United Airlines has grounded its newest Boeing 787-9 Dreamliner, registration N61101, at London Heathrow Airport on 3 July 2026 due to a repeat failure of the Traffic Alert and Collision Avoidance System (TCAS) just days after returning from Boeing maintenance. Both TCAS antennas were replaced at Moses Lake, yet the system failed again immediately upon reentry into international operations, forcing cancellation of UA939 to San Francisco. The aircraft remains parked on the apron with no confirmed return-to-service timeline. United has not disclosed the root cause, raising concerns about TCAS antenna integration quality on new 787-9s and the effectiveness of prior Boeing repairs. This marks the latest operational interruption for the carrier’s flagship premium-heavy Dreamliner featuring Polaris Studio suites.