Why airplanes don’t fly in straight lines…

…and other information about airplane engine failures.

I’ve sometimes wondered why airplanes don’t seem to fly in straight lines. I once saw someone give a seemingly-simple explanation: the Earth is round. While that fact is true, it doesn’t really explain it. When I flew from New York to Hong Kong, “curvature of the Earth” doesn’t explain why we practically flew through the arctic circle. It was certainly not the most efficient path.

I suspect there are many components to the answer, and perhaps the earth’s curvature factors in a bit. I also suspect that weather and wind factor in. But there’s one big, glaring reason that I’ve found: a twin-engine airplane must, at all times, be able to reach an airport on a single engine within a certain period of time. Early on, the limit was 60 minutes, though that figure has gone up over time.

The idea was simple—with only two engines, if one fails, you want to be able to land pretty quickly. So the FAA set a limit of 60 minutes. This surely had all sorts of positive safety implications, but it was also inconvenient, and led to some circuitous routes, plus some areas just not being possible to reach in a twin-engine plane. Over time, apparently, evidence allowed these rules to be relaxed. For one, it turns out that a plane is capable of flying just fine on a single engine.

For example, here is a rather chilling video of an airplane (a Boeing 757) ingesting a bird into one of its engines during takeoff, causing the engine to spew flames until it is shut down:

It continues its takeoff normally, declares an emergency, and lands normally a few minutes later. If you ignore the flames during takeoff and the inspection by the fire department upon landing, it looks entirely normal. Also fascinating to me is how the pilot seems entirely calm during the whole situation, and how half of the radio traffic is just about how they’ll be able to exit the runway normally so other flights shouldn’t need to divert, and how they plan to taxi back.

While a video of an airplane engine spewing fire might not inspire a lot of confidence, what’s intriguing to me is that the plane flew just fine with only one engine operating. It didn’t begin flying sideways or have difficulty landing as I might have naively expected. Hence the initial justification of the 60-minute rule—if an engine failed mid-flight, pilots would be able to safely fly to the nearest airport with only one engine.

Over time, apparently, evidence showed that spontaneous failure of an engine mid-flight was extremely uncommon, and that the 60-minute limit was excessively conservative and made many flights impractical. Over time, allowances of up to four hours have been granted, and newer planes are being certified for times in excess of five hours. But many older planes are still limited by shorter times, hence the seemingly-odd routes they take—they need to stay within range of airports.

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