David Cantrell wrote:

On Mon, Sep 23, 2019 at 04:45:38PM +0100, Someone Somewhere wrote:

But it seems obvious that the best solution would be some kind of
(presumably) electrical tug that could take a plane from the gate to the
point where it needs to switch to using its own engines for takeoff.

Given that the engines (I believe) turn the generators that provide
electrical power, that point is the point at which the plane is
disconnected from ground power. They need electricity to power the
radios that let them talk to the control tower, run air conditioning,
make announcements to passengers, and so on.

Commercial aircraft have an APU (auxiliary power unit) that provides power for
the things you mentioned when ground power is not available and the main engines
are not running. These are small jet engines located in the rear of the
fuselage.

A few years ago there was a big industry focus on reducing ground use of jet
fuel. The two main areas looked at were self propelled sysems (i.e. adding
electric motors to the landing gear) and battery powered robotic tugs that would
bring the aircraft to the runway.

The problem with both approaches was economic. In the case of the electric drive
motor, the cost of the system, reduction in overall reliability of the aircraft,
and most importantly the cost of additional fuel required to carry the weight of
the motor in flight far offset any ground fuel use savings.

The robot tug had similar economic issues, as well as the operational issues
already mentioned in this thread.