B737 uncommanded roll at FL280.

Attributes
ACN	434546
Time
Date	199904
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : rzs.vortac
State Reference	CA
Altitude	msl single value : 31000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zla.artcc
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 60 flight time total : 12000 flight time type : 9000
ASRS Report	434546
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp
Events
Anomaly	aircraft equipment problem : critical inflight encounter other other anomaly other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : declared emergency flight crew : landed in emergency condition
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We departed lax for sfo on a clear evening. Approximately 50 passenger, 5 crew members on board. We were flying direct to rzs at FL310 at about .77 mach in clear and mostly smooth air. Wind was from the northwest, LNAV, VNAV and system B autoplt were engaged, both flight directors were on. Yaw damper was engaged. 2 aircraft were ahead at FL310, 10 mi and 25 mi. We asked for FL280. We began to pick up very light chop at FL310. ATC cleared us to descend and I reset the altitude window, the first officer reset the cabin altitude. As I leaned forward to enter FL280 in the FMC, the aircraft abruptly rolled hard to the left. No flight control input had been made by either pilot. We both grabbed the yoke, disconnected the autoplt and stepped on the right rudder which immediately brought the plane upright. I estimate a maximum of 20-30 degrees of bank was encountered but the roll rate was very fast. No passenger were hurt, 1 flight attendant was thrown to the floor but was not hurt. We declared an emergency and asked to return to lax. ATC complied and we stated the nature of the emergency was a 'flight control problem.' we elected to disconnect the yaw damper manually in case that was the cause of the problem. The flight continued uneventfully to lax. Callback conversation with reporter revealed the following information: reporter states the aircraft involved ia B737-500. After the emergency landing in lax, a flight safety investigation took place. The findings of the investigation indicated a malfunction in the autoplt affecting the roll channel. There was no rudder or rudder valve involvement.

Original NASA ASRS Text

Title: B737 UNCOMMANDED ROLL AT FL280.

Narrative: WE DEPARTED LAX FOR SFO ON A CLR EVENING. APPROX 50 PAX, 5 CREW MEMBERS ON BOARD. WE WERE FLYING DIRECT TO RZS AT FL310 AT ABOUT .77 MACH IN CLR AND MOSTLY SMOOTH AIR. WIND WAS FROM THE NW, LNAV, VNAV AND SYS B AUTOPLT WERE ENGAGED, BOTH FLT DIRECTORS WERE ON. YAW DAMPER WAS ENGAGED. 2 ACFT WERE AHEAD AT FL310, 10 MI AND 25 MI. WE ASKED FOR FL280. WE BEGAN TO PICK UP VERY LIGHT CHOP AT FL310. ATC CLRED US TO DSND AND I RESET THE ALT WINDOW, THE FO RESET THE CABIN ALT. AS I LEANED FORWARD TO ENTER FL280 IN THE FMC, THE ACFT ABRUPTLY ROLLED HARD TO THE L. NO FLT CTL INPUT HAD BEEN MADE BY EITHER PLT. WE BOTH GRABBED THE YOKE, DISCONNECTED THE AUTOPLT AND STEPPED ON THE R RUDDER WHICH IMMEDIATELY BROUGHT THE PLANE UPRIGHT. I ESTIMATE A MAX OF 20-30 DEGS OF BANK WAS ENCOUNTERED BUT THE ROLL RATE WAS VERY FAST. NO PAX WERE HURT, 1 FLT ATTENDANT WAS THROWN TO THE FLOOR BUT WAS NOT HURT. WE DECLARED AN EMER AND ASKED TO RETURN TO LAX. ATC COMPLIED AND WE STATED THE NATURE OF THE EMER WAS A 'FLT CTL PROB.' WE ELECTED TO DISCONNECT THE YAW DAMPER MANUALLY IN CASE THAT WAS THE CAUSE OF THE PROB. THE FLT CONTINUED UNEVENTFULLY TO LAX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE ACFT INVOLVED IA B737-500. AFTER THE EMER LNDG IN LAX, A FLT SAFETY INVESTIGATION TOOK PLACE. THE FINDINGS OF THE INVESTIGATION INDICATED A MALFUNCTION IN THE AUTOPLT AFFECTING THE ROLL CHANNEL. THERE WAS NO RUDDER OR RUDDER VALVE INVOLVEMENT.

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.