Fly By Wire

[HeliStudent]

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?

[Gemini Twin]

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 :).

[HeliComparator]

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.

[JohnDixson]

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?

[HeliComparator]

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.

[HeliStudent]

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?

including power generation to ensure the probability of a complete failure is extremely remote.

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?

I fly a helicopter that requires hydraulic flight control boost - its unflyable without it

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?

[HeliComparator]

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.

[JohnDixson]

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.

[HeliComparator]

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.

[JohnDixson]

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.

[Harry the Hun]

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!

[waterbottle]

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.

[HeliComparator]

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!

[HeliComparator]

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!

[Bravo73]

Im quite surprised at the concern shown by some towards FBW.

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.

[waterbottle]

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?

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.

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?

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.

[waterbottle]

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!

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:

[HeliComparator]

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!)

[heli1]

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!

[HeliComparator]

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!

[Harry the Hun]

Just because NH-90 does not have a civil certification does not mean FBW cannot be certified under civil regulations. There are the options to modify the control laws of the FBW/AFCS/PFCS that are just not possible with conventional mechanical control systems. Depending on flight regime the results of the same control input has different outcomes, which can enhance the handling qualities of a helicopter severely! Up to now I have not met a single pilot who does not like the handling qualities of NH-90!

Who wants to invade North Korea? To me the dark side of the moon is more interesting to invade than this little ****ehole run by that Fatf**k with the funny haircut. But I have to admitt, it is interesting to see how a discussion on FBW drifts off to the invasion of Korea!

[HeliComparator]

Certainly not implying your "an elephant is a mammal, therefore all mammals are elephants" point! I just wonder whether its worth the hassle for a conventional helicopter.

I'm sure the NH90 is nice to fly, that is not in dispute, although I have to say that the cockpit HMI seems quite clunky and old fashioned (yes, I've had a go, but only in the Simulator), but the of course the origins of its designs are last century.

There is already some difficulty with pilots dealing with advanced automation - the "Whats it doing now?" call is still heard. At least we can teach that instead of saying that, you drop down one or more levels of automation ultimately until you are hand flying. If you want to say "Whats it doing now" whilst FBW "hand flying", what do you do then?

FBW didn't help the Air France bods.

N Korea - just to make the point about the EMP since that is probably the most likely source of an EMP at the moment!

[HeliStudent]

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.

In a FBW helicopter would a computer automatically draw power from the most efficient source and keep rotating the power supply in the event of a generator failure?

In terms of relative safety, does anyone know what kind of processors are used to translate the electrical impulses into mechanical movements at the actuator level. What kind of vulnerability do these imply over the existing methods of control? I assume these are circuit boards which then activate servos? What kind of shielding/protection/redundancy do these have?

[Harry the Hun]

Well, I understand your concernes in regard to EMP, but without any experience made in that field all we can do is to speculate.
The AF447 Flight did not have any probs with FBW, as far as I know. The root case were wrong indications due to failing external sensors and apparently the crew failed in the interpretation of cockpit displays. I believe the result would have been exactly the same if that airbus would have had conventional mechanical controls and a conventional AFCS.
The biggest difference in FBW in Planks and Helicopters is that in Planks the FBW will not let you exceed defined limits, while in the helicopters there is no limitation or restriction programmed into the FBW. It is a common misbelieve that NH-90 will limit the pilots inputs when coming close to potentialy dangerous attitudes or flightconditions. In that regard it is like a mechanical control system, it will just feed the pilots input into the rotors, no matter whether that will result in the exceedance of limits or not.
With your remarks on the MMI you are spot on! A nicely sized cockpit, but but the architecture of the MFDs and especially the DKU hold a lot of room for improvements, but then none of the glass cockpits I have flown in the last 10 years (quite a lot) were anywhere close to perfect.

[HeliStudent]

From a comment on Flight Global -

There are times when I think Fly-By-Wire (FBW) enthusiasm is getting out of hand. Some reasons for my concerns:

About 1982 a prototype SH60B Seahawk was performing an automatic approach to hover over the deck of a frigate when the ship turned on its radar. The Automatic Flight Control System (AFCS) went badly wrong, forcing the pilot to shut the system down and take manual control to save the aircraft.

Also in the 1980s a Black Hawk helicopter flying by a 50,000 watt commercial radio tower crashed when the movable horizontal stabilator (the only FBW control on the aircraft, was driven to the full down position forcing the helicopter into an uncontrolled dive. The crew died.

After both these incidents the aircraft systems were successfully hardened to prevent the reoccurrence of the incident. Now for the a present example of concern;

In an military technology magazine in the 1990s was an article on future energy weapons in which was described an incident where an AGISS cruiser locked onto a passing F-16 with a focused beam from its SPY-1 radar (a Phased Array type radar). The F-16 lost control to such an extent, that the pilot declared an emergency and only regained control when the radar operator turned off the radar. The article goes on to note the Navy was continuing studies into the affect of the focused radar beam on FBW systems.

Putting these examples together with the ban on using cell phones and other electronic equipment by passengers in commercial airliners one has to come to the conclusion that outside electronic interference and FBW may not mix very well.

I have worked in the helicopter industry for the past few decades, and I have begun to be concerned about this push to FBW in combat helicopters. It seems to me to be a Let’s-Get-On-The-Bandwagon philosophy, driven by generals and engineers with very little thought about the ramifications on the pilot and crew who will fly in harms way.

Basically flying a helicopter is vintage World War One flying, low altitude and low speed, with no parachute. Besides that helicopters have proven extremely bad at gliding. They autorotate down, this requires a definite control input, which would be lacking in a fried FBW system. Current combat helicopters have redundant control runs and multiple load path linkages, mechanical systems, which are augmented by AFCS to ease pilot work load and the controls have mostly benign failure modes. The benign modes are because a hundred years of aviation trial and error have made mechanical flight controls (MFC) harder to damage and to fail catastrophically (though, of course, it can happen).

Now there are good points to FBW, such control improvements reduce pilot work loads, however modern AFCS do the same. As observed by comparison of the UH60L with the new UH60M, the “M” is much easier to fly than the “L”. The “M” AFCS is 21st century, as compared to the 1980s technology of the “L”, with loads of workload reducing options. Come to think of it, the AFCS computer on board the UH60M is based on FBW without the wire control runs. If this is so with the US Army’s newest combat aircraft, what then is the performance improvement that justifies the costs and risks of adding a FBW system to a helicopter, especially when the result of FBW failure is death.

If the AFCS (the computer gets fried) goes down on the UH60M, the pilot can still fly manually. If he had to, he could even complete his mission. The engines though installed with electronic fuel control still have a redundant mechanical backup. All this designed in redundancy is based on the combat short comings of the Huey in Vietnam.

What I am afraid of is there will come a time when anyone with a powerfull radar and ultimately a designed for purpose energy weapon will start disabling or destroying FBW systems and especially with Helicopters there will be no recovery and no bail out, because the new whizz-bang technology won out to common sense.

Combat Helicopters and Fly-By-Wire - Defence - Defence - Aviation Forums - Flightglobal Airspace (http://www.flightglobal.com/airspace/forums/combat-helicopters-and-fly-by-wire-25405.aspx)

[sierra_sotiropoulos]

In the case of AF447 perhaps the outcome would have been different if the aircraft had a conventional control wheel (yoke) instead of the side stick.

According to the findings, during almost the entire time the aircraft was falling, the F/O (who had the controls as PF) kept pulling the sidestick all the way back, thus never giving the aircraft a chance to get out of the stall.

Should the aircraft had a conventional control column, there is a possibility that the PNF would have noticed this wrong reaction and intervened to correct it in time, thus avoiding the crash.

Of course this will only remain a speculation and never be proven.

[HeliComparator]

HtH the point about FBW was made to show the hazards of over-complicated basic controls. In normal mode, pulling the side stick right back wouldn't stall the aircraft, but with the air data discrepancies the software had gone into fallback mode where the flight envelope protection was inoperative. I guess the pilots didn't know or realise this. As SS says, the fact that one pilot couldn't see what the other was doing to the primary flight controls was probably contributory.

As I said at the beginning, I am not against technological advancement, but it should be incorporated with a clear sense of the benefits and down-sides, (with the latter outweighing the former) not just because it looks all shiny in a glossy brochure.

Glass cockpits: the NH90 felt clunky compared to the 225, though to be fair the latter obviously has much less kit to interface with. Never mind, could be worse, could be like an S92!

[HeliComparator]

HS, yes I can agree with the sentiment of the article you posted!

[HeliStudent]

HC, and I in turn support your view when you said:

As I said at the beginning, I am not against technological advancement, but it should be incorporated with a clear sense of the benefits and down-sides, (with the latter outweighing the former) not just because it looks all shiny in a glossy brochure.

Can I ask - in the 225, these 7 power sources, are they automatically managed to provide power as and where needed in the event that one or more electrical generating source goes offline?

[HeliComparator]

HS, its rather complicated,

2 alternators /TRUs are the normal source of generated power. If one fails, the other can supply all reasonable needs automatically.

If both fail, some non-essential services are automatically shed. The pilot has to select the hydraulic generator manually and some further services are shed but for example its still happy to do a hands-off ILS with auto-level-off at the bottom.

If that fails as well, you are automatically down to the main battery which obviously has limited life. If that fails you are automatically down to the emergency battery. You lose a lot of stuff at that point but at least retain the standby horizon, airspeed, altitude display. Electrical power for the FADECs / Engine governing would be assured by the small alternators on the engines.

These engine alternators normally power the FADECs after start, but if they fail the system automatically reverts to airframe power - any of the power sources above except the standby battery. But these engine FADEC alternators are small and can't be used to power anything else on the airframe.

So although there are 7 power sources, they can't all be used to power everything since the smaller ones have their power limits. However I mentioned it to show that helis already have multiple power sources, so the concept is there even if the detail would have to change for FBW.

[cattletruck]

There was a Qantas A330 that performed a violent uncommanded maneuver that was attributed to a dodgy nav box that was supplying the FBW system with the next level of efficiencies of automation. As I understand it the nav box sent the flight director a bunch of corrupt messages which then told the autopilot to act like a roller coaster.

The bus's side sticks didn't move during the violent maneuver, had it been a helicopter it would probably have broken itself apart.

I don't want to bag FBW, there is good and bad engineering everywhere, and fortunately the engineering of FBW is maturing quite rapidly with a corresponding price tag to match.

Aren't those little flaperons on Kaman helicopter's main rotor blades electrically controlled, is it a FBW system?

[JohnDixson]

HS,

Re the UH-60 anecdote reported in the Flight Global article: unfortunately typical of the apocrypha that went around for a few years in fact regarding the stabilator. There was only one stabilator induced crash, and that was the 1978 crash of one of the prototypes at Sikorsky. That had nothing to do with EMI or the like, but was incomplete maintenance done during an all-nighter to get the ship ready for a VIP ride in the morning, coupled with an inexplicable failure by the crew to utilize the dual, backup straight to the actuators DC manual controls. There was however, an event that took place in Korea in the 80's, wherein a 60A was flying by a Hawk Missile Battery, who thought they would practice on it using their radar, with the result being that the Stabilator electronics reverted from the auto mode to the manual mode.

The Seahawk story is news to me, but after reading the 60A inaccuracies, am skeptical. And skeptical not only because they had the 60A story completely wrong, but also because, being involved in the Seahawk first flight thru development, I am aware of the scope of the EMI invasive testing the USN put the machine thru at PXT. Given the mission of the machine, it was exposed to all of the naval electronic transmission equipment, from the carrier radars on down.
" never say never in aviation " the saying goes, but I would guess the real story about this Seahawk event might be different.

One other observation for HC and the EC community re redundant power sources etc.: it always surprised us at SA that EC never upgraded to installing an APU as standard equipment in the Super Puma line. It really does provide benefits across the board, not only for dispersed field operations and ease of maintenance, but well, take the blade de-icing system. If the ship has one main generator go down, the Hawk or 92 pilot can fire up the APU and continue. Not an end of the world difference, but sure nice to have the all around capability. In the Hawk, up to a pretty decent altitude/ temperature combo, the pilot can hit BOTH engine start buttons simultaneously and start both engines at the same time ( APU bleed air used for start ).

[HeliComparator]

Not having flown a heli with an APU maybe I am biased, but in the examples you give the blade deicing on a 225 continues to work on one alternator (they are big alternators) and you can press both start buttons at the same time, although I grant you that starting will then take place consecuitively not concurrently due to the limitations of the DC supply system. Airstart means the APU must be working, otherwise your bird is dead - ie a single point failure. A few plusses and minuses there but when you include the additional weight and maintenance of what is effectively a third engine, I'm not sure the case for APU is clear cut. Of course if you want to run your aircon before startup, APU is great and that's why EC offer it as an option for VIP / hot weather (not usually a problem in Aberdeen!)

[JohnDixson]

HC, on Hawks and 92 one can start one engine from bleed air sourced at the other, operating engine ( there is a switch labelled " Crossbleed "). On Hawks, there is also an external air source fitting, thus one can either, start using any standard external air cart, or, yes, connect one Hawk to another for a " buddy-start ", using a long air hose ( one or two per lift company ). Vietnam era pilots will readily understand the Army's focus on not repeating the oft-used expedient of trundling a ni-cad from one Huey to another to get the CA on the road.

PM on the way re ambients.