Southwest Airlines said on Friday it is extending the cancellation of flights using 737Max jets from its schedule by another month as Boeing faces increasing hurdles in winning approval to return the plane to service before the end of this year following two fatal crashes.

Last month,Southwesthad extended the grounding of all 34737Max jets in its fleet to February 8. Southwest, the largest operator of the 737 Max 8 jets, said it was now canceling flights through March 6 because of "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."

United Airlines and American Airlines have canceled their 737 Max flights into January.

Reuters reported this week that US and European regulators will need to return to Iowa to complete an audit of Boeing's software documentation after regulators found gaps and substandard documents . Boeing has confirmed it must submit revised documentation.

That has thrown into question when Boeing can complete a certification test flight. The Federal Aviation Administration has said it would not unground the planes until 30 days after that flight occurs. Two US officials told Reuters it is extremely unlikely -- if not impossible -- that Boeing will be able to win approval to return flights to service before the end of December.

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.

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