Southwest Airlines Co.¡¯s pilots union stepped up its criticism of Boeing Co., saying the plane-maker may be trying to rush the return to service of its grounded 737 Max.

In a message to members, the head of the Southwest Airlines Pilots Association said Boeing should never be given the benefit of the doubt again and criticized the manufacturer¡¯s ¡°arrogance¡± over the development and certification of the Max. Union president Jon Weaks also threw his support behind Southwest¡¯s decision to consider flying planes made by a company other than Boeing.

The letter deepened the tension between Boeing and the pilots of the largest operator of the Max as the aircraft¡¯s flying ban hits the eight-month mark. Southwest is the largest operator of Boeing¡¯s best-selling jet, and the grounding reduced the carrier¡¯s operating income by $435 million through September. Southwest has pulled the Max from its flight schedule through March 6 ¡ª almost a year since the jet was grounded worldwide.