Uncommanded roll during straight and level flight.

Attributes
ACN	326795
Time
Date	199601
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : sok
State Reference	HI
Altitude	msl bound lower : 30500 msl bound upper : 31000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : sok tower : pwm
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 4 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute : pacific
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 30000 flight time type : 100
ASRS Report	326795
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : regained aircraft control flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

During cruise flight in moderate turbulence on 'a' autoplt in 'heading command' aircraft rolled abruptly 45 degrees right wing down and would not recover on autoplt. Recovery made manually. Post flight: mechanics found that a rate gyroscope had failed. Callback conversation with reporter revealed the following information: reporter stated the roll into the 45 degree bank was smooth and not abrupt. When it achieved its maximum bank it simply stayed there and did not make any attempt to roll back to level where it was commanded to be. Reporter was PF and stated that as the aircraft rolled into the bank the other flight crew members were saying, 'do something about it.' because of the turbulence and IMC conditions no one else besides the flight crew even knew it ever happened. Reporter recovered from the bank by disconnecting autoplt. After returning to straight and level flight and regaining the 500 ft lost during the rolling bank, he engaged the autoplt again and it rolled slowly into a 20 degree bank. At this time he disconnected the 'a' autoplt again, returned the aircraft to straight flight manually, then engaged the 'B' autoplt. After landing and maintenance made correction, he was informed the rate gyro had failed. This is what caused the uncommanded roll.

Original NASA ASRS Text

Title: UNCOMMANDED ROLL DURING STRAIGHT AND LEVEL FLT.

Narrative: DURING CRUISE FLT IN MODERATE TURB ON 'A' AUTOPLT IN 'HEADING COMMAND' ACFT ROLLED ABRUPTLY 45 DEGS R WING DOWN AND WOULD NOT RECOVER ON AUTOPLT. RECOVERY MADE MANUALLY. POST FLT: MECHS FOUND THAT A RATE GYROSCOPE HAD FAILED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE ROLL INTO THE 45 DEG BANK WAS SMOOTH AND NOT ABRUPT. WHEN IT ACHIEVED ITS MAX BANK IT SIMPLY STAYED THERE AND DID NOT MAKE ANY ATTEMPT TO ROLL BACK TO LEVEL WHERE IT WAS COMMANDED TO BE. RPTR WAS PF AND STATED THAT AS THE ACFT ROLLED INTO THE BANK THE OTHER FLC MEMBERS WERE SAYING, 'DO SOMETHING ABOUT IT.' BECAUSE OF THE TURB AND IMC CONDITIONS NO ONE ELSE BESIDES THE FLC EVEN KNEW IT EVER HAPPENED. RPTR RECOVERED FROM THE BANK BY DISCONNECTING AUTOPLT. AFTER RETURNING TO STRAIGHT AND LEVEL FLT AND REGAINING THE 500 FT LOST DURING THE ROLLING BANK, HE ENGAGED THE AUTOPLT AGAIN AND IT ROLLED SLOWLY INTO A 20 DEG BANK. AT THIS TIME HE DISCONNECTED THE 'A' AUTOPLT AGAIN, RETURNED THE ACFT TO STRAIGHT FLT MANUALLY, THEN ENGAGED THE 'B' AUTOPLT. AFTER LNDG AND MAINT MADE CORRECTION, HE WAS INFORMED THE RATE GYRO HAD FAILED. THIS IS WHAT CAUSED THE UNCOMMANDED ROLL.

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.