United Airlines and Delta Air Lines could be facing maintenance, repair and overhaul costs greater than the industry norm in the next three years, according to an analysis of Aviation Week Intelligence Network (AWIN) data.

Delta is expected to need 416 heavy maintenance checks across its fleet, compared with 289 for American Airlines and 184 for US Airways, the AWIN data show. Among the large U.S. carriers, United ranks second in terms of coming maintenance expense with an expected need for 396 heavy checks in the next three years.

Three aircraft types—Airbus A320s and Boeing 737-800s and 757-200s—dominate the outlook for heavy checks, according to AWIN data compiled from TeamSAI. Delta has 155 757-200s, with an average age of close to 20 years. Delta has 70 737-800s with an average age of 11 years and three that average five years of age. Delta’s 69 Airbus A320s average more than 15 years, the data show.

United has 97 Airbus A320s with an average age of 14 years; 105 Boeing 737-800s at 11 years, and three with an average age of three years. United operates 135 757-200s, the average age of which is older than 15 years, the data show.