Helmets - what you need to know
In a recreational activity like hang gliding , there are significant
risks of injury or death. Helmets on
the market today offer varying degrees of protection. The purpose of this
article is to provide a guide on how helmets work and what to look for when
choosing one.
You need a helmet whenever you fly. Almost all pilots will crash
eventually. Even a low-speed crash can scramble your brains. Gravel rash and
broken bones heal; brains often do not . Exact figures are not available for
hang gliding however research shows that ~90 per cent of cyclists' brain
injuries can be prevented by a properly fitted bicycle helmet. Although helmet use has been shown to reduce
head injuries significantly, there are limits to a helmet's protective
capability. No helmet can protect the
wearer against all foreseeable accidents.
Therefore, injury, death and permanent impairment may occur in accidents
which exceed the protective capability of any helmet. Use your superior
judgement to avoid situations where you will have to use your superior skills
and perhaps your superior helmet.
The physics of how a helmet works
Head injuries aren't caused by speed, but by the sudden stop when our
head hits a hard surface. The way to prevent the injury is to bring the head
(and the brain) to a more gradual stop. A helmet reduces the peak force applied
to the head in a sharp impact. The force applied to an object can be calculated
from the equation :
F = m a where
F = force, m = mass, a = acceleration
As acceleration is the rate of change of velocity we can write:
a = (Vf - Vi) / t where Vi =
initial velocity, Vf = final velocity, and t = time
hence by combining these two
equations we can write:
F = m (Vf - Vi) / t
In a crash situation the initial velocity (Vi) is whatever speed the
pilots head is moving at. The final velocity (Vf) is 0. Thus the equation
becomes:
F = - m Vi / t
As both m and Vi are constants (ie fixed and unchangeable) we have:
F = k / t or
F µ 1 / t
This equation is at the heart of how a helmet works and simply states
that the force in a deceleration is inversely proportional to the time taken
for that deceleration to occur. The only variable in a crash situation is the
time the change in velocity occurs
over, as this determines the deceleration and hence the force applied to the
head. If we can increase this time by a factor of 2 the deceleration (and thus
the force) is halved; If we can increase this time by a factor of 4 the force
is reduced to 25% of what it would have been without a helmet. A human brain
can withstand ~ 400G without ill effect. From 400-700G causes concussion with a
variable period of loss of consciousness. 700G+ results in permanent brain
damage.
To increase the critical time over which deceleration occurs requires a
material that brings our head to a safe stop by gradually crushing under load.
It should not bounce back too quickly. If it does, the energy stored by
crushing will be released and it will not have protected us as well. It also
needs to be thick enough and stiff enough to not totally collapse (bottom out)
before our head comes to a stop. A material which can do this was developed in
the 50s and today nearly all helmets do this with expanded polystyrene (EPS),
the same foam used for Eskis and packing electronics. Once crushed, the foam
does not recover. Spongy foam is added inside for comfort and fit. Another
foam, expanded polypropylene (EPP), does recover, however, it may have some
undesirable "rebound." A stronger EPS called GECET appeared in 1992
and is also widely used. A third foam called EPU (expanded polyurethane) is
used for helmets made in Taiwan. It has a uniform cell structure and good crush
without rebound, but is difficult to manufacture and not used much. The desired
effect is that instead of your head coming to a near instantaneous stop the
crushing of this foam slows the head down over an extended period of time.
But, were not finished. What if the surface we hit is not flat. Rounded
surfaces concentrate the force of the blow in a smaller area. The smaller the
radius, the greater the concentration. To compensate, we can add a hard outer
shell to spread the force over a wider area and reduce the concentration, or we
can increase the stiffness of the crushable material to prevent it from
collapsing, or we can make the crushable material thicker, or combinations of
all three. The hard shell will make it heavier. If the crushable material is
too stiff, it might not crush enough when it hits a flat surface. Thickness has
to be controlled in order to wear the helmet comfortably. Each of these effects
the usefulness of our helmet.
What type of helmet do I need?
A helmet consists of an outer shell, crushable foam, a comfort liner,
and a retention strap system. There are
at least four critical elements that effect a helmet's protective properties:
1. Impact management - how
well the helmet protects against collisions with large objects.
2. Helmet stability - whether
the helmet will be in place, on the head, when it's needed.
3. Retention system strength -
whether the chinstraps are sufficiently strong to hold the helmet on throughout
an impact.
4. Extent of Protection - the
area of the head protected by the helmet.
The EPS foam layer is absolutely critical to impact management as
explained above. The thicker the layer of foam the greater its ability to
absorb impact forces. The firm rubber found in some helmets is a very poor
substitute and not recommended.
The shell also plays an important role in impact management in that it
(1); holds the EPS together during impact/s; (2) helps prevent objects
penetrating the helmet and spreads the load to the foam; and (3) helps the
helmet skid easily on rough surfaces to
avoid twisting your neck.
Obviously a helmet must stay on
even if your head hits more than once
so it needs a strong strap and an equally strong fastener that cannot be
accidentally opened. The comfort liner and straps help hold the helmet in
correct position. With the strap fastened you should not be able to get the
helmet off your head by any combination of pulling or twisting. If it comes off
or slips enough to leave large areas of your head unprotected, adjust the
straps again, add some padding or try another helmet. Keep the strap
comfortably snug when flying.
Open face helmets generally represent an acceptable level of protection
for your brain however provide less protection for your face. Full face helmets
offer some extra facial protection at the expense of extra weight, decreased
peripheral vision and perhaps decreased hearing and tactile sensation. It is
also very important that they fit firmly. I was unfortunate enough to have to
attend a recent hang glider lock-out incident in which the pilot impacted face first. The impact on the chin guard led
to the helmet rotating downward causing the pilots sunglasses to shear his
nasal bridge off, later repaired with plastic surgery. Of concern is the extra
leverage of the chin guard on the neck,
especially some designs in which the chin guard is an excessive distance in
front of the chin.
Bicycle style helmet vents mean less foam in contact with your head in
a crash, which could concentrate force on one point of your skull. These vents
are designed to dissipate heat which is not always optimal for hang gliding where
staying warm is often the main concern. They also tend to have very thin
shells.
"Aero" helmets are not noticeably faster unless you fly at
competition speeds, and the "tail" could snag in a fall twisting your
neck.
Comfort Requirements should be considered. Fit, weight, and
temperature/sweat control are the most critical comfort needs. A snug fit with
no pressure points ensures comfort and correct position on the head if you
crash. It may take a half hour of wearing to feel pressure points. Weight is a
big issue for long flights. Airflow over the head determines warmth. Vented
bicycle style helmets are designed to facilitate heat loss - OK on the coast
but less use at cloudbase. Sweat control can require a brow pad or separate
sweatband.
When do I need to replace a helmet?
Replace a helmet if you crash and hit your head. Impact crushes some of
the foam. The helmet is less protective
but the damage may not be visible. Helmets soften impact, so you may not even
know your head hit unless you examine the helmet for marks or dents. If you can
see marks on the shell or notice any foam crush at all, replace the helmet. You
may be reluctant to replace a helmet that looks almost as good as new, but if
you did hit, you don't want to take chances. If the foam of a bicycle helmet is
cracked under the thin shell, it will be more likely to fly apart in your next
crash. Replace the buckle if it cracks or a piece breaks off.
Can you make it fit correctly? If not replace it.
If you still have a helmet from the 70's without an EPS liner, replace
it immediately. They just do not have the protection of modern helmets.
Finally, the protective capability may diminish over time. Some helmets
are made of materials which deteriorate with age and therefore have a limited
life span. Most manufacturers recommend
helmet replacement after five years. Realistically that depends on usage, and
most helmets given reasonable care should be good for longer than that. Please
note that experience indicates there will be a noticeable improvement in the
protective characteristic of helmets over a five year period. Thus, the
recommendation for five year helmet replacement has some merit.
Is a cheap helmet as safe as an expensive one?
Maybe. Maybe not. Maybe safer. The protective capability of a
particular helmet is difficult to measure. You can quickly judge a helmet for
style, price, fit and comfort. But who knows what that helmet can do when your
skill, experience and every other precaution have failed, and your helmet's the
only thing between your head and a violent impact. Without lab test data it is difficult/impossible to
say. A standards sticker is an indication that the helmet has at least been
tested however the testing may not be hang glider appropriate. If money buys
you a better fit, more stable on your head in a hard crash, then the more
expensive helmet is worth it. A crushable foam liner is absolutely essential - a thick layer will offer more
protection than a thin layer. Shell
strength needs to be adequate to maintain the integrity of the EPS foam and
prevent penetration of objects into the foam and underlying skull. Essentially
the stronger the better, however extra strength = extra weight and may not
represent a significant increase in safety. Carbon fibre or kevlar are stronger
than fibreglass helmets on a weight for weight basis but cost more.
What about helmet standards?
Only the Europeans have a specific standard for hang gliding/paragliding
helmets. The applicable standard is EN966. Many manufacturers of fine helmets
in other countries certify their helmets to other standards. Standards for
activities where accidents involve similar speeds into similar objects may
include: Bicycle standards, Ski helmet standards, Horse Riding standards, Non
motorised sport standards.
To give you an idea why standards certification is important consider
that depending on the application and standard, the helmet must pass all or
some of the following tests:
Impact Test - This test involves a series of controlled impacts where a
helmet is positioned on a metal head form and then dropped in a guided fall
onto various test anvils (Flat, Hemisphere, Kurbstone, Roll bar, etc.) to
simulate different impact surfaces. The head forms are instrumented with an
accelerometer to measure peak G force. The impact energies (determined by drop
height and head form mass) differ between standards. However, in general
if the peak acceleration imparted to
the head form exceeds 300 G's, the helmet will be rejected.
Positional Stability (Roll-Off) Test
- test straps ability to retain helmet on head.
Dynamic Retention Test - tests strap strength.
Chin Bar Test. - tests strength and impact resistance.
Shell Penetration Test - self explanatory.
Faceshield Penetration Test - self explanatory.
Details of many of the standards are available on the internet. Useful
sites are:
www.smf.org (Snell Memorial Foundation)
www.bhsi.org (Bicycle Helmet Safety Institute)
www.standards.com.au (Standards Australia)
www.sph.emory.edu/Helmets (WHO Helmet Initiative)
How to Buy
When you pick up a helmet, look
for a standards sticker inside ie EN966
Check that it has a thick layer of EPS foam
Check that it has a sturdy shell
Put it on, adjust the straps and then try hard to tear it off.
Look at the buckle for long-term durability.
Make sure it is comfortable
Compare the price to the cost of a prolonged hospital stay.
Consider the nightmare of being a vegetable
You will never regret buying and wearing a quality helmet.
Your brain is priceless!