Fly By Wire
 

Could someone please explain what will happen in the fly by wire helicopters which are being planned for the future if they experience a total electrical failure such as a problem with the helicopter alternator/generator or a break in the wire harness which connects to the alternator or anything which could result in a total electrical failure such as a lighting strike or even for combat helicopters if they receive an electromagnetic pulse?

The system will be subjected to an intensive failure modes and effects analysis and designed accordingly with multiple redundacy.

A broken wire on "the alternator" will not be a problem :).

Well I'm no expert but of course there will be a lot of redundancy in the systems, ie multiple computers and circuit paths, including power generation to ensure the probability of a complete failure is extremely remote. I fly a helicopter that requires hydraulic flight control boost - its unflyable without it so if I suffer a double hydraulic failure I'm gonna die. However there is a lot of isolation and 3 pumps to help that not happen.

Core systems like that are protected from lightning effects even if some of the less critical peripherals aren't.

Nevertheless I'm sure that certifying the first civil FBW helicopter will be challenging!

Modern military aircraft have EMP shielding though I suspect it depends on how close / the magnitude of the EMP.

Challenging?
 

Understatement, HC.

Is anyone aware of work on a Part 29 update/amendment to provide design guidance? Same question for work on an Advisory Circular providing guidance for certification testing.

SA has done two FBW machines aimed at production*:

1. Comanche, which had a three axis side-arm cyclic, with limited motion plus force sensors, and an electric proportional motion collective.
2. Canadian MHP with a center two axis limited motion, force sensor cyclic and electric proportional collective and pedals.
* I skipped the FBW UH-60 project because that was ( to me at least ) an exploratory effort. Certainly was managed that way.

So both have some obvious differences, both flew/fly very well indeed, but both are absolutely way out in front of any regulatory overview by the FAA.

When one looks at the certification standards in place now, all of which are based upon fully mechanical controls with all of their dated control characteristics and stability related FAR requirements, the prospect of FAA certification is at least challenging.

Perhaps someone from the Ft Worth Office monitors this forum and can bring us up to date with what is going on within the Rotorcraft Directorate on this subject?

I suppose one has to question "why bother?". With powerful autopilot series actuators the pilot can be well removed from the actual flight characteristics of the basic heli, whilst the retention of control rods/cables allows for ultimate reversion in the event of catastrophe.

I don't mind FBW engine controls (FADEC) but not so sure about the desirability of full FBW flight controls.

Any yet fbw is being talked about (from what I have read) as something which will feature in next generation helicopters.

I understand about redundancy but isn't a wire or even several wires more vulnerable than a control rod? I mean its comparatively easy to damage/disrupt a wire/s compared to a control rod, isn't it?

Probably each engine will have its own alternator but will there be additional back up systems such at the ram air turbine that airliners carry?

But these hydraulic boosts will still continue operating even after an engine failure for example so long at the rotors are turning the pumps?

I suppose what I am asking is, is there more vulnerability to helicopter safety in a FBW system than in conventional control? I understand in a massive airliner that having control rods etc. over long distances probably increases risk but in small to medium size helicopters wouldn't conventional control mechanisms be safer, especially for military operations?

In order to achieve certification, a manufacturer would have to show an equivalent level of safety of FBW compared to pushrods. Pushrods can get jammed, disconnect at the ends or I suppose possibly fracture or be shot in half, therefore the are "quite safe" not "absolutely safe". So similarly an FBW system has to be "quite safe" to the same level, but will never be "absolutely safe". Whereas 1 pushrod achieves that level of safety, it would take several computers probably running different software, multiple power sources and connections.

As you say, there is significant weight saving in large FW, some military fixed wing are so unstable that FBW is the only realistic way to fly. Maybe for x3, tiltrotor etc with complex control mixing but its harder to see the advantages for a conventional heli.

Incidentally the heli I fly already has 7 sources of electrical power, two alternators / TRUs driven by the main gearbox, a hydraulically driven turbine generator for emergency backup, a small alternator on each engine for FADEC supply, a main battery and a standby battery. Although some of these are currently too small to run a FBW system, you get the idea that redundancy of electrical supplies is already extant in modern civil helis.

Times Change
 

In 1970-72, SA and Boeing were getting together competing designs for an Army HLH. Our guys wanted to go full FBW, but I authored a position paper for the Chief Pilot that took the opposite position, using in part the reliability data on all of our Ham-Standard AFCS systems. That data was dynamite* and the memo created a serious stir, all up the line. A bit later, when the ABC first flew and the mechanical controls were so complex as to cry out for a non-existent FBW technology, a very senior Army R&D person told me that he truly accepted the advantages of fly by wire, just as long as all those little wires travelled down the center of the push/pull tubes. That was the mood at the time in many parts of the industry.
* As I recall, the MTBF in the single, analog initial S-64 AFCS system was just under 400 hours, as an example.

But times change. I would wager that if you contacted your friends at EC, they would concur that FBW technology is certainly mature enough to utilize. The advantages of weight saving, cost saving ( not only the elimination of all the extra actuators, rods and the rigging thereof, but your note cites electrical power redundancy that is already there ), vast safety improvement*, and an absolute free hand in designing in handling qualities tailored to the machine/mission, are there for the taking. Ask them.
* Safety advantages derive from several areas of change. The problems of rigging compounded by mechanical mixers, boost actuators, AFCS actuators, cable tension devices, and the like, are eliminated, along with maintenance errors made during the troubleshooting or replacement/re installation of same. FBW offers the ability to integrate smart envelope limiting into the basic control system.

Yes, it is unlikely one could make the argument that FBW makes sense for an R-22/44 type machine, but for the larger, more complex aircraft, it is a technology that is a competitor for inclusion in a new vehicle.

It's an interesting debate John! I take the point about reducing the mechanical and electrical complexity, however its not just complexity and reliability that are the players, its also the consequence of a failure - sorry, I know that you of course know that anyway!

I would rather have any number of failure of actuators etc than one complete FBW failure! Designing systems with true isolation / no single point failure modes is quite hard. Yes you can make fancy and very clever software and certify it to level one, but it can still have bugs (trust me, I know, though fortunately non-critical ones!). Once you get beyond a few thousand lines of code, I remain unconvinced its possible to guarantee bug-free!

By the way, by "bugs" I don't necessarily mean only where the software does something not intended by the writer, but also where the writer/system designer specifies the behaviour inappropriately - after all these guys sometimes live in the virtual and theoretical world of the lab, rather than the real world of flight.

Losing My Touch
 

Just thinking that, I'm nearing 73 years of age and unable to convert a younger pilot who is still in the thick of it, to adopt a newer technology! Uh- oh.

Fly By Wire
 

FBW on NH-90 works flawless with quadruple redundancy. Of course in the event of a complete failure you are fishfood. That is why in case of multiple failures you have to land immediately when operating on the last remaining FBW computer.
Electrical power is not engine related because alternators are driven by transmission!

Im quite surprised at the concern shown by some towards FBW. As a fixed wing pilot Im perfectly comfortable with it, even though my current type is my first and only experience of FBW.

Is it because there haven't really been any examples of FBW in helicopters other than in developmental or test roles?

Is a helicopter any different than a fixed wing aircraft in its need for a reliable flight control system?

When I compare my current type to previous aircraft I have flown that do not have FBW I see that I now have far more redundancy in my flight controls. I also have greater accuracy and efficiency.

When I look at aspects such as power delivery to the flight control system I see that I have six independent sources of electrical power. That is also not taking into account that each of those sources supplies multiple flight control elements thereby giving further redundancy. There are also three flight control modes to allow for reducing levels of system operability. When I compare to another non FBW type I have flown that relied on cables, push rods, and two hydraulic systems I start to wonder how I ever thought that it was safe! Of course it was in fact perfectly reliable, but this new aircraft has so much more redundancy and reliability that you can easily see why fixed wing aircraft are predominantly all now FBW.

Perhaps the experienced helicopter types amongst you can explain to me a fundamental difference with helicopters that would preclude their using FBW. As a fixed wing pilot that has experienced both sides of FBW I certainly cant think of a reason why you wouldn't want it.

Hi John don't despair! I do like new technology, the 225 is one of the most advanced civil helis in terms of its electronics and especially the autopilot and I love it! I just think we have to be careful not to add technology for the sake of it, but only where a clear need or benefit can be demonstrated. As a pilot not paying for the machine, I don't see cost reduction as a need or benefit!

EC have been developing some very clever AFCS functionality which might be considered pretty close to your "FBW with wires inside the control rods" - ie all the advantages of pilot control detachment from swash plate, flight envelope control etc but retaining the ultimate backup of rods. So much easier to certify!

waterbottle, a couple of differences: FW has inherent stability so the aircraft will keep flying with no flight control inputs, eg whilst the system is rebooted following catastrophic software crash, glitch caused by cosmic rays, lightning etc. Helis will typically be upside down within a couple of seconds of swashplate freeze at cruise speed. You couldn't do a Soux City in a helicopter!

Helis suffer from a much less electronics-friendly environment mainly due to vibration levels. Take an item of avionics installed in FW, MTBF is looooong. Identical item in RW, MTBF can be shorter by orders of magnitude.

Oh and yes, we are in the main Luddites!

Don't worry, it's not 'by some'. It's just by 'one'.


And as he's already demonstrated in this thread, in his view, if its not on the EC225, it's not worth having. And what a great aircraft that has turned out to be. :rolleyes:


FBW is the future. Get used to it.

I would say that a control rod is, although very well understood and perfectly safe, more vulnerable than a cable. Think about it this way. What is that cable doing. Effectively nothing. It sits there with those magical little electrical signals passing through it. What does a control rod do. It is constantly moving, it is never still, there are always small loads being applied to it. Now I am not saying that control rods are unsafe, but there are plenty of examples of mechanical control failures, whether they be due to poor maintenance, metal fatigue, design failure etc. Of course there are also examples of cable failure due to similar reasons. Overall though there is potentially less strain and less chance of failure to a cable.

Bear in mind that the RAT is primarily there for hydraulics, not electrical power (on my type anyway). There is so much electrical redundancy that the RAT comes at the end of a VERY long line of electrical back ups.

As has been touched on by another poster if you had any concerns about the flight control system on any aircraft then it should be with the hydraulic side rather than the electrical generation or transmission side.

I think you might be surprised if you look at modern airliner designs with regard to inherent stability. Im sure of course that you are aware that most military fighter types that are FBW are inherently unstable.

Not that it is possible but if you were to turn off the FBW then a fixed wing aircraft will be in a similar position to your hell!

With regard to your swashplate freeze. I take it that the swashplate is hydraulically powered? What happens if the hydraulics lock up? What happens if your control rod linkage separates?

FBW is only a part of the whole system. It replaces parts that are inherently less reliable than the replacement. It reduces costs, build and maintenance, meaning we as pilots are more in demand for the increase in work that can be done :ok:

It also means a joe average pilot can hand fly to a higher standard, which Im more than happy to take advantage of! :ok::ok:

WB - Modern FW are more stable that RW by orders of magnitude. You only have to look and the moment of inertia and radius of gyration given by long wings and a long fuselage. I have no doubt an uncontrolled FW might end up upside down, but it would take a lot longer than 2 seconds or so compared to a heli that has no wings and thus a very low moment of inertia in roll.

As I mentioned earlier, a swash plate freeze (yes, hydraulically powered) is catastrophic but as I also mentioned it boils down to how many failure modes and their probabilities an FBW has compared to pushrods and hydraulic pumps.

As I also mentioned, I don't really "buy" the cost thing. People will fly if they need / want to and cost saving of FBW in the great scheme of things is minimal per passenger mile. Especially for helicopters where the economies of scale apply much less in overcoming the initial design and certification costs

On the "hand flying" thing, FW are different I believe because a non FBW hand flown is just the basic aircraft and aerodynamics, whereas any IFR heli has at least some level of autopilot between the pilot and the swash plate that gives artificial stability and makes it infinitely easier to hand-fly. We never fly without this engaged except in emergency or for training. So that benefit doesn't really apply, the more so on modern types that have very sophisticated autopilot functionality always present when in flight even when "hand flown".

So I still don't really "get" why full FBW would be to my advantage?

Bravo, do you actually have anything to contribute to the discussion or do you just get your kicks out of sniping? Perhaps you could try attacking the arguement rather than the arguer? It might be interesting (though I have to say I somehow doubt it!)

EC did some interesting FBW work on a Dauphin whilst developing the successful NH90 FBW system,including fitting an adjustable elevator and a rudder to offload the main rotor during takeoff/climb. The aircraft flew a series of flights with a safety pilot in the l/h seat with conventional rod controls bur the conclusions were vague.this Dauphin can be seen in the Helicopter Museum whilst the NH90 can be seen in Afghanistan, Australia, France, Germany, Greece, Italy,Netherlands , Spain et al!

Nh90... with no civil certification and no chance of it! I can see an advantage of FBW for military in terms of being able to have a distributed and redundant control system - more tolerant of a bullet than a single control rod. On the other hand more vulnerable to EMP. Don't take them into N Korea when we invade!