Is it feasible to replace brakes on airplanes with electric motor/generator?
On a hybrid or all-electric car, the motors on the wheels supplement or replace the engine, and by regenerating electricity from the braking, it extends the range of the battery.

On an aircraft, the engines would still be needed for the major part of the working life of the aircraft -- the time while it is in flight.  So unlike the case of a car, you don't get any advantage with the motors themselves by adding the motor/generators to the wheels.

Beyond this, on a large aircraft, electricity is in ample supply when you have jet engines turning, so what's gained by regenerating the electricity from the wheels, during the 60-90 seconds of heavy braking on landing, isn't all that significant when compared to the electricity used and produced over the course of a whole flight.  Not nearly as significant as a drive in the city with many cycles of accellerating to speed then decelerating to a stop, over and over.

Beyond this, the major players then are the added weight and complexity of the motor/generators, vs the gains in terms of (a) tire life, from spinning them up prior to landing, and (b) fuel saved by maneuvering on the ground from the wheel motors instead of the jet engines.  When you consider that the costs of jet fuel are on the order of $1000 to multi-thousands of dollars per flight hour, the costs savings of longer tire life will lose out if the extra weight of the motor generators over conventional brakes means that you're spending more money over the course of the flight to carry the extra weight.  Likewise, the fuel burn during taxi in &amp; taxi our on the motors isn't all that great compared to an hour or many of high-power cruise... power settings for taxi are close to or at idle, so fuel consumption isn't all that high.  Plus, it's common to taxi without all engines running, further reducing fuel burn on the ground.

The big killer for this idea is the weight penalty.  Motor generators are significantly heavier than conventional brakes, and when you consider the amount of energy that brakes have to be able to absorb during a high-speed stop, you'd need some seriously beefy generators to accomplish what aircraft brakes can do.  Or else, you'd have conventional brakes installed to supplement the generators, and the weight goes up further.

As a ballpark, on an hour flight in a 737, an extra 1000 lbs of weight increases the fuel burn by about 10 lbs, or about 1.5 gallons.  Doesn't sound like much, but when you multiply the added weight times every flight hour that the aircraft is operated over the course of a year, it really adds up.  Beyond that, motor/generators are a lot more complex than simple brakes, so they'd be more expensive initially and require more maintenance throughout their service life.  Plus, I suspect, they'd need to be supplemented by conventional brakes that would be used in the event of a high-speed max-effort stop (rejected takeoff, landing on a short runway).

The end result of all of this is, no manufacturer I'm aware of has found that the tradeoffs make this sort of project worthwhile.

The best case is probably one of really long taxi times, but in that case a pushback tug could tow the aircraft to a point close to the runway, saving the fuel burn on the aircraft engines until it was time to start them.  I'm sure this has been considered as well, and again, I'm not aware of any carrier that uses this approach (although outside the U.S., somebody might be).

It's an interesting question, but the costs &amp; the math behind the engineering haven't worked out in its favor, at least thus far.