The aircraft mechanics represent a relatively small portion of the company’s workforce of more than 500,000. Of the roughly 1,700 maintenance technicians and related employees who work on the company’s jets, hundreds work at the company’s air hub in Louisville, and about 40 work in Atlanta.

It’s separate from the massive Teamsters contract covering about 350,000 drivers, package handlers and other workers. The Teamsters have threatened the possibility of a strike by those UPS package division workers, even though negotiations aren’t set to start until next year.

The aircraft mechanics are under a separate contract and, as part of the airline industry, are governed by a different labor law than the rest of the UPS workers.

Labor relations for airline workers including the UPS aircraft mechanics are governed by the Railway Labor Act — which covers both airlines and railroads that are crucial to the nation’s transportation infrastructure and is more restrictive on the ability to strike than the broader National Labor Relations Act. Those restrictions can prompt unions to be more likely to agree to a deal, as railroad unions just did to aver a national strike, and as the UPS aircraft mechanics union did.