Passengers on United’s Flight 1175 from San Francisco to Honolulu were understandably concerned this week as one of the 777’s two engines started to fall apart when the plane was still over the open ocean, more than half an hour from landing.

But they didn’t really have to worry, thanks to ETOPS.

Several decades ago, twin-engine jets (like Boeing’s 777) were not allowed to fly the 2,500 miles between the U.S. mainland and Hawaii – or on other long, over-ocean routes. Those routes required jets with three or four engines. The Federal Aviation Administration limited two-engine jet aircraft to routes where they were never more than 60 minutes from the nearest airport. All that changed with the advent of ETOPS.

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The FAA has detailed regulations around ETOPS, which stands for Extended-range Twin-engine Operations.  Those rules dictate how far two-engine jets are allowed to be from the nearest airport, in case they lose an engine and have to make an emergency landing.  The FAA calculates how long it thinks a damaged aircraft can safely fly on one engine before having to land.

The original rule imposed a 60-minute limit on twin-engine jets – the route could never take the plane to a point more than 60 minutes from the nearest airport where it could safely land (i.e., an airport with a runway large enough to accommodate the plane). That’s why several decades back, you only saw three or four-engine jets like the 747, DC-10, MD-11 or L-1011 on trans-oceanic routes.

But as new technology made aircraft engines more powerful, efficient and reliable, the rules were gradually changed to allow twin-engine jets to fly routes that took them farther away from any airports. The 60-minute limit was changed to 180 minutes in 1988, and gradually increased as new aircraft types and engine models were introduced.

For example, depending on the specific model and engine type, Boeing’s 777s and 787s are ETOPS rated at up to 330 minutes (5-1/2 hours), and the new Airbus A350 at 370 minutes (which means it can be up to 2,800 miles from the nearest airport and still land safely if it loses one of its two engines).

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Part of an engine was lost mid-air on a United flight from Hawaii to San Francisco Tuesday afternoon. Passengers showed what it looked like from on board the plane.

Media: Courtesy Aaron Ebert

That’s why twin-engine jets have gradually replaced four-engine planes on most international routes: The newest wide-body twin-engine jets are cleared to fly almost any route on earth, except maybe over Antarctica.

Here's a good video that gives a clear and simple explanation of ETOPS.

Getting FAA’s ETOPS certification for a new aircraft type is not a simple matter, however. Besides subjecting the aircraft to extensive testing, an airline has to show it has everything in place to handle a lost-engine emergency, from properly trained staff and mechanics to plans for accommodating and evacuating passengers from a diversionary airport in case the plane has to land there.

That’s why when Southwest Airlines announced plans last fall to begin flying to Hawaii with its next-generation Boeing 737 MAX 8s, it couldn’t say exactly when the service would begin because it knew it has to go through a months-long ETOPS approval process. Virgin America had to go through a similarly rigorous process before it launched its Airbus A321 flights to Hawaii in 2015.

Related:  Review-Virgin America SFO-Honolulu nonstop A321

So, thanks to ETOPS, those United passengers, who were only half an hour from landing when the engine cover fell off, were pretty safe-- but a full investigation will tell the whole story. The 777 on that flight was said to be almost 24 years old, one of the oldest in United’s fleet.

The question for United now has to be: Why did the cowling on that plane’s Number 2 engine come off – and why didn’t the other one?

UPDATE: Listen to pilots and Air Traffic Control calmly converse as UA1175 lands at Honolulu

What do you think about flying twin-engine jets over oceans? Please leave your comments below.

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