An important group whose members will be called upon to fly on the Boeing 737 Max has concerns about a relaunch.
The Association of Professional Flight Attendants (APFA) made their concerns known in a letter to Boeing CEO Dennis Muilenburg seen by Reuters.
The group represents American Airlines flight attendants.
"The 28,000 flight attendants working for American Airlines refuse to walk onto a plane that may not be safe and are calling for the highest possible safety standards to avoid another tragedy," APFA President Lori Bassani said in the letter.
Bassani wants to make sure her group has all the information needed concerning the safety of sending her crews back to work once the aircraft receives FAA approval.
A Boeing spokesman said Muilenburg had received the letter and will be responding.
Muilenburg spent two days last month on Capitol Hill testifying about the 737 MAX following two crashes that killed 346 people.
The aircraft was grounded worldwide in March.
Boeing's CEO told Congress that the aircraft company knows it made mistakes and is throwing everything into fixing the plane.
Boeing has made what it calls robust improvements to the 737 MAX flight control software.
American Airlines has 24 of the 737 Max in its fleet, making it the second-largest operator of the plane.
Boeing has said it expects a 737 MAX return to service in the current quarter.
American Airlines has removed the 737 MAX from its flying schedule until Jan. 16.