Date: 202012
Local time of day: 1801-2400
Location: IAH.Airport, TX
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent; Initial Approach
Airspace: Class B IAH
ATC / advisory: TRACON I90
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure; Workload
Second reporter function: Pilot Flying; Captain
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; MEL; Aircraft; Procedure
Primary problem: Ambiguous

Narrative: I was Pilot Monitoring on flight [on] DRLLR 5 RNAV STAR to IAH. Aircraft X had one MEL for right FMS inoperative. Single FMS operation. We entered the STAR during preflight and 26R runway per the DRLLR 5 STAR. ATC instructed us to change to 26L just prior to MPORT. This changes the final point on the STAR from SKLER to ZOEEE. I entered the new runway and verified that no duplicate waypoints existed. I also verified and noted that the waypoint sequence on the MFD looked correct; noting the large gap between ZOEEE and BOZZZ on the 26L approach. I also noted the line on the MFD depicted from DOMNO to ZOEEE. For unknown reasons the airplane approached DOMNO; overflew DOMNO; and the autopilot went immediately into ROL mode and continued flying straight. It did not sequence to ZOEEE as expected. We noticed the change immediately; selected heading mode to start the turn to ZOEEE; and then re-engaged LNAV after verifying the FMS was sequencing properly to ZOEEE. As we started to sort out the flight path error; I was distracted when ATC issued heading calls and pilot deviation calls; which removed my attention from assisting the pilot flying. No other deviation occurred that we were aware of. Both pilots were fully engaged in managing the flight. We acted as fast as we could to regain flight path control. [I] suggest verifying the integrity of the FMS arrivals into IAH. Delay pilot deviation calls until on the ground and the airplane has been secured.


Second narrative: We were on the DRLLR 5 arrival and planning for Runway 26R. We had checked the route for no double waypoints and all altitudes were correct. We were assigned Runway 26L and made the appropriate checks of waypoints and altitudes. At DOMNO the plane went into Roll mode and continued south. We saw it right away and began left turn as ATC told us to turn left and gave us a heading (070 I think). A few minutes later the controller gave us a phone number to call about a possible deviation. This plane had the #2 FMS MEL'd which may have contributed since we don't often operate with 1 FMS. Better planning for operations with one FMS. Closer attention while passing DOMNO; which has known issues. However we had no duplicate waypoints so this may be a new issue.


Synopsis: Flight crew reported experiencing an unplanned autopilot/FMS mode shift causing a tract deviation during the DRLLR 5 STAR into IAH. Flight crew cited a runway assignment change; plus one FMS being inoperative as causal factors.