A300
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert
Aircraft
On diversion from ZZZ to COS we were given direct FLOOD Intersection then FSHER join DEBERRY2 arrival. We were inbound from the north to FLOOD; which required a greater than 90 degree turn at that point. In NAV; the aircraft began a turn inside FLOOD to join the selected course. Shortly after we were contacted by Denver Center that we were assigned the DEBRRY2 and missed FLOOD by 9 miles. The plane was verified to be in NAV and operating as selected. This is the second time in a month ATC has asked about the Airbus NAV system. A few weeks back a Controller asked why we missed Glade Springs by 4.7 miles going into GSO. He said his separation is 5 miles and the Airbus logic compromises that. No conflict was reported in either case; but it seems just a matter of time before one is. Airbus NAV logic.
Held over ZZZ VOR for arrivals into ZZZ for around one hour and then started diverting to scheduled alternate of ZZZ1 when Dispatch said there wasn't any parking there and to head for COS. We were in the +30;000 FT MSL range and told Denver Center that we were again changing destination to COS per company request. Turned south and given routing of direct to FLOOD direct FSHER direct Black Forest VOR direct COS. Speed brakes out and descending with autopilot on and in NAV toward FLOOD and then to make the 90 degree turn toward the east to FSHER the FMS/NAV started leading the turn. The First Officer was busy getting ATIS; doing landing data; loading the EFB approach plates; etc. As PF; I saw it set to lead the turn but since we were coming up on the FLOOD fix at around 280 KTS and the airplane had to do a 90 degree turn it didn't surprise me to see the turn being lead. This FMS does that frequently at many airports like DFW and GSO. ATC got upset at us for turning early and said there was a military area there and that is why they sent us to FLOOD prior to turning east. I apologized but told them we were flying on the autopilot and in NAV and the FMS computed the turn lead to make the 90 degree turn and that FLOOD was not showing up as a mandatory fly over point. Once I explained it was the autopilot and our FMS in NAV he backed off but was still not happy with what the airplane did. I have previously had controllers not like how our airplane leads turns. Denver Center perceived it as jeopardy with the military area; but said he was ok with our explanation; but I felt we needed to do the ASAP report since there had been a military area nearby and ATC's need to know that this FMS calculates turns differently than they may be used to seeing and they may need to adjust how they handle the Airbus. FMS system in NAV with autopilot engaged leading the turn like it normally does but ATC did not expect such a big lead and it ran us closer then ATC wanted us to a military area. It leads the turn by close to 5 miles. Maybe a better understanding for ATC of the Airbus FMS system and how big a lead it does in turns. I felt we needed to do the ASAP report since there had been a military area nearby and ATC's need to know and understand that this FMS calculates turns differently than they may be use to seeing from US made FMS systems and they may need to adjust how they handle the Airbus.
A300 flight crew describes the large lead that the FMGC uses to make a 90 degree turn at high altitude. This was not acceptable to the Controller involved who questioned the crew as to why it happened.