Fuel prices are still high and the economy still weak. That’s a bad combination for airlines, even on traditionally strong routes, like those across the Atlantic. Delta Air Lines and its joint-venture partners Air France and KLM — they have anti-trust immunity (ATI) to coordinate schedules and fares — have announced significant (7 percent to 9 percent) transatlantic route cuts come September.

Among the casualties is Delta’s Philadelphia – Paris flight. The backstory: Air France had operated PHL-CDG for years. Once the ATI agreement was approved, Delta took over the route, as it could provide a smaller aircraft (a 757) that to better match the supply of seats to demand than the Airbus A340 Air France had been using. The 757 seems not to have proven the answer either, though it may return next summer.

What besides the economy drove DL/AF off this established route to a top-tier east coast U.S. market? That would be US Airway’s long-established single daily Philly-Paris flight. And even that’s on a smaller plane now than it was two years ago.

Charlotte connection? No competition means more locals on US Airway’s Philly-Paris flight, which is daily year round. The airline also flies CLT-Paris year-round, but that flight is only daily for about half the year (May through October). From November 2010 through April 2011, it operated only four days a week. More traffic on the Philly flight will leave more connecting traffic for the Charlotte flight, helping to make sure it operates this winter too.

Bonus observation: If you still harbored some sort of hope that some carrier besides US Airways and its codeshare partner Lufthansa would fly from the CLT to Europe, think again. It’s just not happening anytime soon.