The 737 MAX is hardly the first case of a commercial aircraft launch or redesign that has run into serious problems. The debut of Boeing’s own 787 Dreamliner, starting in 2011, was marred by everything from faulty supply chains and production delays to onboard fires from overheating batteries — and a grounding for more than three months.

But you have to go back many decades to find the introduction of a commercial aircraft that has involved such a staggering loss of life, or has raised such uncertainties.

“I can’t think of a time since perhaps the 1970s or even the 1960s, when you had two examples of a brand-new aircraft crashing within six months of each other, within the first couple of years of service,” said Todd Curtis, a Seattle-based aviation safety expert who tracks aircraft safety data on the website AirSafe.com.

And although the 737 MAX’s relatively short lifespan — it was first delivered in May 2017 — makes statistical comparisons difficult, at this point the 737 MAX has a crash rate that is roughly 14 times as high as the 737 series as a whole when measured by number of fatal crashes per million flights, according to Curtis’ calculations.

Aircraft introductions and major redesigns are almost never problem-free, said Jim Hall, former chairman of the National Transportation Safety Board, which investigates aircraft accidents. In any new or redesigned aircraft, Hall said, “there may be bugs or things that were engineered that, in actual service, need tweaking and need adjustment.”

What distinguishes the debut of the 737 MAX is not only the sheer magnitude of the tragedies — a total of 346 lives lost on Ethiopian Airlines Flight 302 last Sunday and the Lion Air Flight 610 in October. As noteworthy is the fact that these two successive disasters occurred in what had been one of the safest eras in commercial aviation safety, when decades of work by manufacturers and regulators alike had succeeded in dramatically lowering the risks of serious crashes.

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Investigators don’t yet know exactly what factors led to the crashes of either Ethiopian Airlines Flight 302 last Sunday or the Lion Air Flight 610 in October or to what extent the accidents are related. In grounding the aircraft Wednesday, the Federal Aviation Administration said the two flights “behaved very similarly,” raising “the possibility of a shared cause for the two incidents that needs to be better understood and addressed.” Boeing plans to roll out a software fix next month intended to solve the flight-control problem linked to the Lion Air crash.

Nonetheless, the events have raised daunting questions about an aircraft that is expected to be a major best-seller for Boeing and the next generation of a workhorse jet for the airline sector.

The twin crashes have also invited unwelcome comparisons to commercial aviation disasters from earlier periods, when the industry’s ability to build aircraft — and to understand why some of them failed — was orders of magnitude below what it is today.

The classic case is the Comet, the world’s first jet airliner. Launched in 1952 by British aircraft firm de Havilland, the 36-passenger Comet enjoyed massive commercial and cultural success for barely a year before a series of high-profile fatal crashes.

In 1953, all 43 passengers and crew aboard a British Overseas Airways Corp. Comet died when the aircraft broke into pieces shortly after departing from Calcutta. Eight months later, a second Comet disintegrated after leaving Rome, killing all 35 passengers and crew.

BOAC grounded its remaining Comets, but the flights soon resumed after government investigators were unable to identify any structural or mechanical defects. Four months later, a third Comet exploded in mid-flight, killing all 21 aboard. A second inquiry determined that the hull structure had weakened after repeated pressurization and depressurization and that the doomed planes had simply exploded as the pilots pressurized the ascending aircraft.

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De Havilland scrambled to fix the design flaws in later Comet versions, but the aircraft never recovered commercially and was swept aside in 1958 by the world’s second, and much more successful, commercial jet — the Boeing 707.

But the Comet’s problems were hardly unique. Indeed, although the 7o7 would dominate commercial aviation through the 1960s and seal Boeing’s reputation as the leading aircraft maker, the 707 itself suffered an abysmal safety record, especially in its early years.

Between 1958 and 1966, the 707 recorded 12 major accidents that killed 1,061 passengers and crew –including in one instance, the entire U.S. figure skating team. In a single year — 1962 — four 707 crashes left 435 dead.

Yet because commercial jet aviation was still in its infancy, and because there was far less media coverage of accidents back then, the public’s tolerance for risk was far higher, Curtis said.

Still, manufacturers such as Boeing, along with regulators, airport operators and other members of the aviation sector made huge gains in aviation safety. Between 1959 and 2017, the number of fatal accidents per million flight departures on U.S. and Canadian airlines fell from 40 to zero, according to statistics compiled by Boeing.

Even by 1979, the year of the deadliest airline accident on U.S. soil — the crash of American Airlines Flight 191, which killed all 271 passengers and crew aboard a McDonnell Douglas DC-10 lifting off from O’Hare International Airport in Chicago — flying had become remarkably safe on a statistical basis.

This is the scene of American Airlines Flight 191 which crashed northwest of Chicago’s O’Hare Airport, May 26, 1979.  The DC-10, en route to Los Angeles, carried 258 passengers and 13 crew members, all of whom perished.   (Larry Stoddard / AP)
This is the scene of American Airlines Flight 191 which crashed northwest of Chicago’s O’Hare Airport, May 26, 1979. The DC-10, en route to Los Angeles, carried 258 passengers and 13 crew members, all of whom perished. (Larry Stoddard / AP)

The last crash involving major fatalities on a U.S. airline came in 2009, when the crash of a Colgan Air commuter aircraft near Buffalo, New York, killed 49 passengers and crew and one person on the ground.

That dramatic improvement is one reason that more recent crashes, including the two 737 MAX disasters, are so shocking. In the modern era, there are major commercial aircraft, such as the Boeing 787 or the Airbus A380 and A340, where there have been “no fatal events, not even of a single passenger,” said Curtis. “And [then] you have two planeloads going down with the 737 MAX.”

Still, Curtis warns that it’s far too soon to label the 737 MAX as an unusually dangerous aircraft. The model has racked up less than half a million flights, Curtis estimates, which is a fraction of what is necessary to determine whether an actual pattern exists.

Aviation experts also say that, until the Lion Air crash last year, the 737 MAX had been marred by remarkably few performance problems, indicating that whatever factors led to the crash may not be deep-rooted and systemic.

“Frankly, up until the Lion Air disaster, the MAX was going really smoothly,” said Richard Aboulafia, an aviation industry expert with Teal Group. “Its first year, year and a half was just fine.”

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As important, while Boeing has dragged its feet in acknowledging some technical issues in past crashes, such as 737 rudder problems that led to the 1991 crash of United Airlines Flight 585 in Colorado Springs, the company has been much more responsive to recent problems. The speed with which Boeing found a fix for the Dreamliner battery issues, and then scaled it up across the fleet, shows how effective Boeing can be in fixing problems, said Aboulafia.

That record, coupled with Boeing’s vast financial and technical resources — and substantial motivation to get its cash cow back in the air — make many experts confident that the company will weather this newest setback. As Aboulafia puts it, Boeing has “resilience in scale.”

Still, given the peculiarities of the crashes, some experts warn against assuming that the fix will be straightforward.

“You are talking about events that should be very low probability,” said Adam Pilarski, an aviation consultant and former chief economist with Douglas Aircraft. “And if two events of very low probability happen, that raises flags.”

Boeing 737 MAX