Altitude deviation while on autoplt during ILS approach.

Attributes
ACN	288008
Time
Date	199411
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : oak
State Reference	CA
Altitude	msl bound lower : 1800 msl bound upper : 1900
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : oak artcc : zfw
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : oak
Flight Phase	descent : approach
Route In Use	approach : straight in
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : straight in
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 16000 flight time type : 210
ASRS Report	288008
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : published procedure
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other

Narrative:

Cleared ILS approach to oak runway 11 and to maintain 1800 ft or below until GS intercept point (forgot name of fix). Autoplt captured localizer and GS and started down. Got to about 1/4 DOT low on GS and autoplt started climbing. Went through GS to about 1900 ft where I disengaged and hand flew approach. Thought that something was wrong with aircraft but another crew reported same event. May be a 'bump' in the GS or maybe a navy ship which was in the area caused interference. I did not mention deviation to tower, but now know we should have after learning of other crew having same problem. I believe they needed us at 1800 ft or below for traffic landing sfo. As I said we deviation about 100 plus ft to 1900 ft before taking over manually. Definitely should have mentioned it to tower/approach rather than assuming that it was an autoplt malfunction. Callback conversation with reporter revealed the following information: reporter indicated that he has not been back to that airport and doesn't know if the problem still exists. Reporter indicated that he will find out if other flcs experienced the same problem in an upcoming meeting with the chief pilot. Reporter and other flight crew from the other aircraft mentioned in the report, seem to feel there could have been some interference from an aircraft carrier which was near the final approach course. Reporter stated both aircraft were B737's.

Original NASA ASRS Text

Title: ALTDEV WHILE ON AUTOPLT DURING ILS APCH.

Narrative: CLRED ILS APCH TO OAK RWY 11 AND TO MAINTAIN 1800 FT OR BELOW UNTIL GS INTERCEPT POINT (FORGOT NAME OF FIX). AUTOPLT CAPTURED LOC AND GS AND STARTED DOWN. GOT TO ABOUT 1/4 DOT LOW ON GS AND AUTOPLT STARTED CLBING. WENT THROUGH GS TO ABOUT 1900 FT WHERE I DISENGAGED AND HAND FLEW APCH. THOUGHT THAT SOMETHING WAS WRONG WITH ACFT BUT ANOTHER CREW RPTED SAME EVENT. MAY BE A 'BUMP' IN THE GS OR MAYBE A NAVY SHIP WHICH WAS IN THE AREA CAUSED INTERFERENCE. I DID NOT MENTION DEV TO TWR, BUT NOW KNOW WE SHOULD HAVE AFTER LEARNING OF OTHER CREW HAVING SAME PROB. I BELIEVE THEY NEEDED US AT 1800 FT OR BELOW FOR TFC LNDG SFO. AS I SAID WE DEV ABOUT 100 PLUS FT TO 1900 FT BEFORE TAKING OVER MANUALLY. DEFINITELY SHOULD HAVE MENTIONED IT TO TWR/APCH RATHER THAN ASSUMING THAT IT WAS AN AUTOPLT MALFUNCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT HE HAS NOT BEEN BACK TO THAT ARPT AND DOESN'T KNOW IF THE PROB STILL EXISTS. RPTR INDICATED THAT HE WILL FIND OUT IF OTHER FLCS EXPERIENCED THE SAME PROB IN AN UPCOMING MEETING WITH THE CHIEF PLT. RPTR AND OTHER FLC FROM THE OTHER ACFT MENTIONED IN THE RPT, SEEM TO FEEL THERE COULD HAVE BEEN SOME INTERFERENCE FROM AN ACFT CARRIER WHICH WAS NEAR THE FINAL APCH COURSE. RPTR STATED BOTH ACFT WERE B737'S.

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.