B777-300
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 23
Troubleshooting; Confusion; Situational Awareness
Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Separated Traffic
Procedure; Human Factors; Company Policy; Aircraft
I am a specialist and discovered this event through the Electronic Occurrence Report (EOR) reporting as well as an email that came through about the event. I believe this is a recurring issue that; because NextGen will continue to develop aircraft with radically varying navigational systems (the box in the aircraft are different for different types as well as airlines) this will only get worse. The issue deals with leading a turn at a 'fly-by' fix especially when the turn is 90 degrees or greater. Aircraft are wildly inconsistent with where their FMS systems tell them to lead the turn. Aircraft X and Aircraft Y were both filed ZZZZ./.TOU..AST.. at FL390. Aircraft X was 23 miles in front of Aircraft Y and faster. Aircraft X made his turn at TOU 7 miles prior to the fix (TOU) (FMS leading the turn). Aircraft Y began his turn 21 miles from the fix (TOU). This caused a converging situation south of TOU and the aircraft got within 4.57 miles of each other (reported from ESAT) 85% Measure of Compliance (MOC). The controller took action to turn both aircraft and achieved separation quickly. The pilots both called in to the facility later and explained that they turned where their box (FMS) told them to in order to make the high angle turn. I will add that there was also a Aircraft Z that made a similar turn at TOU just in front of these two (he was at FL410 so not traffic) and he led his turn by about 5 miles; very close to the place where Aircraft X turned. This would lead the controller to reasonably believe that Aircraft Y would turn similarly and remain a safe distance behind Aircraft X.This happens a lot; especially with the amount of flights. Controllers have no way of knowing where an aircraft will commence their turn with fly by fixes. If these aircraft are going to continue to be developed with more sophisticated navigational systems; systems that vary wildly; this needs to be addressed. In this case it was two flights that got together; however; we often have traffic transiting this area west to east at these high altitudes and it's reasonable to expect that separation will be kept if the aircraft turn within a 5-7 mile distance from the fix. If they continue to turn at random distances; we'll have no option but to babysit them each time and put them on headings (i.e. 'fly present heading' or 'don't start your turn without notifying me' etc.); not knowing when they will turn. This adds a level of complexity that adds a definite safety risk.I suggest a national directive (not sure if it's a directive or regulation or what it's called) be issued to require a pilot to advise ATC if their FMS wants them to turn prior to 10(*) miles from a fix before commencing the turn. (*Or any distance that can be set so controllers have a reasonable expectation as to where the aircraft will be on their flight plan.) Controllers need to have a reasonable idea of where an aircraft will be in order to make informed decisions and not control via reaction but by proper planning.
ZSE Controller describes a situation where aircraft are turning at different places in reference to a fly-by fix which lead to an operational error.