A while back I came across the electric taxi system currently being tested. The idea is that electric motors on the wheels move the airplane while on the ground, using electric power from the APU. I think it would be cooler if it harvested power from landing and stored it – but that would probably be the world’s biggest KERS system ever and probably be a huge fire risk. So running it off the APU is fine.
Anyone who knows me knows how I feel about
the stupidity of hybrid cars. I’m generally opposed to carrying around two drivelines, it’s usually a waste of weight and adds needless complexity. I’m fine with a full electric car, and I feel we can be a lot more efficient with our gas cars- but, at least historically, it hasn’t made much sense to have two systems powering the same wheels.
Slowly, racing and high-end sports cars have been able to make good use of hybrid technology, and that’s fine – done right, it can be a good thing – but there are many, many examples of hybrid done wrong out there which keep me from being a believer just yet. The day I see diesel-electric semi-trucks is the day I’ll know hybrid has really arrived.
So how do I feel about a hybrid airliner? At the end of the day, that’s what we’re talking about here.
First, it’s worth pointing out there are small full-electric airplanes that humans can actually ride in already. Full-electric city busses make a lot of sense, too, so a full electric AirBus would be – pretty much the worst idea ever.
This is one of the kinds of statements that kill good ideas – but if there was a more efficient and safer way to move people through the atmosphere than the turbofan jet engine, we would have found it already. Jet fuel is easy to handle, easy to load on an aircraft, and the engines themselves are bulletproof (though not always bird-proof). Using electricity to propel us through the sky would not be practical.
Using it to propel an airliner on the ground makes a lot of sense, though. High torque is required; electric motors can do that. A source of a lot of electricity is required; the already on-board APU, designed to provide electricity, can do that. This technology will save fuel, save engine wear, and make it less likely that something (or someone) gets sucked into a running engine on the ground. It will also save brakes, as taxiing with jet engines usually means riding the brake to keep speed in check.
It’s a good idea all-around.
And now we come to the lesson of the day.
Good solutions typically don’t just solve the problems they are intended to solve, they also typically create opportunities to solve problems which may have never been identified as problems until the solution came along. In this case: Airplanes can’t back up.
When using their engines to taxi on the ground, it’s impossible (or completely impractical) for an aircraft to “back up” – there is no reverse gear.
(Large jets have thrust reversers which could potentially make the aircraft move backwards, but they are very, very inefficient, and require the engines to be at a very high power setting to work – increasing the chance of sucking something into the motor. Additionally, reversers are even less efficient at low speed, due to the likelihood of the engine rebreathing exhaust gas. This is why reversers are only used right after touchdown- they get closed once the aircraft has slowed to a speed where they lose efficiency.)
Without a way to go backwards, airport design, ground traffic patterns, emergency situations, and tons of other things are compromised. It’s an additional constraint that’s simply been a “fact of life” until now.
Nobody has looked into making planes go backwards because it just didn’t seem easily possible. Instead, we have tow trucks, towing equipment, and pushback and startup procedures.
The startup no longer has to happen with the plane standing still. The options that opens is huge.
The joint venture which set out to test and prove electric taxi did so intending to save fuel and engine wear on the ground. That’s going to happen – but additionally, we’re going to end up saving time waiting for a pushback, saving time taxiing to get the right angle to pull into a gate, and be able to completely redesign how traffic moves on the ground and even our airport designs- because airplanes will be able to go backwards.
That’s the indicator of a good solution. It doesn’t only solve the problem, it solves other problems which may not have even been identified as problems- and opens doors to solve even more issues later.
When you’ve found a solution that does all that, it’s time to implement it.