Three of the largest U.S. airlines have extended their suspension of flights involving the Boeing 737 MAX aircraft, indicating no resolution may be expected soon for the flight-control software problem on those jets. Southwest Airlines moved first in this direction, cancelling operations with its 31 737 MAX jets through August 5. United Airlines cancelled flights involving its 14 737 MAX jets through early July, while American cancelled flights on its 24 737 MAX aircraft through mid-August.

Last week Boeing CEO Dennis Muilenburg reported the OEM has completed 96 test flights of the 737 MAX with an updated Maneuvering Characteristics Augmentation System (MCAS) software, testing different in-flight scenarios to correct the causes of two 737 MAX crashes.

Muilenburg did not indicate any schedule for implementing the updated control software, though it’s been reported that the revised programming would be submitted for review by the Federal Aviation Administration, European Air Safety Administration, and other major regulatory  by mid-month.

The “anti-stall” software cited as causing the crashes was developed to off-set the effect of more fuel-efficient engines adopted for the new version of the 737, but which because of their larger size alter the planes’ aerodynamic behavior. The MCAS software apparently responded to inaccurate flight data and sent the two planes into nose dives that the crews were unable to reverse.

Boeing already has halted all 737 MAX deliveries and cut the production rate on all 737 aircraft by 20%, as it diverts resources to correct the cause of two fatal crashes of 737 MAX jets in the past six months.

The FAA and other major regulatory agencies last month suspended all 737 MAX flights, making the carriers’ decisions to cancel flights procedural. However, by extending the cancellations United Airlines, American Airlines, and Southwest Airlines are indicating no resolution may be expected for months, rather than weeks, and so they are revising their reservation systems and other resources accordingly.

A total of 346 passengers and crew members were killed in crashes of a Lion Air flight in October 2018 and an Ethiopian Airlines flight in March.