By Valerie Volcovici

WASHINGTON, June 2 (Reuters) - The U.S. Environmental Protection Agency plans as soon as Friday to determine whether carbon dioxide from aircraft endangers public health, a first step to regulating emissions from the aviation sector, sources familiar with the rulemaking process said.

The EPA has yet to issue its "endangerment finding," despite pressure from environmental groups who first sued the agency to start the rulemaking process in 2010. A federal court in 2011 said the EPA must address aircraft emissions under the U.S. Clean Air Act.

The EPA had initially promised the finding would be ready in 2014.

Most observers expect the EPA to say that aviation emissions endanger public health but are not sure how much the agency and the Federal Aviation Authority will reveal about their vision for a carbon dioxide emissions standard for new aircraft.

"We have efficiency standards for cars, trucks, but we don't have one for airplanes," said Annie Petsonk, international counsel for the Environmental Defense Fund. "We think this is an industry that has great potential in technical terms, and there is nothing like having an ambitious standard to drive innovation."

A domestic rulemaking process would lay the groundwork for the United States to adopt a global carbon dioxide standard currently being developed through the United Nations' International Civil Aviation Organization.

ICAO is also working on an international "market-based mechanism" to push airlines to slash their emissions, with a goal of final approval in 2016.

U.S. airlines, which favor a global industry standard, said they were encouraged that the EPA and FAA are cooperating with ICAO as the UN body works to develop it.

"As aviation is a global industry ... it is critical that aircraft emissions standards continue to be agreed at the international level," said Vaughn Jennings, managing director for government and regulatory communications for U.S. airline lobby group Airlines for America.

Environmental groups hope the EPA's announcement will be more ambitious.

"We hope the EPA can push the envelope beyond what ICAO is looking at," said Ben Longstreth of the Natural Resources Defense Council, one of five green groups that sued the EPA to speed up its rulemaking.

Andrew Murphy, a policy officer at Brussels-based NGO Transportation and Environment, said European regulators might also step up pressure on ICAO to deliver a strong standard.

"The European Aviation Safety Agency has raised the prospect of setting European standards if global ones prove insufficient," he said.

Global aviation emissions are on pace to triple by 2050 if they continue unregulated, according to the International Council on Clean Transportation. (Reporting by Valerie Volcovici; Editing by Lisa Von Ahn)