​​Southwest Airlines is extending cancellation of flights on its fleet of Boeing 737 Max aircraft through 8 February, days after its pilots' union estimated the aircraft would take longer to return to service than the Texas-based airline's previous cancellation through 5 January.

Southwest Airlines is extending cancellation of flights on its fleet of Boeing 737 Max aircraft through 8 February, days after its pilots' union estimated the aircraft would take longer to return to service than the Texas-based airline's previous cancellation through 5 January.

The carrier states 17 October that it is monitoring efforts by Boeing and the US Federal Aviation Administration to modify software on Max aircraft and to develop training requirements. The FAA has not set a timetable for when the Max could return to service, but Boeing has said the aircraft will be granted regulatory approval before 2020.

Extending the cancellation will remove "roughly 175 weekday flights" from Southwest's schedules, out of its total peak-day schedule of around 4,000 daily flights, the airline says. Southwest had 34 737 Max 8s in service when regulators worldwide grounded the aircraft in March. Cirium fleets data shows that carrier is the largest operator of the aircraft, with 234 737 Max 8s and 28 Max 7s on order.

"We remain confident that, once certified by the FAA, the enhancements will support the safe operation of the Max," Southwest says.

In a 14 October letter, the Southwest Airlines Pilots Association (SWAPA) wrote to union members that it does not expect Max aircraft to be ready to return to service before February.

The US National Transportation Safety Board (NTSB) and the Joint Authorities Technical Review Panel (JATR) have published separate reports criticising the FAA and Boeing for not adequately preparing pilots to fly Max jets.

These reports are not officially linked with the FAA's process to prepare Max aircraft to return to service, but SWAPA said in its letter it expects the reports "obviously will have some input indirectly".

The European Union Aviation Safety Agency, for instance, has said it will not rely on the FAA return-to-service approval and will conduct its own review process that will include test flights conducted by its own pilots.

SWAPA is suing Boeing, seeking damages for the 10,000 pilots represented by the union who have lost compensation since the grounding of Max. The lawsuit filed on 7 October in the District Court of Dallas County, Texas, alleges that Boeing deliberately misled the airline and its pilots that 737 Max aircraft were airworthy and not significantly different from its predecessor, the 737NG.

Air Canada announced 16 October it will remove its fleet of 24 Max aircraft from flight schedules until 14 February. American Airlines has extended cancellation of flights on Max aircraft through January.