B747 crew had track deviation.

Attributes
ACN	451831
Time
Date	199910
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : plado
State Reference	AK
Altitude	msl single value : 33000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zan.artcc
Operator	common carrier : air carrier
Make Model Name	B747-100
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute : pacific enroute : on vectors enroute airway : a590.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 200 flight time total : 10000 flight time type : 2000
ASRS Report	451831
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance other spatial deviation
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance controller : issued alert flight crew : returned to original clearance flight crew : overcame equipment problem none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were in cruise flight along airway A590 from seoul to anchorage above a cloud deck. The captain was the PF and company procedures called for #1 INS and 'a' autoplt to be engaged, which they were. It was dusk but we could see another aircraft's contrail. The INS showed we were tracking along the airway centerline. Visually, we were tracking left (upwind) of the preceding aircraft's contrail. At approximately intersection plado, the INS's began updating from sya (shemya) VOR. At that instant, the xtrack error showed us 0.5 left of track and the autoplt made a right turn to an intercept angle of 15 degrees. We noted that this seemed to be too large a correction and began to confirm the xtrack deviation with the other system (#2 and #3). They also showed us left of track. Visually, we crossed the contrail of the preceding aircraft and expected a turn back to track. The autoplt did not make a turn and we noted that the #1 INS still showed us 0.2 left of track with the same intercept angle. Oddly, the INS xtrack error did not decrease any further, even though we had a 15 degree intercept angle, so the captain selected a course direct to pinso, the next point along the route. The aircraft turned and tracked direct to pinso. I selected the track plado-pinso in the #2 INS and it showed us still 0.2 left of course. We knew this could not be possible so all INS's were changed to track direct to pinso. As we were making the turn to pinso, zan notified us they showed us 8 NM right of track and diverging. We informed them that we were turning direct to pinso and checking the equipment. We verified the INS position at pinso with the sya VOR and all were in agreement. After that, there were no discrepancies with the INS's. The INS's updated normally (automatically) with bethel (bet) VOR and showed us on course. We were unable to explain how all 3 system showed the same deviation left of track, then not re-intercept track even though there was more than a sufficient intercept angle. This entire incident seemed to have taken less than 2 mins. To be 8 NM right of track, would have required a divergent heading of at least 25 degrees. The captain submitted a crew report to the company.

Original NASA ASRS Text

Title: B747 CREW HAD TRACK DEV.

Narrative: WE WERE IN CRUISE FLT ALONG AIRWAY A590 FROM SEOUL TO ANCHORAGE ABOVE A CLOUD DECK. THE CAPT WAS THE PF AND COMPANY PROCS CALLED FOR #1 INS AND 'A' AUTOPLT TO BE ENGAGED, WHICH THEY WERE. IT WAS DUSK BUT WE COULD SEE ANOTHER ACFT'S CONTRAIL. THE INS SHOWED WE WERE TRACKING ALONG THE AIRWAY CTRLINE. VISUALLY, WE WERE TRACKING L (UPWIND) OF THE PRECEDING ACFT'S CONTRAIL. AT APPROX INTXN PLADO, THE INS'S BEGAN UPDATING FROM SYA (SHEMYA) VOR. AT THAT INSTANT, THE XTRACK ERROR SHOWED US 0.5 L OF TRACK AND THE AUTOPLT MADE A R TURN TO AN INTERCEPT ANGLE OF 15 DEGS. WE NOTED THAT THIS SEEMED TO BE TOO LARGE A CORRECTION AND BEGAN TO CONFIRM THE XTRACK DEV WITH THE OTHER SYS (#2 AND #3). THEY ALSO SHOWED US L OF TRACK. VISUALLY, WE CROSSED THE CONTRAIL OF THE PRECEDING ACFT AND EXPECTED A TURN BACK TO TRACK. THE AUTOPLT DID NOT MAKE A TURN AND WE NOTED THAT THE #1 INS STILL SHOWED US 0.2 L OF TRACK WITH THE SAME INTERCEPT ANGLE. ODDLY, THE INS XTRACK ERROR DID NOT DECREASE ANY FURTHER, EVEN THOUGH WE HAD A 15 DEG INTERCEPT ANGLE, SO THE CAPT SELECTED A COURSE DIRECT TO PINSO, THE NEXT POINT ALONG THE RTE. THE ACFT TURNED AND TRACKED DIRECT TO PINSO. I SELECTED THE TRACK PLADO-PINSO IN THE #2 INS AND IT SHOWED US STILL 0.2 L OF COURSE. WE KNEW THIS COULD NOT BE POSSIBLE SO ALL INS'S WERE CHANGED TO TRACK DIRECT TO PINSO. AS WE WERE MAKING THE TURN TO PINSO, ZAN NOTIFIED US THEY SHOWED US 8 NM R OF TRACK AND DIVERGING. WE INFORMED THEM THAT WE WERE TURNING DIRECT TO PINSO AND CHKING THE EQUIP. WE VERIFIED THE INS POS AT PINSO WITH THE SYA VOR AND ALL WERE IN AGREEMENT. AFTER THAT, THERE WERE NO DISCREPANCIES WITH THE INS'S. THE INS'S UPDATED NORMALLY (AUTOMATICALLY) WITH BETHEL (BET) VOR AND SHOWED US ON COURSE. WE WERE UNABLE TO EXPLAIN HOW ALL 3 SYS SHOWED THE SAME DEV L OF TRACK, THEN NOT RE-INTERCEPT TRACK EVEN THOUGH THERE WAS MORE THAN A SUFFICIENT INTERCEPT ANGLE. THIS ENTIRE INCIDENT SEEMED TO HAVE TAKEN LESS THAN 2 MINS. TO BE 8 NM R OF TRACK, WOULD HAVE REQUIRED A DIVERGENT HEADING OF AT LEAST 25 DEGS. THE CAPT SUBMITTED A CREW RPT TO THE COMPANY.

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.