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Theoretically, I know it is possible. I know the benefits of fly-by-wire too.

My question(s) most revolve around:

  • why it hasn't been done?
  • What challenges1 would there be?
  • What benefits are there by not implementing it even for new light GA aircraft2 being produced?
  • I know that many smaller airplanes (e.g. Cessna 172) do have fly-by-wire flaps. Why the remaining flight controls don't use this technology?

1: Not considering money as an issue.
2: I couldn't find any reference of it for Cessna Skyhawk.

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  • $\begingroup$ I know that FCS (Flight Control System) is really expensive to develop, so only affordable for big production rates... so if money is not an issue I don't see any limitation, but I can not tell you for sure this statement $\endgroup$ – Trebia Project. Apr 30 '15 at 18:12
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    $\begingroup$ DA-42 converted to fly-by-wire: flightglobal.com/news/articles/… [en], ils.uni-stuttgart.de/forschung/projekte/safar/index.html [ger/en, technical], avweb.com/avwebflash/news/… [en], $\endgroup$ – JulianHzg Apr 30 '15 at 18:59
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    $\begingroup$ I do not consider the 172 flaps to be fly-by-wire. To me, FBW means the PIC provides input, which a computer interprets, modifies, and limits before acting on. The 172 has electrically controlled flaps. When I move the lever to flaps-10, the motor moves the flaps, but with zero authority to do anything other than what I commanded. $\endgroup$ – abelenky Apr 30 '15 at 19:24
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    $\begingroup$ "What challenges would there be, not considering cost as an issue" ends up being pretty close to a null-set of issues. Most challenges in engineering & design end up being matters of cost. "We could do that, but it will cost..." is generally the reason that all sorts of otherwise cool ideas aren't implemented. With hundreds of engineers, full scale test facilities, and unlimited prototypes, not much remains as a challenge. As you say, it's clearly possible, and between things like the A-320, the B-2, and the F-16, the theoretical issues are solved, so the remaining challenges amount to $$. $\endgroup$ – Ralph J May 1 '15 at 0:55
  • $\begingroup$ @abelenky FBW only means that surfaces are controlled "by wire" as opposed to by mechanical/hydraulic linkage. I believe the first FBW Airbus aircraft had no interpretation or limitation in the line $\endgroup$ – raptortech97 May 8 '15 at 15:15
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There are multiple factors here:

The first and main one is of course weight. Fly-By-Wire requires computers and hydraulics to actuate the surfaces in a small plane weight is critical and a FBW system may be the difference between 3 and 4 passengers in total capacity.

The next that comes to mind is certification, I'm not sure exactly how this works but I would think that FBW implementation in a current airframe would require a recertification of sorts. With that being said many GA planes flying out there like the 182, Piper Arrow and other similar things still made today are sporting designs first flown more than 30 years ago. It's a simple matter of, "if it ain't broke don't fix it".

I would think that basic service and maintenance would also become more complex and worst of all more costly. I don't know much about the cost of these systems so I wont comment father than speculation like this.

There is a safety concern in small planes (and I'm sure even bigger planes) if control feedback is not present. This Article outlines some serious concerns with the Cirrus side stick and its lack of feedback (not fly by wire but some of the ideas carry over to the FBW systems that don't have feedback).

A Note on the 172:

The way I understand FBW leads me to think the 172 does NOT have FBW flaps. While they are electronically actuated I consider FBW (and by definition):

...flight control computers determine how to move the actuators at each control surface to provide the ordered response

There is no computation done in a 172's flaps to my knowledge, if you move the switch they will move. While the flaps are not directly cable driven I would consider the lack of computer to keep the 172 out of the FBW category but thats just personal opinion.

Also of note, the 172 did not always have electronic flaps - that was a later addition to the aircraft in '65.

A Note On Cost:

Although you note that one must ignore cost in this case you really can't. A FBW system would make a GA plane even more expensive than they are. This is a very real and very limiting factor in aviation. With an unlimited budget you could develop, certify, and regularly fly a FBW small plane that was chock full of gadgets but the FAA approval process is what it is and cost is a very real limit for aircraft makers.

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    $\begingroup$ +1, especially for the last paragraph. We'd do a lot of things different in GA if cost weren't an issue. For instance, there would be a lot more people flying jets around and hours would be much easier to come by. :) I don't know about you, but, if cost weren't an issue, I'd be learning to fly a 777. $\endgroup$ – reirab Apr 30 '15 at 21:16
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    $\begingroup$ Re the 172's electrical flaps, I found I much preferred the manual flaps on my Cherokee. I think that would go across the board: unless there was a MAJOR performance/efficiency advantage to be had from FBW, I wouldn't want it, any more than I'd want a car with automatic transmission. $\endgroup$ – jamesqf May 1 '15 at 6:42
  • $\begingroup$ @reirab: Yeah, and let's see you land that 777 on an Idaho backcountry strip :-) $\endgroup$ – jamesqf May 1 '15 at 6:43
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The biggest reason that fly-by-wire hasn't been implemented yet is the complexity of the change required. Larger aircraft already started using hydraulics to move their control surfaces, and a fly-by-wire system adds to this by actuating them with a computer. Smaller aircraft do not have the need for hydraulics, so they must move from direct linkages to computer control. As ratchet freak says, this adds a lot of weight and complexity.

Flaps are much more simple than a whole control system. They don't move very often, and if they fail, the plane is still able to take off and land safely. The pilot directly controls them on a simple scale of up to down. The rest of the flight controls must be able to move throughout the flight, be quick to respond, and implement computer control logic. Failure is much more critical than the flaps as well.

There is also a large inertia of history to overcome. The industry is used to traditional control linkages. Moving away from this will incur a large cost in developing the system, testing it, and getting it approved by regulators. With sales at their current levels, adding this to older designs may not be worth the cost of development, and new designs would have to make the option attractive compared to other options.

A big hurdle to overcome is also safety. While larger aircraft have managed to implement these systems, the challenges are greater with smaller aircraft. Mechanical controls are generally more simple to inspect and repair, and the GA maintenance community would have to learn many new skills to work with the new systems. Redundancy and safe back-up controls would have to be included in a smaller airframe. All of this when regular controls work perfectly fine and are efficient in cost, weight, and complexity.

As reirab commented, stall training is required for a PPL. It makes sense that pilots should still know how to handle a stall, in case the protections fail. But this means the system must have an option to be inhibited. So although the plane should usually prevent stall, pilots will also need to know how to recognize and handle system failures, which even experienced pilots with an ATPL can struggle with.

So what benefits are there? Safety is one area where fly-by-wire can offer great improvements. A fly-by-wire system can automatically prevent the pilot from stalling the plane or otherwise putting the plane in a position where it is out of control. These kinds of incidents are much more common in general aviation than with airliners, so there is a huge potential for reducing the number of accidents. The plane can also trim itself automatically and deal with many of the aerodynamic idiosyncrasies that different small planes have.

There is progress being made here though. Fly-by-wire technology has been used on business jets like the Dassault Falcon 7X. Diamond is working on a fly-by-wire option for the DA-42, as well as the future DA-62. As of 2012 they were hoping to have this available around now, but there are at least manufacturers working on bringing this technology to market.

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  • $\begingroup$ One note on envelope protection: It's often not desired in GA. While you certainly don't want your pilot to stall an airliner in which you're riding, it's actually very common for GA pilots to intentionally stall the aircraft. At least in the U.S., demonstrating stalls and recovery is required in order to get even a private pilot certificate. At the very least, the pilot would have to be able to manually disable envelope protection if it were implemented in a GA aircraft. $\endgroup$ – reirab Apr 30 '15 at 21:12
  • $\begingroup$ @reirab Good point, and that has its own issues. $\endgroup$ – fooot Apr 30 '15 at 21:20
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Fly by wire costs weight in actuators for the control surfaces and power to move those actuators. It's simpler if there are hydraulics already needed but many small GA use pulleys and control-rods to link the control surfaces to the controls.

Then you need a few computers (redundancy) and sensors feeding into them.

Also the plane should still be safe to fly and be able to make a controlled landing after the fly-by-wire system fails entirely.

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COST like most things that could be added to light aircraft and are not because it is not cost effective to do so. Especially given that most light aircraft are based on 50-60 year old designs. The Cessna 172 entered service in the late 1950's. They would have to do a complete redesign of the aircraft which would cost millions to develop and certify. The finished aircraft would be very expensive and people may not be willing to pay 700,000+ for a standard performance 4 seat light aircraft.

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Let's not forget the liability issue. Practically every GA accident that results in injury or fatality results with a lawsuit. That's one of the biggest reasons we have such old technology for which we pay such high costs.

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A bit of history helps here. As planes got larger, manually activating control surfaces became more and more difficult, so mechanical assists, such as hydraulics came into use. As in operating a hydraulic shovel or a hydraulic lift, the inputs are still controlled by moving a lever or turning a knob by hand. Indeed, even a remote control radio signal turning an electric servo is still a mechanical extension of a physical command by the pilot.

"Fly by wire" implies a physical input, changed to an electrical signal, transmitted to a control surface servo, which moves the control surface. Electrical flaps could be considered fly by wire with that definition.

But the modern "Wikipedia" definition also implies that a computer determines the amount of movement the control surface makes, putting us two steps away from knowing if and how much a control surface will actually respond.

For very large aircraft, mechanical boost is needed (imagine trying to open a barn door in a hurricane). For borderline stable aircraft, like modern fighter planes, computer "artificial stability" must be added to make the plane flyable.

Neither are needed for a small, soundly designed GA plane like the 172, being flown in VFR conditions. However, applications for higher technology do exist for collision and terrain avoidance and flying in less than ideal conditions. This would represent a step up from purely recreational flying, which is generally done in fair weather. An autopilot can certainly be a life saver.

But with a smaller aircraft sensor inputs go to the pilots brain and controls are moved with hands and feet. Mechanical and computer systems, which are expensive, complex, and require sufficient redundancy to be safe, are not needed.

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It is actually technically possible to implement FBW in light aircraft. Technologies like Horizon Hobby ASX, SAFE & A3D already provides a FBW for RC planes, drones and helicopters. Likewise, the drones like DJI do have "follow me" mode where the drone is able to follow a person on bike or on foot at a set altitude and attitude. So, technically, it is possible. It's already implemented there. Just need to find some GA aircraft producer to bite the bullet and get these stuff and certificate and integrate in their products.

By the way, with FBW, you generally use electric servo motors, not hydraulics for control surfaces.

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  • $\begingroup$ I'm not clear what this adds to what the OP already acknowledged... theoretically possible, etc. The question has some rather specific inquiries, and "drones do this already" doesn't address those. $\endgroup$ – Ralph J Aug 24 at 23:06
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    $\begingroup$ "you generally use electric servo motors": maybe in drones. FBW aircraft like A320 or B777 use hydraulic actuators. At this size, electric servo motors are no longer feasible. $\endgroup$ – Bianfable Aug 25 at 7:22
  • $\begingroup$ F16 uses electric motors and fiber optic control lines. The OP asked about general aviation planes, where hydraulics would be totally unfeasible but electric servos would perfectly provide a control force comparable to a human. Thus, in my opinion, it is only a matter of time for a GA producer to come up with FBW to improve handling and safety. $\endgroup$ – flatline Aug 26 at 19:30

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