On November 15, 2017 Frontier Airlines announced a $15 billion order for 134 additional A320neo family aircraft. The order, slightly revised under new owner Indigo, consists of 100 A320neo and 34 A321neo. The order also includes the conversion of the remaining A319neo to A320neo. With this order, Frontier Airlines fleet has industry-leading fuel efficiency. The fleet is also one of the most modern and young, particularly in comparison to other low-cost carriers,[nt 1] at an average age of 5 years as of 2018.
Allegiant Air has been closely monitored by the FAA due to many emergency landings and aborted takeoffs. ABC interviewed a former Allegiant mechanic, who said "Dedicated steps were not being performed with maintenance manuals or even with general practices, before an aircraft is released." Many of these incidents had involved Allegiant's aging MD-80 aircraft, which as of November 2018 have been replaced by newer Airbus A320 family aircraft. 46 of the 86 aircraft have made emergency landings, all of which were on MD-80s. The average age of Allegiant's MD-80 fleet was 29 years compared to less than 13 years for its Airbus fleet.