Possible compass malfunction. Unable callback phone out of order.

Attributes
ACN	137126
Time
Date	199001
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw
State Reference	TX
Altitude	agl bound lower : 500 agl bound upper : 500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : dfw tracon : psp
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 50 flight time total : 6000 flight time type : 175
ASRS Report	137126
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	other

Narrative:

While making an ILS 17L approach on localizer at dfw in an (glass cockpit) both compass system indicated a heading of 150 degree while the aircraft was visually confirmed to be heading 170 degree. I believe this was due to an error in the software of the system. The serious problem I see is that while each navigation system is completely separate to allow comparison (i.e. Comparator system) to detect malfunction, the software is always exactly the same. Therefore, any error in the software will produce the same erroneous indication on both sides and no error can be detected through the comparator system or by the pilots. I suspect this may be a common fault in all computer navigation systems.

Original NASA ASRS Text

Title: POSSIBLE COMPASS MALFUNCTION. UNABLE CALLBACK PHONE OUT OF ORDER.

Narrative: WHILE MAKING AN ILS 17L APCH ON LOC AT DFW IN AN (GLASS COCKPIT) BOTH COMPASS SYS INDICATED A HDG OF 150 DEG WHILE THE ACFT WAS VISUALLY CONFIRMED TO BE HDG 170 DEG. I BELIEVE THIS WAS DUE TO AN ERROR IN THE SOFTWARE OF THE SYS. THE SERIOUS PROB I SEE IS THAT WHILE EACH NAV SYS IS COMPLETELY SEPARATE TO ALLOW COMPARISON (I.E. COMPARATOR SYS) TO DETECT MALFUNCTION, THE SOFTWARE IS ALWAYS EXACTLY THE SAME. THEREFORE, ANY ERROR IN THE SOFTWARE WILL PRODUCE THE SAME ERRONEOUS INDICATION ON BOTH SIDES AND NO ERROR CAN BE DETECTED THROUGH THE COMPARATOR SYS OR BY THE PLTS. I SUSPECT THIS MAY BE A COMMON FAULT IN ALL COMPUTER NAV SYSTEMS.

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.