FMGC starts turn early, 9 mi according to the center controller.

Attributes
ACN	269069
Time
Date	199404
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : trm
State Reference	CA
Altitude	msl bound lower : 29000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude
Route In Use	departure other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 15000 flight time type : 800
ASRS Report	269069
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument
Events
Anomaly	non adherence : clearance other anomaly other other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Flight was sna to dtw. Cleared musel 5 departure, thermal transition then trm direct tnp, etc. Approaching trm, the FMGC started a left turn to tnp. Center said we started the turn 9 mi early and directed us back to a 070 degree heading. He said there was traffic to our left and was not pleased with the way we led the turn. The controller said this feature of FMGC aircraft is causing the ATC system problems. The FMGC computer seems to be working as describes in far 91.181 and the aim. That is, it leads the turn to avoid flying out of airway boundaries on the far side. I suggest the software be changed to sharpen the turns. The FMGC is trying to be too smooth. Another possible change would be to use the 'overfly' function when flying direct between fixes instead of airways.

Original NASA ASRS Text

Title: FMGC STARTS TURN EARLY, 9 MI ACCORDING TO THE CTR CTLR.

Narrative: FLT WAS SNA TO DTW. CLRED MUSEL 5 DEP, THERMAL TRANSITION THEN TRM DIRECT TNP, ETC. APCHING TRM, THE FMGC STARTED A L TURN TO TNP. CTR SAID WE STARTED THE TURN 9 MI EARLY AND DIRECTED US BACK TO A 070 DEG HDG. HE SAID THERE WAS TFC TO OUR L AND WAS NOT PLEASED WITH THE WAY WE LED THE TURN. THE CTLR SAID THIS FEATURE OF FMGC ACFT IS CAUSING THE ATC SYS PROBS. THE FMGC COMPUTER SEEMS TO BE WORKING AS DESCRIBES IN FAR 91.181 AND THE AIM. THAT IS, IT LEADS THE TURN TO AVOID FLYING OUT OF AIRWAY BOUNDARIES ON THE FAR SIDE. I SUGGEST THE SOFTWARE BE CHANGED TO SHARPEN THE TURNS. THE FMGC IS TRYING TO BE TOO SMOOTH. ANOTHER POSSIBLE CHANGE WOULD BE TO USE THE 'OVERFLY' FUNCTION WHEN FLYING DIRECT BTWN FIXES INSTEAD OF AIRWAYS.

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.