How safe is to fly a Boeing 737?

Air travellers nervous about flying on a Boeing 737 after two crashes in close succession might take comfort in thinking of aeroplanes as herd animals: There is some safety in numbers.

Every hour of every day, thousands of 737s have the opportunity to prove the old cliché that anything that can go wrong will go wrong. But each time a flaw is found in a 737, there is a corresponding opportunity to correct it in all planes — because a cornerstone of aviation safety is that you don’t ever make the same mistake twice. Or hardly ever.

In October, a Lion Air jet crashed into the Java Sea off Indonesia, killing all 189 people aboard. And on Sunday, an Ethiopian Airlines jet crashed en route from Ethiopia to Kenya, killing all 157 aboard. Both planes were the Boeing 737 MAX 8 model. Both planes crashed not long after take-off. A sensor and software problem are believed to have contributed to the Lion Air crash, although a final cause has not been determined. The Ethiopian Airlines crash investigation is in its early stages. Most countries, including the United States, have grounded the MAX 8s in their fleets.

The 737 is the most popular airline model in aviation history. So many 737s traverse the skies — Boeing has built more than 10,000 over 50 years, with more than 5,000 still in service — that opportunities to learn are ample. When something goes wrong with one, the news spreads fast.

For example, 737s have fuel tanks in the wings. A conduit typically runs through the tank, with an electric cable inside that carries current to a fuel pump. In April 1998, a mechanic found fuel leaking into the conduit of a Continental Airlines 737. Sparks from the electric cable had burned a hole in the conduit. Once you have fuel and a spark, all you need is oxygen to have a catastrophe. In nothing flat, the Federal Aviation Administration ordered inspections — immediately for older planes and promptly for younger ones. Sure enough, other aircraft showed signs of wear on the same conduit. The flaw was corrected. No accident resulted from it.

The system isn’t perfect. In 1991, a 737 crashed near Colorado Springs. At first, investigators attributed the crash to an extraordinary meteorological phenomenon. But another 737 crashed on approach to Pittsburgh in 1994, and after a lengthy investigation, the National Transportation Safety Board diagnosed a jam in a hydraulic system controlling the rudder. And that was the probable cause of the Colorado Springs crash, too, the board said.

The problem was exceedingly rare; at the time it was discovered, the 737 had completed about 77 million flights. It took two crashes (and several close calls) to find it.

At this point, “737” is a bit of a catchall, because the plane has evolved over the decades, with a different wing, different engines and, now, different software. In the 1990s, when Boeing introduced new models, it called them “next generation” and renamed the earlier ones “classic.” The “next generation” has advanced to become today’s MAX series planes.

Answers are at hand for the two MAX crashes. Both planes had highly capable digital flight data recorders, as well as cockpit voice recorders. Until the Ethiopian Airlines flight’s digital data is read out and interpreted, it won’t be clear to what degree the crashes were related. Grounding the planes has a downside, and the case for doing so was by no means crystal clear. It’s certainly not as straightforward as it was in 2013 when the FAA grounded the then new Boeing 787 after battery problems appeared in two planes. In 1979, when the FAA grounded DC-10s after several crashes, it turned out that the crash that prompted the action wasn’t related to the earlier crashes, or even to the design of the plane.

That is most likely why the Federal Aviation Administration did not lead the way in grounding this plane.

When the dual investigations determine the circumstances of the Lion Air and Ethiopian Airlines crashes, the time-tested aviation safety system will work to eliminate the cause.

Matthew Wald is a transportation safety specialist. He writes about technology and safety.