Does anyone have enough knowledge of commercial aviation to explain why this hasn't been done before? To an outsider, it seems like kind of an obvious feature.
Airbus 320 pilot here. This is something I've wondered myself for a long time. This same idea of a electric motor on the wheels powered by the APU(auxiliar small turbine for electricity and air presure, is the loud noise you hear when boarding the plane), is what some other pilots and myself have talked about for a long time.
Just taking in to account that it makes you autonomous when pushing back and taxing, the long times you spend waiting for take of at the runway, it makes a lot of sense for this kind of planes that spend a lot of the daily time taxing on congested airports.
I don't know exactly the reason why it hasn't been done before, I don't think is a weight or technology reason. It surely has seated on a TODO list for a long time, just take into account that in aviation we are way behind in aplication of the latest technology. For example we are still using single chanel radio for comunications with control( this is terribly frustrating and ineficient, it's pretty common to have comunication jams due to excess of traffics on the same frequency).
Airplane computers even in new models are still 90s models (very slow but i suppose that stability is worth the boredom). There are a lot of other vintage technologies in aviation, after all every single release to production must be tested and approved by the FAA and EASA. It's the same for all major changes in systems and procedures. That makes all changes and improvements very slow and expensive.
Yep. The air transport industry is very very change (read: risk) averse.
The current APUs (except for 787) have traditionally supplied power at 28 VDC and 115 VAC due to the worries about high voltage arcs. They would rather have more current over more copper than higher voltage.
I suspect they have added significant wiring and electronics to this A320 to get this to work. I haven't been able to find out if this aircraft has even flown with this rigging much less gone through certification.
I bet they could slightly improve this system and get rid of reversers as well (active braking).
That being said, I'd imagine ATC to be very wary of the current 130hp system: there is no way you could expedite a move across an active runway at almost 80,000kg MTOW.
I imagine that the APU generator must be reinforced or redesigned to be able to withstand the charge of the motors.
Revereser are quite efective and I don't think you can get rid of them, because when you have slippery runways the braking through the wheels is going to be compromised.
It's also another sys to fail on toP of the normal braking system (that already has quite a bit of it's own problem), so it's better to have paralel systems for the same function.
If you are going the active braking route you'll need a place to send that electricity and the actual batteries are not big enough for that.
You are right that the motor are not very powerful, 20kts max is a bit on the slow side as top speed. Given that in a big airport when you taxi at less than 15 kts you are slowing the rest of traffic (and they get nervous indeed when they are following a turttle!).
Also to really be autonomous with this sys I imagine they'll install cameras to see the rear and the engines's nacelle (to be able to see that the engine is starting without issues, witha normal pushback you have ground crew observing). That's another level of systems that need new procedures and testing, not very complex initially but things can become really weird very fast in aviation and all unusual user cases must be taken into account.
I think it's an oversimplification to say the reason for this innovation not happening sooner is because of risk. The Boeing 777 flies on only 2 engines which sounds far riskier but didn't stop it being approved. Also there's a race for building planes from carbon materials.
Robert Witwer, vice-president of advanced technology at Honey-well of Morristown, cited some other reasons [1]:
- Increased congestion: "Short hauls do a lot of taxiing time, especially in a place like Newark, where you can be 20th in line"
- Legislation: “And there are a number of airports in Europe that have tough emission standards that would make the Green Taxiing System appealing. I believe it’s a game-changing technology."
- Fuel costs: "The prospect of annual savings of $200,000 per jet from lower fuel use and less ground time has sparked interest from Airbus and airlines such as EasyJet and Alitalia. Airlines face the highest sustained prices ever for jet fuel. United Continental, the world’s biggest carrier, says it burns $25,000 of fuel per minute. Jet fuel delivery in New York averaged $3.12 per gallon in 2012, more than four times as much as a decade ago."
- Technological advances: "What’s new in today’s technology is the convergence of airlines’ search for more efficiency and recent advances in miniaturizing electric motors to propel a plane at the 32 kilometres per hour it may need for taxiing."
It sounds risky to operate with only 2 engines, and in 1953 twin engine planes were only allowed to be within 60 minutes of a diversion airport, mostly because of the unreliability of piston engines. It was a fairly long journey to allow twin engine planes to fly the kinds of routes they are allowed today. http://en.wikipedia.org/wiki/ETOPS
As jet engines turned out be much more reliable than their piston counterparts, the restrictions were loosened over time, starting with waiving the 60 minute rule for tri-jets in 1964. The original concept for the 777 was actually a tri-jet design. However the 767 turned out to be very reliable as a twin-engine plane and in 1985 it became the first twin engine plane to be allowed to fly 120 minutes away from a diversion airport (ETOPS-120) and in 1989 it was allowed to fly 180 minutes away from a diversion airport (but planes had to first complete 1 year with 120 minutes before they could be certified for ETOPS-180). Since twin engine designes were suddenly much more capable than anticipated Boeing dropped the tri-jet design of the 777, and eventually came back with a twin-engine design. By 1995 there was enough experience with operating twin engine planes with the 180 minute rule in place and Boeing was able to get the 777 certified to 180 minutes from the start.
So it basically took from 1953 to 1995 to allow a brand new twin-engine plane to fly most transoceanic routes (with a 180 minute rule in place), based on increasing evidence that the jet engines themselves are fairly safe, and learning more about additional safety mechanisms and procedures required to safely operate twin engine planes at extended distances from diversion airports.
It's more like risk vs gain. There was a lot of testing and previous experience leading up to the 777 launch ETOPS rating. A better example is the 787 battery system. Likewise, you want to be very certain that the new electrical motor system won't interfere with normal braking, so something that sounds simple becomes very complex and difficult once you put it on an airlplane.
Weight added to the aircraft (660lb). Another factor is reliability. Wheel or tire damage can bring down an airliner. The Concorde crash. Also a DC-8 crash in Jeddah in 1991 was due to a fire in the wheel well.[1] Even things like entertainment systems cause risk. Swissair Flight 111 went down due to a fire that started in the entertainment system rack, behind the cockpit. [2]
Maybe one day they can forget about landing gear and have airport based landing drones which fly up and attach under the plane. Less to go wrong on the plans and if airports have them, them certainly spares to cover situations. Though a long way from now still.
Would help too isolate a important system in a fuel effecient way with redundancy added in for landing. But when they put low priority entertainment systems in a position that worst case can effect critical systems does highlight the added risk of being able to watch a movie on the chair infront of you.
Wrong solution to wrong problem. The emergency gear extension works in a vast majority of cases. 99% of gear emergencies are minor, and either false warnings, or easily fixed on the ground, with minor or no damage to the jet.
Newer jets have wheel well fire "bottles" or extinguishers and sensors to detect problems so the crew can respond appropriately. Aircraft tires are filled with nitrogen for safety. The DC-8 ground crew topped up with air, and not nitrogen, and also flew with under-inflated tires. "Hot brakes" is a relatively common cause of aircraft incidents, and responded to by airport fire services, to avoid major disasters.
In the Swissair 111 crash, the entertainment system didn't have the proper isolating breaker installed, and wasn't fully shutdown once it malfunctioned. Placing the racks in a critical cockpit location near pilot oxygen supply lines, was a major flaw in the system design.
PS. Running out of fuel waiting for "ground landing gears" would be significantly more likely, as commented below.
I don't think that is a good idea. A simple half hour system fail could let tens of planes in emergency fuel. Also in turbulent weather is not very wise to have something trying to dock into your plane.
Since aviation became an industry, adding more "things" is always circumspect. If you think about it, that makes sense since every new feature, by virtue of being "new", still has to go through a gauntlet of testing, retesting and independent verification (plus Aviation Authority approval from several different countries).
There may have been previous attempts that were possibly abandoned until now when technology has finally caught up to the expected reliability.
Well, we shall see. If the Boeing battery problems[1] are a hint, it will take actual major use to bring out any bugs. But I'm still hopeful.
I'm guessing the added weight of electric engines and batteries/generators weren't worth the fuel savings until now, thanks to advances in batteries and rising fuel costs.
It's powered by the APU, a small jet fuel burning generator that already supplies the plane with electrical power when the engines are off, so there's no need for extra batteries.
I think I like the sound of the Taxibot over installing electric engines which weight 660 lbs on the wheels. It's like a Roomba for push backs :-)
I assume the bot autonomously 'docks' with the plane, then the pilot takes over control and pushes back. Then the bot autonomously returns back to its docking station by the jet bridge.
I suppose a single bot could do push backs for 3 or 4 neighboring jets, if they're just doing the push back. If it's taxing the jet all the way down the runway, I guess it's more of a 1:1 ratio, and probably a lot more complicated to work the returning units into ground traffic control, to the point where you need to work it into the overall airport design?
On the other hand, a taxibot would be working most of the time, while this system would be sitting idle most of the time. Not just sitting sitting idle, it would be weighting on the airplane while flying.
EDIT: And this system, like the other one, would be just an alternative solution to the plane main engines, so if they were not working, not enough at peak hours, or not available at every airport, it wouldn't be such a big problem.
No they wouldn't, because planes could still taxi under engine power. If you land at an airport with taxibot, you get a taxibot. If not, just taxi as usual.
Does this take away from being able to warm the engines up to prevent thermal shock and/or ensure give them enough run time before takeoff to ensure they are running well?
Wouldn't they also have to place an electric air conditioning system on the plane as well? When they disconnect the shore power at the terminal, the air does not come back on until the engines spin up.
Air conditioning and electricity are suplied by the APU, not only by the ground units. For engine start up you must use the air presure(the startup motor is pneumatic not electric), that's why you don't have air conditioning till the engines are running, but you can have it till the moment that you begin pushing back.
Also low cost companies try to save every cent so they don't use the APU till they have to start the engines. This can be pretty unconfortable when you have more than 28C inside, but they don't care too much.
The APU was a nice advance in that it enabled getting rid of the ground cart - which provided power to the airplane when its engines were off, and also provided the electric power needed to start the engines.
I wonder if they could spin up the tires before they hit the tarmac on landing? Think about it: the landing gear tires go from 0 rpm to spinning rapidly enough to carry the plane going about 140 mph in a fraction of a second. That puts the tire under enormous strain, which causes them to blow out all the time. A pre-landing spin-up system might be able to save a lot of petroleum by conserving synthetic rubber.
They don't blow out "all the time," that's why you've never experienced a landing with a blow out. Any way this is a FAQ and has been investigated before:
> While I was in college at the University of Cincinnati, I was watching television and saw a shot of an aircraft landing. There was the normal boil of tire smoke at touchdown. I had the same thought as your reader. Thinking that I had a great invention on my hands, I did some research in the library. I found some U.S. Government (Air Force or Navy) documents that addressed this issue. They were complete with photographs of a very elaborate test sled designed to accelerate a test tire over various surfaces and measure the effects of load and spinup time. Tests were done also of pre-spinning the tires. Much to my surprise, tire spinup was not a significant factor in tire wear.
> Flight crews did not like having almost 1000 lb. of spinning mass ~15 ft below the center of gravity. The special tires made the aircraft very hard to turn, especially at low airspeeds associated with the approach and landing phase. The crews wanted gyroscopes in the cockpit, not under the wings. But it seemed like a good idea at the time.
I believe the US air force did extensive studies on this back in the 60's and found that tire ware from spinup only accounted for ~<10% of overall wear on the tires. They found most of the stress comes from the act of braking and turning. Also I believe they found the gyroscopic effects of the spinning wheels actually made it more difficult to land the plane... although that may not be an issue anymore with modern avionics and autopilots.
The Taxibot system, where a small tug vehicle positions itself under the plane's front wheels, would be very promising if it could be self-driving and fully automated.
And think of the cost saving when we get rid of the pilot and cockpit. And heck, let's get rid of the landing gear too, and have the plane drop into a cradle running along underneath the plane on a track to catch the plane. The cradle would also provide a lot of the acceleration of liftoff, and the cradle's liftoff track could climb a few hundred feet like a roller coaster, so maybe the plane's engines could be smaller.
We could get rid of those pesky complaining meat sacks too. I never quite understood the fiscal arguements for pilotless passenger aircraft. It's not like a single seat fighter jet where you can ditch the pressurisation gear, you've got to still keep the cargo alive no matter how much noise that they make.
> It's not like a single seat fighter jet where you can ditch the pressurisation gear, you've got to still keep the cargo alive no matter how much noise that they make.
Furthermore, my understanding is that modern fighter aircraft are generally limited in performance to operational envelope of the human driving it, i.e. most modern fighter airframes are capable of performance that could literally kill a human pilot. Getting rid of the pilot opens up the possibility of gaining significant manoeuvrability.
At the tail end of the war, those Luftwaffe pilots were flying some pretty damn advanced planes, with some pretty awful inherent problems. But the follow on designs (Mig-15 et al) were in service for quite some time after the war. The rate of advance of German industry despite massive aerial assault and savage ground wars is fascinating. I recommend reading Gitta Sereny's biography of Albert Speer. Among other rather less impressive acts (extent debated, but likely involvement in genocide, definitely involved in slave labour organisation etc) he rearranged German industry such the production increases were occurring right up until the very final stages of the war. The major industrial output declined started about early '45 or late '44 from memory, which given the volume of allied bombs dropped speaks to German organisation, allied bomb accuracy, or both!
Was the fuel shortage due to bombing or Russians taking the oil fields? I had thought it was nearly entirely due to Russian advance, but the below article makes it seem otherwise. It quotes Speer saying how effective allies bombings were, but it is dated May 1944, which is quite late in the piece given that the tide turned, although this may have been less clear as it was still pre Normandy (just). Interesting fact from that article - the treaty with Russia gained Germany 4m barrels a year, and the subsequent invasion only gained them about the same. Are the numbers misleading? Did Germany still need to process the oil it imported, making relatively small domestic production less relevant and the processing facilities critical?
http://vanrcook.tripod.com/Germanfuelshortage.htm
Been also watching WheelTug[1] as a result of Andrew Tobias' interest (and investment) in it[2]. Sounds like they have more customers lined up, but competition already in this market should be a good thing!
I wonder how much weight this adds to the plane. A full jet is often right at the edge of it's envelope and adding a few hundred pounds of electric motors could mean that they lose a row of seats. I would love it if extra leg room was an unintended consequence of electric ground propulsion.
Airframe companies know exactly what a pound of extra weight on an airliner is worth. It's large, well into the 6 figures. 660 pounds needs a lot of fuel savings to make up for it. Remember, it takes extra fuel to hold that 660 lbs up in the air.
The context makes it sound like they included the extra weight in fuel calculations, but it's not clear whether they included overall weight capacity. As the parent points out, this probably isn't worth it if they have to cut a row of paying passengers to do it.
An A320 burns about a half pound of fuel per second when taxiing. [1] So it seems plausible that the amount of weight you save on fuel could go a significant part of the way toward offsetting the weight penalty from carrying the extra motors.
I suppose if you're pushing the envelope precisely, and push back overweight, count on the taxi burn to get you down to max take-off weight, and plan to land with the legal minimum fuel reserve, then yeah, you'd still pay the full weight penalty. But I'm guessing that most airlines, as a policy, anticipate a range of possible taxi times in calculating their weight and fuel budgets, so significant weight savings should be realized.
Guessing that an A320 carries about 150 passengers, losing 3-4 of them would still make this worth it, and if they weren't flying 100% full in the first place the lost seats don't cost them anything. It's a neat idea, seems like a relatively safe mod and a way to catch some low hanging fruit.
smoyer, thanks for catching the weight. Seems like I have gotten a little too "efficient" with my reading.
Just carry a bit less cargo. I don't think passenger airplanes are typically limited by weight, but rather volume, when it comes to the number of passengers.
"The companies claim the result is a potential savings of more than 150 gallons during a multi-flight day for an airplane like the A320 or Boeing’s 737. Overall an airplane equipped with the EGTS could cut fuel consumption by four percent over the course of a year."
Another step to help reduce the costs of air travel. Follows the trend of airlines and manufacturers choosing to provide more efficient air travel vs faster air travel to consumers: https://medium.com/lift-and-drag/7885a299bca2
It's a pretty funny trend since, as both articles suggest, speed is the cooler/sexier technology. Goes to show that it isn't always the cooler inventions that have the greatest impact.
As we've agreed, this is not a new or novel idea, but it does illustrate why the patent system is broken. A quick, cursory search yields a patent from 1994 for the exact same thing. " Preferably, a motor (27, 44) powered by the auxiliary power unit of the aircraft is used to drive the wheels of the landing gear."
I think the Concorde was the worst offender when it comes to taxing to the runway, if I remember correctly, because its jet engines were hugely inefficient at low speeds, it would burn almost 2 tonnes of fuel just to get to the runway
> if I remember correctly, because its jet engines were hugely inefficient at low speeds
Yes, turbojet vs turbofan. Turbojet only become efficient over Mach 1.
> the Concorde was the worst offender
No, I the SR 71 was worse, it literally leaked fuel:
"Similarly, the fuselage panels were manufactured to fit only loosely on the ground. Proper alignment was achieved only when the airframe heated up and expanded several inches. Because of this, and the lack of a fuel sealing system that could handle the thermal expansion of the airframe at extreme temperatures, the aircraft would leak JP-7 jet fuel on the runway."
That was the next logical steps after Boeing electrifying most of the 787 functions (I'm still wondering why Airbus didn't go with bleedless engines for the 350).
My impression was that Airbus quite consciously decided to be a bit more conservative with the A350 after seeing all the problems Boeing had with the 787 switching to a zillion new technologies, all at once...
Airbus had also said that the Boeing was using too much carbon fibre on the 787, saying that it was too new a technology, but they ended up using more of it on the A350.
I bet all the newer airplanes will switch to bleedless engines in the near future.
Could they also use he motors to spin the wheels up to speed right before landing? I always see a puff of smoke when the wheels first touch the runway. I imagine that wears down the tires pretty quickly?
