF BACKING UP IRS GENERATED INFO WITH RADIO NAV AND DOING AN ACCURACY CHK JUST BEFORE TKOF.

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.