The airline last  month stopped flying to Brussels, Belgium, from Philadelphia — or from anywhere. The last Brussels flight was Aug. 21.

American will discontinue a seasonal flight to Zurich, Switzerland, on Sept. 30. The flight will  not resume in the spring.  A Zurich flight will still operate from John F. Kennedy International Airport in New York.

Another change: American's year-round Philadelphia-Frankfurt flight will become seasonal.  "We are suspending Philly-Frankfurt from the beginning of November through March," Brown said. "It's a reduction based on route performance. It will resume in April."

Despite the trims, Philadelphia will remain American's fourth-busiest hub with more than 430 daily flights, and is the airline's transatlantic gateway, said American spokeswoman Victoria Lupica.

After the terrorist attacks in Europe — Paris last November, Brussels and Istanbul in July, and Nice on Bastille Day —  travel to Europe suffered a shock. "Clearly, there is impact from terrorism. Brexit to some degree has had some impact," said Seth Kaplan, managing partner of Airline Weekly, an industry publication.

All airlines are trimming transatlantic capacity, but American has trimmed the most seats, based on an Airline Weekly analysis of Diio Mi data, Kaplan said.

Delta plans to cut seats to Europe 1.5 percent in the first quarter of 2017 compared with the period this year.  United's seats to Europe will be down 0.6 percent.  American's seats to Europe will be down 7.1 percent in the first quarter of 2017, Kaplan said, based on scheduled flights.

Among American's nine hubs,  Philadelphia's seat count to Europe will be down 16.5 percent  in the first quarter next year, compared with the same period in 2016, Kaplan said.

Philadelphia is not the only American hub losing seats to  Europe: A flight from JFK to Manchester, England, will become seasonal, as will one of American's two JFK-Paris flights.  A Chicago-Paris flight will be suspended between January and March.   "Our seasonal reductions, while they are going away through March, will return in April," Brown said.