Autoplt abruptly rolls to intercept on course.

Attributes
ACN	317465
Time
Date	199510
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zmp
State Reference	MN
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 120 flight time total : 6000 flight time type : 200
ASRS Report	317465
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe aircraft equipment problem : critical other anomaly other
Independent Detector	other other : unspecified cockpit
Resolutory Action	none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

FL330 initial problem was malfunctioning yaw damper. Problem isolated and damper turned off. Turbulence in flight without yaw damper on caused some ride problems. At some point autoplt was turned off to hand fly. Upon re-engagement and selection of 'LNAV' back on course, aircraft entered rapid right bank. Autoplt disconnected. Aircraft control maintained. Again B autoplt reselected. LNAV again reselected with aircraft off left navigation course one bar width. Aircraft again entered rapid right bank. Autoplt again disconnected. Manually using control wheel steering established on course. Autoplt 'B' system reselected and no problems on remainder of flight. Crew discussed divert into omaha, but as no other problem, continued on. Crew did not understand reason for rapid right roll, but monitored carefully. In retrospect, should have selected 'a' autoplt, which might have prevented second right roll. Our initial thoughts were of pittsburg problem and caused some concern. Took about 1 hour to narrow down to autoplt as yaw damper had been eliminated. But we still had doubt as rate of roll was inappropriate for flight situation. At no time did we deviation from assigned altitude or route, but it was an interesting time. Callback conversation with reporter revealed the following information: the reporter stated they were flying a B737-300 from sfo to ord. The reporter had 23 yrs in the military much of that time as an instructor in aerobatics and unusual attitudes. The visibility in flight was about 1 mi and the horizon was not visible. At the time of the occurrence the flight crew was focused on flying the airplane with the existing turbulence and yaw damper off the ride was not comfortable. The airplane was well within its performance capability and cg limits. When selecting autoplt on and LNAV mode the aileron control wheel abruptly turned to produce a rapid right bank and the autoplt disconnected. At no other time in his 4500 hours in the B737 had he experienced an upset or a problem like this.

Original NASA ASRS Text

Title: AUTOPLT ABRUPTLY ROLLS TO INTERCEPT ON COURSE.

Narrative: FL330 INITIAL PROB WAS MALFUNCTIONING YAW DAMPER. PROB ISOLATED AND DAMPER TURNED OFF. TURB IN FLT WITHOUT YAW DAMPER ON CAUSED SOME RIDE PROBS. AT SOME POINT AUTOPLT WAS TURNED OFF TO HAND FLY. UPON RE-ENGAGEMENT AND SELECTION OF 'LNAV' BACK ON COURSE, ACFT ENTERED RAPID R BANK. AUTOPLT DISCONNECTED. ACFT CTL MAINTAINED. AGAIN B AUTOPLT RESELECTED. LNAV AGAIN RESELECTED WITH ACFT OFF L NAV COURSE ONE BAR WIDTH. ACFT AGAIN ENTERED RAPID R BANK. AUTOPLT AGAIN DISCONNECTED. MANUALLY USING CTL WHEEL STEERING ESTABLISHED ON COURSE. AUTOPLT 'B' SYS RESELECTED AND NO PROBS ON REMAINDER OF FLT. CREW DISCUSSED DIVERT INTO OMAHA, BUT AS NO OTHER PROB, CONTINUED ON. CREW DID NOT UNDERSTAND REASON FOR RAPID R ROLL, BUT MONITORED CAREFULLY. IN RETROSPECT, SHOULD HAVE SELECTED 'A' AUTOPLT, WHICH MIGHT HAVE PREVENTED SECOND R ROLL. OUR INITIAL THOUGHTS WERE OF PITTSBURG PROB AND CAUSED SOME CONCERN. TOOK ABOUT 1 HR TO NARROW DOWN TO AUTOPLT AS YAW DAMPER HAD BEEN ELIMINATED. BUT WE STILL HAD DOUBT AS RATE OF ROLL WAS INAPPROPRIATE FOR FLT SIT. AT NO TIME DID WE DEV FROM ASSIGNED ALT OR RTE, BUT IT WAS AN INTERESTING TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THEY WERE FLYING A B737-300 FROM SFO TO ORD. THE RPTR HAD 23 YRS IN THE MIL MUCH OF THAT TIME AS AN INSTRUCTOR IN AEROBATICS AND UNUSUAL ATTITUDES. THE VISIBILITY IN FLT WAS ABOUT 1 MI AND THE HORIZON WAS NOT VISIBLE. AT THE TIME OF THE OCCURRENCE THE FLC WAS FOCUSED ON FLYING THE AIRPLANE WITH THE EXISTING TURB AND YAW DAMPER OFF THE RIDE WAS NOT COMFORTABLE. THE AIRPLANE WAS WELL WITHIN ITS PERFORMANCE CAPABILITY AND CG LIMITS. WHEN SELECTING AUTOPLT ON AND LNAV MODE THE AILERON CTL WHEEL ABRUPTLY TURNED TO PRODUCE A RAPID R BANK AND THE AUTOPLT DISCONNECTED. AT NO OTHER TIME IN HIS 4500 HRS IN THE B737 HAD HE EXPERIENCED AN UPSET OR A PROB LIKE THIS.

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.