CLEARWATER — Allegiant Air has cancelled at least 12 upcoming flights over the next two days to and from St. Pete-Clearwater International Airport , according to an airline travel advisory.That scrambled travel plans for hundreds of passengers from Florida to as far west as Tulsa, Okla.The problem, the airline told the Tampa Bay Times, is a delay in receiving new aircraft to add to its fleet."We are in the process of transitioning to an all-Airbus fleet," Allegiant spokeswoman Krysta Levy said in an email sent Wednesday. "Unfortunately, some of the delivery dates for aircraft scheduled to join our fleet have fallen short of the projections, leaving us with limited resources available and the very difficult choice to cancel certain flights."The cancellations even extend into next week, with four flights affected on June 21 as of early Thursday morning.David Eckerson, 52, of Muskogee, Okla., arrived in the bay area on Sunday and found out Wednesday that his flight back home had been cancelled just hours before he was supposed to board. He and his wife had to grab last-minute seats on another airline’s flight."It just stinks that they’re going to give us $225 and we had to pay $435," he told the Times, referring to the price of a voucher Allegiant sent him compared to the price of his new ticket.TAMPA BAY TIMES INVESTIGATION: Thousands of people flew Allegiant last year thinking their planes wouldn’t fail in the air. They were wrong.The budget carrier had long depended on an aging fleet of McDonnell Douglas MD-80s. In 2016, a Times investigation found that mechanical issues made Allegiant four times more likely to have unscheduled landings than other airlines in 2015.In August, the airline announced that it had replaced all of its MD-80s at St. Pete-Clearwater International with newer Airbus 320s. In fact, Allegiant plans to replace its entire fleet of MD-80s by 2019 because the older planes are more prone to mechanical issues and also fuel inefficient."We have offered options to our passengers on canceled flights," Levy said, "including re-accommodation to other flights or refunds."Jeff Jones, 39, and his wife travelled with the Eckersons. Each couple came to Pinellas County to celebrate their 20-year anniversaries. If they come back, they said, it won’t be on an Allegiant plane."We’re never going to trust them to get us anywhere and back," he said.Other disgruntled passengers took to social media to express their frustration.Hey @Allegiant my mom was going to visit us on Friday but her flight was cancelled. She's been on hold for 1.5 hours trying to resched. What can she do? Any chance ya'll will answer the phone?— Dr Anastasia Bodnar (@geneticmaize) June 13, 2018Bad news: flight cancelled <24 hours before leaving, no reason given. Good news: flight rebooked, $150 flight voucher, and a $225 "apology" check. Thanks, @Allegiant— Scott Hooker (@PapaHooks) June 13, 2018Had to hang up after 2.5 hours on the phone trying to reach @Allegiant. Hold time was supposed to be 45 minutes. Thanks for ruining my last day of vacation.— Laura Merchant (@LMerchantDesign) June 14, 2018Never flying @Allegiant again, they canceled my flight within 48hrs, no reason why, then send me a voucher, im on hold for over an hour, the voucher doesn’t even cover the cost of a one way flight! Amateur hour. Thanks for ruining a family trip.— Bertaglius (@ILikeBertls) June 13, 2018Contact Justin Trombly at [email protected] Follow @JustinTrombly.