What you need to know about the Boeing 737 Max 8 and why it's still grounded

The CEO of Boeing begins two days of testimony before U.S. Congress on Tuesday, the one-year anniversary of the first deadly crash of a Boeing 737 Max 8.

Dennis Muilenburg, who was stripped of his Boeing chair title earlier this month, is expected to acknowledge that the aircraft manufacturer made mistakes in the wake of two crashes in 2018 and 2019 involving the aircraft, whichkilled a total of 346 people.

"We have learned and are still learning from these accidents," Muilenburg said in comments prepared for delivery Tuesday to a Senate committee. "We know we made mistakes and got some things wrong. We own that, and we are fixing them."

Boeing is also facing a criminal probe by the U.S. Justice Department and is being sued by the families of some of the people who died in the crashes.

Here's what you need to know as Muilenburg speaks to the U.S. Senate commercecommitteeTuesday and the U.S. House transportation and infrastructurecommittee Wednesday.

The crashes

On Oct. 29, 2018, Lion Air Flight 610 crashed into the Java Seaminutes after takeoff from the Indonesian capital of Jakarta. All 189 passengers and crew on board died.

Flight data recordings suggestedthe pilots struggled to maintain control of the jet as its automatic safety system repeatedly pushed down the plane's nose.

Less than five months later, on March 10, 2019, another Boeing 737 Max 8 crashed, this one operated by Ethiopian Airlines. It also went down shortly after takeoff, this time from Addis Ababa.

All 157 people on board died, including 18 Canadians.

Grounded

The second crash raised serious concerns about the safety of the jet, and airlines and aviation administrations began grounding the Boeing 737 Max.

Ethiopian Airlines grounded its entire 737 Max fleet on the day of that crash. China's aviation authority ordered all jets in China grounded the next day.

Also on March 11, the U.S. Federal Aviation Administration (FAA) issued a Continued Airworthiness Notification to the International Community.

The U.K., the European Union, Australia and other countries moved quickly to ban the jet from their airspace.

Canada, one of the final holdouts, acted on March 13, grounding all of its Boeing 737 Max 8 aircraft and banning the jet from entering its airspace until further notice.

Investigations and reports

Early investigations into the two crashes focused on the jet's anti-stall systemthat kicked in and pushed down the noses of the planes. Boeing has acknowledged that both crashes involved the repeated activation of the Manoeuvring Characteristics Augmentation System (MCAS) after the safety system received faulty sensor input.

Indonesian investigators issued their final report into the Lion Air crash last week. It found that Boeing, acting with inadequate oversight from U.S. regulators, failed to grasp the risks of the design of the 737 Max cockpit software.

And it concluded that the pilots had not been trained in how to respond to the jet's automated flight-control system.

CBC News found that theBoeing 737 Max 8 manual only mentions MCAS once:

A month earlier, the U.S. National Transportation Safety Board concluded that Boeing underestimated how long it takes pilots to diagnose and react down to precise seconds when they are being bombarded by multiple, cascading warning alerts. It said that in both crashes,the pilots' control columns would have shaken to warn of an impending stall, and that the pilots would have received several visual and audio alerts about things like altitude and speed.

Ethiopian investigators released preliminary findings into the Ethiopian Airlines crash in April. It said the crew of the doomed jet did everything Boeing recommended when their plane started to nosedive, but that they could not save it. Theirfinal report is expected next year.

Boeing self-regulation

Earlier this year, severalU.S. media outlets investigated whether the FAA allowed Boeing too much power to self-regulate. In March, the acting head of the FAA was grilled by a U.S. Senate panel on the subject.

Daniel Elwell told senators it would cost $1.8 billion US and take 10,000 new employees for the U.S. aviation regulator to handle all aircraft certification internally.

He also said the FAA "is directly involved in the testing and certification of new and novel features and technologies."

Part of what U.S. Congress is currently looking into is whether there should bechanges to how the FAA delegates some certification tasks to manufacturers for new airplanes.

In the 737 Max certification, it initially delegated 40 per centof the work to Boeing and later shifted more work to Boeing, including work on the MCAS.

Last week, the U.S. Department of Transportationissued a report into the practice following its owninvestigation. It said the FAA faces a "significant oversight challenge" to ensure that companies conducting those tasks "maintain high standards and comply with FAA safety regulations."

Returning to flight

The Boeing 737 Max has not flown commercially since the March crash.

Air Canada recently updated its plans for the jet, saying it will remain grounded until at least February 2020. Sunwing and WestJethave also confirmed they will not fly the jet before next year.

U.S. airlines have cancelled 737 Max flights into January and February.

The FAA has said it will not return the 737 Max to serviceuntil the regulator has determined the jetto be safe.