B737-400 crew got a MSAW while conducting a CAT 3 approach at sea.

Attributes
ACN	631998
Time
Date	200409
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : sea.airport
State Reference	WA
Altitude	msl single value : 800
Environment
Flight Conditions	IMC
Weather Elements	Fog
Light	Daylight
Aircraft 1
Controlling Facilities	tower : sea.tower
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 16r
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	631998
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	632388
Events
Anomaly	aircraft equipment problem : critical inflight encounter : weather
Independent Detector	atc equipment : msaw atc equipment other atc equipment : radar
Resolutory Action	controller : issued alert flight crew : executed go around other
Consequence	other other
Supplementary
Problem Areas	Weather Aircraft
Primary Problem	Aircraft

Narrative:

WX in sea was 1/4 SM br (mist) VV001 30.15 RVR 16R 4500-6000-6000. I elected to do a CAT 3 autoland approach with all hgs (heads up guidance system) monitoring. We briefed the approach plate and CAT 3 card per the flight handbook. All checklists and callouts were completed per our pros. The approach was normal on speed, both autopolts engaged. All enunciated and both navs showing on GS and localizer. At around 800 ft MSL sea tower gave us an altitude alert, as I initiated a go around I noticed we were still on localizer and GS. During the missed approach we again compared altimeter settings and readings. Both altimeters matched and tower gave us a new barometric of 30.16 but that was only .01 off what we had set 30.15. Since we could not find any problems with any aircraft navigation system a second CAT 3 autoland was accomplished. Since altitude reporting is the only system in common with ATC, I switched the transponder altitude source to the #2 side and left the transponder in the 2 position where it was in the first approach. No further problems were reported by ATC and the approach was normal. I wrote up the problem and maintenance MEL'ed the #1 transponder. Tower indicated that their system is very sensitive. Aircraft operation normal suspected altitude reporting or ATC system problem.

Original NASA ASRS Text

Title: B737-400 CREW GOT A MSAW WHILE CONDUCTING A CAT 3 APCH AT SEA.

Narrative: WX IN SEA WAS 1/4 SM BR (MIST) VV001 30.15 RVR 16R 4500-6000-6000. I ELECTED TO DO A CAT 3 AUTOLAND APCH WITH ALL HGS (HEADS UP GUIDANCE SYS) MONITORING. WE BRIEFED THE APCH PLATE AND CAT 3 CARD PER THE FLT HANDBOOK. ALL CHKLISTS AND CALLOUTS WERE COMPLETED PER OUR PROS. THE APCH WAS NORMAL ON SPD, BOTH AUTOPOLTS ENGAGED. ALL ENUNCIATED AND BOTH NAVS SHOWING ON GS AND LOC. AT AROUND 800 FT MSL SEA TWR GAVE US AN ALT ALERT, AS I INITIATED A GAR I NOTICED WE WERE STILL ON LOC AND GS. DURING THE MISSED APCH WE AGAIN COMPARED ALTIMETER SETTINGS AND READINGS. BOTH ALTIMETERS MATCHED AND TWR GAVE US A NEW BAROMETRIC OF 30.16 BUT THAT WAS ONLY .01 OFF WHAT WE HAD SET 30.15. SINCE WE COULD NOT FIND ANY PROBS WITH ANY ACFT NAV SYS A SECOND CAT 3 AUTOLAND WAS ACCOMPLISHED. SINCE ALT RPTING IS THE ONLY SYS IN COMMON WITH ATC, I SWITCHED THE XPONDER ALT SOURCE TO THE #2 SIDE AND LEFT THE XPONDER IN THE 2 POS WHERE IT WAS IN THE FIRST APCH. NO FURTHER PROBS WERE RPTED BY ATC AND THE APCH WAS NORMAL. I WROTE UP THE PROB AND MAINT MEL'ED THE #1 XPONDER. TWR INDICATED THAT THEIR SYS IS VERY SENSITIVE. ACFT OP NORMAL SUSPECTED ALT RPTING OR ATC SYS PROB.

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.