Narrative:

I was given a clearance direct to bouno followed by a clearance to cross bouno at 17000 ft MSL. To the best of my knowledge; I believe we complied with the clearance as it was programmed into the autoplt coupled to LNAV and VNAV. The local altimeter setting was 30.35 driven by a remarkably high pressure WX system that was dominating the area with spectacular clear skies. For the most part; it was your basic non-eventful; routine flight. 11 days later; an ATC controller from new york TRACON called my house and wanted to discuss my flight -- in particular; the 17000 ft crossing at bouno. The controller stated that radar showed our aircraft 1.5 mi left of bouno at 17400 ft which created a loss of separation with another aircraft in our vicinity at FL180. We spent the rest of the conversation discussing the modern lateral and vertical navigation modes used in honeywell's planeview avionics. I described to controller the company's checklists; philosophies; and cockpit altimeter error displays associated with the transition level. Given the bouno crossing and the local altimeter setting (approximately 400 ft divergence from qne); industry practice dictates that the pilot set the altimeter early before arming VNAV. This technique does 2 things -- provides for a more accurate and smoother vertical profile to the 17000 ft restr at bouno. I am confident that we programmed all of the equipment in accordance with company SOP's and monitored the descent clearance. Had I felt that something was amiss with regard to making the crossing restr; I would have advised ATC that we would be unable to make the crossing restr. The lateral displacement is not unusual due to the turn anticipation features of many modern day FMS system. The avionics system is configured so that it normally leads the turns between waypoints within the limits of the airspace or airway. Another factor that can attribute to slight lateral deviations is that gulfstream incorporated a software change that reduced the aircraft bank angle used in turns at waypoints in FMS flight plans. As a consequence of the reduced bank angle available; the aircraft will initiate a turn at a greater distance from the waypoint in order to smoothly intercept the next leg after the waypoint. This feature has caused some problems in certain stars and sids when the aircraft does not closely align with the required flight path. In order to correct this shortcoming; the flight crew should change the bank angle used for en route operations. Gulfstream issued a temporary fix in an alert bulletin to change the default software bank factor from 70 to 150. Once the 150 bank factor is set in the FMS (on the ground); it supposedly becomes the new default setting after 1 takeoff and landing. This setting cannot easily be adjusted while in-flight. During this flight; I noticed that the airplane was making shallow turns; so I checked this setting and it was at 70; not 150.

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Original NASA ASRS Text

Title: GLF4 SUFFERS LOSS OF SEPARATION WHILE TURNING OVER AN INBOUND RNAV WAYPOINT FOR WHICH A CROSSING RESTRICTION HAD BEEN GIVEN BY ATC.

Narrative: I WAS GIVEN A CLRNC DIRECT TO BOUNO FOLLOWED BY A CLRNC TO CROSS BOUNO AT 17000 FT MSL. TO THE BEST OF MY KNOWLEDGE; I BELIEVE WE COMPLIED WITH THE CLRNC AS IT WAS PROGRAMMED INTO THE AUTOPLT COUPLED TO LNAV AND VNAV. THE LOCAL ALTIMETER SETTING WAS 30.35 DRIVEN BY A REMARKABLY HIGH PRESSURE WX SYS THAT WAS DOMINATING THE AREA WITH SPECTACULAR CLEAR SKIES. FOR THE MOST PART; IT WAS YOUR BASIC NON-EVENTFUL; ROUTINE FLT. 11 DAYS LATER; AN ATC CTLR FROM NEW YORK TRACON CALLED MY HOUSE AND WANTED TO DISCUSS MY FLT -- IN PARTICULAR; THE 17000 FT XING AT BOUNO. THE CTLR STATED THAT RADAR SHOWED OUR ACFT 1.5 MI L OF BOUNO AT 17400 FT WHICH CREATED A LOSS OF SEPARATION WITH ANOTHER ACFT IN OUR VICINITY AT FL180. WE SPENT THE REST OF THE CONVERSATION DISCUSSING THE MODERN LATERAL AND VERT NAV MODES USED IN HONEYWELL'S PLANEVIEW AVIONICS. I DESCRIBED TO CTLR THE COMPANY'S CHKLISTS; PHILOSOPHIES; AND COCKPIT ALTIMETER ERROR DISPLAYS ASSOCIATED WITH THE TRANSITION LEVEL. GIVEN THE BOUNO XING AND THE LOCAL ALTIMETER SETTING (APPROX 400 FT DIVERGENCE FROM QNE); INDUSTRY PRACTICE DICTATES THAT THE PLT SET THE ALTIMETER EARLY BEFORE ARMING VNAV. THIS TECHNIQUE DOES 2 THINGS -- PROVIDES FOR A MORE ACCURATE AND SMOOTHER VERT PROFILE TO THE 17000 FT RESTR AT BOUNO. I AM CONFIDENT THAT WE PROGRAMMED ALL OF THE EQUIP IN ACCORDANCE WITH COMPANY SOP'S AND MONITORED THE DSCNT CLRNC. HAD I FELT THAT SOMETHING WAS AMISS WITH REGARD TO MAKING THE XING RESTR; I WOULD HAVE ADVISED ATC THAT WE WOULD BE UNABLE TO MAKE THE XING RESTR. THE LATERAL DISPLACEMENT IS NOT UNUSUAL DUE TO THE TURN ANTICIPATION FEATURES OF MANY MODERN DAY FMS SYS. THE AVIONICS SYS IS CONFIGURED SO THAT IT NORMALLY LEADS THE TURNS BTWN WAYPOINTS WITHIN THE LIMITS OF THE AIRSPACE OR AIRWAY. ANOTHER FACTOR THAT CAN ATTRIBUTE TO SLIGHT LATERAL DEVS IS THAT GULFSTREAM INCORPORATED A SOFTWARE CHANGE THAT REDUCED THE ACFT BANK ANGLE USED IN TURNS AT WAYPOINTS IN FMS FLT PLANS. AS A CONSEQUENCE OF THE REDUCED BANK ANGLE AVAILABLE; THE ACFT WILL INITIATE A TURN AT A GREATER DISTANCE FROM THE WAYPOINT IN ORDER TO SMOOTHLY INTERCEPT THE NEXT LEG AFTER THE WAYPOINT. THIS FEATURE HAS CAUSED SOME PROBS IN CERTAIN STARS AND SIDS WHEN THE ACFT DOES NOT CLOSELY ALIGN WITH THE REQUIRED FLT PATH. IN ORDER TO CORRECT THIS SHORTCOMING; THE FLT CREW SHOULD CHANGE THE BANK ANGLE USED FOR ENRTE OPS. GULFSTREAM ISSUED A TEMPORARY FIX IN AN ALERT BULLETIN TO CHANGE THE DEFAULT SOFTWARE BANK FACTOR FROM 70 TO 150. ONCE THE 150 BANK FACTOR IS SET IN THE FMS (ON THE GND); IT SUPPOSEDLY BECOMES THE NEW DEFAULT SETTING AFTER 1 TKOF AND LNDG. THIS SETTING CANNOT EASILY BE ADJUSTED WHILE INFLT. DURING THIS FLT; I NOTICED THAT THE AIRPLANE WAS MAKING SHALLOW TURNS; SO I CHKED THIS SETTING AND IT WAS AT 70; NOT 150.

Data retrieved from NASA's ASRS site as of May 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.