A B737 descends 800 ft below its assigned altitude during a 25 degree bank turn onto localizer runway 27, 20 mi from bos, ma. Autoplt had disconnected in the turn along with associated 'a' system lights illuminating. Bos, ma.

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Attributes
ACN	493430
Time
Date	200011
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bos.airport
State Reference	MA
Altitude	msl bound lower : 5200 msl bound upper : 6000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : a90.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 27 Other
Flight Phase	descent : intermediate altitude
Route In Use	approach : instrument precision arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument pilot : multi engine
ASRS Report	493430
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : overshoot non adherence : clearance other anomaly
Independent Detector	aircraft equipment other aircraft equipment : hyd sys wng lites other flight crewa other flight crewb
Resolutory Action	aircraft : equipment problem dissipated controller : issued new clearance controller : issued advisory flight crew : regained aircraft control
Consequence	faa : reviewed incident with flight crew other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Just vectored and cleared for ILS runway 27 at logan and in clouds, autoplt kicked off at 6000 ft MSL, in cloud, and in about 25-30 degrees bank. Captain took control manually. I evaluated problem. Overhead panel indications were both system 'a' hydraulic lights on and related system 'a' lights on. I reached for abnormal book and noticed altitude was 5200 ft as bos approach said 'what is your assigned altitude? You are not in trouble.' we responded '6000 ft and correcting.' we requested a vector for time to solve problem. Bos asked if we were an emergency. We said no. Then 'a' hydraulic system recovered all system worked normally. We got vectors for approach and landed uneventfully.

Original NASA ASRS Text

Title: A B737 DSNDS 800 FT BELOW ITS ASSIGNED ALT DURING A 25 DEG BANK TURN ONTO LOC RWY 27, 20 MI FROM BOS, MA. AUTOPLT HAD DISCONNECTED IN THE TURN ALONG WITH ASSOCIATED 'A' SYS LIGHTS ILLUMINATING. BOS, MA.

Narrative: JUST VECTORED AND CLRED FOR ILS RWY 27 AT LOGAN AND IN CLOUDS, AUTOPLT KICKED OFF AT 6000 FT MSL, IN CLOUD, AND IN ABOUT 25-30 DEGS BANK. CAPT TOOK CTL MANUALLY. I EVALUATED PROB. OVERHEAD PANEL INDICATIONS WERE BOTH SYS 'A' HYD LIGHTS ON AND RELATED SYS 'A' LIGHTS ON. I REACHED FOR ABNORMAL BOOK AND NOTICED ALT WAS 5200 FT AS BOS APCH SAID 'WHAT IS YOUR ASSIGNED ALT? YOU ARE NOT IN TROUBLE.' WE RESPONDED '6000 FT AND CORRECTING.' WE REQUESTED A VECTOR FOR TIME TO SOLVE PROB. BOS ASKED IF WE WERE AN EMER. WE SAID NO. THEN 'A' HYD SYS RECOVERED ALL SYS WORKED NORMALLY. WE GOT VECTORS FOR APCH AND LANDED UNEVENTFULLY.

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.