On Feb 8th, 2026, a United Airlines Boeing 737-800, registration N11206, operating flight UA-1125 from Denver, Colorado, to Boise, Idaho, encountered a concerning in-flight technical issue while cruising high above the Rockies.
The aircraft was at FL380, approximately 220 nautical miles northwest of Denver, when the crew reported high vibration levels in the left CFM56 engine. Taking a precautionary approach, the pilots decided to discontinue the flight and return to Denver. The aircraft drifted down to a lower altitude as part of the abnormal procedure before setting course back to its departure airport.
About 45 minutes after turning around, the 737 landed safely on Denver’s runway 34R. Emergency services were on standby, and the aircraft vacated the runway without further incident. There were no reports of injuries to passengers or crew.
To minimize disruption, United arranged a replacement Boeing 737-9 MAX, registration N37555, which later completed the journey to Boise, arriving with a delay of roughly three hours.
The affected aircraft was withdrawn from service and, approximately 21 hours after landing, remained on the ground in Denver while maintenance teams carried out inspections to determine the cause of the abnormal engine vibrations.
