Narrative:

ATC issued clearance 'fly the hudson 1 arrival; after mad you are cleared direct to pwl.' arrival was already loaded into FMS as it was our filed route. I then cleared the briss waypoint; then I cleared the discontinuity; and the flight plan showed dpk-mad-pwl as was cleared. I noticed that it seemed to show a turn on the pfd display prior to mad as is customary; and with the briss intersection removed it is at least a 110 degree course change. As we were in a descent; with a speed greater that 270 KTS I had anticipated the FMS would lead the turn to prevent an overshoot of the mad VOR. As previously mentioned; the pfd had a pictorial view of this early turn; but the scale cannot be relied upon to determine exactly when (in terms of a DME distance) this lead turn will commence. I rely completely on this honeywell FMS to execute the turn at the proper DME and have never encountered any problems with its calculations in this regard. It appeared to start the turn a little early; but a very slow turn; so I let it continue. ATC then queried us on why we had turned early; and rather than explain the workings of the FMS; I elected to get a phone number and talk with him on the ground. The controller supplied me with radar tracks of me; air carrier A320; and a prior flight. I'm going to supply a cut and paste email from the controller that he emailed to me. Here it is: 'thanks for your call on saturday regarding your concerns about an underflown turn on the HUD1 arrival to stewart. As you requested; I am providing you with some written notes about our conversation; as well as some radar plots that illustrate the concerns we discussed. As we discussed; the FMS system algorithms look ahead at waypoints and consider speed; altitude and magnitude of course change when planning a turn. In ATC; we understand that either the aircrew using conventional navigation; or the aircraft using FMS; will lead a turn; and may not directly overfly the waypoint. This is especially true when the course change is a steep one; as on the HUD1 arrival. In the case that prompted your call; the controller queried you because the turn was much earlier and wider than we typically expect in a case like this. Thankfully; because the controller involved was immediately aware of the situation; took appropriate internal action; and no other aircraft were nearby; there was no immediate safety concern. As I looked further into the occurrence; I came up with some interesting findings. As I explained to you; I looked at the radar track from your flight to see just what happened. The data showed that your turn started almost 10 NM before mad VOR; and your closest proximity to the VOR was 6.7 NM. This was in sharp contrast to the flight that had just proceeded you on a similar profile; and remained within 3 NM of the VOR. This caused me to wonder if there could be a difference in the way your company programs the FMS; or if there is a difference in the equipment between the companies; or if there is an application issue on the flight deck. I then looked back at radar plots of the same trip from other recent days. Yesterday; the aircraft also turned early; an in fact flew an even wider turn. Two other trips in the last week however; the turn was entirely within 2 to 3 miles of mad.'callback conversation with reporter revealed the following information: the reporter believes that the honeywell FMC on the B717 uses airspeed over the previous waypoint to calculate the turn radius; and produced and a much wider turn than necessary even with fuel conservation in mind. The aircraft had slowed 60 KTS by the time the turn was initiated. The fact that the radar plots that the controller examined showed other instances of wide turns; indicates that this was not a unique occurrence.

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Original NASA ASRS Text

Title: B717 CAPT REPORTS FMS PRODUCED AN EXTREMELY WIDE TURN PASSING MAD ON THE HUD1 TO SWF.

Narrative: ATC ISSUED CLEARANCE 'FLY THE HUDSON 1 ARRIVAL; AFTER MAD YOU ARE CLEARED DIRECT TO PWL.' ARRIVAL WAS ALREADY LOADED INTO FMS AS IT WAS OUR FILED ROUTE. I THEN CLEARED THE BRISS WAYPOINT; THEN I CLEARED THE DISCONTINUITY; AND THE FLIGHT PLAN SHOWED DPK-MAD-PWL AS WAS CLEARED. I NOTICED THAT IT SEEMED TO SHOW A TURN ON THE PFD DISPLAY PRIOR TO MAD AS IS CUSTOMARY; AND WITH THE BRISS INTERSECTION REMOVED IT IS AT LEAST A 110 DEGREE COURSE CHANGE. AS WE WERE IN A DESCENT; WITH A SPEED GREATER THAT 270 KTS I HAD ANTICIPATED THE FMS WOULD LEAD THE TURN TO PREVENT AN OVERSHOOT OF THE MAD VOR. AS PREVIOUSLY MENTIONED; THE PFD HAD A PICTORIAL VIEW OF THIS EARLY TURN; BUT THE SCALE CANNOT BE RELIED UPON TO DETERMINE EXACTLY WHEN (IN TERMS OF A DME DISTANCE) THIS LEAD TURN WILL COMMENCE. I RELY COMPLETELY ON THIS HONEYWELL FMS TO EXECUTE THE TURN AT THE PROPER DME AND HAVE NEVER ENCOUNTERED ANY PROBLEMS WITH ITS CALCULATIONS IN THIS REGARD. IT APPEARED TO START THE TURN A LITTLE EARLY; BUT A VERY SLOW TURN; SO I LET IT CONTINUE. ATC THEN QUERIED US ON WHY WE HAD TURNED EARLY; AND RATHER THAN EXPLAIN THE WORKINGS OF THE FMS; I ELECTED TO GET A PHONE NUMBER AND TALK WITH HIM ON THE GROUND. THE CONTROLLER SUPPLIED ME WITH RADAR TRACKS OF ME; ACR A320; AND A PRIOR FLT. I'M GOING TO SUPPLY A CUT AND PASTE EMAIL FROM THE CONTROLLER THAT HE EMAILED TO ME. HERE IT IS: 'THANKS FOR YOUR CALL ON SATURDAY REGARDING YOUR CONCERNS ABOUT AN UNDERFLOWN TURN ON THE HUD1 ARRIVAL TO STEWART. AS YOU REQUESTED; I AM PROVIDING YOU WITH SOME WRITTEN NOTES ABOUT OUR CONVERSATION; AS WELL AS SOME RADAR PLOTS THAT ILLUSTRATE THE CONCERNS WE DISCUSSED. AS WE DISCUSSED; THE FMS SYSTEM ALGORITHMS LOOK AHEAD AT WAYPOINTS AND CONSIDER SPEED; ALTITUDE AND MAGNITUDE OF COURSE CHANGE WHEN PLANNING A TURN. IN ATC; WE UNDERSTAND THAT EITHER THE AIRCREW USING CONVENTIONAL NAVIGATION; OR THE AIRCRAFT USING FMS; WILL LEAD A TURN; AND MAY NOT DIRECTLY OVERFLY THE WAYPOINT. THIS IS ESPECIALLY TRUE WHEN THE COURSE CHANGE IS A STEEP ONE; AS ON THE HUD1 ARRIVAL. IN THE CASE THAT PROMPTED YOUR CALL; THE CONTROLLER QUERIED YOU BECAUSE THE TURN WAS MUCH EARLIER AND WIDER THAN WE TYPICALLY EXPECT IN A CASE LIKE THIS. THANKFULLY; BECAUSE THE CONTROLLER INVOLVED WAS IMMEDIATELY AWARE OF THE SITUATION; TOOK APPROPRIATE INTERNAL ACTION; AND NO OTHER AIRCRAFT WERE NEARBY; THERE WAS NO IMMEDIATE SAFETY CONCERN. AS I LOOKED FURTHER INTO THE OCCURRENCE; I CAME UP WITH SOME INTERESTING FINDINGS. AS I EXPLAINED TO YOU; I LOOKED AT THE RADAR TRACK FROM YOUR FLIGHT TO SEE JUST WHAT HAPPENED. THE DATA SHOWED THAT YOUR TURN STARTED ALMOST 10 NM BEFORE MAD VOR; AND YOUR CLOSEST PROXIMITY TO THE VOR WAS 6.7 NM. THIS WAS IN SHARP CONTRAST TO THE FLIGHT THAT HAD JUST PROCEEDED YOU ON A SIMILAR PROFILE; AND REMAINED WITHIN 3 NM OF THE VOR. THIS CAUSED ME TO WONDER IF THERE COULD BE A DIFFERENCE IN THE WAY YOUR COMPANY PROGRAMS THE FMS; OR IF THERE IS A DIFFERENCE IN THE EQUIPMENT BETWEEN THE COMPANIES; OR IF THERE IS AN APPLICATION ISSUE ON THE FLIGHT DECK. I THEN LOOKED BACK AT RADAR PLOTS OF THE SAME TRIP FROM OTHER RECENT DAYS. YESTERDAY; THE AIRCRAFT ALSO TURNED EARLY; AN IN FACT FLEW AN EVEN WIDER TURN. TWO OTHER TRIPS IN THE LAST WEEK HOWEVER; THE TURN WAS ENTIRELY WITHIN 2 TO 3 MILES OF MAD.'CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR BELIEVES THAT THE HONEYWELL FMC ON THE B717 USES AIRSPEED OVER THE PREVIOUS WAYPOINT TO CALCULATE THE TURN RADIUS; AND PRODUCED AND A MUCH WIDER TURN THAN NECESSARY EVEN WITH FUEL CONSERVATION IN MIND. THE ACFT HAD SLOWED 60 KTS BY THE TIME THE TURN WAS INITIATED. THE FACT THAT THE RADAR PLOTS THAT THE CONTROLLER EXAMINED SHOWED OTHER INSTANCES OF WIDE TURNS; INDICATES THAT THIS WAS NOT A UNIQUE OCCURRENCE.

Data retrieved from NASA's ASRS site as of January 2009 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.