FMS logic contributed to flight crew's failure to intercept the localizer track deviation.

Attributes
ACN	317586
Time
Date	199510
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : dfw
State Reference	TX
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	common carrier : air carrier
Make Model Name	Fokker 100
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	arrival other arrival star : star enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	317586
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other anomaly other other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On scy arrival into dfw and set up for the ILS runway 35R approach between seago and gladd I was given a heading to intercept runway 31R localizer and cleared approach. I had approach selected on the fmp and armed VOR/localizer for the autoplt to intercept the localizer. I switched approach plates and briefed the new approach and noticed that the autoplt was not intercepting the localizer. I punched off the autoplt and turned to re-intercept the localizer. I saw that VOR was now displayed -- not localizer. I rearmed the VOR/localizer. Approach control called with a heading and cleared us for the stadium visual runway 31R. I think the autoplt did not capture and the fmp switched to VOR because of the change in ILS frequency while armed.

Original NASA ASRS Text

Title: FMS LOGIC CONTRIBUTED TO FLC'S FAILURE TO INTERCEPT THE LOC TRACK DEV.

Narrative: ON SCY ARR INTO DFW AND SET UP FOR THE ILS RWY 35R APCH BTWN SEAGO AND GLADD I WAS GIVEN A HDG TO INTERCEPT RWY 31R LOC AND CLRED APCH. I HAD APCH SELECTED ON THE FMP AND ARMED VOR/LOC FOR THE AUTOPLT TO INTERCEPT THE LOC. I SWITCHED APCH PLATES AND BRIEFED THE NEW APCH AND NOTICED THAT THE AUTOPLT WAS NOT INTERCEPTING THE LOC. I PUNCHED OFF THE AUTOPLT AND TURNED TO RE-INTERCEPT THE LOC. I SAW THAT VOR WAS NOW DISPLAYED -- NOT LOC. I REARMED THE VOR/LOC. APCH CTL CALLED WITH A HDG AND CLRED US FOR THE STADIUM VISUAL RWY 31R. I THINK THE AUTOPLT DID NOT CAPTURE AND THE FMP SWITCHED TO VOR BECAUSE OF THE CHANGE IN ILS FREQ WHILE ARMED.

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.