Narrative:

We were in cruise, at FL390, and passing lrp. When the computer sequenced lrp, the autoplt began a turn to the left even though the next waypoint was essentially straight ahead. The PF allowed the turn to continue for a little to see if the computer/autoplt would correct itself, and when it didn't, he selected a heading to resume the proper course. Center inquired, and I belatedly realized that I should have said something to him earlier. The computer displayed an anomaly even though the map presentation was entirely normal. The computer anomaly was the 'to' waypoint bearing was 100 degrees in error, although with one exception -- the aircraft had been correctly sequencing waypoints. That exception was an inappropriate turn initiation over a waypoint during the prior leg. That time I immediately selected a heading, and everything was normal thereafter. On the ground, I discussed the event with a mechanic who consulted one of our in-house experts at company headquarters. At their direction we cycled data bases back and forth although everything looked normal before and afterward. Realize that 'to' waypoints are not displayed on the ground. I also called our A320 fleet captain/director who said he never heard of such a turn and 'to' waypoint bearing error. He asked us to carefully examine the data base during the next leg to see if we could find any errors. Although everything looked normal on the ground, after takeoff we realized the 'to' waypoint bearing error was still with us, although this time it was 100 degrees in the opposite direction. After the turn at lrp, we noticed that not only the 'to' waypoint bearing was in error, but all bearings on the progress page were also in error. I wrote it up at the next stop and the mechanic (who had never heard of this problem) cleared it by resetting the IRU's. No one seems to understand the anomaly, but they say they'll discuss it with the manufacturer. Callback conversation with reporter revealed the following information: this A320 captain with 6 months experience on the aircraft had situation with computer anomaly which had not been experienced before or after. The problem was brought to the attention of maintenance technicians as well as the fleet captain. Data base error was suspect, but checking all data bases found no errors. Why did this isolated incident occur when subsequent flts did not have the anomaly? The laser gyroscope inertial reference system provides attitude, flight path vector, heading, track, acceleration, ground speed and airplane position. The inertial reference system does not navigation, rather it provides inertial position input to the flight management and guidance system. After initial power up of the adirs present position, latitude and longitude must be entered correctly during the alignment process prior to each flight. If the flight crew failed to enter the correct latitude/longitude, it would explain subsequent bearing errors. The reporter captain mentioned another anomaly the air carrier had with the A320 -- that the aileron had unexplained jerks or twitches bordering on uncommanded roll. It was determined to be caused by a rheostat in the sidestick. Changing the software appears to have corrected the problem.

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Original NASA ASRS Text

Title: COMPUTER ANOMALY WITH LNAV IN USE TURNED ACFT OFF COURSE AFTER PASSING WAYPOINT. FLC WAITED TO SEE IF IT WOULD CORRECT AND WHEN IT DID NOT, USED HDG SELECT TO RETURN TO COURSE.

Narrative: WE WERE IN CRUISE, AT FL390, AND PASSING LRP. WHEN THE COMPUTER SEQUENCED LRP, THE AUTOPLT BEGAN A TURN TO THE L EVEN THOUGH THE NEXT WAYPOINT WAS ESSENTIALLY STRAIGHT AHEAD. THE PF ALLOWED THE TURN TO CONTINUE FOR A LITTLE TO SEE IF THE COMPUTER/AUTOPLT WOULD CORRECT ITSELF, AND WHEN IT DIDN'T, HE SELECTED A HDG TO RESUME THE PROPER COURSE. CTR INQUIRED, AND I BELATEDLY REALIZED THAT I SHOULD HAVE SAID SOMETHING TO HIM EARLIER. THE COMPUTER DISPLAYED AN ANOMALY EVEN THOUGH THE MAP PRESENTATION WAS ENTIRELY NORMAL. THE COMPUTER ANOMALY WAS THE 'TO' WAYPOINT BEARING WAS 100 DEGS IN ERROR, ALTHOUGH WITH ONE EXCEPTION -- THE ACFT HAD BEEN CORRECTLY SEQUENCING WAYPOINTS. THAT EXCEPTION WAS AN INAPPROPRIATE TURN INITIATION OVER A WAYPOINT DURING THE PRIOR LEG. THAT TIME I IMMEDIATELY SELECTED A HDG, AND EVERYTHING WAS NORMAL THEREAFTER. ON THE GND, I DISCUSSED THE EVENT WITH A MECH WHO CONSULTED ONE OF OUR IN-HOUSE EXPERTS AT COMPANY HEADQUARTERS. AT THEIR DIRECTION WE CYCLED DATA BASES BACK AND FORTH ALTHOUGH EVERYTHING LOOKED NORMAL BEFORE AND AFTERWARD. REALIZE THAT 'TO' WAYPOINTS ARE NOT DISPLAYED ON THE GND. I ALSO CALLED OUR A320 FLEET CAPT/DIRECTOR WHO SAID HE NEVER HEARD OF SUCH A TURN AND 'TO' WAYPOINT BEARING ERROR. HE ASKED US TO CAREFULLY EXAMINE THE DATA BASE DURING THE NEXT LEG TO SEE IF WE COULD FIND ANY ERRORS. ALTHOUGH EVERYTHING LOOKED NORMAL ON THE GND, AFTER TKOF WE REALIZED THE 'TO' WAYPOINT BEARING ERROR WAS STILL WITH US, ALTHOUGH THIS TIME IT WAS 100 DEGS IN THE OPPOSITE DIRECTION. AFTER THE TURN AT LRP, WE NOTICED THAT NOT ONLY THE 'TO' WAYPOINT BEARING WAS IN ERROR, BUT ALL BEARINGS ON THE PROGRESS PAGE WERE ALSO IN ERROR. I WROTE IT UP AT THE NEXT STOP AND THE MECH (WHO HAD NEVER HEARD OF THIS PROB) CLRED IT BY RESETTING THE IRU'S. NO ONE SEEMS TO UNDERSTAND THE ANOMALY, BUT THEY SAY THEY'LL DISCUSS IT WITH THE MANUFACTURER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS A320 CAPT WITH 6 MONTHS EXPERIENCE ON THE ACFT HAD SIT WITH COMPUTER ANOMALY WHICH HAD NOT BEEN EXPERIENCED BEFORE OR AFTER. THE PROB WAS BROUGHT TO THE ATTN OF MAINT TECHNICIANS AS WELL AS THE FLEET CAPT. DATA BASE ERROR WAS SUSPECT, BUT CHKING ALL DATA BASES FOUND NO ERRORS. WHY DID THIS ISOLATED INCIDENT OCCUR WHEN SUBSEQUENT FLTS DID NOT HAVE THE ANOMALY? THE LASER GYROSCOPE INERTIAL REF SYS PROVIDES ATTITUDE, FLT PATH VECTOR, HDG, TRACK, ACCELERATION, GND SPD AND AIRPLANE POS. THE INERTIAL REF SYS DOES NOT NAV, RATHER IT PROVIDES INERTIAL POS INPUT TO THE FLT MGMNT AND GUIDANCE SYS. AFTER INITIAL PWR UP OF THE ADIRS PRESENT POS, LATITUDE AND LONGITUDE MUST BE ENTERED CORRECTLY DURING THE ALIGNMENT PROCESS PRIOR TO EACH FLT. IF THE FLC FAILED TO ENTER THE CORRECT LATITUDE/LONGITUDE, IT WOULD EXPLAIN SUBSEQUENT BEARING ERRORS. THE RPTR CAPT MENTIONED ANOTHER ANOMALY THE ACR HAD WITH THE A320 -- THAT THE AILERON HAD UNEXPLAINED JERKS OR TWITCHES BORDERING ON UNCOMMANDED ROLL. IT WAS DETERMINED TO BE CAUSED BY A RHEOSTAT IN THE SIDESTICK. CHANGING THE SOFTWARE APPEARS TO HAVE CORRECTED THE PROB.

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.