No plane. No game.

It has been 12 days since the Federal Aviation Administration grounded the Boeing 737 MAX 8, and airlines are warning of more flight cancellations to come. American Airlines and Southwest Airlines are offering customers a cushion for how long they can cancel or rebook their flights. And though it's still unclear how long the FAA order will last, other carriers are planning long term: Air Canada told its passengers to brace for cancellations heading into July.

American Airlines, which has 24 MAX planes in its fleet, said Sunday it would extend cancellations through April 24. The carrier said it expects to cancel roughly 90 flights per day based on its April schedule, and that it was waiting on the FAA, the Department of Transportation and the National Transportation Safety Board, along with Boeing, before reintroducing the grounded planes.

The flight disruptions are the latest ripple effects of a mounting crisis of confidence for Boeing, the manufacturer of the 737 MAX 8 jet. Those concerns started when a brand-new Boeing 737 MAX 8 crashed in Indonesia in late October, killing 189 people.

Weeks later, Boeing disclosed the existence of a new flight control system called MCAS, which can push the plane's nose downward in certain circumstances. Pilot groups in the U.S. were incensed that Boeing had built the system into its new jets without detailing it in training courses, raising concerns that pilots might not know how to take control at a crucial moment.

Those criticisms reached a fever pitch earlier this month when another Boeing 737 MAX 8 crashed in Ethiopia under similar circumstances. Regulators in China, Europe, Indonesia and Ethiopia quickly grounded the planes; the FAA held off for several days before idling all 737 MAX 8 and 9 planes in the U.S. Investigators have yet to directly assign blame for either crash.

The FAA's relative slowness in grounding the 737 MAX, along with reports of a criminal investigation into how the new plane was certified, has shaken international confidence in the agency. Numerous international regulators committed to conducting their own regulatory reviews of Boeing's MCAS fix rather than trust the FAA's analysis. This could extend the financial pain for Boeing, Cowen investment analyst Cai Von Rumour said in a note Monday to investors, because about 40 percent of Boeing's 737 MAX backlog comes from China, Canada and Europe.

"Because both the FAA and Boeing were slow to call for grounding the MAX, they've both suffered a loss of reputational credibility," the note said.

For wary passengers, that doesn't mean that every flight originally scheduled on a MAX aircraft will be dropped. The company is swapping other types of planes for canceled MAX flights. Still, a flight that wasn't originally scheduled on a MAX plane might be canceled to cover for the rest of the fleet. Affected customers should be contacted by the airline's reservations team, or a travel agent. Customers also can request refunds if they don't want to rebook.

Southwest Airlines said Monday that it's handling an average of 130 cancellations a day, out of more than 4,000 flights throughout its system. The airline has idled 34 MAX aircraft and is making cancellations five days out. Last week, the airline said that any customer booked on a canceled MAX flight could rebook without any additional fees or fare differences within 14 days of the original travel date.

Boeing, meanwhile, is working with pilots and regulators on its software and training updates for its 737 MAX. Two hundred pilots and regulators are expected to join a training session on Wednesday at its Renton, Washington, plant. Boeing executives also met with pilots and trainers on Saturday.

On March 13, the FAA grounded all 737 MAX jets in the U.S. Agency officials said that airplane tracking data and new evidence from the wreckage of the Ethiopia crash showed similarities to the crash in Indonesia, leading the agency to ground both the 737 Max 8 and the Max 9, another aircraft in the series. As recently as that morning, Boeing and the FAA maintained that the MAX planes were safe to fly.

There's some question of how long the FAA will continue to ground planes in the United States. Boeing announced on March 12 ― a day after the Ethiopian Airlines crash ― that it would carry out a new "software enhancement" designed to "make a safe aircraft even safer," and would also set up new crew training related to the MCAS system. The software fix had been in the works for months.

The FAA expects the software fix to be ready some time this week, an agency spokesman said Saturday, after which point the FAA would evaluate it. The new software is to include new indicators of the plane's "angle of attack," a measure of the direction of the plane's nose. It also is to include a new warning system so pilots know when the automated MCAS system has been activated, and limits on how far down the system can point the plane.

Separately, Boeing is rolling out new pilot training courses to cover the new flight control systems.

The training and software fixes are part of a full-court press by Boeing to restore confidence in its 737 MAX 8 jets. To return to business as usual, the company will have to satisfy not only the FAA but also global regulators, skeptical airlines in the U.S. and elsewhere, and the flying public.

It could be months before some airlines will fly the 737 MAX 8: Air Canada announced last week it would remove all 24 MAX aircraft from its flight schedules through at least July 1.

"Given the magnitude of our 737 MAX operations which on average carry nine to twelve thousand customers per day, customers can expect delays in rebooking," the airline said.