Airplanes already use "regenerative braking". When you're coming in to land the airspeed and altitude is used to move the aircraft forward, with reduced throttl

Airplanes already use "regenerative braking". When you're coming in to land the airspeed and altitude is used to move the aircraft forward, with reduced throttle on the engine. In fact it could be reduced to exactly idle, but that's just not the optimal setting, and in the final phase of the flight also not the safest setting (spooling up time of a turbojet engine depends a lot on the initial rpm, in case you need to go-around). There's no need for a capacitor when you have gravity and altitude.