When the Boeing (NYSE:BA) 737 MAX was first grounded back in March, it seemed likely to resume commercial service within a few months -- possibly in time for the peak summer season. However, since then, the timeline for getting the aircraft recertified has slipped multiple times. Some airlines have removed the 737 MAX from their schedules until early 2020.

Fortunately, the schedule for recertifying the 737 MAX has been fairly stable for the past two months. Boeing remains on track to get the Federal Aviation Administration's blessing at some point in October, based on recent reports. That would enable airlines to start putting their Boeing 737 MAX fleets back in service before the end of 2019.

Why the 737 MAX's return has been delayed repeatedly

Initially, industry analysts had good reason to believe that the Boeing 737 MAX grounding would be short-lived. After all, at the time of the grounding order, Boeing had already been working for months on a software update to address the risk of an erroneous activation of the "MCAS" system that played a key role in two fatal crashes over the past year.

By April 11, Boeing had already completed 96 test flights using the updated software, totaling more than 159 hours of flying time. Even as of May 23, FAA officials thought that the grounding order could be lifted as early as late June, according to the New York Post.

However, in the months after the second Boeing 737 MAX crash, the entire FAA certification process became the subject of intense scrutiny. Rather than just requiring Boeing to fix the flaws that led to the two 737 MAX crashes, regulators went back to the drawing board in an attempt to identify any other potential risks in the aircraft's design. This included examining scenarios deemed extremely unlikely to occur. Boeing engineers have had to respond to hundreds of questions, slowing the process of finalizing the software update.

A rendering of a Boeing 737 MAX 9 flying over clouds.

The Boeing 737 MAX has undergone an extensive safety review in recent months. Image source: Boeing.

The FAA identified at least one potential safety issue unrelated to the MCAS system. In June, test pilots triggered a problem with a key microprocessor during a simulator session, causing a severe slowdown in data-processing speeds that made it hard to regain control of the aircraft. This forced Boeing to design a new software update to address this scenario, adding months to the timeline for recertifying the 737 MAX.

Boeing is finally getting close to the finish line

The good news for Boeing shareholders and 737 MAX customers is that no significant new issues have been identified since June. Furthermore, the company is still on schedule to submit its proposed software update to the FAA next month. An FAA panel is also expected to issue new guidelines for Boeing 737 MAX pilot training in early September.

All of these factors are consistent with Boeing's current goal of getting the 737 MAX back in the air by early in the fourth quarter. The FAA is likely to schedule an official test flight for early October, according to Bloomberg. This would put the regulator on track to recertify the jet by the end of that month.

Of course, the FAA has consistently said that it will only recertify the Boeing 737 MAX when it is convinced of the model's safety, so this timeline isn't set in stone. Additionally, international regulators aren't bound to follow the FAA's lead.

The next two months will be critical

If all goes well, the Boeing 737 MAX will be cleared to fly in the U.S. and most other major markets by the end of October. Boeing's ability to stick to this timeline -- roughly speaking, anyway -- will be critical to shoring up the investment case for the U.S. aerospace giant.

As long as the 737 MAX stays grounded, Boeing will continue to accrue liabilities, due to the need to compensate customers for their losses. Boeing is also relying on a return to service this fall so it can begin the process of delivering the hundreds of 737 MAX jets it has built and stored since March. Until it starts to deliver those jets, free cash flow is likely to remain in negative territory, further straining Boeing's balance sheet. Indeed, if the 737 MAX's return to service is delayed much longer, Boeing may have to pause production, which could cause irreversible damage to its supply chain.

Furthermore, December has become a huge month for aircraft orders in recent years, but it would be hard for Boeing to win new firm orders for the 737 MAX if the planes were still out of service then. Conversely, if the Boeing 737 MAX is cleared to fly again in October, the company will have an opportunity to assuage investors' remaining concerns with a big year-end order haul.