Air carrier large transport capture GS low and started down during an ILS approach with no apparent ...

Attributes
ACN	497657
Time
Date	200101
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lax.airport
State Reference	CA
Altitude	msl bound lower : 3380 msl bound upper : 3500
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 25l
Flight Phase	descent : vacating altitude descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : instrument pilot : multi engine pilot : commercial
Experience	flight time last 90 days : 150 flight time total : 5200 flight time type : 1400
ASRS Report	497657
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : crossing restriction not met altitude deviation : overshoot non adherence : clearance non adherence : published procedure
Independent Detector	aircraft equipment other aircraft equipment : glide slope indication other flight crewa
Resolutory Action	none taken : insufficient time none taken : unable
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Established at 3500 ft on ILS runway 25L approach 1/2 NM outside of hunda intersection, cleared for the approach. ADI indications showed us 1 1/2 dots below GS when I armed the MCP for the approach. Confirmed the white GS annunciation at 4 NM from hunda. In anticipation of starting down to 2200 ft MSL inside hunda for limma intersection, I selected vertical speed to start down at 2 NM from hunda when the GS annunciation turned green and aircraft pitched down to capture GS. I hadn't anticipated the GS capture so soon and crosschecked GS indication -- only to find that we were still about 1/2 DOT below GS. As a result of the aircraft capturing and starting down on GS too soon, we ended up crossing hunda intersection 120 ft too low. This all happened so quickly that I was unable to have enough time to correctly assess and prevent the aircraft from capturing GS and starting down. I don't understand why the aircraft would capture GS and start down when still below GS.

Original NASA ASRS Text

Title: ACR LGT CAPTURE GS LOW AND STARTED DOWN DURING AN ILS APCH WITH NO APPARENT REASON.

Narrative: ESTABLISHED AT 3500 FT ON ILS RWY 25L APCH 1/2 NM OUTSIDE OF HUNDA INTXN, CLRED FOR THE APCH. ADI INDICATIONS SHOWED US 1 1/2 DOTS BELOW GS WHEN I ARMED THE MCP FOR THE APCH. CONFIRMED THE WHITE GS ANNUNCIATION AT 4 NM FROM HUNDA. IN ANTICIPATION OF STARTING DOWN TO 2200 FT MSL INSIDE HUNDA FOR LIMMA INTXN, I SELECTED VERT SPD TO START DOWN AT 2 NM FROM HUNDA WHEN THE GS ANNUNCIATION TURNED GREEN AND ACFT PITCHED DOWN TO CAPTURE GS. I HADN'T ANTICIPATED THE GS CAPTURE SO SOON AND XCHKED GS INDICATION -- ONLY TO FIND THAT WE WERE STILL ABOUT 1/2 DOT BELOW GS. AS A RESULT OF THE ACFT CAPTURING AND STARTING DOWN ON GS TOO SOON, WE ENDED UP XING HUNDA INTXN 120 FT TOO LOW. THIS ALL HAPPENED SO QUICKLY THAT I WAS UNABLE TO HAVE ENOUGH TIME TO CORRECTLY ASSESS AND PREVENT THE ACFT FROM CAPTURING GS AND STARTING DOWN. I DON'T UNDERSTAND WHY THE ACFT WOULD CAPTURE GS AND START DOWN WHEN STILL BELOW GS.

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.

Air carrier large transport capture GS low and started down during an ILS approach with no apparent reason.