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What makes a fly-by-wire airplane fly-by-wire? The term is broadly used nowadays, but not often explained. Since we have many GA colleagues here, and fly-by-wire is a concept more common to the military and airline world, it might be interesting to share some pilot perspective about its way of working and our way of flying it. Since I spent the last couple months transitioning between the only two Boeing fly-by-wire models, I thought it would be worth sharing it with you.
Well, conventional airplanes have control surfaces—ailerons, elevators, spoilers, rudder—that are controlled by cables, just like a bicycle gear or brakes. You pull a yoke or stick and the cable, directly connected to the control surface, moves it. Even huge airplanes, like the Boeing 747, work that way, although obviously assisted by hydraulic actuators, since the commands are too heavy for the pilots to move by muscle force alone. Fly-by-wire is somewhat different: the control surfaces are not directly connected to the control column, nor the rudder pedals; instead they are linked by electrical wires, through which pilot inputs, once translated into electronic signals, travel to actuators at the control surface.
Fly-by-wire systems include lots of parts, but no physical cables.
Airbus introduced the FBW concept to civilian airliners with the A320 in the 1980s, and little by little, pretty much all other major manufacturers followed. FBW itself is nothing new—it was tested in late 1930s in Russia—and the Apollo Lunar Module and Concorde used it many decades ago. But was it was only in the 1990s, with its first clean sheet design in over a decade, that Boeing decided to adopt it in its commercial projects.
So, the first thing we need to understand are the modes of a FBW system: usually there is a normal, a degraded, and a direct mode. Airbus calls the degraded “alternate” and Boeing calls it “secondary.” In normal mode, the system works as it is designed to, with all enhancements and protections. In degraded, the system uses simplified data to operate, and many things are lost. In direct, well, you guessed: whatever pilot inputs it gets, it sends directly to the control surfaces, kind of like a conventional airplane (albeit without cables).
As you can see from the diagram at right, we have four ACEs and three PFCs. The ACEs (Actuator Control Electronics) are responsible for sending the signals to move the control surfaces; the PFCs (Primary Flight Computers) receive the inputs either from the ACEs during manual flight or straight from the autopilot system. The PFCs then process those inputs along with inertial, air data, radio altimeters, engine controls, etc., and produce enhanced signals that are sent to the ACEs (back to them if in manual flight, because they came from the ACEs in the first place, remember?) and then, from there, to the control surfaces.
The fact that we have four ACEs—none of which has authority alone to move a surface unless all other ACEs that share that control surface with it have failed—and three PFCs brings a lot of redundancy to the system. But still, in the 777 there are a couple spoilers (and the stabilizer) that can be controlled in a quite conventional way. The 787 has no such kind of old school backup—more on that later.
If something happens, like unreliable air data, the system falls into the secondary mode: the PFCs still process the data, but in a simplified way, with the old or remaining data. If the failure is on all PFCs (highly unlikely) or if the pilots disconnect them through a switch (more realistic, since some non-normal checklists would guide them to), the airplane reverts to the direct mode. Now whatever inputs the pilots make go directly to the ACEs, and from there to the control surfaces. So, fly-by-wire itself means that there are no cables or pulleys, hence no direct mechanical connection between the pilot controls and the control surfaces. But… why do it like that? What’s the point? And what else can we get from it?
From what we have covered so far, it looks a bit complex this FBW stuff. So why would the engineers go down that road instead of keeping it simple? Well, first of all weight. Nothing in an airplane hurts more than weight, and if you have to make profits with it, that’s even more true: in a typical 12-hour passenger flight, about 40% of the weight of a 777 will be the airplane itself, another 40% fuel, and only the remaining 20% is going to be the payload. Not pretty, at least not in pounds or kilograms. So, since electronics are lighter than mechanical controls, there you go.
The second reason is reliability. Once again we get down to costs, and the more reliable a system is, the cheaper it is to keep it working—and why not, the safer it is. Now, the third reason is more like a bonus than a reason itself.
The pilot is in control of those ailerons and spoilers, but not completely.
Because you need to transform pilot inputs into digital data anyway, why not use computers to make them better? Why not including envelope protection? Why not provide a smoother and more efficient ride? The fact that—at least in normal mode—all the signals go through the PFCs allows for them to enhance the flying qualities of the airplane. A Northrop Grumman B-2 Spirit, for example, is inherently unstable and can only exist because of fly-by-wire. This is an extreme example; all airliners are inherently stable, including the 737MAX (which is not FBW, by the way). But that doesn’t mean their flying or pilots’ handling of them doesn’t have room for improvement.
So, every time the pilot commands something, the computers decide how to translate that to the control surfaces; instead of giving what you asked, the system gives you what you want. On my first flights in the 787, I felt like I was being “filtered;” three thousand hours later I can’t notice anymore. Something else it does is to compensate for things like changes in trim, power, or turbulence. These are all events that would require some input from you, as a pilot, but thanks to FBW are handled automatically. You’re maintaining a 25 degree bank? No trim needed. You are a bit above the glideslope? Here goes some autodrag!
But by far the coolest of fly-by-wire features are the protections: roll protection, yaw protection (in both 787 and 777 they are similar but not the same), overspeed, stall, and more. The list is long. Want to trim up until you stall it? Maybe so, sir, but not today. Feeling like Tex Johnson? Go ahead, but remove the bank angle protection first—it will make your life easier. Lost an engine? Let the robot kick the rudder for you. The downside is that all those nice things are mostly or entirely gone once the system automatically reverts to secondary mode or you revert it to direct mode. And I forgot to mention, the autopilot goes away too.
Flying characteristics of the 787 in direct µode are quite respectable. I would go far so far as to say that it feels like a 777 in normal mode—I don’t want to be too harsh, but… we are talking about almost 20 years of development. But the real puzzle is, how safe is it? What if you lose all hydraulics? Once again, is highly unlikely—history accounts for only one case (Sioux City, anyone?)—and in fly-by-wire airplanes that would be… ugly. (I recommend you check the QRH for this one; it is self-explanatory.)
But once again, except for maybe the 737, no recent big airplane can fly completely without hydraulics, therefore it doesn’t make sense to ask as much from a fly-by-wire aircraft, which are, otherwise, full of redundancies, usually three independent and fully capable systems.
The thing is, we are not going back, and backup cables—like the Concorde had and the 777 has—are being abandoned. They are a layer of protection so deep that it is only useful in extremely unlikely scenarios. Flying a fly-by-wire airplane is really good once you understand it, and it is not even the future anymore—it is a well stablished present. Embrace it, and it will prove vital.
Ah… how is it flying the 777 after flying the 787? Well, it is big, heavy—but I miss the HUD.
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Thank you Enderson for this excellent write up and explanation of FBW. I’ve always wondered what it really meant and your detailed articulation yet with plain and simple language for novices like me was extremely helpful. I learned today and Thank You!
I thank you, Ray, for reading and leaving your impression! That was the idea, to share something that for me wasn’t clear years ago too.
Enderson, this is something that I have worried about ever since FBW first came in: What happens if there is an electrical fire on board and the FBW system wires are affected? In theory, the pilots would have no way to control the craft. Or is there a backup system that I don’t know about?
Thanks
Carson
Hi, Carson! Yes, that used to be one of my concerns as well. But the fact is that with fly by wire we have many more redundancies than with control cables, because the wire go along the fuselage in different places, therefore even if you lose some, you will not lose all. And if you lose all, well, than it is some catastrophic failure with the fuselage in which having control or not of the flight surfaces is already irrelevant. Thank you for your question!
It’s interesting that commercial and military aircraft have 3, redundant computers for flight controls. But GA and experimental aircraft have only one in the autopilot, and some have no manual elevator trim.
Even Cessna has a single switch to control the flap position, with a number of limit switches and follow up switches.
I’ve had a limit switch fail, and the flaps were stuck in the 30 degree position, so after landing, I was stuck there until an A&P could find the bad limit switch and replace it.
My current 1961 Cessna has manual, Johnson bar flaps….no electrics needed.!
And that gets me remembering of all the fuzz around single pilot operations in the airlines. Having two human pilots in Part 121 ops is also part of the redundancy. But simplicity is key, even in complex systems – I like the electrical flap switch on Cessnas, tho.
Very interesting and educational. I am not a pilot but was an avionics specialist at General Dynamics Fort Worth for 30 years. As I recall, the F-16 was probably the first plane to incorporate FBW. It certainly made sense because of the relatively small airframe and single engine. I believe that Airbus has FBW on their aircraft. However, Boeing has resisted.
Hi, Joe! You must have expertise in some areas I am certainly oblivious about. Well, we tend to have the idea that Boeing was behind because, I believe, of the longevity of its projects. The 737, 767, 757, 747… all them came before the first FBW Airbus, and are still flying. The first clean sheet design after A320 and A330/40 was the 777, and it was already FBW, less than a decade after the A320. That is, at least, how I see it.
Not even close. The Avro 707C was the first FBW aircraft, in 1956. The production Avro Canada CF-105 mk 1 was flown in 1958 with FBW. The Lunar Landing Research Vehicle was FBW in 1964, whilst Concorde was FBW in 1969. The first DFBW aircraft was an F-8C modified to use a full authority digital Apollo spacecraft PNGSC, with first flight in 1972. The YF-16 and YF-17 both flew in 1974 with analog FBW. First production DFBW aircraft was the F-18 in 1978, long before DFBW came to the F16C/D block 40 aircraft.
Interestingly, the Airbus A320 used FBW control laws which were developed on the Boeing “Dash 80”
Essentially its small amounts of energy whether its electricity or hydraulics to control larger amounts of that same energy. The F14 used 3000 psi low volume servo controller which you could input with your hand pressure and control those two main vertical stabilizer actuators which exerted 50,000 pounds of force. Incredible engineering and production methods. Small electric torque motors control huge volumes of hydraulic fluid on submarines using servo valves.
That’s the idea, and that is why the B787 (and the A350 too, I believe) use 5000psi, so the systems can be more compact.
Fly by Light is another good one too.
Next step for sure!
Very nice explanation of FBW, but I would like to add one important point if I might. I have not flown the Boeing version, but I do have a few thousand hours in the Airbus A-330. The thing they never taught in school, but the most important characteristic of FBW, in my opinion, is to never forget that “the stick tells you nothing.” To explain, every airplane we have flown before FBW (at least the Airbus version) “talks” to us through control feedback. Move the controls further and resistance is greater, etc. Because of this, pilots can judge their control inputs by how the stick or the yoke “feels.” With FBW this is not true. There is no resistance to stick movement and no difference in feel no matter how far or fast one moves the stick. This is important because if one is distracted by fault messages, alarms, etc. while hand flying it is very easy to over-control and have no idea that you are doing so. I saw it and experienced it multiple times and I am quite convinced that the Air France accident over the Atlantic was the result of pilots holding the stick full back while distracted by multiple alarms and having no idea that they were doing so because there was no “feel” telling them they had put in full control. I always made sure this was the one lesson I taught every first officer: Always remember, the stick tells you nothing!
Hi Raymond, thank you for your insights on the Airbus FBW. Well, that is a big difference among the two manufacturers approach: in Boeing they want you to fly it like you would fly a non-FBW acft – and honestly the 777 “feels” a lot like a non-FBW acft. So, there is feedback (when the airplane turns, the yoke moves, even in autopilot – the same way the throttle levers move according to the engine rpm), and they get harder the faster you are. In fact, the ailerons are locked in high speeds, and only flaperons move on 777/787 models during high speed flight. Still, every time you do an input, the computers are going to deliver what you asked for, not what you did. So, PIO is a less likely possibility – and many of the pilots I’ve seen doing it were actually ex-Airbus guys new to the fleet, who apparently lost the “feeling” of having a feel. But of course, everyone not taught on this subject is prone to doing PIO, and the 777 is particularly rough on it: the 787 sophistication tends to be more forgiving.
I did know that the 777 provided an artificial feel for feedback but assumed that the 787 did not. Interesting. While I finally got used to the non-moving autothrottles on the 330, I always missed the moving version on the 757/767. Autothrottles are great, but they don’t understand minor disruptions, so being able to push them around or hold them in place was a major advantage to the Boeing system. I had a love/hate relationship with the 330. You had to treat it more like a video game than an airplane. There was no easier way to cross an ocean, but my description of the aircraft was “non-intuitive and inherently dangerous.”
Oh, sure. The not moving thrust levers of Airbus is something I really do not get. The Auto-throttle of the 777/787 is way more precise than those from previous Boeing models, and very rarely require some override from the pilots. But Boeing wants you to do it if you feel like. For me, that’s the beauty of it. See you over the Atlantic! Cheers!
Good article. However, I would argue that the primary benefit of FBW is that it takes away all the non-intuitive “quirkiness” of controlling an aircraft. The ideal for aircraft control is positively stable in the longitudinal axis (i.e., seeks its trimmed AOA), positively stable in the directional axis (i.e., seeks to zero out sideslip), and neutrally stable in roll (i.e., doesn’t resist changes in angle of bank but also doesn’t seek to increase it exponentially). There are other ideals, like increasing stick resistance with increasing AOA, but there’s not enough space here to go into all the details.
Before FBW, aircraft designers had to incorporate these features via aerodynamics. Because of physics, this would usually mean compromising some other aspect of aircraft performance. When performance features were more important, this typically lead to aircraft with “quirks” where they departed from the ideal of aircraft stability and control. This is particularly evident in the transonic region, where aerodynamics change enough that it is difficult to design a basic control that functions the same at all Mach numbers.
After FBW, we no longer have to choose between an aircraft that “behaves well” aerodynamically and performs its job well. We can have both. While this may seem small, it’s huge in terms of safety. Lots of accidents in the early days of aviation could be traced to a pilot being overwhelmed by trying to control an aircraft that isn’t doing what he expects.
Very nice insights, Ryan! Indeed, the design features incorporated into FBW goes well beyond most pilots – and I include myself – comprehension. We are, in this sense, like the guy who uses Word to write a nice article, and knows the resources of the tool bar, but does not know the codes to build a writing software himself.
Cheers!
A side note concerning the DC-10 crash: A good friend of mine was responsible for one of the Sioux City emergency rooms that fateful day in 1989. “Tragedy & miracle” was his observation. His employer relocated him to Oklahoma City for expanded duties; he was there when the Murrah building was bombed in 1995. I met him when we both lived in Boulder City NV and worked in the Las Vegas valley. We that I needed to get relocated before whatever was following him around happened again.
Oh my! Well, it’s been said that “a great man doesn’t seek to lead. He’s called to, and he answers”.
Nice article. I am fascinated by the technology that the 787 has and I love the fly by wire system and amazing redundancy it has. One thing I’d like to say. You mentioned how if you lose the hydraulics on FBW aircraft, it would be ugly. On most FBW aircraft or conventional aircraft bigger than the 737, all the flight controls are finished if that happens but as far as I’m aware, the 787 you flew has Electro-Mechanical Actuators, requiring no hydraulics to operate, that will operate one spoiler surface per wing and the horizontal stabilizer for control, but not for too long as I recall one pilot who was training on the 787 said. I’d also like to say that the A350 and A380 can also survive without the 2 main hydraulic systems thanks to Electro Backup Hydraulic Actuators that will operate most of the flight controls as long as you have engines or the RAT is working.
Hi there, Clayson, thank you for your insight. Well, the 787 in fact uses the EMAs, but to a very limited extent, more like a compensation for certain partial failures, not something capable of maintaining the controllability of the aircraft in case you lose all your hydraulic fluid on the 3 systems. Both 777 and 787 can’t be flown in such a scenario. I am not sure of the Airbus, but I believe it is a similar case.
Thank you once more,
Enderson
Wow. Didn’t know the EMA wasn’t for if all 3 systems failed on the Dreamliner. Now I know. The A350 and A380 are different. According to pilots who fly both, the EBHAs will power most of the control surfaces should both hydraulic systems fail. The autopilot will continue to work even. Thanks for the reply.
That’s right, in the A350/80, according to typed pilots I’ve talked to, the EHA (Electronic Hydrostatic Actuators) will handle the problem in case you lose the two hydraulic systems; in the 777/787 Boeing chose to put a third hydraulic instead as the redundancy for the loss of two systems. Always nice to learn, thank you for bringing it up!
Hey again. One question. The 787 uses a roll rate feature in normal mode like the Airbus in the roll axis. Does the 787 provide artificial force feedback in the roll axis in normal mode and make the wheel harder to turn as airspeed although roll rate is being used or is there no feedback like the FBW Airbus?
Very interesting read
Airbus did not introduce FBW to commercial aircraft. That was done in the sixties by Concorde, and in many ways, it was done better. Airbus had to introduce envelope protections to make up for some rather extreme deficiencies in their form of FBW which would not allow certification otherwise. This was because of the control laws they used, which in turn, they used to avoid what they estimated as about a tonne of weight, if they provided Q feel etc… Some accidents have been caused by ABs approach to FBW, but it can’t be denied that some have been prevented or greatly mitigated as well. We had a pilot who had to retire as he could not handle the stress of flying an Airbus again after one tried to kill him and completely ignored his perfectly correct control inputs. The B777 and B787 have a far more preferable set of flight laws.
Airbus did not introduce FBW to airliners, Concorde did, in 1969.
Hello! I have a question that is not directly related to FBW, but to something that came with it, in the case of Airbus, namely the side-stick, as opposed to a regular yoke. I’m asking myself: Supposed there’s a stressy, severe emergency (e.g. multiple system failures, physical, electrical, mechanical damage to the aircraft, night flight with severe turbulence/sheer winds during an approach, i.e. a combination of bad cases, making even “aviate!” a challenge), would one rather want to pilot an Airbus A350 with a (passive) side-stick in direct law, or e.g. a degraded 787/777 with a (active) yoke?
Considering psychological effects (high stress levels, difficult situational awareness, “degradation” of brain functions to instinctive “fight or flight” in worst case), I imagine that a regular active yoke would be preferable? As it feels more “natural” and “intuitive” (most pilots started flying with a yoke), gives active feedback on input (even if it’s just simulated one). And as one can operate it with both hands – I imagine this gives finer and more confident control over difficult manoeuvres in a difficult situation? Also, I suppose that even after thousands of flight hours, it can’t be advantageous in an emergency (degraded brain) if one has to operate a side-stick with the left hand as a naturally right-handed person (or vice-versa). A yoke can be operated with whatever hand one prefers, and with both, there’s a less steep learning curve when transitioning from the right to the left seat (or vice-versa).
How do pilots see this? Is it even a concern, i.e. is this even discussed as part of the pilot training? Or does it get neglected in the sense of: “you’d have to fly 100 mio. miles to ever encounter such a situation, in short: it won’t happen, don’t even think of it” or “pilots are trained not to panic in emergencies” (well, yes), or “99% of so-called emergencies are still well manageable by following check-lists, there’s hardly ever a need to prioritise plain flying skills”.
Are there statistical studies with evidence whether one or the other is statistically advantageous in a really bad emergency with multiple system failures (i.e. ruling out well-manageable single-system failures like an “engine out” or “cabin depressurisation” incident with otherwise fully working systems – in such cases, I expect the above question to be largely irrelevant, as it’s more of a question how well the plane can still automatically adapt to such situations without significantly increasing the crew’s workload)?
I wonder whether there’s any scientific basis (regarding overall safety only) that lead Airbus to decide for FBW with (passive) side-sticks, and Boeing to decide for FBW with (active) yokes. And whether Airbus ever considered to at least switch to active side-sticks (but perhaps dismissed it for consistency/compatibility, although the safety data would be in favour of it).
Thank you.