B737 had erratic localizer signal and minor map shift on approach to dtw.

Attributes
ACN	423086
Time
Date	199812
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dtw
State Reference	MI
Altitude	msl bound lower : 3000 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Operator	common carrier : air carrier
Navigation In Use	ils localizer & glide slope : dtw Other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial
Experience	flight time last 90 days : 200 flight time total : 10000
ASRS Report	423086
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	other anomaly other other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted other
Consequence	Other
Supplementary
Air Traffic Incident	other

Narrative:

Approach: vectored for ILS runway 21L at dtw captain flying. As first officer, I tuned idented localizer for runway 21L. Non eventful as we intercepted and tracked inbound on the localizer autoplt flying the ship, clearance received for ILS runway 21L. Engaged approach on MCP (mode control panel), GS intercept normal. Shortly thereafter localizer began scalloping or moving erratically consequently the autoplt tracked these oscillations in an effort to eliminate the aircraft oscillation tracking the localizer. The captain attempted to select heading mode on MCP the autoplt will not default to heading once GS and localizer are captured. Captain then disengaged autoplt momentarily, re- engaged autoplt and selected LNAV to coincide with LNAV waypoint data selected on fmcs. Aircraft began aggressive turn to right which resulted in a departure from localizer. First officer was in raw data as was captain and captain quickly corrected with heading commands and reselected approach. Almost immediately the aircraft was reestablished on the localizer and GS. Normal landing followed. My only thoughts are that it is very common for crew members to attempt to change MCP modalities while autoplt is engaged in approach with localizer and GS captured and find nothing happening. It would be a nice design consideration to be able to deselect any MCP function regardless of phase of flight and reselect the desired modality without either turning off flight directors then back on, or what usually happens, autoplt off a moment of disorientation then reengagement recaptured or break off.

Original NASA ASRS Text

Title: B737 HAD ERRATIC LOC SIGNAL AND MINOR MAP SHIFT ON APCH TO DTW.

Narrative: APCH: VECTORED FOR ILS RWY 21L AT DTW CAPT FLYING. AS FO, I TUNED IDENTED LOC FOR RWY 21L. NON EVENTFUL AS WE INTERCEPTED AND TRACKED INBOUND ON THE LOC AUTOPLT FLYING THE SHIP, CLRNC RECEIVED FOR ILS RWY 21L. ENGAGED APCH ON MCP (MODE CTL PANEL), GS INTERCEPT NORMAL. SHORTLY THEREAFTER LOC BEGAN SCALLOPING OR MOVING ERRATICALLY CONSEQUENTLY THE AUTOPLT TRACKED THESE OSCILLATIONS IN AN EFFORT TO ELIMINATE THE ACFT OSCILLATION TRACKING THE LOC. THE CAPT ATTEMPTED TO SELECT HEADING MODE ON MCP THE AUTOPLT WILL NOT DEFAULT TO HEADING ONCE GS AND LOC ARE CAPTURED. CAPT THEN DISENGAGED AUTOPLT MOMENTARILY, RE- ENGAGED AUTOPLT AND SELECTED LNAV TO COINCIDE WITH LNAV WAYPOINT DATA SELECTED ON FMCS. ACFT BEGAN AGGRESSIVE TURN TO R WHICH RESULTED IN A DEP FROM LOC. FO WAS IN RAW DATA AS WAS CAPT AND CAPT QUICKLY CORRECTED WITH HEADING COMMANDS AND RESELECTED APCH. ALMOST IMMEDIATELY THE ACFT WAS REESTABLISHED ON THE LOC AND GS. NORMAL LNDG FOLLOWED. MY ONLY THOUGHTS ARE THAT IT IS VERY COMMON FOR CREW MEMBERS TO ATTEMPT TO CHANGE MCP MODALITIES WHILE AUTOPLT IS ENGAGED IN APCH WITH LOC AND GS CAPTURED AND FIND NOTHING HAPPENING. IT WOULD BE A NICE DESIGN CONSIDERATION TO BE ABLE TO DESELECT ANY MCP FUNCTION REGARDLESS OF PHASE OF FLT AND RESELECT THE DESIRED MODALITY WITHOUT EITHER TURNING OFF FLT DIRECTORS THEN BACK ON, OR WHAT USUALLY HAPPENS, AUTOPLT OFF A MOMENT OF DISORIENTATION THEN REENGAGEMENT RECAPTURED OR BREAK OFF.

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.