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Why do passenger aircraft not fly at the speed of fighter jets? What factors stop them from achieving such speed?

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There are 2 big reasons:

  1. Speed costs lots of money. Fighter jets can go much faster than passenger jets but they spend most of their time cruising as the same speeds because it is more fuel efficient. Faster airplanes need more exotic materials and would be more expensive to build. The engines would need to be bigger and more powerful and would cost more, and to go very fast uses a lot more fuel as air resistance increases exponentially with speed. Concorde tickets were very expensive not because of any sort of exclusivity, but because that's what it cost to operate it. Newer technology could probably decrease that somewhat, but it would still be very expensive. Air forces can operate fast jets because they have the funding, airlines have to make a profit
  2. Passenger jets fly pretty close to the speed of sound already, to go much faster they would need to break the sound barrier, which creates a very loud sonic boom that disturbs people on the ground and can break windows in some cases. Supersonic travel over land has largely been banned because of that, so unless you are over water you can't go any faster anyway
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Passenger airplanes do fly at the cruising speeds of most fighter jets. Most fighter jets cruise subsonically between Mach 0.8-0.9 (to extend range), which is a typical cruising speed of jet transport category airplanes:

  • A320: long range cruise 0.78M, high speed cruise 0.8M
  • B777: cruise speed 0.84M
  • Global 7500: long range cruise 0.85M, high speed cruise 0.9M

Any faster, you'd reach supersonic speeds, which is prohibited over continental territories of most countries due to sonic boom noise concerns. This is aside from the much higher fuel costs of cruising supersonically, and an often prohibitive costs of developing a supercruise capable airliner.

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