The Federal Aviation Administration is conducting a more detailed review of Mirant Corp.'s plan to raise the height of the five stacks at its troubled Alexandria power plant, a plan the FAA initially rejected as potentially hazardous to air navigation.

Mirant requested the expanded "aeronautical study" because the company believes increasing the stack height could eliminate the environmental problems that forced the plant's shutdown in August, company officials said. The five brick smokestacks are the source of the polluted downwash from the coal-fired facility that an analysis showed considerably exceeds what federal standards allow.

"Increasing stack height is a traditional and historical way to alleviate downwash," said Steve Arabia, a Mirant spokesman. "That being the case, it makes sense for us to look seriously at all of our options. We're essentially going to leave no stone unturned . . . to find the right solution."

Bill Hayden, a spokesman for the Virginia Department of Environmental Quality, agreed that increasing stack height "is one way to reduce air pollution" because it pushes emissions higher into the atmosphere where they can be more efficiently dispersed.

But only the FAA can approve Mirant's plan to raise the stacks, which are now 164 feet high, to 214 feet. In a Sept. 30 letter to Mirant, the FAA said that any height exceeding 164 feet "will result in a substantial adverse effect" on air navigation. The letter did not spell out the problem but said it would involve "an adverse physical or electromagnetic interference effect upon navigable airspace or air navigation facilities."

William E. Merritt, an FAA airspace specialist who wrote the letter, said this week that the FAA is now conducting a more detailed review, at Mirant's request. "We will put an x in every square before this is all done," he said.

The intensified review comes as Mirant continues to search for a solution to its environmental problems and state officials closely monitor the air quality at the plant. The plant recently restarted one of its generating units and Virginia's Department of Environmental Quality has said it found "no indication" that the limited emissions exceed air quality standards.

But it remains unclear when the plant, which provides electricity to homes in the District and Maryland, but not Virginia, might reopen on a full-scale basis. Mirant acknowledged that a full-scale reopening could be more than a year away.

Mirant decided on Aug. 24 to shut down the plant in response to an order from Virginia officials to cut potentially harmful pollution. The directive by the environmental department came after it reviewed the results of an analysis that showed that some pollutants in the plant's vicinity were found at rates considerably higher than federal standards allow.

Mirant's neighbors have complained for years about what they believed to be high levels of contaminants emanating from the plant. The 56-year-old facility was supplying enough electricity to serve about 400,000 homes in the District and Maryland, and officials with Pepco and the D.C. Public Service Commission have described it as a vital link in the Washington power grid.

Alexandria officials have been campaigning to have the plant shut down as a health hazard, and the city recently issued a news release in which Mayor William D. Euille (D) blasted Mirant for scrambling to reopen it. "Mirant has operated with callous indifference to the health of Alexandria's families," Euille said. "This outmoded coal plant is a hazard to the environment and to all who live, work or visit here."

Asked to respond, Arabia said the city "has gotten more filled with hyperbole over the months.'' Other than the city, he said, "a lot of parties are working in earnest to try to find a solution as quickly as possible because they understand the plant is needed for regional electric reliability.''

City officials also put out a news release hailing the FAA's rejection of Mirant's proposal to raise the stacks. "For obvious reasons, we wholeheartedly support the FAA in their determination,'' Euille said. "Mirant's effort to end-run local and state officials has been met with a resounding 'no' from the federal government.''

But Merritt, the FAA official, said that rejection was only preliminary. To help the agency make a final determination, the FAA recently issued a public notice asking for comments on the air navigation issue from "interested persons." Any comments must be received by the FAA's office in Fort Worth, Texas by Dec. 14.

Merritt said a final decision might take one to two months after that.

Mirant hopes the FAA will allow its plant's stacks to go up. The plan was once rejected.