If air travel has been a part of your life, chances are one of the planes you've flown on has been made by American manufacturer Boeing.

Key points: Boeing's 737 MAX series was involved in two fatal crashes killing over 300 people

Boeing's 737 MAX series was involved in two fatal crashes killing over 300 people The issue appears to stem from an automated system that pilots were unable to override

The issue appears to stem from an automated system that pilots were unable to override Boeing is testing a software patch, but questions are being raised about certification

From the world's best-selling commercial jet, the 737, to the world's first jumbo jet, the 747, Boeing's aircraft have defined the modern aviation era and international travel for many around the globe.

But in recent months, the company has hit unprecedented turbulence after two separate air crashes involving its 737 MAX jets killed over 300 people within five months, which resulted in global suspensions of the fleet.

Each month the fleet continues to be suspended it's estimated that Boeing will lose between $US1.8 billion and $US2.5 billion in revenue (roughly $2.5–3.5 billion), according to analyst estimates, though it could recoup those amounts once the plane is back in the air.

The preliminary investigation reports into the Lion Air and Ethiopian Airlines disasters both noted that the pilots fought repeatedly against automated plane controls that pushed the plane down.

"This is quite spectacular given that it involves Boeing's most popular aircraft [series] in history," said Dr Chrystal Zhang, a senior lecturer and research director in aviation at Swinburne University.

"No one is immune from brand and reputation damage, especially after these fatal crashes … bad memories always last much longer."

She explained that the MAX series only makes up 2 per cent of the global fleet of narrow-body aircraft — the ones that fly between Australia's capital cities on a regular basis — but it may grow to 5 per cent.

It's unclear if the MAX will still reach that estimated market share, as some carriers are reviewing their orders while Garuda Indonesia has cancelled its order of 49 jets altogether, citing passenger fears.

But how did the MAX 737 slip through regulators? Who is to blame? And what lies ahead for the aviation giant?

From the start: Competitors spark race for better planes

Boeing's direct competitor, Airbus, spurred Boeing into action by announcing an efficient update to its A320 short-haul fleet. ( Flickr: Kevin Hackert )

To understand how Boeing has found itself in this predicament, it pays to get familiar with the story of the 737's direct competitor, the Airbus A320.

In 2010, the European manufacturer announced plans to create a new version of the aircraft, the A320neo (short for "new engine option"), which promised airlines greater fuel efficiency at a time when the Boeing 737 models in service — the Next Generation series launched in 1993 — could not compete.

Advanced manufacturers such as Boeing add to the US's industrial prestige. ( Flickr: US State Department )

So, a race quickly ensued after Airbus fired the starting gun.

At this point, Boeing found itself at a crossroads: make a new version of the 737 from scratch or retrofit the existing Next Generation series with newer technologies?

Boeing went with the latter, which meant that an existing 737 frame was fitted with larger, more fuel-efficient engines that altered its aerodynamics in a way that made it prone to tilt up during flight.

Boeing engineer and cockpit designer Rick Ludtke told the New York Times that the 737 MAX's designers were told they "could not drive any new training that required a simulator".

"They wanted the minimum change to simplify the training differences, minimum change to reduce costs, and to get it done quickly," he told the paper.

When contacted by the ABC about Mr Ludtke's claims, a Boeing representative said they "followed a [design] process that was absolutely consistent with introducing previous new airplanes and derivatives".

'High angle of attack': The suspected MCAS fault

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In the weeks after the Ethiopian Airlines crash, Boeing has come under fire from pilots who alleged that they were not told about an automated system, known as the Manoeuvring Characteristics Augmentation System (MCAS), when the plane was introduced.

This system is activated by sensors on the fuselage designed to automatically tilt the plane down if is detects the nose is pointing at too steep of an angle which risks a stall, known as an angle of attack.

A report from the Canadian Broadcasting Corporation (CBC) found that the system was only mentioned once in the aircraft manual, which was in the glossary, explaining the MCAS acronym — an omission Boeing did not deny to the CBC.

A Boeing spokesperson told the ABC that MCAS's function was referenced in the MAX's flight crew operations manual, where it outlined what the plane would do "in the rare event that the airplane reaches a high angle of attack".

They added that they had discussed MCAS's functionality with more than 60 airline operators since 2016.

But some want Boeing to do more, as others have pointed to the lack of backup MCAS sensors mounted on the 737 MAX's fuselage.

While the jet has two sensors, a report from the Seattle Times noted that "the plane may not be able to automatically determine which of the two readings is correct" in the event of one failing.

The Airbus A320 relies on three sensors as a fail-safe.

A former Boeing engineer told the Seattle Times that relying on one sensor in the event of another failing "is an absolute no-no".

'Weaknesses' in the certification process: Who's to blame?

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 40 seconds 40 s The acting US FAA chief explains why the 737 MAX fleet was grounded.

Subsequent reports have found that the US Federal Aviation Authority's (FAA) safety approvals of the 737 MAX were done by Boeing staff, and this proximity has given cause for concern about who is doing the regulating.

"It's not just the product that has caused the [reputational] damage at this stage, because people are now questioning the relationship between Boeing and the FAA," Dr Zhang said.

As part of the fallout, the European Union Aviation Safety Agency (EASA) and Canada's Transport Safety Board announced they would no longer heed the advice of the FAA, while China has set up a task force to review any further Boeing changes to the 737 MAX.

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At a US Senate hearing into the 737 MAX crisis, Calvin Scovel, the inspector-general of the US Department of Transportation, acknowledged that the FAA had "weaknesses" in the oversight of manufacturers.

"Clearly, confidence in the FAA as the gold standard for aviation has been shaken," Mr Scovel said.

In response, the FAA has built an international coalition to review the 737's MCAS system.

However, Dr Geoffrey Dell, a safety scientist and associate professor at Central Queensland University, said the close relationship between Boeing and the FAA is to be expected.

"There are no regulators on the planet who get the funding to have [a level of] expertise that is greater than [Boeing's]," he said.

"The regulators do not have the capability to have someone who can second-guess the entire design, or sit around to wait for another plane to be produced."

When asked by the ABC if Boeing believed it was reasonable for the company to review the 737 MAX's safety on behalf of the FAA, it said:

"The long-standing collaborative engagement between the FAA, Boeing, its customers and industry partners has created the safest transportation system in the world.

"The regulatory requirements defined by the FAA provide the requirements, policies and procedures which ensure that Boeing employees serving in this capacity act independently on behalf of the FAA when performing in this role."

Last month, acting administrator of the FAA Daniel Elwell told US senators that parts of the 737 MAX's safety approval was delegated to Boeing representatives.

The FAA echoed Boeing's sentiments when asked by the ABC about whether it was appropriate to delegate safety checks to an aircraft manufacturer.

"The FAA's aircraft certification processes are well-established and have consistently produced safe aircraft designs," a statement read.

"FAA has never allowed companies to police themselves or self-certify their aircraft.

"With strict FAA oversight, delegation extends the rigor of the FAA certification process to other recognised professionals, thereby multiplying the technical expertise focused on assuring an aircraft meets FAA standards."

Software fix: Temporary solution or permanent resolution?

Boeing's Seattle factory has been a home for modern aviation, with its first production lines making the 747. ( Flickr: Jetstar Airways )

At the time of writing, Boeing chief executive Dennis Muilenburg said the company had tested a software fix for the 737 MAX across 120 flights, and added that 85 per cent of carriers using the plane had tested the update in simulator sessions.

It remains unclear when the 737 MAX suspension will be lifted, but Boeing told the ABC that it was "confident in the safety of the 737 MAX and is continuing to take the necessary actions to ensure that going forward".

While questions will continue to be asked about why a manufacturer known for its admirable safety record let a crucial flaw through its design process, Dr Dell said more questions needed to be asked about how automated systems are tested.

"On the face of it, you could say that [Boeing] did not test [the 737 MAX] long enough, but then you get into a situation where you ask, 'How long is a piece of string'?," he told the ABC.

"It raises a question of how do you effectively test software to ensure there is not a rogue code or a programming anomaly that will only reveal itself under specific circumstances.

"So you could theoretically test for an infinite amount of time and still not have those [MCAS] circumstances pop up."