United Airlines glided through the first part of summer, as strong travel demand pushed average fares higher, but the grounding of Boeing 737 Max jets will become a bigger challenge in the months ahead.

United said Tuesday that second-quarter profit soared 54 percent, to $1.05 billion. The results beat expectations, and United slightly raised its forecast of full-year profit.

The Chicago-based airline, however, faces uncertainty because of the grounding of Boeing 737 Max jets after two deadly accidents.

The airline was counting on the Max to become a bigger part of its fleet, growing from 14 planes to 30 by the end of September, and 28 more of them next year. That plan is now in jeopardy, as Boeing has halted Max deliveries.

The missing planes will leave a huge gap in United’s schedule and cut into the airline’s revenue. United has already canceled more than 8,000 flights.