American Airlines, Philadelphia's largest carrier, is trimming some service to European cities from Philadelphia International Airport, as weaker travel demand to Europe leads airlines to pull back transatlantic flying.
For American,  some cuts will be for the winter season, and some will be permanent.

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

American suspended its year-round  PHL-Brussels flight after the Brussels bombings in March.  The route resumed in June for the summer, but due to lackluster passenger demand, the airline decided to end the route and close its Brussels station, said American spokeswoman Lakesha Brown.

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."

In January, American will stop flying to Halifax, Nova Scotia. US Airways had operated the twice-weekly flights since 1995, but more recently "the market did not perform well,"  said  Brown.  American will exit the Halifax market altogether, and  close its station there.

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.

The seat drop reflects the loss of the Frankfurt and Brussels flights, which had operated year-round. The year-over-year decline includes a daytime London flight that American tried for a year in March 2015, but discontinued because it was not popular.  Travelers to London preferred an evening departure, with the flight arriving the next morning . (American still has an evening flight to London Heathrow from PHL.)

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.