Despite being unionized as members of the Independent Pilots Association (IPA), UPS pilots will in effect join the strike against UPS if one occurs at the end of the month. In a letter to Teamsters President Sean O'Brien earlier this month, IPA President and Captain Robert Travis pledged that:
[...]the IPA is committed to exercising our contractual rights, both domestically and internationally, to honor any potential IBT [International Brotherhood of Teamsters] strike and act in sympathy with our fellow workers at UPS by not working. 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.
The IPA previously did the same with the 16-day Teamster strike of 1997, during which all of its pilots respected the Teamster picket line. The same is expected if a strike happens at the end of the month.
This sort of thing is, to be clear, typically not legal—and it's an unusual arrangement that seems to make it legal in this case. For most workers, my understanding is that taking action like this would constitute a sympathy strike or a solidarity action—those are legally prohibited under the Taft-Hartley Act. However, IPA's collective bargaining agreement allows them to honor primary picket lines, and a Teamster picket line would constitute one of these. Consequentially—because doing any work would constitute crossing the picket line here—this means they can in this specific context suspend their work in a way that isn't technically a strike by law, but functions as one in practice. A detail that gives this arrangement teeth is that all IPA members work for UPS, and IPA's bargaining unit cover all UPS pilots (meaning there's no immediate, realistic replacement for them if they walk off the job).