Correction: An earlier version of this story incorrectly said the speed of sound is related to air density; it is related to air temperature.
Over the past few months, I’ve read stories of commercial airliners that fly at supersonic speeds. However, the last passenger plane that could fly faster than the speed of sound was the Concorde.
That supersonic aircraft had a cruise speed of more than twice the speed of sound but was retired from service back in 2003.
However, on Dec. 11, a Boeing 777 airliner reported a groundspeed of 765 mph, faster than the speed of sound at sea level, on its flight from Hawaii to California.
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So what is the speed of sound? Well, the transmission of sound happens when air molecules in our atmosphere hit each other and allow the transfer of kinetic energy that we hear and sometimes feel as sound.
The speed of sound at sea level is approximately 761 mph, depending on air temperature.
Interesting to note, a rise in air temperature will decrease its density and will increase the speed of sound because of the air molecules’ greater elasticity.
It may seem counterintuitive, but humid air is lighter, or less dense, than dry air. Generally, the higher your altitude, the lower the air temperature and the slower the speed of sound. For example, the speed of sound is about 100 mph slower at 50,000 feet.
By the way, the speed of sound in water is about 3,140 mph. Sound travels much faster in water than air because of its greater density.
When an object moves faster than the speed of sound, it produces a shock wave that forms a cone of pressurized air that moves across Earth’s surface. In 1947, Chuck Yeager became the first person to fly faster than the speed of sound in the X-1 rocket-powered aircraft. He had the courage to break through the “wall” of intense pressure before achieving supersonic speeds.
So how can a present-day airliner fly faster than the speed of sound? Relatively speaking, it’s the winds.
Let me explain. The Boeing 777 that traveled from Hawaii experienced 220 mph tailwinds courtesy of the jet stream. In other words, its groundspeed was 765 mph, but its true airspeed, the speed at which it traveled through the air, was just 545 mph. If a passenger on this particular aircraft walked from the back of the plane to the front at about 3 mph, his or her groundspeed would be approximately 768 mph relative to Earth’s surface.
Today, commercial airline companies and governments pay meteorologists to track and forecast these upper-level winds. A good upper-level wind forecast can save fuel and drastically cut flight time.
I learned this firsthand while flying in a propeller-driven, P-3C Orion maritime patrol aircraft with VP-65 at Naval Air Station Point Mugu near Ventura. The primary mission of our squadron was to track submarines and surface vessels. To perform this mission, our squadron would sometimes deploy to Japan.
On our journey to Japan from California, we would fly the “great circle” to Alaska to refuel and then southwestward to Japan. You see, on a globe, the shortest distance between two points is a curved line. On our trip back to California, we would fly in an easterly direction, to take advantage of the upper-level winds that commonly flowed from west to east. Even though the distance was longer, the tailwinds made it a shorter flight. We would still have to get fuel in the Hawaii to make it home.
On one occurrence, we flew the entire distance from Japan to California in one single flight without the need to refuel because of the extraordinarily strong tailwinds; for a propeller driven aircraft, that was quite a feat.
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