Air carrier issued low altitude alert by tower.

Attributes
ACN	325141
Time
Date	199512
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : sfo
State Reference	CA
Altitude	msl bound lower : 2000 msl bound upper : 2000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : sfo
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 140 flight time total : 10700 flight time type : 1000
ASRS Report	325141
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot other anomaly other other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

After 12 hour pacific crossing, we were vectored for an ILS runway 28R approach at sfo. Normal vectors were given for intercept at 2000 ft (GS1A 1800 ft). WX on ATIS reported as 800 ft and 4 NM. After localizer capture and normal tracking, I observed GS capture, announced it, and set map altitude in mode control panel. Then given frequency change to tower. While changing frequency, aural alert sounds. GS unusable by autoplt. Aircraft continues descent at stabilized attitude, approximately 800 FPM descent. I called for PF to disconnect autoplt, I cycled flight directors off then on as required and rearmed approach mode. Aircraft descended to 1200 ft MSL (550 ft low). Prior to climb to glide path, were alerted by approach control of low altitude alert. On breaking out with runway approach light in sight at 600 ft MSL, I observed another B747-400 holding between runways 28L and 28R. Obviously, the ILS was not protected (not required 800 ft/4 NM or better) but every aircraft landing sfo with a 1200 ft thick overcast was relying on the ILS completely. I believe the 800 ft/4 NM rule is not valid. If an aircraft must penetrate an overcast layer on an approach, the primary approach facility should be reliable, ie, protected.

Original NASA ASRS Text

Title: ACR ISSUED LOW ALT ALERT BY TWR.

Narrative: AFTER 12 HR PACIFIC XING, WE WERE VECTORED FOR AN ILS RWY 28R APCH AT SFO. NORMAL VECTORS WERE GIVEN FOR INTERCEPT AT 2000 FT (GS1A 1800 FT). WX ON ATIS RPTED AS 800 FT AND 4 NM. AFTER LOC CAPTURE AND NORMAL TRACKING, I OBSERVED GS CAPTURE, ANNOUNCED IT, AND SET MAP ALT IN MODE CTL PANEL. THEN GIVEN FREQ CHANGE TO TWR. WHILE CHANGING FREQ, AURAL ALERT SOUNDS. GS UNUSABLE BY AUTOPLT. ACFT CONTINUES DSCNT AT STABILIZED ATTITUDE, APPROX 800 FPM DSCNT. I CALLED FOR PF TO DISCONNECT AUTOPLT, I CYCLED FLT DIRECTORS OFF THEN ON AS REQUIRED AND REARMED APCH MODE. ACFT DSNDED TO 1200 FT MSL (550 FT LOW). PRIOR TO CLB TO GLIDE PATH, WERE ALERTED BY APCH CTL OF LOW ALT ALERT. ON BREAKING OUT WITH RWY APCH LIGHT IN SIGHT AT 600 FT MSL, I OBSERVED ANOTHER B747-400 HOLDING BTWN RWYS 28L AND 28R. OBVIOUSLY, THE ILS WAS NOT PROTECTED (NOT REQUIRED 800 FT/4 NM OR BETTER) BUT EVERY ACFT LNDG SFO WITH A 1200 FT THICK OVCST WAS RELYING ON THE ILS COMPLETELY. I BELIEVE THE 800 FT/4 NM RULE IS NOT VALID. IF AN ACFT MUST PENETRATE AN OVCST LAYER ON AN APCH, THE PRIMARY APCH FACILITY SHOULD BE RELIABLE, IE, PROTECTED.

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.