^{A TWA Boeing 707}

Consider these two facts:

• In 1957, the first Boeing 707 took flight. It had a cruising speed of 600 mph (Mach 0.78).

• In 2009, Boeing unveiled its 787 Dreamliner. It has a cruising speed of 650 mph (Mach 0.85).

Wow, two planes released by the same manufacturer 52 years apart, and the new one is only 8% faster than the old one? Doesn’t it seem strange that technological advancements haven’t moved the needle on the speed of commercial airliners more?

The reason why commercial airplanes aren’t getting faster is due to a combination of physics and economics.

Reason One: Physics

On Oct. 14, 1947, when Chuck Yeager broke the sound barrier in the rocket-powered Bell X-1 over the Mojave Desert. The speed of sound is 767 mph at sea level at 68 degrees Fahrenheit. Crossing the “sound barrier” (also known as Mach 1) is the point at which an aircraft equals and then surpasses the speed of sound. As an aircraft approaches the barrier, increases in drag and other effects make it difficult to continue accelerating. Flying below, at, and above the sound barrier are well summarized as follows:

1) For an airplane moving at subsonic speed, sound waves (denser and less dense phases of air) get to escape. No problems.

2) For a plane moving at the speed of sound, the sound waves no longer get to escape but form a dense ‘wall’ of air. Energy consumption increases dramatically.

3) When an airplane travels at supersonic speed the drag is immense, and so is energy consumption. And noise. Source.

Importantly, when a plane breaks the sound barrier, a sonic boom is generated (and continues to be generated). Here’s the Air Force’s explanation of why a sonic boom occurs:

An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship’s bow. When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves that travel forward from the generation or “release” point.

As an aircraft flies at supersonic speeds it is continually generating shock waves, dropping sonic boom along its flight path, similar to someone dropping objects from a moving vehicle.

Sonic booms are a problem because they are loud and can be damaging (breaking windows, knocking items off shelves, etc.).

Here’s a photo of a military jet breaking the sound barrier.

Because of the sonic booms created by flying faster than the sound barrier, the FAA prohibits supersonic civilian aircraft from flying over land. Thus, even if a commercial airliner could fly faster than the speed of sound, they are prohibited from doing so in the continental United States.

Reason Two: Economics

The only effective supersonic passenger jet was the Concorde. It could fly at Mach 2.02 (about 1,340 mph), which meant a flight from NYC to London took just about 3 1/2 hours. Wow. Only 14 Concordes were placed in service, and flew only two routes for two airlines (British Airways and Air France). The first commercial flight of the Concorde was in 1976, and the last flight was in 2003.

While the Concorde effectively whisked passengers across the pond in record time, it was an economic bust. Why? Two main reasons:

• To fly supersonically, the plane needed to be highly aerodynamic which meant that it needed to be narrow, thus limiting the number of passengers. The Concorde could only hold about 100 passengers (compare that to the 330 that a 787-10 will hold). Plus, it was noisy and cramped. And,
• Operating costs were huge. To fly supersonically required a great deal of fuel.

High operating costs spread over relatively few passengers isn’t a viable economic model and meant that flying on the Concorde was super expensive: By the end of the 1990s, it cost $6,000 to fly one-way on the Concorde — a huge premium to what a ticket on a regular passenger jet cost. The Concorde was retired by British Airways and Air France because they had trouble getting enough passengers to pay the premium to fly supersonically.

Airlines are driven by profits, not by how fast they can fly and flying fast is expensive. As such, airplane technology has focused on increasing efficiency, not on increasing speed. “Airliners fly at the so-called “sweet spot” speed. They fly at the speed which is fast enough to be fast, but where the drag (and hence the fuel consumption) does not impair the revenue. The speed at which they fly is the optimum between performance and revenue. This speed was attained already in the 1960s.” Source.

Even though the 707 and 787 have similar cruising speeds, the 787 is a much better plane from a cost perspective as the 707 consumed about 6,800 kilograms of fuel per hour, and the 787 only consumes about 5,000 KG per hour even though the 787 carries about 140 additional passengers. Modern planes focus on getting the most passengers to their destination for the least amount of fuel and overhead costs as possible.