Narrative:

I was pilot flying. While on skees 3 slc center told us to cross skees at 16;000 feet. Descending in VNAV; we reached 16;000 feet at skees and then were told to contact slc approach. Approach told us to descend via skees 3 for the ILS 16L approach. The FMS was set up for 16R so I instructed the pilot monitoring to change the runway to 16L. When he selected 16L in the FMS; the arrival dropped out completely; and the aircraft started a right turn. Passing about 20 degrees of right turn; I selected heading sel and spun the heading back to approximately where the next fix on the arrival (booot) had been until it disappeared during the FMS runway change. As we began to reload the arrival and runway; ATC said he noticed we didn't turn right away and told us to just go direct irron fix. We acknowledged the new clearance and requested a heading until we could get irron in the FMS. No loss of separation was noted and flight continued normally to slc.I've seen an uncommanded turn like this once before during a late runway change. I didn't see the fix LNAV was trying to navigate to during the undesired right turn; as I focused instead on trying to keep the aircraft as close to the arrival as possible using heading sel when the aircraft turned right off the arrival path. Usually; just altitudes drop out when making runway changes; but this one was complicated by the complete loss of all the arrival's fixes in the FMS until the first officer (first officer) entered the arrival with the new runway the second time. I wish I could replay a video of the key strokes the first officer used to enter the runway change; but when I confirmed his entry with the new runway and arrival; it looked good. Three things might help. First; do everything possible to get the assigned runway information as soon as possible. In this case; we got handed over just at the point on the arrival (skees) where the FMS has to make a choice. Sooner would be better; but sometimes you just don't descend into approach control's airspace until a critical place like skees. Second; have your finger on either the heading hold or heading sel button to enable you to immediately overcome an undesired LNAV turn. Having the heading bug centered helps; but in this case; we were supposed to turn left just as we got the clearance; so the best we could do was guess at the correct heading to stay near the arrival. (Turns out; I guessed heading 127; and slc approach assigned 130 to stay on the arrival pending FMS entry completion.) finally; perhaps our FAA liaison's can work strenuously to get our approach control friends to assign runways either well outside; or well inside points on the arrival where the LNAV has to make a choice; such as at skees.

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Original NASA ASRS Text

Title: B757 Captain experiences a track deviation when ATC issues a runway change during the SKEES 3 arrival. When the runway is changed in the FMC; the arrival disappears.

Narrative: I was pilot flying. While on SKEES 3 SLC Center told us to cross SKEES at 16;000 feet. Descending in VNAV; we reached 16;000 FEET at SKEES and then were told to contact SLC approach. Approach told us to descend via skees 3 for the ILS 16L approach. The FMS was set up for 16R so I instructed the pilot monitoring to change the runway to 16L. When he selected 16L in the FMS; the arrival dropped out completely; and the aircraft started a right turn. Passing about 20 degrees of right turn; I selected HDG SEL and spun the heading back to approximately where the next fix on the arrival (BOOOT) had been until it disappeared during the FMS runway change. As we began to reload the arrival and runway; ATC said he noticed we didn't turn right away and told us to just go direct IRRON fix. We acknowledged the new clearance and requested a heading until we could get IRRON in the FMS. No loss of separation was noted and flight continued normally to SLC.I've seen an uncommanded turn like this once before during a late runway change. I didn't see the fix LNAV was trying to navigate to during the undesired right turn; as I focused instead on trying to keep the aircraft as close to the arrival as possible using HDG SEL when the aircraft turned right off the arrival path. Usually; just altitudes drop out when making runway changes; but this one was complicated by the complete loss of all the arrival's fixes in the FMS until the First Officer (F/O) entered the arrival with the new runway the second time. I wish I could replay a video of the key strokes the F/O used to enter the runway change; but when I confirmed his entry with the new runway and arrival; it looked good. Three things might help. First; do everything possible to get the assigned runway information as soon as possible. In this case; we got handed over just at the point on the arrival (SKEES) where the FMS has to make a choice. Sooner would be better; but sometimes you just don't descend into approach control's airspace until a critical place like SKEES. Second; have your finger on either the HDG HOLD or HDG SEL button to enable you to immediately overcome an undesired LNAV turn. Having the heading bug centered helps; but in this case; we were supposed to turn left just as we got the clearance; so the best we could do was guess at the correct heading to stay near the arrival. (Turns out; I guessed heading 127; and SLC approach assigned 130 to stay on the arrival pending FMS entry completion.) Finally; perhaps our FAA liaison's can work strenuously to get our approach control friends to assign runways either well outside; or well inside points on the arrival where the LNAV has to make a choice; such as at SKEES.

Data retrieved from NASA's ASRS site 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.