Deferring Re-Tirement
Making tires
live longer also
increases safety
By TIM kern
A
ircraft tires spend most of
their time doing nothing more
than keeping the metal parts
off the ground. In flight, all they do
is take up space and reduce useful a cut in the sidewall can lead to a blowout.
load; but for those short periods when
they’re rotating, their health means the
difference between a good day and lose 1%. If a tire is inflated with air Typical overinflation (from, say, 10
a bad one. Understanding the wheel/ or its nitrogen is contaminated with to 50%) reduces traction in braking
tire system is critical to safety. Unlike in water, a temperature drop can cause and increases wear; and specialty tires
many light general aviation applications, underinflation. An altitude change, can have particular problems of their
where a tire may have an increased load from a high-density-altitude takeoff to own. Ask anyone who has landed on
rating as a safety margin, commercial a lower-density-altitude landing, can overinflated tundra tires: five landings
and military aircraft tires are optimized have dramatic effects. per takeoff, minimum!
for their tasks. Sloppiness in monitoring Even correctly-fitted new tires lose Tires are sometimes better-able
and maintenance, or in mounting and up to 10% of their pressure during to cope with overinflation than their
inspection, cannot have a good outcome. their first 24 hours. Tube tires can rims. According to the FAA, tires
The single most-important factor appear to lose even more, since some must withstand four times their rated
in tire performance and longevity is air is typically trapped between the pressure (which is higher than their
inflation. Underinflated tires lengthen tire and tube during installation; and recommended pressure). Often, the
the takeoff roll, make braking it leaks away. Engineers say that, if wheel or wheel bolts fail first, but
unpredictable, and impact handling. you can avoid it, don’t use the tire for sometimes they fail simultaneously,
Underinflation naturally increases the first 24 hours after mounting; one or the tire goes first. Any explosion/
wear, but few understand how said to not even set the weight of the failure is dangerous.
quickly it can fail a tire. Misalignment, airplane on it – mount it early (off the People often disregard helpful
dragging brakes, and FOD are all airplane); then fit the assembly the clues. A red dot on a tire’s sidewall
dangers, but we instinctively know next day. marks the light spot: align the red dot
that. Underinflation is so common, Underinflation causes with the valve stem. On some tubes,
and leakage so natural, that we tend overdeflection, which increases there may be a yellow stripe marking
to underestimate its importance. exponentially as pressure falls below the tube’s heavy spot, so align the
Nitrogen is used to inflate high- optimum. Aircraft tires deflect yellow stripe with the red dot. White
performance tires. It’s not a panacea, 32-35%, two or three times more than dots mark the vents on a tube-type
but Rob Robson, Goodyear’s aviation automobile or lorry tires. This builds tire; green dots mark the vents on a
tire Product Support Manager for heat fast. The cure for underinflation tubeless tire.
North America, said, “Dry nitrogen is to check pressures before every Do not wash your tires unless
is a good thing; it reduces corrosion takeoff, and top up with dry nitrogen necessary (such as when they have
on wheels; oxygen also can affect whenever necessary. oil on them). Tires’ anti-ozone slime
the rubber.” He added, “Nitrogen Overinflation, though uncommon naturally oozes out, looks bad, and
has the additional benefit of being except during mounting or when works great. Without it, ozone attacks
non-combustible.” filling a tire, can fail the tire or the the rubber, cutting the long-string
Underinflation is insidious wheel itself. An exploding wheel or molecules in the rubber carcass.
and hence more common than tire can kill. Severe overinflation can The black dust – that was your tire.
overinflation. Industry standards occur when using a high-pressure air Manufacturers use a longer-string
(following FAA guidelines) allow or nitrogen source without a regulator, molecule in the sidewalls, and shorter
pressure leakage of 5% of pressure, or (more-commonly) when the person molecules in the thicker tread area.
per day; even “perfect” tires can filling the tire is distracted. Long-string molecules degrade faster.
18 Aviation Maintenance | avmain-mag.com | February/March 2010
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