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Amazon’s promise of one-day shipping has led it to increasingly rely on its own air cargo division, Amazon Air. But as the number of shipments pushed through the cargo arm multiplies, the pilots who fly those packages continue to voice that they are overworked and underpaid.

The pilots don’t work for Amazon directly, but are employed by the contractors Air Transport Services Group (ATSG) and Atlas Air. More than 200 cargo pilots who fly for ABX Air, which is a division of ATSG, cast a vote of “no confidence” against management’s ability to resolve ongoing labor disputes, reported Reuters earlier this week. In total, all but one member of the pilot’s union voted “Yes” on a resolution that asked if they had “no confidence in management’s willingness to negotiate or reach an agreement for the benefit of all stakeholders to include the shareholders, the customers, and the employees.”

The pilot union, the Airline Professionals Association, has battled with the management of ATSG for five years to negotiate new work rules for its pilots. Issues involving how pilots are scheduled for their routes, salaries, and retirement still remain unresolved.

Those who fly for Atlas Air, another cargo operator used by Amazon, recently lost a three-year bid to secure a new work contract. Atlas pilots protested outside an airport in Cincinnati, Ohio for better workplace conditions in April. And in February, Atlas pilot crashed an Amazon Air flight, killing all three of its occupants, which some have suggested was linked to its staff being overworked.

Pilots at both airlines have complained about low morale, low pay, and poor workplace retention. Such troubles are brewing during a global pilot shortage, and many well-trained pilots who work for Atlas and ABX have simply left for better opportunities. A union survey earlier this year found that 60% of the pilots who work for the three airlines employed by Amazon (ABX Air, Atlas Air, and another called Southern Air) are looking for work elsewhere.

“We still have very experienced high quality pilots leaving for other carriers because they have better pay and better schedules,” Rick Ziebarth, an ABX Air pilot and executive council chairman of Airline Professionals Association Teamsters Local 1224, told Quartz. Ziebarth argued that as a result of well-trained pilots leaving, ABX is forced to hire people with far less experience who require more training. Quartz has reached out to Amazon for comment on the matter.

Worker grievances appear to have cropped up in every leg of Amazon’s supply chain. Amazon’s warehouse workers were found to suffer injuries at twice the national average of other warehouse workers, according to an investigation from Reveal. Delivery drivers for Amazon Flex, who are considered independent contractors by Amazon, work long hours with no chance of earning overtime or benefits.

ABX Air pilots won’t be able to strike until released from the US National Mediation Board (NMB), the federal agency which oversees aviation industry labor relations. Ziebarth said the board is still in the middle of holding negotiations with ATSG.

According to data from Flightpath Economics, a consulting firm, pilots who work for ABX Air and Atlas Air are amongst the lowest paid in the industry.

Company Pilot pay per hour ATSG $152 Atlas $139 Fedex $243 UPS $288

As a whole, cargo pilots face tougher working conditions than their passenger pilot counterparts. Cargo pilots were left out of a 2014 law that required passenger pilots to get a minimum of 10 hours’ rest between flights. Aviation experts also say that lax safety standards and pilot fatigue has lead to a higher number of fatal crashes on cargo flights compared to passenger flights.

Meanwhile, Amazon’s air shipments are only likely to continue rising. FedEx in June announced it would no longer assist Amazon in its air delivery. That same month, Amazon announced it would roll out free one-day shipping for millions of new products. These combined factors have led to the e-commerce giant to rely on its own delivery services than ever before. In July, Amazon Air flew 136 million pounds of goods across in the US, a 29% increase from the same period in 2018, and only 9 pounds less than the December 2018 holiday rush, according to data from ATSG and Atlas Air analyzed by Cargo Facts Consulting. And the growth simply won’t die down this holiday season, when Amazon expects to invest $1.5 billion into one-day shipping costs alone.

Amazon founder Jeff Bezos has said that his company will rely less on airplanes as it builds out its local warehouses. But for now, Amazon is continuing to grow its air cargo operations: it added 15 more planes to its fleet this year, and says it expects to have 70 planes by 2021. It is working on a $1.5 billion expansion of its Amazon Air Hub in Cincinnati—essentially an Amazon cargo airport—which is expected to finish in 2021. When complete, the Air Hub will be able to handle 200 daily takeoffs and landings. It recently opened an Air Hub in Fort Worth, Texas. It seems that as long as demand is high, the future of Amazon’s fast and free shipping will rely heavily on air freight.