Small aircraft crashworthiness

[Dafydd Llewellyn]

I imagine - I don't KNOW - that an airbag might be a substitute for a hans device or equivalent head-neck support - and indeed if you use a crash helmet, the added mass of the helmet makes neck injury more likely in a sudden deceleration event. However, that's supposing the major impact is "on the nose" (as in a motor vehicle). The aircraft requirements for impact testing are more directed towards impacts from below. I do know that the impact testing for the Whitney Boomerang showed that whiplash protection is necessary (i.e. a headrest behind the head). However an air bag cannot cope with multiple impacts, whereas a full safety harness with a hans device can.

 

If one were flying low-level air racing or something like the Red Bull air racing, I expect going to this extent would be advisable. However it will not protect your spine from an impact from below; and it does little to correct basic structural collapse of the cockpit; so I would not consider it a substitute for proper basic crashworthiness design. Simply designing the lower front corner of the firewall so it slides rather than digging into the ground, would achieve at least as much benefit as airbags, I would imagine.

 

I'd advise people not to purchase aircraft that have obvious design deficiencies of this sort, rather than trying to compensate for the deficiencies by the addition of airbags.

 

We all make compromises in these regards - I'm setting up a Blanik, which has anchorages for 9 G four-point harnesses, and no possibility of more than about a 2 inch cushion of temperfoam. I'm counting on the primary safety. But at least I understand what I'm choosing. I hope this thread will help other people to understand what they choose.

 

 

[dazza 38]

A few years ago a bloke in Europe crashed a Savage cub into a lake. The airspeed indicator was recovered and the airspeed indicator revealed that the aircraft hit at 120 kilometres per hour. The pilot escaped relatively unscathed.

 

 

[bexrbetter]

Airbags built into the seatbelts are available for aircraft, although I suspect not for LSA yet. With no other structural modification do you think these would significantly improve the crash worthiness of LSA or small GA aircraft?/

Seat belt tension is critical and inertia reels found in all cars are a poor compromise to get people to wear belts. Prior to airbags Volvo and BMW had explosive belt retractors that actually pulled the belt tight in an accident.

 

So yes, the airbag seatbelts are a great idea but just as good as a harness that is worn properly tight - again evidenced by what racing drivers walk away from.

 

Earlier open cockpit aircraft had padding around most of the places you would contact.

I'll find the specs for foam on racecar rollcages for you later. Rollbars killed many a driver before padding and distances from head were made mandatory.

 

 

[Old Koreelah]

...trying to fight your way out of a cockpit with inflated airbags would be difficult.

Airbags only fully inflate for an instant; there are bloody big holes at the back to let the gas out.

 

 

[SDQDI]

Airbags only fully inflate for an instant; there are bloody big holes at the back to let the gas out.

And the powder lol.

It always interests me that movies always portray airbags as balloons that stay inflated and it is quiet a large misconception that can be dangerous.

 

I think the likes of a seatbelt airbag would be great even if it does no more than keep the stick out of your guts, once it has done its job it would be no hassle as like OK says they deflate almost as quick as they blow so you would only have to wrestle with a rag.

 

 

[metalman]

Airbags only fully inflate for an instant; there are bloody big holes at the back to let the gas out.

The steering wheel and pax side work like that, the side curtain bags inflate like a thin air mattress , I've fired dozens of them!

Front impact bags are inflated by an explosive charge, side curtains are inflated by a compressed gas, they stay inflated to protect the occupants incase of a roll over

 

 

[SDQDI]

The steering wheel and pax side work like that, the side curtain bags inflate like a thin air mattress , I've fired dozens of them!Front impact bags are inflate by an explosive charge, side curtains are inflated by a compressed gas, they stay inflated to protect the occupants incase of a tool over

You are right with a lot of curtain airbags but I don't think that would be the case with the seatbelt airbag but I haven't seen them in operation or the aftermath but I would suspect they would be more like your steering wheel/dash airbags.

 

 

[metalman]

These ones appear to stay inflated ,although they may deflate at a slower rate than vehicle front impact bags

 

 

[metalman]

 

Here's a Toyota bag being deployed,

 

 

[Jabiru Phil]

Got me thinking that an airbag installed in the seat would probably save spinal damage. It would need to be designed thin enough on inflation so as to not propel your head through the canopy.

 

Just a thought.

 

Phil.

 

 

[Dafydd Llewellyn]

I suggest you read this first: http://www.dtic.mil/dtic/tr/fulltext/u2/a221557.pdf

 

 

[M61A1]

I suggest you read this first: http://www.dtic.mil/dtic/tr/fulltext/u2/a221557.pdf

Interesting. I think I stumbled on that document once previously, when there was some discussion about helmets.

In reading that, I think the best bang for your buck, would be to go out and spend $2k on a good flight helmet, yes they increase the mass of your head, but a proper flight helmet is much lighter than your average motor vehicle helmet, and they are specially designed not to restrict your peripheral vision. Hearing protection and eye protection are also built in.

 

 

[Oscar]

Caveat: I am NOT an engineer and what follows is a series of observations, and I invite correction and comment that is based on the principle of trying to assist understanding of this whole issue of occupant safety. Dogmatic denial of what I offer based on nothing more than personal bias against specific aircraft does not help this matter; however qualification / questioning / additional information most certainly would.

 

I believe that a great deal of secondary safety comes (mainly) from two aspects: the dissipation of energy at a rate survivable by the occupants and a lack of intrusion to vital organs from structural members / unsurvivable collision of the human frame with the airframe.

 

Most small aircraft have a very limited amount of material around the occupants available to dissipate the energy in a crash - and that depends very much on the nature of the impact. As far as I am aware, for a near-vertical descent, (without impact absorbing seats), the only source for dissipation of energy is the u/c, which needs to be capable of collapse at a vertical speed at a rate that imparts non-lethal stresses on the lumbar and neck. Once the u/c has gone, any residual energy is pretty much guaranteed to kill / render the occupants at best, paraplegics. Some temper-foam on the seat is an addition to safety and though the compressibility characteristics are good, the secondary possibilities of belts loosening, 'submarining', coming into contact with bits of structure are all things that need to be considered - just adding some temper-foam may give you very little but confidence.

 

However, in an impact with decent forwards velocity, there's a lot more opportunity for 'good' characteristics in design and materials to come into play. Rather than get into an argument with anybody who feels that comparisons between specific aircraft are intended to disparage one vs the other, can we just look at the 'good' aspects of one type and people can muse on that and think about how other types match up.

 

I'm going to use Jabirus as my example of the combination of 'good' characteristics ( no surprise to those on this site that have read my previous posts).

 

I think that it is fair to say that the statistics show that Jabirus have an extremely good ratio of 'survivable' crashes (and a statistically meaningful incidence of crashes, let's be up-front there!). The idea that most of those crashes have been 'small' ones does not invalidate the observable results from significantly more serious ones.

 

http://www.jabirucrash.com/images-crash-site/Dscn1888.jpg

 

http://transform.fairfaxregional.com.au/transform/v1/crop/frm/silverstone-feed-data/ba98490d-a4d7-4765-9ea5-53f129e87037.jpg/r222_268_1432_989_w1200_h678_fmax.jpg

 

http://cdn-www.airliners.net/aviation-photos/middle/5/2/6/0656625.jpg

 

http://i859.photobucket.com/albums/ab160/bc_j400/mgt.png

 

What characterstics do Jabiru's have that provides a demonstrably decent level of occupant safety?

 

I suggest there are two major factors, and as almost always, these actually combine into a 'system'.

 

The first, is an effective 'occupant cage'. By the nature of a strut-braced high-wing single-tractor-engine design, there are three major load-bearing locations: the mainspar, the lift-strut and main u/c attachment and the engine mount. The occupants sit inside the triangle formed by those three connected points of strength. So, what is needed for occupant safety AND what is needed to keep the aircraft structurally sound as a conjunction of the three critical areas of load, work together. That has the advantage of crash worthiness being integral to the design structural requirements; or, in very simplistic terms - occupant safety and structural necessity coincide.

 

By comparison, a low-wing aircraft - and particularly one with a large 'cut-out' in the fuselage structure necessary to fit two occupants into a small package - in crude terms resembles a plank connecting the mainspar, the u/c and the engine, on top of which sit the occupants. All the occupant safety structure is essentially 'additional' to the basic structure that connects the main load-carrying structure - and that means, (again in crude terms), additional weight for the required characteristics. I'm not suggesting that a low-wing aircraft can not offer both good basic structural strength AND good occupant safety - but - due to the bloody stupid weight limitation restrictions for our class of aircraft - we pay an occupant safety penalty for 'redundant' weight in terms of 'usable' weight - and that translates to reduced range, carrying capacity and performance.

 

The second 'advantage' that Jabiru have over some other aircraft in terms of occupant safety comes from the selection of a low-tech composite structure. Jabirus are made from an ambient-cure, hand-laid glass-epoxy matrix. Not even vacuum-bagged to achieve a high glass-resin ratio. What does this mean?

 

Well, it means that in order to achieve the required stiffness, it is fundamentally excessively strong in terms of deflection to failure stresses. What is this? OK, let's try to develop some understandable examples.

 

Let's take a series of beams, of various materials, of 25mm wide and 300mm long. We'll put them in a vyce set vertically and put a weight hung from the outer end. Let's say, that weight is 5 kgs and the required deflection of the beam (for other structural reasons) has to be no more than 5mm. A beam of carbon fibre will almost certainly be the lightest construction that meets those specifications (leaving out 'unobtainium' materials such as Titanium, highly-exotic fibre reinforcements such as boron-fibre and (probably) materials involving nano-technology). High-end aluminum alloys will be next, I think, and good wood (sitka spruce, hoop pine) will be fairly well up there. 4130 steel will come into the picture.

 

If you are looking to materials that optimise the stiffness-to-weight ratio, the Jabiru composite will come in quite badly. However, if you then load that beam to the point where it fails to remain a beam and buckles, there is a different story. The Jabiru composite beam will deflect by a very large amount (and a very considerable weight) before it finally tears apart. The rise of resistance to force is reasonably linear, which gives the human frame a better chance to accommodate the circumstances up to the point of failure, akin to the old experiment of dropping an egg from 20 feet onto concrete or a thick down pillow. A structure that is 'excessively strong (rigid)' and does not deform until beyond the point of the body's tolerance simply transfers unsurvivable forces; a structure that is excessively 'weak' and deforms catastrophically while the occupants are still surviving is pretty obviously inadequate protection.

 

The next thing to consider is the behaviour of the structure surrounding the occupants when it does - eventually - fail. Jabs are basically a monocoque and a pair of cones with the join being at the occupants' shoulder region. If you take two cones and join them at their bases and then push from the pointy ends inwards, they will tend to fail at the join by it deflecting outwards - or AWAY from the occupants. There is no structure around a Jabiru cockpit that inherently will collapse inwards, crushing the occupants (provided, obviously, there isn't something trying to force its way in, such as a tree), and the curved shape of the 'cones' of structure will tend to ensure they fail outwards, away from the occupants. That curved shape is not only aerodynamically useful but also structurally useful, as the compund curves tend to diminish drumming of the structure - so that's a bit of a win-win as a result of the material choice. Mostly the same applies to a well-designed steel tube structure provided the tube(s) don't detach; however it needs to be recognised that steel tubes act mostly in direct compression /tension and so if there is a bending load applied on a tube acting in compression it will eventually collapse in the direction of the bend. And - generally speaking - once one member of a steel tube structure fails, others will follow until there is insufficient energy left, as they become over-stressed.

 

The final point that should be made is that, particularly in a low-tech composite structure, load paths have by necessity to be taken out over a fairly large area - you can't just put in small areas of high-stress to accommodate attachment of things because the nature of the material won't allow you to. That has the beneficial effect of transferring failure-level loads over a larger area and that larger area provides more material to progressively dissipate energy as it fails. The 'classic' Jabiru overturn accident shows repeatedly that the firewall (and therefore everything forwards of it) will eventually detach, torn away from the 'A' pillars, with (usually) relatively little damage to the actual occupant cell structure, absorbing quite a bit of the energy as it does.

 

Jabirus tend to 'bounce' rather well in the case of an accident, with the combination of the strength of the material and its ability to deflect quite considerably before failing serving to absorb at least some of the energy into deflecting the entire aircraft away from the 'hard' bits of the scenery. If you like, think of the difference between throwing an egg - for which the shell is a very stiff structure for its weight- against a wall vs throwing a squash ball at the same velocity, angle etc. Anybody who has owned a conventional fibreglass yacht will know how well they 'bounce' when you misjudge the approach to a wharf - for instance..

 

Jabirus are not 'perfect' - quite obviously. I would not be so silly as to suggest that they are; however they have a reputation for occupant safety that is enviable and statistically significant in this class of aircraft. If you simply examine closely a Jabiru while thinking about what may happen in a crash and compare that with the same situation for different aircraft with an appreciation of the 'system' that each aircraft offers in terms of occupant safety, I believe that you may have a better chance of, at least, being able to evaluate how well the aircraft you are considering (or flying now!) MAY protect you in a crash vs. what a Jabiru can deliver.

 

I'm not trying to set Jabirus up as 'the gold standard' here; however, statistically they are about as good as it gets in this class of aircraft. We all know that there is a price in terms of performance, usable weight etc. that Jabirus pay for their low-tech composite structure; nothing comes for free.

 

Absolutely everything we fly, drive, use in our daily lives is a compromise; what one chooses SHOULD be the set of compromises that you evaluate best suits your needs and situation. To make that choice, you need as much information as you can get (and understand). Jabirus offer a set of compromises that -and this is borne out by the statistics - tend to offer pretty good occupant safety in the operational environment in this class of aircraft. At the very least, understanding why that is may be of value.

 

 

[Head in the clouds]

A lot of good stuff has come out of this discussion already to stimulate further thoughts I've put together a very short summary of the main points and issues, these can be toyed with at will and sensibly divided into things that might be retrofitted to existing aircraft, and those that have to be taken into account at the conceptual design stage.

 

I'm sure the list is by no means complete yet, so folks please add more items as they come to mind - Old K, you mentioned you had made a dozen or so safety mods to your Jodel, can you expand on that a bit further please?

 

1. Harnesses - 4&5 point, consider the angles to the shoulders and to the lap strap

 

2. Securing of loose and heavy objects

 

3. Open cockpits/low wing canopy designs generally not as structurally crashworthy as high-wing cabin designs

 

4. Steel cage fuselages considered strongest/most crashworthy. Jabirus have good record for glass structures

 

5. Energy absorbing seats, stroking room etc

 

6. Helmets - also provide eye protection and incorporate comms, helmet weight may be a risk of injury in itself

 

7. Temper-foam

 

8. Internal padding as is compulsory in motorsports

 

9. Personal airbags, harness airbags, side airbags, motorcycle helmet airbags (see Gizmag), bum airbag

 

10. BRS - has the advantage that a non-pilot pax (your wife?) can operate it if you are incapacitated

 

11. Molded seat inserts

 

12. Chamfered/bevelled bottom edge of firewall

 

13. Rudder pedals - avoid inverted L shape welded tube pedals - risk of broken ankles

 

14. Fuel bladders and/or cross-linked polyethylene fuel cells as compulsory in motorsports

 

15. Engine reliability - more reliability equals less forced landings (but would people then practice less??)

 

16. Automatic electrical Master cutout - how does that work? Does it cut the power at the source i.e. at the battery?

 

17. Fixed seat and adjustable pedals rather than vice versa

 

18. Keeping the crash more horizontal than vertical regardless of ground aversion - vis Seabird Seeker Airflow system

 

19. Attention to overhead structure collapse eg Cub rear spar intrusion into cabin - X bracing rather than K bracing

 

20. Reference - USAF Crash Survival Design Guide Post #57

 

21. Reference Don Morgan's Cone-Head Post #58

 

22. Hans device #76

 

23. Headrests - considered essential in Whitney Boomerang tests, slot between headrest and seat-back corrects harness angle

 

That's a pretty good list so far and I find it quite surprising that several of the things have been considered essential and have been compulsory in cars and racing for many years, but have been completely neglected in our potentially far more dangerous activity of flying.

 

Something that hasn't been mentioned is that of 'rotation injury' which our US expert says is statistically the main way that people get killed, whether in stroker seats or not, and more so than hitting the panel in folded up low-wings, apparently. 'Rotation' being the actual mechanism of the fatality. I'm not a medical person so I only know what little our friend has told but it has to do with the head, neck or internal organs staying still while the rest of the body is rapidly rotated, as I understand it, a bit like the classic commando style of silent killing by twisting the neck suddenly and violently. This can occur from the plane cartwheeling, one wing striking the ground first, or as simply as by not wearing a three point harness correctly. In fact it provides a very good case for four point harnesses as there is little else that can be done to reduce this type of force except keeping the wings level at the last moment - the 'Airflow kit' would be a boon in this regard I would think.

 

Regarding helmets - I have a Gentex that I used for the early part of my mustering and I liked it mainly for the excellent visors and good soundproofing particularly because it was before ANR headsets were readily available. Statistically speaking we (the mustering industry in general) had a regular percentage of machines crash through one reason or the other and I'm not sure that I can recall anyone who survived purely as a result of the helmet - no doubt there was the occasional one though. The fatal crashes tended to be unsurvivable and the others often resulted in quite minor damage to the machine or occupant.

 

There's a bit of a stigma to wearing a helmet when going for a casual fly in an enclosed cabin aircraft too, either it looks like a pose or that the occupants aren't very confident in their ability to complete the flight safely. That Cone-Head thing, and modern sports headwear got me thinking though. I'd be quite happy to wear head protection if it didn't look like a helmet, and would be more a case of padding than a hard bone-dome. I'm thinking of something like some NRL players and all kids wear to play contact sports (think Johnathan Thurston, for NRL or SoO fans), then add a peak like a cap, a microphone and ANR earpieces, it'd get rid of that hard spring band over the top of the head for one thing - what do you think, would you wear something like that?

 

 

[Old Koreelah]

Good summary, HITC.

 

I'm completely self-taught, so my efforts are often a bit on the rough side, but they are the result plenty of thought and seeking advice from the wise.

 

Jodel safety features:

 

Battery mounted behind firewall, away from fuel system. A remote arm next to the pilot's leg automatically disconnects the earth in the event of a hard landing or crash.

 

Fire extinguisher plumbed into engine bay

 

Fuel tanks in the wings behind spar, protected in rubber-lined cradles. Tanks made from vinyl ester (resists all common fuels) and wrapped in layers of kevlar, which resists penetration and returns to shape after a major trauma.

 

Tanks vented from top inboard end to bent-up wingtips. Tanks can only drain thru the vents if upside down, dumping fuel (hopefully to soak into the ground) far from the upturned cockpit.

 

A sudden impact could cause a pressure shock in the fuel tanks. The large diameter vent tubes are designed to dissipate the pressure so the fuel caps are not blown out.

 

Fuel taps on each tank and also before fuel lines pass through the firewall.

 

Duplicated fuel lines in case one pump fails.

 

Ballistic Recovery System mounted ahead of the instrument panel with harness anchored on spar bolts. If rocket is launched, it tears out harness buried under outside plywood skin. Steel cables anchored on rear wing bolts hold parachute harness back so that aircraft descends nose down to ensure undercarriage absorbs most impact.

 

Trim system acts as backup elevator control circuit.

 

Flat plywood rudder pedals which won't injure feet in the event of a prang.

 

Windows in floor to improve visibility before startup and in circuit.

 

Wingtip strobes spring-mounted in sockets to allow movement if struck, reducing chance of damage.

 

Head protection: (the cheap approach) a bicycle helmet with Dave Clark earpieces bolted to each side. Light, easy to clip strap, you always have to wear it because that's the only headset.

 

A substantial rear bulkhead and two chrome-moly steel canopy hoops. Whiplash protection: deeply padded head rest.

 

Fibreglass seat moulded to pilot's backside, with hinged lumbar support. Seat mounted behind spar with provision to slide down, progressively crushing thick styrene blocks.

 

Sliding/folding canopy that seals shut, but which can be opened in flight.

 

Screen of polycarbonate (resists bird impact) and canopy of acrylic (easy to smash your way out of if inverted).

 

Cockpit lined inside with plywood to improve strength and reduce the risk of structural spruce timber splintering in a crash.

 

Carbon monoxide detector with independent power supply.

 

Fold-out chart table for maps and flight plan.

 

Fold-out iPad mount. Fixed iPhone running GPS.

 

GPS locator beacon carried on pilot's belt, can be activated in flight, and is not left aboard aircraft if pilot exits a wreck.

 

Humidity, outside air temperature and carby throat temperature readouts on panel.

 

The aircraft folds up and is transported on a carrier that lives in a secure shipping container.

 

 

[bexrbetter]

Wow OK, that's awesome!

 

 

[turboplanner]

Sorry, I started out to make this a comprehensive list, but have an urgent project. However this gives some alternative ideas for a shopping list of subjects.

 

The US has been very slow to covert to using seat belts, having tried "automatic seatbelt entry where, when you open the door the sash is moved and when you sit down and close the door, all you have to do is buckle up, but the fact is that a sizeable part of the population, including cops in pursuit still don't wear them, so the US air bag performance has to decelerate total body weight, and at the US 95th percentile.

 

In Australia, with almost universal seat belt use, our airbags only have to cope with extraneous movement, such as arms and head, so ours are quite a bit more benign, and a number of people I've spoken to believe their airbag failed in the accident they had, when it really had deployed and deflated to suit the g forces before they even realised they had been hit.

 

So, while I don't have any contact for airbag manufacturers, I think they could probably come up with a specification to suit likely common g forces.

 

However, in looking at secondary safety possibilities, I think it would be well worthwhile to look at the type of potential impacts and the directions they come from, and what g force the human body can stand.

 

Some are compatible with and can benefit from, study of types of motor racing, some are not.

 

Examples:

 

What can the human body stand:

 

I had this in mind when I posted the medical causes of motor racing fatalities in post #3, particularly the points where the brain suffers internal damage and the aorta is severed by g force.

 

In a quick search I couldn't find a definitive g force we could a figure we could use.

 

I also haven't yet found the design changes introduced on Indy Cars after a safety wall fatality, which could be useful to us, and they have now moved on to polyethylene safety barriers containing air cells, a bit like the coke can suggestion, mounted on safety fence walls which we don't have the option of using.

 

Finding an upper limit would allow an engineer to calculate a structure which would be effective up to this limit, at the same time minimising weight.

 

What is going to kill us regardless of workable fuselage/cockpit safety equipment:

 

(a) An uncontrolled spin

 

(b) A stall from x height

 

etc.

 

Coming up with a comprehensive list here gives us a cutoff point above which there is no value in providing additional structure because it plays no part.

 

We can then move those items to Primary designation and work on training, recency, behaviour etc.

 

What direction is the threat going to come from?

 

In road cars the predominant threat is from the front, either in a head on collision or hitting an object such as a tree, followed by a multiple rollover.

 

In most forms of motor racing the threat is from being hit from the side, bumped sideways into a safety barrier or hitting a safety barrier, stationary car etc

 

In both these forms the threat is primarily from the front.

 

Motor Racing has generally arrived at two principles for survival in the secondary stage:

 

• Siting hard barriers to deflect the car or sand traps, both of which absorb momentum
   
   
  
• Restraining the driver inside a roll cage and allowing the car to disintegrate around him.
   
   
  

 

 

 

There has been a lot of success, even though crash speeds are very high

 

• Average speed on a Nascar at Talladega - 343 km/hr
   
   
  
• 850 hp Sprintcar - top speed 230 km/hr (There has only ever been one fatality in Australia)
   
   
  
• Formula 500 speedway car Australia - top speed 190 km/hr (No fatalities in Australia since 1964)
   
   
  
• Drag Racing terminal velocity - 540 km/hr
   
   
  

 

 

 

Chassis weight of a Sprintcar, which holds a 410 cu in V8 engine is just 86 kg. Note the two bars running from the top of the frame down to the front. These two bars stop the cage folding up (ref RV tail discussion)

 

Minimum legal weight of an Australian Formula 500 complete car with engine os 200 kg, and maximum legal is 290 kg dry weight.

 

• So Motor Racing presents a lot of possibilities, but one primary difference is which direction the threat will come from.
   
   
  
• Front - covered well in the above posts
   
   
  
• Side - In low wing aircraft the wings provide excellent progressive crumple rate and the ability to use them against trees etc to progressively slow the aircraft, high wing may have a problem with stumps, rocks and fences.
   
   
  
• Below - some good posts
   
   
  
• Rear - in some stalls near the ground the fuselage may provide some protection, but could be improved.
   
   
  

 

 

Fire

 

We've seen a few RA fatals from fire in an otherwise survivable crash

 

• Fuel cells or controlled release of petrol confine the fire to a manageable level, rather than a fireball
   
   
  
• A burstproof design to reduce the chance of a fracture, split, fittings breakaway has been succesfully achieved in the transport industry by specifying a drop test for the tank type
   
   
  

 

 

In Motor racing a 100 litre (approx) fire can be contained withing the 30 seconds survival time in a race suit and head gear by the combination of hand held fire extinguished to snuff the flames, and a foam pump to remove the chance of a reflash. It's quite spectacular to see a two man team moving towards a fireball in a powder-then-foam walk right up to the car as the fireball diminishes, and fire deaths are very rare in world motor racing today. This system could be introduced at virtually every airfield.

 

 

[Dafydd Llewellyn]

Yes, very good thinking. A lot of those aspects are in the area of primary safety - i.e. accident prevention by design - which should really be the next major thread on this overall safety subject.

 

However there is a bit more to be said on the subject of spinal protection:

 

The first requirement has been defined by the various authorities as limiting the compressive force in the lumbar spine to not more than 15 00 lbs force, whilst reducing the body's velocity in the direction parallel to the spine from 31 feet per second to zero; so the problem is one of energy absorption (i.e. force times distance) with an upper limit on the force involved.

 

Various of the research efforts that are documented in the bibliography of the report whose link is in post #86, show that the dynamic response of the aircraft structure, the seat structure, and any cushions between the pilot and the seat, can all have major modifying effects on the spinal load/time history; and for that reason, the authorities require a dynamic test using an instrumented dummy* fitted with a load cell at the base of its "spine" - in other words, in this particular area, the design requirement has not been reduced to a simple, readily measured "code", but it instead a prescription for what amounts to basic research.

 

People doing such tests have been able, by "fine-tuning" the seat structure and a temperfoam cushion, to meet the 31 ft/second impact requirement without having to resort to a "stroker" seat. The Whitney Boomerang seat is one example; it achieved the result by allowing the temperfoam cushion to partly extrude through calibrated perforations in the seat pan.

 

It is not clear to what extent this sort of thing constitutes complying with the letter of the requirement rather than the spirit of it; after all, the intent of FAR 23.562 was to provide an overall level of protection against a wide range of impacts; tailoring a minimal system to just comply with the precise impact conditions prescribed for the design standard may not provide good protection for impacts in general; insufficient research exists at present to answer that question.

 

It is known, from the research, that adding a soft cushion over the top of either a "stroker" seat, or of one of the "finely-tuned" rigid seat systems, can completely nullify the protection provided by the seat, because it introduces a "secondary collision" - between the pilot's backside and the seat. "Energy-absorbing" foam tends to be decidedly on the hard side for comfort; but it is unwise to add more than 25 mm of soft "comfort" cushioning over it.

 

A real "stroker" seat with around 150 mm of travel or more, can overcome these secondary effects to a considerable degree, provided it is sufficiently light that the inertia of the seat itself does not overload the pilot's spine in order to start the collapse process. Such a seat brings with it issues of maintaining harness tension, and avoiding injury from the controls - so it becomes a very considerable design challenge in its own right. Thinking about ways to design around these issues may well be less costly than throwing money at impact sled testing.

 

* A very expensive instrumented dummy - around $35K if you can get hold of one - but they were out of production, last time I looked.

 

 

[Dafydd Llewellyn]

Turbs, you will find data on the accelerations that a human body can stand in the reference for which I gave the link in post #86. It's also given in the USAF Crash Survival Design Guide, and in the early Vulcan and Sarraihle report, for which I have not been able to find a link so far. This work goes back to the NACA rocket-sled research performed by Colonel Stapp, in the early 1950s. http://en.wikipedia.org/wiki/John_Stapp We do not need to infer it from road or race car practice, although some of the design techniques devised in racing may be of value.

 

 

[Old Koreelah]

Wow OK, that's awesome!

Thanks Bex. Most of those features are untested. The best safety feature of all is pilot skill and caution- I've got to work on that part.

 

 

[rgmwa]

Temperfoam and related products are made by Dynamic Systems. There is some useful information on their website that might help in selecting an appropriate density and thickness for any given seating situation: http://www.sunmatecushions.com/pages/how-to-select-a-sunmate-cushion

 

rgmwa

 

 

[aj_richo]

For info if anyone wants confor/temper foam these guys have it in WA.

 

http://specialtyfoams.com.au

 

I bought pink and blue in 25mm, and sewed my own cushions for my project.

 

Expensive but quite comfortable.

 

 

[M61A1]

A real "stroker" seat with around 150 mm of travel or more, can overcome these secondary effects to a considerable degree, provided it is sufficiently light that the inertia of the seat itself does not overload the pilot's spine in order to start the collapse process. Such a seat brings with it issues of maintaining harness tension, and avoiding injury from the controls - so it becomes a very considerable design challenge in its own right. Thinking about ways to design around these issues may well be less costly than throwing money at impact sled testing.

.

Just wondering...If the seat is on top of the deforming structure, then in a vertical deceleration, surely some weight may help it begin the deformation? As your mass and that of the seat would be moving as one.

 

 

[Dafydd Llewellyn]

Quote: "If the seat is on top of the deforming structure, then in a vertical deceleration, surely some weight may help it begin the deformation? As your mass and that of the seat would be moving as one."

 

Once your mass and the seat are moving as one, yes. However if you had, say, a three inch cushion on top of the seat (not uncommon when a woman wants to see over the instrument coaming), your body will collide with the seat quite hard before the seat starts to move. This showed up in the various impact tests. This is why people need to read and understand the technical data before setting out to "improve" their seats.

 

Attached is the force curve I expect to see from my "folding parallelogram" seat base. As you can see, it has a "soft" start. This is the force produced by plastically stretching two pieces of soft stainless-steel rod; the shoulder harness adds something to this, increasingly towards the end of the stroke.

 

There's another issue with seats that use a deforming metal element (e.g. the S-bend seat legs on the Cessna 208), and that is, that the deforming elements must get fairly close to their yield stress in flight if the aircraft is manoeuvring hard or in turbulence; and that means they eventually start to develop fatigue cracks; I think you will find there's an AD on Cessna 208 seat legs for this reason. It needs clever design to avoid this.

 

In regard to the overall summary by HITC, I think it's important to put the various items listed into the following context:

 

The FIRST step is to achieve a non-deforming cockpit structure. The idea of any form of "crash cage" in any form of vehicle, is to keep the immediate space in which the body moves under impact, free of hard objects. Everything outside the crash cage should, if possible, act as energy-absorbing material; but since energy absorption is the product of the crushing resistance of the material and the distance it crushes under that load, the scope for that is very limited in a small aeroplane.

 

The main object of motor vehicle barrier-crash testing is to demonstrate that the cabin does not deform significantly, up to the limit specified.

 

Until you have achieved a non-deforming cockpit, none of the other secondary safety devices is going to achieve anything, really.

 

The SECOND step is to absorb the velocity of the crash cage without exceeding its structural capability. Since we can't do this by building a long, crushable snout on the thing, we have to do things to allow the aircraft to skid along the ground rather than to dig in. So the detail design of the lower front corner of the crash cage is absolutely critical; the "bevel" needs to act like a belly-board on steroids. Also, the stall behavior needs to be such that the aircraft does not pitch nose-down and spear into the ground - although that's a piece of primary safety, but it's of vital importance in an emergency landing situation.

 

The THIRD step is to use any or all of the things in the summary, to prevent the occupant from colliding with the crash cage itself. THIS is where safety harness, air bags, temperfoam etc all come into play. They are a waste of time, pretty much, unless they are used in this context.

 

I should add - a fully-triangulated steel space-frame makes an excellent non-deforming structure, and it's not particularly heavy - but when it does collapse, it absorbs little energy. The cabin structure of the Jabiru does absorb significant energy by minor elastic deformation, and to that extent it is arguably superior to an equivalent rigid steel space-frame; however designing such a structure is largely a matter of trial and error, so unless you have a dynamic test facility handy, it's very difficult to beat a welded steel space-frame for the crash cage.

 

 

 

[Jabiru Phil]

LINE-X spray makes anything unbreakable. http://wimp.com/sprayunbreakable/

 

My next paint job.