The Boeing 737 MAX accounts for about 25% of domestic aircraft production. If MAX production is halted, it would knock about 0.5%-point off quarterly annualized GDP growth. For context, 19Q1 GDP growth is currently expected to be 1.5%, ignoring Boeing. If Boeing keeps producing but temporarily can’t deliver the MAX, inventories will rise and GDP would not be affected. If, however, orders are cancelled, that would directly drag down GDP.