When
was the last time you gave your tires a second thought: while
performing your last annual, or maybe a kick on a walk around? They may
not even be on the pre- or post-flight checklists, but they are an
integral part of the aircraft. Tire performance and lifespan can be
heavily influenced by the care and maintenance they receive. The
following questions will test how much you know about maintaining
aircraft tires:
1. Q: Which of the following best describes an aircraft tire?
A. Round, black, and dirty
B. Same as a car tire but more expensive
C. Dead weight
D. A highly engineered composite structure designed to carry heavy
loads at high speeds in the smallest and lightest configuration
practical
While
answer A is true, answer D is in fact the most accurate description of
an aircraft tire. Despite this, they are one of the most underrated and
most misunderstood parts on the aircraft. They are, in fact,
considerably different from an automobile tire. For example, a 737 nose
tire is nearly identical in size to a typical passenger car tire, but
it is rated for almost 10,000 pounds load at 200 pounds per square inch
and 225 miles per hour. Four main differences help the aircraft tire to
accomplish this:
1. More plies: car tires
typically have one or two plies (layers of rubber-coated fabric cords),
while aircraft tires contain as many as 20 plies. The 737 nose tire in
the example has 6 plies.
2. Different components: aircraft tires
use nylon fabric (most car tires use polyester) and highly specialized
rubber compounds designed for their particular function in the tire.
3. Simpler tread design: tread patterns consist only of tread ribs
between several deep, wide cirumferential grooves.
4. Higher deflection: an aircraft tire typically runs at 32 percent
deflection, while a car tire runs at only 11 percent deflection.
Another
major difference is that 80 percent of aircraft tires are bias-ply
tires, which have multiple layers with ply cords at alternating angles
substantially less than 90 degrees to the tread centerline. Automobiles
use radial tires, in which all plies run in the same direction and 90
degrees to the tread centerline. Radial tires are becoming more common,
as aircraft manufacturers take advantage of their lighter weight and
additional landings per tread. Radials are rarely retrofitted onto
existing airplanes due to the high cost of certification for the
airframer.
Cut-away view of bias tire construction.
Cut-away view of radial tire construction.
These
differences are important to understand because they dictate different
procedures for properly maintaining and caring for aircraft tires. Most
people check their car tire inflation once a month, if that. Aircraft
tires should be checked for proper inflation DAILY. There are several
reasons for this. Tubeless aircraft tires lose inflation pressure more
quickly than car tires; up to 5 percent in 24 hours is allowable per
FAA limits. The higher deflection that aviation tires use is designed
to carry high loads while limiting heat generation from sidewall
flexing to an acceptable level. However, that means that small amounts
of additional deflection bring rapidly increasing heat generation and
thus, rapidly diminishing tire durability. When that fact is coupled
with the faster inflation loss, it means that pressures should be
checked daily to ensure that the tire is not overdeflected.
Overdeflection,
which can be caused by underinflation and/or overloading, is very
damaging to the tire. It produces uneven tire wear and greatly
increases stress and flex heating in the tire, which shortens tire life
and can lead to tire damage and eventually blowouts or other serious
problems. This might lead one to “play it safe” and overinflate the
tire, but this is not the correct answer, since overinflation can cause
uneven treadwear, reduce traction, make the tread more susceptible to
cutting, and increase stress on the wheels. Hence, the only acceptable
solution is to check inflation pressures daily, which is the single
most important maintenance item for aircraft tires.
The above test results for a 40x14 tire show the consequences of even slight underinflation.
Following are the eight steps of proper inflation procedures (When
servicing assemblies, always use a bottle or supply line with a
pressure regulator, which should be set 50 percent higher than the
rated pressure of the tire. Extreme overinflation from an unregulated
pressure supply can cause the wheel or tire to burst, leading to injury
or death):
1. CHECK DAILY WHEN TIRES ARE COOL
Tire pressures should always be checked with the tire at ambient
temperatures. Tire temperatures can rise in excess of 200o F (93o C)
above ambient during operation. A temperature change of 5o F (3o C)
produces approximately 1 percent pressure change. It can take up to
three hours after a flight for tire temperatures to return to ambient.
2. INFLATE TO WORST CONDITIONS
When tires are going to be subjected to ground temperature changes in
excess of 50o F (27o C) because of flight to a different climate,
inflation pressures should be adjusted to worst case prior to takeoff
so that the minimum required inflation is maintained for the cooler
climate.
3. USE DRY NITROGEN GAS (WHEN REQUIRED)
Nitrogen will not sustain combustion and will reduce degradation of the
liner material, casing plies and wheel due to oxidation.
4. INCREASE PRESSURE 4% FOR TIRES UNDER LOAD
A tire under the load of an aircraft has 4 percent higher pressure than
it does when unloaded. Therefore, if unloaded pressure has been
specified in the manual but the tire is being checked on aircraft, that
number should be increased by 4 percent to obtain the equivalent loaded
inflation pressure.
5. ALLOW 12-HOUR STRETCH AFTER MOUNTING
All tires, particularly bias tires, will stretch (or grow) after
initial mounting and inflation. This increased volume of the tire
results in a pressure drop. Consequently, tires should not be placed in
service until they have been inflated a minimum of 12 hours, pressure
rechecked, and tires re-inflated if necessary.
6. NEVER REDUCE PRESSURE ON A HOT TIRE
Excess inflation pressure should never be bled off from hot tires. All
adjustments to inflation pressure should be performed on tires cooled
to ambient temperature.
7. EQUAL PRESSURE FOR DUALS
To prevent one tire on a gear from carrying extra load, all tires on a
single gear should be inflated equally. The mate tires will share the
load, allowing individual tires to run underinflated or overloaded even
though all tires on the gear are deflected identically.
8. CALIBRATE INFLATION GAUGE REGULARLY
Use an accurate, calibrated gauge. Inaccurate gauges are a major source
of improper inflation pressures. Gauges should be checked periodically
and recalibrated as necessary.
Remember, keeping tires correctly inflated is the single most important factor in any maintenance program.
2. Q: Why do tires lose pressure?
A. High inflation pressure
B. Weep holes
C. Using natural rubber tubes
D. All of the above plus numerous other sources in the assembly
There
are numerous sources of pressure loss in tubeless wheel/tire
assemblies, including the valve, fuse plug, pressure plug, wheel base,
o-ring seal, tire growth, temperature change, tire damage, improper
bead seating, or an inaccurate gauge. In tube-type applications,
natural rubber tubes lose pressure faster than butyl tubes. Tire casing
vents, a.k.a. weep holes, relieve pressure buildup in the casing plies
and by design will continually allow air or nitrogen in the casing to
escape. This pressure loss should be within the allowable 5 percent in
24 hours. If not, a systematic troubleshooting process can be followed
to determine the root cause.
3. Q: What is the most common reason for early tire removal?
A. Flat spots
B. Sidewall cracking
C. Low pressure
D. Cuts and other foreign object damage (FOD)
FOD
is indeed the number one cause of early tire removal. Nuts, bolts,
rocks, etc. will easily cut aircraft tires. While not completely
controllable, FOD can be minimized by keeping hangar floors clean and
reporting pavement breaks and debris to the airport authorities. A good
general rule for cut or damage limits is to remove any tire with damage
that extends to the fabric, whether it is in the tread or sidewall.
Removal criteria for cuts in the tread are more specific: any cut into
the casing plies on bias tires, any cut into fabric on radial tires,
any cut to fabric which extends across one or more rubber tread ribs,
or rib undercutting at the base of any cut. While inspecting inflated
tires, do not probe cracks, cuts, or embedded foreign objects.
FOD collected from tires at initial inspection at Goodyear’s Atlanta retread plant.
4. Q: How do you determine when to remove a tire for wear limits?
A. When the pilot/mechanic/owner says so
B. When the second layer of fabric is visible
C. When there is a minimum of 2/32” of tread left
D. When the tire is worn to the bottom of the groove at any point
Tires
should be removed when tread has worn to the bottom of any groove at
any spot. Yes, there are military tires that wear through numerous
layers of fabric, but these are designed specifically for this and the
limits are marked on the sidewall. Tires have varying numbers of plies,
and on some tires, wearing through any layers leaves too small of a
safety margin. Wearing to the bottom of the groove is the best
compromise point between safety and obtaining the most landings.
5. Q: What are the age limits for aircraft tires?
A. Use until the tire turns brown
B. Remove when the sidewall starts to crack
C. 10 years
D. There are none
There
is no age limit for aircraft tires, provided that they are properly
stored and maintained and all inspection criteria are met. Proper
storage practices include keeping tires in a cool, dry place out of
direct sunlight and away from fluorescent lights, electric motors, and
other ozone producers. Care should be taken that tires do not come in
contact with oil, gasoline, jet fuel, hydraulic fluids, or similar
hydrocarbons. If desired, the age of the tire can be determined from
the tire’s serial number, where the first number is the year of
production and the next three are the Julian date of manufacture (for
example, a tire with serial number 20341111 was built on the 34th day
of 2002, i.e. February 3rd). Age combined with ozone or excessive
service conditions can cause cracks to form on the sidewalls or in the
bottoms of the grooves (known as ozone cracking, weather checking, or
groove cracking). This is not a cause for removal unless fabric is
visible in the cracks.
6. Q: What is the cause of flat spots?
A. Landing with the brakes on
B. Hydroplaning
C. Parking the aircraft for a long time
D. All of the above
There
are three different types of flat spots: skid spots, skid burns, and
nylon sets. Skid spots are caused by skidding the tire across the
runway, usually from overzealous braking. Skid burns are caused by
hydroplaning on water or skidding on ice. Although skid burns result in
ugly reverted rubber on the flat spot, removal criteria for both of
these types of flat spots are the same: remove the tire only if fabric
is exposed or unacceptable vibration results. Nylon sets, a.k.a. cold
sets or casing flat spots, are a different matter. They result from
parking the aircraft with load on the tires, and the severity depends
on several factors including temperature, inflation, load, and length
of time under these conditions. Nylon sets usually disappear by the end
of the taxi roll. They can be prevented by occasionally moving a parked
aircraft or by jacking up an aircraft expected to be parked over 30
days.
7. Q: How many times can a tube be reused?
A. None, tire companies need your money
B. Once
C. Until they start to crack
D. None, they grow in service and don’t fit into a new tire
While
answer A is of course true, it’s not the real reason to avoid reusing
tubes. Tubes grow in service just like tires, and a grown tube is too
large to fit well in a new tire. This leads to a chance that the tube
will wrinkle or crease, which can cause vibration or tube failure.
Typical example of tube fold.
8. Q: Why does my landing gear vibrate?
A. The gear needs to be serviced
B. The tire is excessively out of round or out of balance
C. The tire and wheel were not assembled properly
D. Any of the above
There
are numerous causes of gear vibration, most of which are not the fault
of the tire. They include gear alignment, worn or loose gear
components, out of round or out of balance tires from flat spotting,
improperly assembled tires, improperly installed tubes, improperly
torqued axle or wheel nuts, incorrectly balanced wheel assemblies, etc.
9. Q: What are the numerous V-shaped cuts on the tread of some tires?
A. FOD from a thumbtack spill
B. Defective tread compound
C. Damage from rolling over a cheese grater
D. Chevron cuts
Chevron
cuts occur frequently on higher pressure tires operating on runways
with cross-cut grooves. They happen primarily when the tread is still
deep and will often wear off and disappear before the tire is fully
worn. Despite their unattractive appearance, they are not cause for
removal unless they cause tread chunking down to the fabric.
Typical example of chevron cuts.
10. Q: How can I operate my aircraft to get the optimal life from my tires?
A. Always make the first turn
B. Get to the ramp as quickly as possible
C. Use heavy braking to bring the aircraft speed down as soon as possible
D. None of the above
To
get the maximum life from tires, follow the opposite of choices A, B
and C. Land at the correct point on the runway using the correct speed.
Select a later turnoff; making the first turn often requires heavy
braking and high-speed cornering, both of which are hard on tires. Taxi
slowly, take wide turns, and don’t ride the brakes while taxiing. These
practices will help get better life from wheels and brakes as well.
By
now, you have probably realized that answer D is the correct answer to
each question. Understanding these questions and their answers will
help you to achieve the best performance from your aircraft tires. The
most important factor is maintaining the proper inflation in the tires.
Tires are often the last thing an owner, pilot, or mechanic thinks
about when it comes to aircraft maintenance, yet it’s pretty hard to
get off the ground without them. Following these guidelines will
enhance tire performance and ensure extended tire life.
The
procedures and standards mentioned in this article are intended to
supplement the specific instructions issued by aircraft and wheel
manufacturers.
Bio:
Robert Robson is a Product Support Manager for the Goodyear Tire &
Rubber Company. He has five years experience in aircraft tire design
and product support. Goodyear is the world’s leading aircraft tire
manufacturer, supplying tires for everything from piston singles to
jumbo jets, helicopters and fighter jets. For more information, visit www.goodyearaviation.com or contact Rob at 330.796.1596 or email: robert_robson@goodyear.com. |
