Narrative:

During cruise at FL410, autoplt engaged with 'INS' and altitude hold selected on this B747-200, aircraft began a roll to the right. Aircraft was not at or even near the waypoint we were proceeding to. Roll continued through 40 degrees of bank, at which time captain changed autoflt selector from 'INS' to 'heading.' aircraft then rolled left to establish a normal (25 degree bank) left turn to capture the selected heading. Roll rate was normal at all times. We were fortunate to be daylight/VFR, and so detected the uncommanded roll quickly. There have been previous industry occurrences of this on B747 which were at night or IFR that resulted in a much higher bank angle and in at least 1 case a jet upset. The clue provided here that might help to isolate the cause of these uncommanded rolls is that we did not have to disengage the autoplt to stop it -- simply changing the mode selector from 'INS' to 'heading' restored normal operation. Once back on proper heading, we reselected 'INS,' and no further anomalies were noted. At time of occurrence, we were using autoplt 'a,' had been touched for at least 10 mins prior to start of roll. Aircraft was a B747-200F. Callback conversation with reporter revealed the following information: callback to reporter revealed that the air carrier changed the roll rate computer on this aircraft. Reporter was not 100 percent certain of this and also had a belief that this type event happened on this aircraft prior to his. Reporter, the PIC, stated that he was going to check with the maintenance department to determine the actual components that were changed. The maintenance personnel were not able to duplicate the event. After being questioned on the modes of flight used it was determined that nothing unusual had been done prior to this roll to the right of 40 degrees. Reporter was concerned that if this had happened to a crew at night, tired from a long flight, they may not have noticed it as soon as he had.

Google
 

Original NASA ASRS Text

Title: B-747 ACFT PERFORMS UNCOMMANDED STEEP TURN AT CRUISE ALT.

Narrative: DURING CRUISE AT FL410, AUTOPLT ENGAGED WITH 'INS' AND ALT HOLD SELECTED ON THIS B747-200, ACFT BEGAN A ROLL TO THE R. ACFT WAS NOT AT OR EVEN NEAR THE WAYPOINT WE WERE PROCEEDING TO. ROLL CONTINUED THROUGH 40 DEGS OF BANK, AT WHICH TIME CAPT CHANGED AUTOFLT SELECTOR FROM 'INS' TO 'HDG.' ACFT THEN ROLLED L TO ESTABLISH A NORMAL (25 DEG BANK) L TURN TO CAPTURE THE SELECTED HDG. ROLL RATE WAS NORMAL AT ALL TIMES. WE WERE FORTUNATE TO BE DAYLIGHT/VFR, AND SO DETECTED THE UNCOMMANDED ROLL QUICKLY. THERE HAVE BEEN PREVIOUS INDUSTRY OCCURRENCES OF THIS ON B747 WHICH WERE AT NIGHT OR IFR THAT RESULTED IN A MUCH HIGHER BANK ANGLE AND IN AT LEAST 1 CASE A JET UPSET. THE CLUE PROVIDED HERE THAT MIGHT HELP TO ISOLATE THE CAUSE OF THESE UNCOMMANDED ROLLS IS THAT WE DID NOT HAVE TO DISENGAGE THE AUTOPLT TO STOP IT -- SIMPLY CHANGING THE MODE SELECTOR FROM 'INS' TO 'HDG' RESTORED NORMAL OP. ONCE BACK ON PROPER HDG, WE RESELECTED 'INS,' AND NO FURTHER ANOMALIES WERE NOTED. AT TIME OF OCCURRENCE, WE WERE USING AUTOPLT 'A,' HAD BEEN TOUCHED FOR AT LEAST 10 MINS PRIOR TO START OF ROLL. ACFT WAS A B747-200F. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK TO RPTR REVEALED THAT THE ACR CHANGED THE ROLL RATE COMPUTER ON THIS ACFT. RPTR WAS NOT 100 PERCENT CERTAIN OF THIS AND ALSO HAD A BELIEF THAT THIS TYPE EVENT HAPPENED ON THIS ACFT PRIOR TO HIS. RPTR, THE PIC, STATED THAT HE WAS GOING TO CHK WITH THE MAINT DEPT TO DETERMINE THE ACTUAL COMPONENTS THAT WERE CHANGED. THE MAINT PERSONNEL WERE NOT ABLE TO DUPLICATE THE EVENT. AFTER BEING QUESTIONED ON THE MODES OF FLT USED IT WAS DETERMINED THAT NOTHING UNUSUAL HAD BEEN DONE PRIOR TO THIS ROLL TO THE R OF 40 DEGS. RPTR WAS CONCERNED THAT IF THIS HAD HAPPENED TO A CREW AT NIGHT, TIRED FROM A LONG FLT, THEY MAY NOT HAVE NOTICED IT AS SOON AS HE HAD.

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.