B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 142; Flight Crew Type 12000
Human-Machine Interface; Situational Awareness
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Human Factors; Procedure
Reprogramming selected approach caused route discontinuity that resulted in track deviation. On WLEEE 5 Arrival to AUS; just prior to XWING waypoint; we reprogrammed approach from RNAV (RNP) Z 17L to ILS 17L. (Lowering weather made ILS a better choice.) Selected 17L JEDYE transition in CDU; examined LEGS page for route from JEDYE to runway; and confirmed with First Officer. When executed; it caused route discontinuity (discontinuity on CDU scratch pad); and FMC mode changed to Control Wheel Steering (CWS). I attempted to follow magenta track; which still displayed desired route; on navigation display (ND) using Heading Select; but was unable to maintain track after XWING. Approach Control assigned a heading. We continued with radar vectors to ILS 17L. The remainder of flight was uneventful. I did not expect that a change to the approach for the same runway as previously selected would result in a route discontinuity. When I examined the LEGS page prior to executing the change; I was focused on the new portion of the route; from JEDYE to ILS 17L to runway. I failed to note that the route in the LEGS page contained a discontinuity. Obviously; we changed runways and approaches very frequently during arrival. I'm unsure of which conditions (combination of arrival; approach; and runway) will cause a discontinuity and which will not. During the three remaining flights of this trip; each arrival involved a runway or approach change; reprogramming the FMC/CDU did not result in a discontinuity in any of these situations. I understand; now; that the most important thing is to ensure; prior to executing the change; that the first point on the LEGS page is the next point on the arrival; because that's where the aircraft will go. [I should] confirm that the LEGS page accurately reflects the remainder of the route to be flown prior to executing a route change. Training on FMC system to make pilots aware of which conditions; when changing runway or approach; will cause a route discontinuity and which will not. Modify FMC software so that modifications to runway or approach behave consistently.
B737-700 Captain reported a track deviation occurred on arrival into AUS when a late runway change required FMS reprogramming; during which a route discontinuity resulted.