A 13-person crew of United Airlines flight attendants based at San Francisco International Airport were fired last year following an incident in which they refused to fly aboard an aircraft that had a threatening message (pictured above) scrawled in the grime below the tail engine. As you can see it said "Bye Bye" with a couple of creepy faces and what looks like an image of something falling into water. As NPR and the SF Chronicle is reporting, the flight attendants have filed a federal whistleblower complaint against the airline protesting their firing, and they are contemplating a lawsuit.

The incident happened on July 14 at SFO when 13 of the Hong Kong-bound flight's 15 flight attendants — two pursers (managers) were not fired — were shown the photo of the "menacing" threat that one of the pilots had seen scrawled below the tail. The pilot guessed it was put there by mechanics in South Korea, where the plane was before it landed at SFO, or perhaps while it was on the ground at SFO. The pilots then requested a maintenance check in which the tail engine compartment was thoroughly examined, and the flight was delayed well over an hour with passengers already aboard.

The flight attendants, though, refused to fly until United provided another plane, saying that a thorough safety sweep needed to occur with all the passengers taken off. Management arrived, assured them the aircraft was safe to fly, and gave them a direct order, which they refused to comply with.

United has issued a statement saying that "All of FAA’s and United’s own safety procedures were followed, including a comprehensive safety sweep prior to boarding, and the pilots, mechanics and safety leaders deemed the aircraft entirely safe to fly."

The captain allegedly told one of the flight attendants, Evans Francisco Jesus Mendonca, "I'm with you," after Mendonca said he wasn't comfortable flying aboard the plane, but the captain subsequently told other flight attendants, following the safety check, that everything was OK.

The flight ended up getting canceled with 300 people aboard due to "crew availability," and the crew subsequently dismissed. Their complaint says that the airline then took the plane out of service for two days, but the airline "told them that time was needed for the aircraft to resume its rotational schedule."

Most of the crew lived in Asia and worked this regular SFO to Hong Kong route, with one of the flight attendants living in San Francisco.

They're now planning a lawsuit either in California or Illinois (United's headquarters) state court. Their whistle-blower complaint has just been filed with the U.S. Occupational Safety and Health Administration.