The Federal Aviation Administration has told employees that they face 11 furlough days due to sequestration, starting as early as April 7 and spread out through September.

“Please know that I sincerely regret the impact that even a single furlough day will have on you,” FAA Administrator Michael P. Huerta said in an e-mail to employees dated Tuesday.

Huerta said that the timing of formal furlough notices, which trigger a 30-day waiting period before the forced unpaid days off could begin, would vary within the agency. A number of unions have bargaining units in FAA, some of which have requirements governing such notices in their contracts.

FAA’s parent Transportation Department two weeks ago said the vast majority of the FAA’s nearly 47,000 employees faced furloughs of about one day per pay period “until the end of the fiscal year in September,” although it did not specify a number of days or a starting date.

The furloughs could force the closure of some smaller air traffic control centers, shifts would be eliminated at others, and maintenance would be deferred, resulting in travel delays especially at major city airports, the department has said. Some Republicans have challenged those projections as too dire, arguing that the agency has budgetary room to maneuver to lessen the impact.

Huerta’s message acknowledged that there has been “much public discussion about the effect of sequestration on the FAA” and added, “We will continue to explore ways to reduce expenditures and minimize the number of total furlough days required across the agency to meet our budget requirements.”

Doug Church, spokesman for the National Air Traffic Controllers Association, said in an e-mail that “We continue to implore Congress to act now to stop the very negative effects of these budget cuts before they take full effect. This does not need to happen. Travelers may face unneeded delays and the closure of nearly 200 towers would be very bad for the aviation system and the economy and communities that depend on those airports.”

Church said the union, which represents more than 20,000 controllers, engineers and other employees, is continuing to work with agency management on the impact.