US airlines are pulling the 737 Max from their schedules until March, suggesting they're losing faith in Boeing's plans to get the plane flying in 2019

• Southwest Airlines and American Airlines are pulling the Boeing 737 Max from their schedules until at least the first week of March.
• American had previously said that it planned to reintroduce the Max starting January 16.
• Boeing is facing increasing hurdles getting the plane returned to service, putting its goal of having the Max flying by the end of 2019 into doubt.
• The plane type has been grounded worldwide since March.
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Southwest Airlines and American Airlines said on Friday that they were extending the cancellation of 737 Max flights until at least the first week of March, 2020, fas Boeing faces increasing hurdles and turmoil as it seeks approval to return the plane to service before the end of this year.

Southwest had previously extended the grounding of its 34 737 Max jets to February 8, the latest of any US airline. It now said that it would cancel 737 Max flights through March 6. The airline cited "continued uncertainty around the timing of Max return to service" and added it is "unable to provide an update on first quarter capacity guidance at this time."

American had last stated that it planned ot begin reintroducing the jet to service on January 16, assuming that Boeing would meet its expected timeline of winning US approval during the fourth quarter of this year. American now plans to cancel flights through March 5.

United Airlines did not extend cancellations of the jet past January as of Friday night.

Earlier this week, regulators found gaps and substandard documents in the recertification paperwork submitted by Boeing, and asked for revisions. Reuters reported that Boeing confirmed it must resubmit revised documentation

The documentation, which was the latest setback, has raised questions into when Boeing can complete a certification test flight and get the plane approved to resume flying, and whether that could happen before 2020. The FAA said it would take 30 days after the certification flight before it would unground the plane. according to Reuters.

The 737 Max, Boeing's best-selling plane, has been grounded since March after crashes in Indonesia and Ethiopia killed 346 people.

Investigations into the two crashes suggest that an automated system called MCAS, or Maneuvering Characteristics Augmentation System, erroneously engaged, forcing the planes' noses to point down, and that pilots were unable to regain control of the aircraft.

The system could be activated by a single sensor reading - in both crashes, the sensors are thought to have failed, sending erroneous data to the flight computer and, without a redundant check in place, triggering the automated system.

MCAS was designed to compensate for the 737 Max having larger engines than previous 737 generations. The larger engines could cause the plane's nose to tip upward, leading to a stall - in that situation, the system could automatically point the nose down to negate the effect of the engine size.

Since the grounding, other potential safety issues have been found in the plane, leading Boeing to make major changes to how its onboard flight computer functions.

Do you work for Boeing, or one of the airlines affected by the Boeing 737 Max grounding? Contact this reporter at dslotnick@businessinsider.com.