Wichita ? Boeing Co. rejected the latest overture from the second-largest union at its Wichita plant, but the two sides scheduled more talks for this week.

On Friday, the Society of Professional Engineering Employees in Aerospace’s professional and technical unit — who last month voted to authorize a strike if an agreement cannot be reached — proposed negotiating an 18-month contract.

“We’re not interested in a contract of that duration,” Boeing Wichita spokesman Fred Solis said, declining to elaborate.

The next round of negotiations was set for Tuesday, and both sides said they remained hopeful that a contract could be negotiated successfully.

The union, which has already voted down two three-year offers from Boeing, wanted a shorter time frame to align the contract with three other bargaining units in Seattle and Wichita, officials said.

Besides the duration of any contract, the two sides also disagree over money, with Boeing insisting that it must operate within a specific economic package.

“We have flexibility within that package,” Solis said, “but we’re not going outside that package. Everything that SPEEA brings us is outside that package.”

The union countered that its latest proposal was reasonable.

It “may have been a little outside the box, but is as close as we’ve been in matching their envelope of money,” SPEEA Midwest director Bob Brewer said. “I think we were real close with that package.”

About 3,400 technical and professional employees are continuing to work under the terms of the last contract, which expired in February.

In balloting that ended May 28, SPEEA members voted 70 percent to 30 percent to reject Boeing’s three-year offer. They also voted 75 percent to 25 percent to authorize union negotiators to call a strike if needed.