B737 crew reports failure of autoplt to capture localizer on approach to den runway 35L.

Attributes
ACN	708489
Time
Date	200609
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : den.airport
State Reference	CO
Altitude	msl single value : 10000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : d01.tracon
Operator	common carrier : air carrier
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 35l
Flight Phase	descent : intermediate altitude
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 266 flight time type : 7500
ASRS Report	708489
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 253 flight time type : 1900
ASRS Report	708476
Events
Anomaly	aircraft equipment problem : less severe non adherence : clearance other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

While on an assigned heading to join the localizer for runway 35L at den with VOR/localizer armed we expected the aircraft to capture the localizer. As the aircraft began to fly through the localizer; both pilots announced the deviation and the autoplt was disengaged and the aircraft was turned back to recapture the localizer. At about this time; ATC also became aware that we were east of the localizer and issued a heading to recapture the localizer. Callback conversation with reporter revealed the following information: the captain reports that he doesn't understand why the autoplt did not capture the localizer. After manually flying back to the localizer; the autoplt was engaged in the localizer mode and functioned normally.

Original NASA ASRS Text

Title: B737 CREW RPTS FAILURE OF AUTOPLT TO CAPTURE LOC ON APCH TO DEN RWY 35L.

Narrative: WHILE ON AN ASSIGNED HDG TO JOIN THE LOC FOR RWY 35L AT DEN WITH VOR/LOC ARMED WE EXPECTED THE ACFT TO CAPTURE THE LOC. AS THE ACFT BEGAN TO FLY THROUGH THE LOC; BOTH PLTS ANNOUNCED THE DEV AND THE AUTOPLT WAS DISENGAGED AND THE ACFT WAS TURNED BACK TO RECAPTURE THE LOC. AT ABOUT THIS TIME; ATC ALSO BECAME AWARE THAT WE WERE E OF THE LOC AND ISSUED A HDG TO RECAPTURE THE LOC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT RPTS THAT HE DOESN'T UNDERSTAND WHY THE AUTOPLT DID NOT CAPTURE THE LOC. AFTER MANUALLY FLYING BACK TO THE LOC; THE AUTOPLT WAS ENGAGED IN THE LOC MODE AND FUNCTIONED NORMALLY.

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.