It has been over a month since one of Boeing's 737 Max 9 aircraft, operated by Alaska Airlines, suffered a structural blowout mid-flight. Between aircraft groundings, inspections and increased FAA scrutiny, Boeing is still trying to recover from the incident.

Now, the aircraft manufacturer is facing another challenge from the International Association of Machinists and Aerospace Workers (IAM), the company's largest union, which is still irked over a deal that barely received approval in 2014.

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