United Airlines has grounded its newest premium Boeing 787-9 Dreamliner, registration N61101, at London Heathrow on 3 July 2026 following a repeat failure of the Traffic Alert and Collision Avoidance System (TCAS). The aircraft, delivered in late February 2026 and featuring Polaris Studio suites, returned to service on 30 June after Boeing replaced both TCAS antennas at Moses Lake, yet the system failed again before the scheduled UA939 return to San Francisco. This marks the third major operational interruption for the jet, including an emergency diversion in April 2026, and threatens United’s rollout of its premium-heavy long-haul network. United has not disclosed the root cause or timeline for return to service, substituting older widebodies for the route while Boeing and the airline investigate the persistent TCAS anomaly.