U.S. regulators will tell airlines to quickly make changes aimed at preventing ice from building up in fuel lines of Boeing 777s, which...

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DALLAS — U.S. regulators will tell airlines to quickly make changes aimed at preventing ice from building up in fuel lines of Boeing 777s, which British investigators said today probably caused one of the jets to make a jarring emergency landing last January in London.

The Federal Aviation Administration’s formal directive by week’s end will require changes in the way ground crews prepare planes and pilots fly them in extreme cold weather, FAA spokeswoman Alison Duquette said today.

Currently 777 pilots are required to rev their engines when the fuel temperature falls to 14 degrees Fahrenheit. That would conceivably dislodge any ice that might be in the fuel line.

The FAA directive will apply to more than 50 U.S.-registered Boeing 777s with Rolls Royce engines, mostly operated by American Airlines and Delta Air Lines.

FAA spokesman Les Dorr said the directive is viewed as an interim measure, and Rolls Royce will be expected to make design changes to its engines.

The U.K. Air Accidents Investigation Branch recommended that the FAA and European regulators immediately consider whether the same problem could occur in planes with different engines — a move that could affect many more carriers.

Duquette said, however, that the FAA’s initial review “has not revealed the same vulnerability to ice buildup in the fuel lines” of Boeing 777s with different engines.

The report by the British investigators also recommended that regulators review certification requirements to make sure fuel systems can cope with possible ice buildup.

The investigators said water — normally present in aircraft fuel — probably froze in the fuel lines and caused a British Airways jet to lose power and make an emergency landing just inside the Heathrow Airport boundary on Jan. 17. More than a dozen people suffered injuries, including one listed as serious.

The flight from Beijing had been uneventful until its final approach. Just 720 feet above the ground, the jet began losing power in the right engine and, seven seconds later, in the left one, investigators said in a report issued today. They said those events were consistent with a drop in fuel flow.

“Although the exact mechanism in which the ice has caused the restriction is still unknown in detail, it has been proven that ice could cause a restriction in the fuel feed system,” the report said. “The risk of recurrence needs to be addressed in the short term whilst the investigation continues.”

The report called for the FAA and the European Aviation Safety Agency to work with Boeing and Rolls-Royce to develop measures to reduce the risk of ice forming.

Jim Proulx, a Boeing spokesman, said the company was recommending several procedural changes and a new checklist for operators of 777s with Rolls Royce engines — about 30 percent of the 736 planes in service.

American Airlines operates 47 Boeing 777s with Trent engines. The nation’s largest airline had no immediate comment on the British findings.

Delta spokeswoman Betsy Talton said the airline flies 10 777s, all with Trent engines. She said the airline would comply with any safety directive from the FAA.

Boeing’s Proulx said 777s with Pratt & Whitney engines are designed differently and do not seem prone to ice buildup. United Airlines flies 52 of those, and Continental Airlines has 20.

The Rolls Royce Trent 800 engines have fuel oil heat exchangers that cool engine lubricating oil and warm fuel to prevent the formation of ice.

However, there are no regulatory requirements that address the possibility of a sudden release of a large amount of ice, which could disrupt fuel flow.

Ice crystals start to form when the fuel temperature dips to 31 to 27 degrees Fahrenheit, but generally remain suspended as discrete particles, the report said. At 0 degrees Fahrenheit, the crystals can stick to each other and form clumps.

The investigators said little is known about the mix of ice crystals and fuel at even colder temperatures — they called for further research.

The fuel temperature during the Beijing-London flight fell to minus-29 degrees Fahrenheit for 80 minutes as the outside air temperature at high altitude dipped to 49-below, investigators said.

The National Transportation Safety Board in Washington said it supported the recommendations made by the British aviation-safety agency. NTSB acting chairman Mark Rosenker said the investigation shows that international cooperation can lead to safety improvements.

Associated Press reporters Robert Barr in London, Joan Lowy in Washington and Daniel Lovering in Pittsburgh contributed to this story.