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Realizing that there are other issues with which to contend, why not just start slowing down airline flights?

Drag, and therefore fuel used, is a function of velocity squared. https://en.wikipedia.org/wiki/Drag_(physics)#Aerodynamics

Slowing down from 500 to 400 knots adds a half hour to a 1000 nm flight, but could provide maybe 25-30% fuel savings?

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Jet aircraft typically already fly at their most fuel efficient speeds rather than their max speeds

No, they typically fly at the most profitable speeds within the constraints of the flight and airspace parameters. Which results designing the planes to be efficient at those speeds.

https://www.boeing.com/commercial/aeromagazine/articles/qtr_...

It's more complex than that. There factors other than drag. Planes wouldn't be more efficient if they simply used roads at 60mph all the way to the destination (no takeoff)! Fuel is expensive so they do a lot of work to figure out the optimal speed.

Use roads at 60mph? I guess they're most efficient if they don't start the engines at all and you put the plane on a boat or a train.

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