Federal experts are investigating the possibility that the United Airlines Boeing 767 that temporarily lost power in both engines over the Rocky Mountains Friday evening was flying so efficiently under computer command that the engines were not running fast enough to keep themselves free of ice.
There is little question that ice caused both engines to overheat and forced the captain of United Flight 310 to turn them off while descending through a storm into Denver's Stapleton International Airport after a trip from Los Angeles, aviation sources said yesterday.
The engines were restarted, and the plane, carrying 197 passengers and a crew of eight, landed without further problems.
The issue for investigators is whether this is a one-time occurrence because of a malfunction or represents something that could happen again in the highly automated 767, one of Boeing's two new airplanes. United has been flying the 767 for almost a year and has 19 of them. Fifty-six 767s had been delivered worldwide as of June 30.
The 767 is considered a "clean" airplane by aviation experts, which means that in flight it presents very little resistance to the atmosphere. This is generally desirable because it means increased fuel efficiency, a major selling point for the 767.
As a result, little power is needed to control the 767 on descent. According to sources, Flight 310's controls and throttles were being handled by computer as the plane dropped gradually from 41,000 feet to 24,000 feet.
"The question is what happens when the computer just idles back the engines," one source said. "Pretty soon you're below what you need" to keep the anti-icing equipment functioning to clear the air intakes.
The pilot, not identified by United, noticed "hot" warnings on both engines and elected to shut down one at 19,960 feet and the other at 17,400 feet. Air traffic controllers cleared the path into Denver and, United said, the plane could have glided to a landing had the captain been unable to restart the engines. The plane had cleared the peaks of the Rockies, United said.
After the engines cooled, they were restarted at 15,600 feet and 14,500 feet, respectively, investigators said.
Investigators and United discounted earlier reports that lightning had struck the plane. Temporary losses of electrical power accompanied the loss of the engines, but emergency power was used for essential cockpit instruments.
The National Transportation Safety Board began an inquiry Friday and is using experts from the Federal Aviation Administration, United, Boeing and the Pratt & Whitney Division of United Technologies Corp. The engines on the 767 are Pratt & Whitney JT9D7R4s, also common to the Boeing 747 and Airbus Industrie A300.