Normally U.S. airlines compete to sell tickets and fill seats during the peak summer travel season. But operators of the grounded Boeing 737 Max are facing a different problem: scarce planes and booming demand.

The grounding of Boeing Co's fuel-efficient, single-aisle workhorse after two fatal crashes is biting into U.S. airlines' Northern Hemisphere spring and summer schedules, threatening to disarm them in their seasonal war for profits.

"The revenue is right in front of them. They can see it, but they can't meet it," said Mike Trevino, spokesman for Southwest Airlines Pilots Association and an aviation industry veteran.