As Republic Airways Holdings was in the process of bidding to acquire Frontier in 2009, it was also in the process of acquiring Milwaukee-based Midwest Airlines. Through the fall and winter of 2009, Republic operated its two new acquisitions as separate brands. However, to improve efficiency by better matching aircraft capacity to route demand, Republic began to intermix the fleets of the two airlines, swapping a portion of its higher-capacity planes from Frontier with its smaller-capacity planes from Midwest and vice versa. However, the move caused some confusion amongst the public, as the two brands did not offer the same amenities and did not match the amenities mentioned on the airfare. As a result, in the Spring of 2010, Frontier and Midwest Airlines announced that their brands would merge, with Frontier being the surviving brand. This was a merger of brands only—no Midwest Airlines aircraft was ever operated by Frontier, as by this time, all Midwest Airlines flights were operated on its behalf by other Republic Airways Holdings subsidiaries.
In summer 2015, a rash of midair breakdowns drew federal scrutiny. "Before the night was finished on June 25, 2015, five Allegiant flights had been interrupted in four hours, all because different planes had failed in midair," reported the Tampa Bay Times. Since October 2015, the Federal Aviation Administration has kept Allegiant under close supervision. In July 2015, Allegiant Air announced bases would be established at the Asheville Regional Airport and Cincinnati/Northern Kentucky International Airport, becoming the first base at a non-vacation destination.
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Allegiant aims primarily to serve leisure travelers, particularly those in colder northern climates, going to warm-weather tourist destinations such as Punta Gorda, Tampa Bay, Las Vegas, Orlando, Los Angeles and Phoenix. It also serves smaller destinations that see few direct flights by major carriers. Many of the airline's markets, such as Peoria, Illinois, are served only by commuter service requiring a connection at an airline hub. In October 2009, Allegiant had competition on just five of its 136 routes.
On April 10, 2008, Frontier filed for Chapter 11 bankruptcy in reaction to the intent of its credit card processor, First Data, to withhold significant proceeds from ticket sales.[better source needed] First Data decided that it would withhold 100% of the carrier's proceeds from ticket sales beginning May 1. According to Frontier's press release, "This change in practice would have represented a material change to our cash forecasts and business plan. Unchecked, it would have put severe restraints on Frontier's liquidity..." Its operation continued uninterrupted, though, as Chapter 11 bankruptcy protected the corporation's assets and allowed restructuring to ensure long-term viability. After months of losses, Frontier Airlines reported that they made their first profit during the month of November 2008, reporting US$2.9 million in net income for the month.
In 1997, Maverick Airways was operating code share service for Frontier with de Havilland Canada DHC-7 Dash 7 STOL capable turboprops between Denver (DEN) and two destinations in Colorado: Grand Junction (GJT) and Steamboat Springs (SBS). However, the service was short lived as Maverick encountered financial challenges and then ceased all flights.
In February 2011, Allegiant proposed to sell two types of tickets to passengers: advance tickets at a fixed higher rate and time-of-departure tickets that cost less but may have fees added based on the price of aviation fuel. In 2012, the U.S. Department of Transportation banned the practice as part of wider regulations that also require taxes and fees to be included in airfares. Allegiant, along with Spirit Airlines and Southwest Airlines, sued the DOT to overturn these new rules. The United States Court of Appeals for the District of Columbia Circuit ruled in favor of the DOT on 24 July 2012 and the US Supreme Court denied certiorari on 1 April 2013.
jetBlue previously had its headquarters at 80–02 Kew Gardens Road, and then in the Forest Hills Tower, both in Forest Hills, Queens, New York City, 6 miles (9.7 km) from the current office in Long Island City. In 2001 the airline had announced that it wanted to take 74,000 square feet (6,900 m2) of space in the Forest Hills Tower, and by December 2002 announced that it planned to increase its leased space.
On October 25, 2016 JetSuiteX announced that JetBlue had made a minority equity investment in JetSuiteX. Part of the agreement also gave JetBlue a seat on JetSuite's board of directors. Reasons for the investment was outlined by CEO Robin Hayes "Our investment in JetSuite makes sense as we continue to execute on our west coast plan and invest in innovative ideas that reflect the disruptive spirit of JetBlue." In JetBlue's 1st quarter 2018 investor call JetBlue's CFO Steven Priest Confirmed they currently hold about 10%  of JetSuiteX.
jetBlue's founders had set out to call the airline "Taxi" and therefore have a yellow livery to associate the airline with New York. The idea was dropped, however, for several reasons: the negative connotation behind New York City taxis; the ambiguity of the word taxi with regard to air traffic control; and threats from investor JP Morgan to pull its share ($20 million of the total $128 million) of the airline's initial funding unless the name was changed.
In May 2017, Allegiant Air took delivery of its first brand-new A320. Allegiant took delivery of ten new A320s in 2017 and is scheduled to accept two in 2018. All new aircraft will be painted in Allegiant's new livery at the time it is delivered. Older Airbus aircraft will be repainted during their C-checks. The new Airbus aircraft will have fuel-efficient sharklets. The new A320s seat 186 passengers, an increase of the 177 seats that are found in the rest of the Allegiant A320 fleet. To fit the additional nine seats, Allegiant opted for the Airbus Space-Flex V2 Lavatory. 
In 2008, jetBlue partnered with Irish flagship carrier Aer Lingus to allow passengers to switch between airlines on a single ticket for flights with connections in New York–JFK or Boston Logan. Unlike traditional codeshare agreements, the companies cannot sell seats on each other's flights, so customers initiate the purchase on one airline's website and then are transferred to the other site to complete the transaction.