One of United Airlines' new automated jet bridges at Denver International Airport collapsed Friday onto the wing of a plane that had just arrived from Boston.

The bridge, which passengers walk through when boarding and getting off airplanes, buckled as it was being connected to the rear door of a United plane at Gate B22, the airline said.

The bridge was empty at the time, and no injuries were reported.

"We are currently investigating what happened and (assessing) the damage," United spokeswoman Megan McCarthy said.

The aircraft involved - a Boeing 757 - was carrying 176 passengers and eight crew, United said. The plane's next scheduled flight to La Guardia International Airport was canceled, but DIA said the incident didn't affect other airport operations.

The incident involved one of United's new fully automated dual-end bridges, which allow passengers to use both the front and rear doors of an airplane. The nation's second- largest carrier is testing the technology in Denver as it decides whether to roll it out on a broader basis.

The bridges essentially form a "Y" shape, with one arm connecting to the front of the plane and the other branching out over the aircraft's wing to the back door. Sensors and software, rather than humans, help guide the bridges to the jets.

In Friday's incident, the arm that hooks to the rear of the aircraft collapsed. Passengers left the plane through the bridge's front arm, United said.

A spokesman for the union that represents ground workers at United said that the new bridges have had mishaps in the past, sometimes scraping against planes.

"We are very concerned that our members and passengers could be in danger by the safety of these jet bridges," said Joe Tiberi, a spokesman for the International Association of Machinists.

United, Denver's dominant carrier, said it has had occasional problems with the bridges' sensors but that the previous incidents didn't cause damage.

The carrier began using the bridges in August and operates five at DIA. Dewbridge Airport Systems, a division of Ottawa, Ontario-based Dew Engineering and Development Ltd., developed the bridges, and DIA maintains them.

United said it will use only the front arms of the remaining four dual-end bridges while it investigates Friday's incident.

Dewbridge Vice President Neil Hutton said the company also is investigating. He said there have been no indications of safety issues with the bridges and no other collapses. Three bridges also are used in Vancouver, British Columbia, he said.

walshc@RockyMountainNews.com or 303-954-2744.