B767 aircraft pitches down away from GS while on autopilot; generating warning from ATC.

Attributes
ACN	737559
Time
Date	200705
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lszh.airport
State Reference	FO
Altitude	agl single value : 2400
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : lszh.tracon
Operator	common carrier : air carrier
Make Model Name	B767-300 and 300 ER
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 34
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : atp pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 150 flight time total : 13000 flight time type : 9500
ASRS Report	737559
Events
Anomaly	aircraft equipment problem : less severe conflict : ground less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

ATIS reporting ILS runway 34 in zurich; WX 2000 meters; light rain; 500 ft scattered; ceiling 1000 ft broken; wind light and variable. ATC gives vectors and a 010 degree heading to intercept the localizer for the ILS to runway 34 'cleared for the approach.' between utixo (13.2D) and milni (10.3D) we get a good GS capture; which was prior to localizer capture. Then we get a good localizer capture just outside milni (10.3D). Just inside milni we get an amber line through the GS on the fdi. Then the aircraft pitched down away from the glide path raw data. Simultaneously approach control states; 'low altitude alert switch to tower;' and captain (flying) disconnected the autoplt and corrected back to glide path. Estimate maximum of 200 ft below glide path; and at this time 2400 ft above ground. Reset both flight directors and rearmed the approach mode; got the GS and localizer back. Re-established stabilized approach. Landing uneventful. I think the problem was a fault in the GS sub-mode of the autoplt. To correct the problem I would recommend installation of GPS in all air carrier aircraft.

Original NASA ASRS Text

Title: B767 ACFT PITCHES DOWN AWAY FROM GS WHILE ON AUTOPILOT; GENERATING WARNING FROM ATC.

Narrative: ATIS RPTING ILS RWY 34 IN ZURICH; WX 2000 METERS; LIGHT RAIN; 500 FT SCATTERED; CEILING 1000 FT BROKEN; WIND LIGHT AND VARIABLE. ATC GIVES VECTORS AND A 010 DEG HDG TO INTERCEPT THE LOC FOR THE ILS TO RWY 34 'CLRED FOR THE APCH.' BTWN UTIXO (13.2D) AND MILNI (10.3D) WE GET A GOOD GS CAPTURE; WHICH WAS PRIOR TO LOC CAPTURE. THEN WE GET A GOOD LOC CAPTURE JUST OUTSIDE MILNI (10.3D). JUST INSIDE MILNI WE GET AN AMBER LINE THROUGH THE GS ON THE FDI. THEN THE ACFT PITCHED DOWN AWAY FROM THE GLIDE PATH RAW DATA. SIMULTANEOUSLY APCH CTL STATES; 'LOW ALT ALERT SWITCH TO TWR;' AND CAPT (FLYING) DISCONNECTED THE AUTOPLT AND CORRECTED BACK TO GLIDE PATH. ESTIMATE MAX OF 200 FT BELOW GLIDE PATH; AND AT THIS TIME 2400 FT ABOVE GND. RESET BOTH FLT DIRECTORS AND REARMED THE APCH MODE; GOT THE GS AND LOC BACK. RE-ESTABLISHED STABILIZED APCH. LNDG UNEVENTFUL. I THINK THE PROB WAS A FAULT IN THE GS SUB-MODE OF THE AUTOPLT. TO CORRECT THE PROB I WOULD RECOMMEND INSTALLATION OF GPS IN ALL ACR ACFT.

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.