The union representing pilots flying cargo planes for UPS say they will not cross picket lines if Teamsters strike next month, a spokesperson for the Independent Pilots Association confirmed to CNN.

The union representing 3,400 pilots has vowed solidarity "by not 'turning an aircraft wheel' on behalf of the company," which could spell even deeper disruptions for the worldwide shipper. The heads of the two unions say in letters to each other that their support proved pivotal during a previous Teamsters strike in 1997.

"The IPA is committed to exercising our contractual rights, both domestically and internationally, to honor any potential IBT strike and act in sympathy with our fellow workers at UPS by not working," said pilot union head Captain Robert Travis. "No one wants a work stoppage, but should a legal IBT strike be initiated, you and the IBT can count on the IPA for support."

This does not mean UPS pilots are going on strike, rather there will be a work stoppage as they vow not to fly UPS cargo. UPS has said negotiations with Teamsters will continue next week and says it "prepared to increase our industry-leading pay and benefits."

The loss of UPS' air unit will make it very difficult for the company to operate with thousands of non-union members they are training to move packages in the event of a strike.

"With losing UPS's air force, it effectively eliminates time sensitive shipments," said Jason Miller, Interim Chairperson for the Department of Supply Chain Management at Michigan State. "Time sensitive shipments will become impossible to handle. Such as high value spare parts, pharmaceuticals, medical devices, anything of high value or high opportunity cost"

Without UPS' Pilots -- the company will not be able to move anything from California to New York in a day.

"This certainly is even more pressure on ups management to come to a deal with teamsters," said Miller. "This is one more leg of transportation stool that will be lost if there is an authorized strike."

When asked about the pilots' vow not to cross the picket line, a UPS spokesperson pointed to their previous statement: "We are pleased to be back at the negotiating table next week to resolve the few remaining open issues. We are prepared to increase our industry-leading pay and benefits, but need to work quickly to finalize a fair deal that provides certainty for our customers, our employees and businesses across the country."

The loss of UPS Air will likely shift packages to FedEx Express, which handles air transport, Miller said.