American Airlines has canceled Boeing 737 MAX services until April 24 and says the worldwide grounding of the troubled plane is affecting about 90 flights a day. The extension from the previous date of March 28 comes as Boeing and the US Federal Aviation Administration are working on changes to the plane's flight control software. American said canceling in advance provided certainty to its customers by providing them with additional availability and rebooking options. READ: Reports that 737 MAX safety options re lucrative are nonsense. It said both international and domestic flights were available but it had balanced cancelations across its system and the 6700 daily flights it offers on an average day. American has 24 737 MAX 8 jets similar to two that crashed within five months of each other in Ethiopia and Indonesia. American pilots said last week they were expecting to test Boeing's software fix on simulators over the weekend. Some media predicted a statement from Boeing on progress would come as early as Monday, although the company dowplayed this. Other pilots from affected airlines were also due at Boeing facilities at the home of the 737 in Renton, Washington, US media reported. Boeing said it had been "working diligently and in close cooperation with the FAA on the software update. "We are taking a comprehensive and careful approach to design, develop and test the software that will ultimately lead to certification,'' it said. "We have been engaging with all 737 MAX operators and we continue to schedule meetings to share information about our plans for supporting the 737 MAX fleet.'' The Wall Street Journal reported Saturday that US regulators had already "tentatively approved" the software and training changes but a Boeing spokesman said the software could still go through revisions. Boeing has also put the enhanced software through its paces on an 80-minute test flight. It began working on changing pilots displays, operations manuals and crew training relating to the new software after the crash in Indonesia last October of a Lion Air plane with 189 passengers and crew. The pilots were still fighting the aircraft as it plunged into the ocean off Jakarta after the software, known as the  Maneuvering Characteristics Augmentation System (MCAS), repeatedly pushed down its nose because of incorrect information from a single angle of attack sensor. However, they failed to follow an established procedure that would have allowed them to shut off the system by flicking two switches. MCAS is now being updated to allow access to more than one angle of attack sensor input, limit the number of stabilizer trim commands when the software receives an erroneous AoA reading and provide a limit to the stabilizer command to allow pilots to retain elevator authority. Boeing is also understood to be making standard on the MAX a system that tells pilots when the two angle of attack sensors are not giving the same reading. That system had previously been an option. The move comes as the company and the FAA face audits of the process used to certify the aircraft and questions about its lobbying in Washington.