Washington (CNN) -- The Federal Aviation Administration this week is sending out scores of letters to small- and medium- size airports nationwide saying the agency may soon close their control towers because of budget cuts, unless they can convince the government that the action will harm the national interest.
"Negative impact on the national interest is the only criterion the FAA will use," the letters read. "The FAA is unable to consider local community impact."
The FAA says it has not yet identified which towers will be closed, or the date of closure.
But Spencer Dickerson, the head of a contract tower association, said top FAA officials made clear in a Monday conference call that the agency intends to close contract 173 towers on April 7, and another 16 towers on September 30, the end of the fiscal year. In addition, 49 FAA-staffed towers are on a list that may be closed.
In the letter to airports, the FAA says they must make a case for their tower to remain open by March 13. The list of closures will be finalized March 18, the FAA said.
The FAA, in talking points obtained by CNN, says it has adopted a philosophy of cutting in a way "to minimize impact to the greatest number of passengers."
All of the towers on the list have fewer than 150,000 operations (an operation is a takeoff or landing) or 10,000 commercial operations a year.
The 189 contract towers handle approximately 20% of all airport operations, but less than 2.5% of commercial operations, according to the talking points. The 49 FAA-staffed towers handle approximately 26% of all tower operations, but less than 3.5% of commercial operations.
Combined, the 238 towers handle 5.8% of all commercial operations during the last fiscal year, the talking points say.
Tower closures would not necessarily result in airport closures, because some aircraft can land without air traffic control help, and those that need controller help can communicate with more distant FAA facilities. Further, individual airports could pay for contract controllers, the FAA said.
"We're extremely discouraged and disappointed that the FAA is taking this action," said Dickerson, of the contract tower association. "The rest of the FAA's budget is getting a 5% haircut; the contract towers are getting a 75% cut, because the FAA is cutting 189 of the 251 contract towers."
"It's hard for us to see the fairness in the budget cuts. It seems the contract tower program is taking a high, disproportionate cut. We have serious concerns about the safety, efficiency and loss of jobs in almost 150 communities across the country," he said.
The towers are part of the FAA's contract tower program, in which 251 towers are staffed with contractors instead of FAA employees.
Though little-known, contract towers are widely used by the FAA to manage air traffic. FAA employees staff busier towers.
John Cozart, CEO of Robinson Aviation Inc., which staffs contract towers in the South and Southwestern United States, said the decision was "not unexpected."
"I didn't think they'd resolve it in favor of the contract tower program. I kind of expected that they would continue on their course," he said.
Asked for his reaction, he said, "You're asking a guy who's having to lay off a ton of people because of this."
A 2011 report by the Department of Transportation's Office of Inspector General said contract towers cost on average $537,000 a year to operate, compared with $2 million for an FAA-staffed tower with a similar workload. The lower costs were chiefly from lower staffing and salary levels at contract towers, which had an average of six controllers, while FAA towers had 16.
And a typical contract controller near Tampa, Florida, received a base pay of $56,000 per year, compared with a base pay ranging from $63,000 to $85,000 a year for an FAA controller in Sarasota, Florida, the study said.
Dickerson said contract towers are carrying the brunt of the cuts, despite having comparable safety records and being more cost efficient.
But the forced spending cuts, known as the sequestration, are also affecting FAA staff. Most of the agency's 47,000 workers, including its 14,700 controllers, have been told to expect one or two furlough days every two-week pay period. The FAA also has proposed eliminating overnight shifts at some towers.