B757 flight crew reports anomalous automatic pilot operation during ILS approach to runway 8 at tjsj...

Attributes
ACN	779571
Time
Date	200803
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : tjsj.airport
State Reference	PR
Altitude	msl single value : 1600
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : tjsj.tracon
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 8
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
ASRS Report	779571
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	779572
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : eicas other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : regained aircraft control
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Level at 1600 ft; we were given a heading to intercept the runway 8 localizer and cleared for the ILS to runway 8. First officer was flying and used heading select to go from LNAV to heading control. He also selected localizer. We both noticed the heading select did not respond right away and then was very slow to turn to assigned heading. As we are turning to intercept the localizer; the first officer armed 'app' and a few seconds later the autoplt disengaged uncommanded and I noticed an amber line through my GS indication on my ADI. The next thing I notice is that we are now at 1200 ft. First officer corrected back and went on to an uneventful landing. Points: 1) this is the second time something very similar has happened to me at the same time on the same approach. 2) the GS was full scale up when this happened. It very much seemed like we intercepted a false GS. The first officer said he saw a fly down command on the flight director. 3) I have altered my procedure for arming the localizer/approach procedure based on my last experience here; but the first officer had already armed 'app' before I saw it and things were happening very fast. 4) something is not right just outside the marker on the ILS runway 8 approach at tjsj.

Original NASA ASRS Text

Title: B757 FLT CREW REPORTS ANOMALOUS AUTO PILOT OPERATION DURING ILS APPROACH TO RWY 8 AT TJSJ.

Narrative: LEVEL AT 1600 FT; WE WERE GIVEN A HDG TO INTERCEPT THE RWY 8 LOC AND CLRED FOR THE ILS TO RWY 8. FO WAS FLYING AND USED HDG SELECT TO GO FROM LNAV TO HDG CTL. HE ALSO SELECTED LOC. WE BOTH NOTICED THE HDG SELECT DID NOT RESPOND RIGHT AWAY AND THEN WAS VERY SLOW TO TURN TO ASSIGNED HDG. AS WE ARE TURNING TO INTERCEPT THE LOC; THE FO ARMED 'APP' AND A FEW SECONDS LATER THE AUTOPLT DISENGAGED UNCOMMANDED AND I NOTICED AN AMBER LINE THROUGH MY GS INDICATION ON MY ADI. THE NEXT THING I NOTICE IS THAT WE ARE NOW AT 1200 FT. FO CORRECTED BACK AND WENT ON TO AN UNEVENTFUL LNDG. POINTS: 1) THIS IS THE SECOND TIME SOMETHING VERY SIMILAR HAS HAPPENED TO ME AT THE SAME TIME ON THE SAME APCH. 2) THE GS WAS FULL SCALE UP WHEN THIS HAPPENED. IT VERY MUCH SEEMED LIKE WE INTERCEPTED A FALSE GS. THE FO SAID HE SAW A FLY DOWN COMMAND ON THE FLT DIRECTOR. 3) I HAVE ALTERED MY PROC FOR ARMING THE LOC/APCH PROC BASED ON MY LAST EXPERIENCE HERE; BUT THE FO HAD ALREADY ARMED 'APP' BEFORE I SAW IT AND THINGS WERE HAPPENING VERY FAST. 4) SOMETHING IS NOT RIGHT JUST OUTSIDE THE MARKER ON THE ILS RWY 8 APCH AT TJSJ.

Data retrieved from NASA's ASRS site as of May 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.