As an engineer, I spend my days buried deep inside aircraft maintenance manuals writing courses to train mechanics on business jet maintenance. I’ve read a lot of the answers, but none of them have convinced me completely. Let me just address some of the recurring themes:
“The pilots don’t like it because they won’t know when the plane has touched down
“: Planes are already fitted with WoW (weight-on-wheels) switches, that trigger when the
landing gear shocks are compressed by the weight of the plane. They’re used by many of the avionics onboard to switch them between “flight” mode and “ground” mode (for example, on some aircraft types the WoW signal triggers the ground spoilers upon landing and kills the auto-throttle). Adding a cockpit indication when the aircraft is on the ground would be a trivial matter. You could add an indication on the PFD (primary flight display) or an aural alert that speaks the word “touchdown” to the pilot when it happens. The hardware is already there; what’s another indication to add to the other 500 already there, or another aural alert to add to the existing 50?
“The gyroscopic forces affect the handling of the plane“: Two issues here. First of all, while it’s true that spinning wheels generate gyroscopic forces, the weight of the wheels is so small relative to the weight of the plane that I doubt gyroscopic precession is going to make much of a difference. Remember you have two or more engines weighing multiple tonnes onboard, and their high-pressure turbines and compressors are spinning at 15,000 RPM. No one complains of precession effects from those, so what difference are a few wheels at 1000rpm going to make? Secondly, even if they did pose a precession problem, most modern airliners use fly-by-wire (with the exception of the Boeing 737). Since that uses data from the IRS (Inertial Reference System) and other sensors to determine what the plane is actually doing, then makes the appropriate control surface motions to get the plane to do what the pilot is commanding it to, the existing system, as fitted to current planes, will automatically compensate for any gyroscopic effects of the spun wheels without any software changes.
“You’ll need lots of power to spin up the wheels“: Nope. You only need the system to operate briefly before landing (short duty cycle), and it only needs to accelerate a few hundred kilos (weight of rim and tyre) to ~1000rpm, which isn’t that big a deal. So you can use small components and work them hard (but briefly). Fortunately, there’s already component on every plane that does something very similar. Presenting… the ATS (Air Turbine Starter):
The ATS is the unit that I’ve indicated with a red circle.
Here’s what it looks like from the other end:
It is responsible for starting the entire engine you see in the picture (in this case it’s a CFM56 – used on Boeing 737s and Airbus A320s). It’s a small turbine that uses compressed air from the APU (auxiliary power unit), a ground start cart, or a running engine (if the plane has one already started) to spin up the engine during startup. That little thing accelerates the entire high pressure (N2) spool of the engine (weighing several tons) from zero to over 7,000 rpm in about 30 seconds. The downside is that you can only use it for about 5 minutes at a time before it needs to cool down (plenty of time to start an engine though). Looking at the size of the starter relative to the engine, I reckon a proportionately scaled-down turbine, sized for spinning just a wheel and tyre, would weigh not more than 20 kilos. A hydraulic or electric solution would be similar in size and weight.
“What if it breaks? Then the plane would be grounded and the airline would lose money!“: Nope. Your plane has what’s called an MEL (Minimum Equipment List). It’s a list of items that have to be working or you can’t legally fly. Since this item is optional (you can still land fine with wheels not pre-spun up), and it would be coupled via a one-way clutch so the wheels can still spin if it jams (same protection fitted to current ATS units), then it won’t be an MEL item; i.e. planes are free to take off and land normally if it’s broken.
I suspect what’s really holding things back is steep certification requirements. You need time and lots and lots of money to prove to the authorities that something is safe enough for use on an aircraft. It’s why gasoline piston engines for aircraft are still based on 1960s designs… and why the 787 uses lithium-cobalt batteries, when safer chemestries have been invented since it was certified. It’s just too expensive to certify these things and there isn’t enough return on investment to cover that cost. Ironically it sometimes leads to a worse-performing and less safe aircraft, but it’s the best system humanity could develop in 110 years of aviation.
There are some advantages of the wheel-spinning-up system no one else seems to have mentioned. Imagine you make the motors big enough to propel the plane at slow speeds (0-30mph). And you make the APU generator a little bigger. You now have a plane that can taxi without starting the engines.
Imagine that – the passengers board the plane (with the APU already started up, to supply the air conditioning), the pilot “drives” it to the runway using electric power alone (engines still off, APU supplying the electricity for the motors in the wheels), then starts the engines while waiting near the runway. You’ve just saved a good 20 minutes of fuel burn and wear & tear on the main engines, and all it cost you was a few wheel motors and a larger APU (which planes like the 787 already have). So I think it’s not a matter of “if”, but a matter of “when” the economic advantages of engine-off taxiing outweigh any disadvantages of this system, and it becomes a standard feature of all passenger airliners.