United Airlines, already under increased scrutiny by the FAA for maintenance issues, says it’s investigating the loss of one of eight main gear wheels on one of its Boeing 757s on Monday. The 30-year-old plane was taking off from LAX when the wheel dropped. The flight continued to Denver and landed uneventfully. Although the plane is designed to land safely with one wheel missing, cabin crew ordered passengers to brace during the landing in Denver.This incident comes a few months after a spate of mishaps, including a United 777 dropping a main gear wheel on takeoff from San Francisco International, prompted the FAA to freeze any expansion of routes by the airline pending a review of maintenance. Meanwhile United says it’s investigating. “The wheel has been recovered in Los Angeles, and we are investigating what caused this event,” the airline said in a statement. United didn’t say if the wheel damaged anything on the ground.Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.