B737 crew had an altitude deviation.

Attributes
ACN	424560
Time
Date	199901
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : tnv
State Reference	TX
Altitude	msl bound lower : 32600 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	arrival other arrival star : star
Flight Plan	IFR
Aircraft 2
Make Model Name	B737-300
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : cfi pilot : flight engineer
Experience	flight time last 90 days : 120 flight time total : 6500 flight time type : 300
ASRS Report	424560
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial
Experience	flight time last 90 days : 100 flight time total : 15000 flight time type : 10000
ASRS Report	424254
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Consequence	Other
Supplementary
Air Traffic Incident	Pilot Deviation

Narrative:

Captain flying in cruise at FL330. 'A' autoplt, LNAV and VNAV were engaged. FL330 was set in MCP window. '250 KTS' and '10000 ft' were preset on descent page in FMS for hoagi intersection. We did not have descent clearance yet. Aircraft began an uncommanded descent with FL330 still set in MCP window. Captain was briefing approach at the time and I was looking at approach plate. The descent could not be felt, but I noticed the throttles retarding and realized we were descending just as the altitude alerter went off at FL327. Both of us reached for the control wheel and the descent was stopped by FL326. We immediately returned to FL330. Prior to this flight, 'a' autoplt had been written up as 'not holding altitude.' it was, however, signed off as fixed by our maintenance department. The controller never commented on the altitude loss and there was no traffic conflict. Supplemental information from acn 424254: prior to this flight, 'a' autoplt was written up as 'not holding altitude.' both maintenance write-ups had been written off by our maintenance department. 'A' autoplt had been functioning properly for approximately 2 hours. ZHU did not comment on this 400 ft altitude loss.

Original NASA ASRS Text

Title: B737 CREW HAD AN ALTDEV.

Narrative: CAPT FLYING IN CRUISE AT FL330. 'A' AUTOPLT, LNAV AND VNAV WERE ENGAGED. FL330 WAS SET IN MCP WINDOW. '250 KTS' AND '10000 FT' WERE PRESET ON DSCNT PAGE IN FMS FOR HOAGI INTXN. WE DID NOT HAVE DSCNT CLRNC YET. ACFT BEGAN AN UNCOMMANDED DSCNT WITH FL330 STILL SET IN MCP WINDOW. CAPT WAS BRIEFING APCH AT THE TIME AND I WAS LOOKING AT APCH PLATE. THE DSCNT COULD NOT BE FELT, BUT I NOTICED THE THROTTLES RETARDING AND REALIZED WE WERE DSNDING JUST AS THE ALT ALERTER WENT OFF AT FL327. BOTH OF US REACHED FOR THE CTL WHEEL AND THE DSCNT WAS STOPPED BY FL326. WE IMMEDIATELY RETURNED TO FL330. PRIOR TO THIS FLT, 'A' AUTOPLT HAD BEEN WRITTEN UP AS 'NOT HOLDING ALT.' IT WAS, HOWEVER, SIGNED OFF AS FIXED BY OUR MAINT DEPT. THE CTLR NEVER COMMENTED ON THE ALT LOSS AND THERE WAS NO TFC CONFLICT. SUPPLEMENTAL INFO FROM ACN 424254: PRIOR TO THIS FLT, 'A' AUTOPLT WAS WRITTEN UP AS 'NOT HOLDING ALT.' BOTH MAINT WRITE-UPS HAD BEEN WRITTEN OFF BY OUR MAINT DEPT. 'A' AUTOPLT HAD BEEN FUNCTIONING PROPERLY FOR APPROX 2 HRS. ZHU DID NOT COMMENT ON THIS 400 FT ALT LOSS.

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.