Pilots at Deutsche Lufthansa AG plan another walkout tomorrow, continuing a series of strikes even after Chief Executive Officer Carsten Spohr pledged to amend an offer for retirement benefits.

The pilots flying Airbus Group NV A380, A340 and A330 models as well as Boeing Co. 747s will strike tomorrow from 9 a.m. to 5 p.m., their union said in a statement. Spohr last week said he’d present cockpit crews with a “concretization” of the company’s existing offer for early retirement benefits, even as he vowed to stand firm in a conflict that has crippled operations for six days already this year.

The walkouts have led to spiraling costs for Lufthansa and disrupted traffic at Germany’s largest airports. With tomorrow’s planned strike, the pilots are hitting the company’s most prestigious, long-haul services.

“As Lufthansa’s management has so far not made an offer that’s fit for compromise, we feel forced to additional actions,’ the union said in a statement.

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The pilots staged a three-day walkout in April, their first since 2010, forcing the airline to cancel 3,900 flights and trimming profit by 60 million euros ($77 million), with bookings taking about six weeks to recover. Three recent walkouts at the Germanwings low-cost unit, at Lufthansa’s main Frankfurt hub and in Munich, caused a total of about 500 suspended flights, and will reduce profit by about the same amount as the April action, Lufthansa said.

Pilots could so far retire as early as age 55, with Lufthansa paying benefits of as much as 60 percent of the last salary to bridge the gap until the government pension scheme kicked in. Lufthansa aims to lift the age threshold to 60, and canceled the existing agreement at the end of last year.

To contact the reporter on this story: Richard Weiss in Frankfurt at rweiss5@bloomberg.net. To contact the editors responsible for this story: Benedikt Kammel at bkammel@bloomberg.net. 

Photo Credit: A Lufthansa plane in Frankfurt, Germany. Aero Icarus / Flickr