Anyone buying a ticket on one of the super-low-cost, low-fare airlines knows not to expect any frills. But in recent months, flying on big-bargain airlines like Spirit and Allegiant has often meant delayed or canceled flights as they’ve struggled to run their operations reliably.

Each has unique issues to address, from Spirit Airlines‘s labor meltdown with its pilots to a wholesale effort at Allegiant Travel to sharpen its aircraft maintenance policies and practices. Allegiant also replaced some employees at various airports this spring with “new faces and fresh blood,” said Scott Sheldon, the airline’s interim chief operating officer.

The low fares continue to fill the planes, and all three of the U.S. ultra low-cost airlines remain profitable. But the hassle factor has been as real for fliers as jet fuel bills are for the airlines.

“We sincerely apologize to our customers who were affected by the flight disruptions during the quarter,” Bob Fornaro, Spirit’s president and chief executive, said in a statement Thursday accompanying the airline’s latest results. Allegiant Travel Co. President John Redmond told analysts on Wednesday, “When you look at our operations [in the second quarter], we did not meet our expectations at all, and surely not those of our customers.”

An ultra-low-cost carrier will never, ever try to be as punctual as a big legacy airline. Being on time all or most of the time costs money. Delta Air Lines Inc., for example, has spent enormous sums in recent years to move toward the head of the industry pack in quashing delays. ULCCs can’t go all-out on that without risking their business model. Most try to settle somewhere in the middle—not so punctual that it boosts costs and not so delay-prone that they get a reputation and incur long-term customer wrath.

At privately held Frontier Airlines Holdings Inc., “we don’t necessarily believe that it’s cost-effective to end up in the top quartile for on-time performance,” Daniel Shurz, a senior vice president, said Friday in an interview. Frontier plodded along in 10th place in the latest federal on-time performance data, for May, as measured over the prior 12 months.

Spirit’s 1,500 Airbus pilots earn roughly 60 percent of their peers’ salaries at other airlines, according to their union, the Air Line Pilots Association. The company and pilots continue to meet with federal mediators. The labor rancor began to affect customers in April, when the airline says pilots began refusing to pick up open trips as part of their push for a new labor contract.

That led to as many as 25 flights a day being canceled by pilot shortages, a number that has swelled to about 800 and cost $25 million in the second quarter. In May, more than half of Spirit’s nearly 500 flights were canceled on 5 percent or more of the days they were scheduled to operate. Police at the Fort Lauderdale, Fla., airport had to handle hundreds of unruly or just plain enraged passengers after Spirit canceled flights amid the pilot contretemps.

That incident, and others, sent Spirit to federal court for a restraining order against its pilots, citing an “illegal work slowdown” by the group. The airline has said the problem has since receded, although Spirit still scrubs two or three flights a day due to pilot staffing. It is also forecasting a lower flight completion factor, 98 percent, for the rest of year than it posted in 2016.

Spirit officials say the labor dispute marred what had been notable gains in higher on-time performance and reduced customer complaints, dating to April 2016. In May of this year, customer complaints about Spirit to U.S. regulators soared. “We were clearly showing the most improvement on those two areas of any other airline, and we were rocking and rolling until this happened,” spokesman Paul Berry said.

Travel glitches on a carrier like Spirit or Frontier are often compounded because those airlines typically don’t fly their routes as frequently as the larger, network airlines. That means there may be only one flight a day, or even just a few a week, reducing passengers’ options. In cases of non-weather delays, the airlines will at times buy a customer a ticket on another carrier.

That’s why the carriers try to minimize the cancellations and keep completions as high as possible. Even if a flight is delayed by many hours, the airline wants to get customers to their destination and avoid expensive rebookings.

Frontier is risking more operational woes in the coming months as it embarks on a major national expansion, with additional connections in its hometown of Denver and a much larger flight schedule from Florida. The airline will add 21 new cities by next spring and nearly double the number of nonstop routes it flies, topping 300.

At Allegiant, many of the problems stem from maintenance procedures and the pains of transitioning from a fleet of aged McDonnell Douglas MD-80 aircraft to newer Airbus jets. Allegiant established a “forward recovery base” in Cincinnati to help reduce the number of aircraft out of service with a special team of techs available for quick dispatch to 18 stations in the Northeast when a plane breaks down.

Allegiant has another incentive to make its flight schedule more reliable. It has grown large enough that next year it must begin reporting monthly operating data to the Department of Transportation, which lays bare each month which airlines run late, lose bags, and draw customer complaints.

“Obviously we want to be aggressive,” Sheldon said, “to really pull this thing up” in the next six months or so.

Pull it up, yes, but not by outspending the ULCC cost advantage. As Einstein said of space and time, it’s all relative.

This article was written by Justin Bachman from Bloomberg and was legally licensed through the NewsCred publisher network. Please direct all licensing questions to [email protected].

Photo Credit: Flying on big-bargain airlines like Spirit and Allegiant has often meant delayed or canceled flights as they’ve struggled to run their operations reliably. Airbus