As Amazon readies for what will likely be two of its busiest days of the year, the pilots who transport its cargo are releasing a digital ad campaign on Facebook to highlight “concerns about how they are being overworked, underpaid and disrespected by their carriers.”

It’s the latest move in an increasingly bitter logistics saga as Amazon appeals to customers with faster and faster delivery options. The tactics have left pilots complaining about challenging schedules and low pay, and the airlines involved are accusing the pilots’ labor union of spreading lies—and delaying negotiations.

The new ads will direct viewers to the pilots’ website, PilotsDeserveBetter.org, which a union spokesman said “provides background from the pilots’ perspective about serious issues at their carriers.”

Here are examples of the ad creative the union will be running online during Prime Day:

In addition, he said the pilots are standing in solidarity with the Amazon warehouse workers in Minnesota who are planning a Prime Day strike. A representative from the pilots’ union will be on the ground to show striking workers they have the support of Teamsters Local 1224.

“As we know firsthand, Amazon’s business model too often neglects the wellbeing of the workers who make the ecommerce giant so incredibly successful,” said Daniel Wells, an Atlas Air pilot and president of APA Teamsters Local 1224, in a statement. “We’re proud to be the airline professionals who fly the planes that deliver Amazon’s packages to millions of Americans, but we want to make sure we’re engaged in a sustainable, long-term operation.”

A union spokesman, however, said the pilots themselves are not striking during Prime Day—despite an incredibly, increasingly complicated relationship.

How Amazon takes to the air

Amazon is many things, but it is not an air carrier—at least not yet. So it subcontracts the cargo planes it leases to at least two carriers—Atlas Air Worldwide and Air Transport Services Group (ATSG)—which, in turn, are responsible for providing pilots and ensuring flights meet Federal Aviation Administration requirements.

At the International Paris Air Show in June, Amazon announced it is expanding its 42-plane fleet with an additional 15 Boeing 737-800 cargo aircraft through a partnership with financial services firm GE Capital Aviation Services. It has commitments for 13 more through 2021.

An Amazon spokesperson said these planes are former passenger aircraft that are no longer viable for commercial flights for reasons like interior wear and tear. GE buys the planes, guts them and retrofits them for cargo. The “new” planes will become part of Amazon’s network once they have been converted.

These planes will then be used to support one- and two-day delivery for Prime members, she added.

According to Boeing, the 737-800 cargo aircraft can hold a total volume of 185 cubic meters. That’s roughly 49,000 gallons of milk—or, as the Amazon spokesperson put it, each Prime Air plane can hold over one million Echo Dots, 10,000 Instant Pots or 5,000 teddy bears.

But, as previously reported, pilots for these carriers are in the midst of a long-running contract dispute.

Robert Kirchner, an Atlas Air pilot and executive council chairman for the Atlas Air pilots of Teamsters Local 1224, called the new planes “good news,” but said experienced pilots are still leaving airlines that fly for Amazon as a result of a toxic work environment and higher pay from competitors.

“There’s so many jobs out there, they’re not able to get the kind of pilots they need to do this challenging flying without a new contract and atmosphere,” he added.

Kirchner said Amazon flights are particularly challenging because of frequent takeoffs and landings in bad weather, at night and with older aircraft. They are also complicated by small airports and short runways at some of Amazon’s “air gateways,” which are in locations like Allentown, Pa., and Stockton, Calif.

Kirchner said Atlas subsidiary Southern Air has reduced its pilot qualifications below the minimum for most airlines in order to attract pilots—and they are flying two empty “beat-up old 737s” around the country just to get those pilots trained and qualified. “I’ve never heard of such a thing,” he said.

A growing divide

In response to Kirchner’s criticisms, Debbie Coffey, vice president and chief communications officer at Atlas Air Worldwide, said Atlas’ fleet and workforce “meet or exceed all government safety standards,” including multiple reviews, evaluations and proficiency checks during pilot training. (And, of course, the airline has its own website to tell its side of the story.)

She said Atlas has doubled its roster of pilots in the past four years to 2,000 and “we continue to attract highly qualified, top tier talent who are attracted to work for Atlas because of our global and diverse operation.”

In another statement, Amazon said it is eager to see the carriers and pilots “reach a mutually agreeable resolution to their lengthy contract negotiations soon.”

The statement also noted that a lack of willingness on either side to come to a reasonable compromise “could result in a change to the allocation of our current and future aircraft.”

It’s not yet clear what that means, but the Amazon spokesperson said: “We are very happy to have the delivery capacity our carrier partners can provide. They provide a high quality service and our own delivery efforts are needed to supplement that capacity rather than replace it.”

And yet, according to a regulatory filing, Amazon has the option to buy up to 39.9% of Atlas. And, Kirchner said, it has a similar relationship with ATSG.

Will Amazon launch an airline?

“Amazon has been threatening to form its own logistics company and its own airline, Prime Air,” Kirchner said. “I think it was eventually coming, and their plan is to get away from both FedEx and UPS, but they’re going to have a heck of a time doing it here under the current situation.”

This may be buoyed by FedEx’s recent decision to no longer fly for Amazon.

Amazon declined comment on the potential for launching its own airline, noting its “longstanding practice of not commenting on rumors and speculation.”

Kirchner, however, described Amazon as already operating “a shadow airline” in which its own management builds and assigns schedules and decides what kind of freight is carried—including how much fuel each plane has. Such control, he said, should be in the hands of the airline operator.

Amazon referred operational questions to its partner airlines, adding it repeatedly hears claims from the union about Atlas’ service for Amazon that “when investigated, are factually inaccurate.”

Atlas Air spokeswoman Coffey said Kirchner’s comment about Amazon micromanaging flight operations is false and customers like Amazon “play no role governing our operations or setting work rules.” She said, “Atlas and Southern … maintain operational control over their own operations in strict accordance with FAA regulations.”