History of the flight
On 19 December 2004, a Boeing Company 737-86N aircraft,
registered VH-VOH, was being operated on a scheduled passenger
flight from Melbourne, Victoria to Canberra, ACT. At about 1655
Eastern Daylight-saving Time, while landing on runway 35, the right
inboard (No. 3) main landing gear tyre failed. The aerodrome
controller reported hearing a loud bang, then a piece of rubber was
seen 'flying from under the aircraft'. The controller advised the
crew that the right inboard tyre had failed. The crew completed the
landing roll, and stopped the aircraft on the adjacent taxiway.
The airport Rescue and Fire Fighting Service (RFFS) attended the
aircraft and confirmed that the other three tyres appeared to be
undamaged. The crew then elected to taxi the aircraft to the
terminal with the RFFS following. Approximately 20 pieces of rubber
ranging in size from several centimetres to over a metre and a half
in length were subsequently recovered from the runway.
The Australian Transport Safety Bureau (ATSB) attended the scene
to inspect the damage to the tyre and aircraft. The ATSB retained
the pieces of rubber for further examination. The operator's
maintenance facility removed the tyre from the wheel rim for later
examination by the ATSB.
Damage to aircraft
The aircraft structure sustained minor damage, which consisted
of several dents in the lower surface of the right wing and flap,
and deformation and cracking of a bracket in the right wheel well.
A hydraulic line running through the wheel well also sustained a
small dent.
The number-3 tyre had shed a significant portion of the tread and
had a diagonal tear through the casing1 (Figure 1). The diagonal tear ran
continuously from one sidewall to the other. There was also
scuffing running circumferentially around the shoulders of the
tyre.
Figure 1: Failed number-3 tyre
Aircraft operation
A review of the recorded data from the flight indicated that the
landing was within the operating limitations. The take-off and
landing weights were within the allowable limits, as was the
aircraft's centre of gravity.
Examination of the taxi speed data recorded over a set of previous
flights from VH-VOH and another 737-800 series aircraft in the
operator's fleet (that also had a tyre failure), indicated that the
straight line taxi speeds were not exceeded, but a number of turns
were recorded slightly above the manufacturer's recommended 10 kt
maximum taxi speed whilst turning through 30º or more.
Meteorological information
At the time of the occurrence, the Terminal Area Forecast (TAF)
for Canberra indicated moderate to severe turbulence below 5000 ft
AGL. The Canberra Automatic Terminal Information Service (ATIS)
reported the wind as 280 degrees at 25 kts with turbulence over the
runway. A maximum gust of 32 kts was reported 5 minutes prior to
the landing. However, the approach and landing was within the
allowable wind limits for the aircraft.
Similar occurrences
Between early October and late December 2004, the ATSB noted an
increase in the number of tyre failures on Boeing 737 aircraft in
Australia. In this 3-month period, the ATSB received seven reports
from operators of Boeing 737 aircraft, as compared with four
reports in the preceding 21 months.
Six of the seven failures had occurred on the High Gross Weight
variant of the Boeing 737-800 series aircraft and with tyres on the
fourth (R4) and fifth (R5) retread. As a result, the ATSB issued
safety recommendation R20040093 on 23 December 2004, recommending
that operators of Boeing 737-800 series aircraft review the
practice of fitting retread tyres of R4 (fourth retread) or above,
until their serviceability limitations could be identified.
There were two commercial operators of Boeing 737-800 aircraft
in Australia. Of the six failures on these aircraft, five were from
one operator.
The ATSB obtained five of the failed tyres for further
examination. All tyres were tubeless H44.5x16.5-21 28 PR2 tyres, which were approved by the aircraft
manufacturer for use on the aircraft type, and met the applicable
US Federal Aviation Administration (FAA) Technical Standard Order,
TSO-C62d. All failed tyres were from the same original manufacturer
(or brand) and retreaded by the same retread facility. Both
operators had about 50% of this brand in their tyre pool. The other
two brands used on this aircraft type making up about 30% and 20%
of each pool. All brands had representative tyres at retread levels
R4 and R5.
Each operator had separate tyre pools that had operated since
new and were of about the same age and R-level distribution. All of
the tyres in these pools had been retreaded by the one retread
facility since new.
The failures were not common to any position on the main landing
gear, phase of flight, location, or retread date.
Examination of failed tyres
Five of the failed tyres were examined by the ATSB and by the
tyre manufacturer in the United Kingdom (UK). The ATSB performed a
visual examination and took fibre samples for detailed laboratory
examinations, prior to shipping the tyres to the manufacturer. The
tyre manufacturer examined the tyres under the supervision of a
representative from the UK Air Accidents Investigation Branch
(AAIB). A description of the tyre section components is contained
in Figure 2.
Figure 2: Tyre section components
Examination by the ATSB
All tyre failures were similar in respect to the manner of
casing break-up, with all presenting transverse ruptures along the
sidewall ply axes. The stripping and loss of the tread from the
tyre crown was limited to the two tyres that had failed at higher
rotational speeds (landing and take-off) and was attributable to
the effects of centrifugal forces on the tread belts as fracture
and tearing progressed across the tyre crown. Bulk tread separation
was not evident on the three tyres that had failed at lower (taxi)
speeds.
Examination of the sampled fibres showed a combination of
failure modes, attributable to tensile overload and possible cyclic
loads stemming from structural flexure.
The failures were all consistent with the effects of a
progressive failure and breakdown within the sidewall structure of
the tyres. There was no evidence of the involvement of individual
events such as foreign object impact, wheel skidding, or similar
damage.
Examination by the tyre manufacturer
The tyre manufacturer's examinations found that the five tyres
had failed in a similar manner. The conclusion made for each of the
items examined was:
Cyclic fatigue in the inner ply turn-ups
3, leading to casing break-up, fracturing of the inner
liner construction, ply separation and a localised sidewall rupture
along the bias angle of the cord layers.
The tyre manufacturer also indicated that the fatigue in the
inner ply turn-ups was due to cyclic flexing of the inner plies,
which created a local looseness and reduction in strength. This led
to a small fracture in the inner liner, resulting in a slow loss of
pressure. The rate of pressure loss increased as the fracture in
the inner liner grew, until it reached a point where, when combined
with the operational loads, exceeded the venting capacity
4 of the casing and a rupture occurred.
A review of the original manufacture records for each of the
tyres indicated that all had been manufactured from the correct
material that were checked and passed laboratory sample testing.
The destructive examination confirmed that the tyres were
constructed to the approved design and process specifications.
Due to the extent and severity of the fatigue damage in the
sidewall, and the low number of landing cycles accumulated by some
of the failed tyres since the last retread, the manufacturer
considered that the damage was possibly present, to some degree,
during the last retreading process.
Approved tyres
At the time of the failures there were three tyre manufacturers
approved to produce H44.5x16.5-21 28 PR tyres for the Boeing
737-800 series aircraft. A review of the specifications found that
all three complied with the requirements of FAA Technical Standard
Order TSO-C62d. The only notable difference between the brands was
the average weight. The average weight for the tyres was 85.7 kg,
90 kg and 93 kg. The failed tyres were from the brand having the
higher average weight.
TSO-C62d prescribed the minimum performance standard that
aircraft tyres must meet to be identified with the applicable TSO
marking. This performance standard included an overpressure and a
dynamometer test
5. The TSO also dealt with changes to the tyre design
with the following statement:
7.0 Requalification tests. A tire shall be
requalified unless it is shown that changes in materials, tire
design, or manufacturing processes could not affect performance.
Changes in material, tire design, or manufacture processes that
affect performance or changes in number or location of tread ribs
and grooves or increases in skid depth, made subsequent to the tire
qualification, must be substantiated by dynamometer tests in
accordance with paragraph 6.0.
Retreading of tyres
Operational experience has shown that the tread of an aircraft
tyre wears away at a much higher rate than the basic casing of the
tyre and can be safely replaced multiple times before the casing
reaches its service limits. As with all aircraft components, a
controlled and regulated process was required to retread tyres. The
FAA required a retread facility to have an approved process to both
qualify tyres for retread, and for physically performing the
retread. In 1982, to assist retread facilities in producing a
process specification, the FAA released guidance material in the
form of Advisory Circular AC 145-4.
In 1985, the Australian Civil Aviation Authority (the
predecessor to the Civil Aviation Safety Authority, CASA) mandated
the use of the guidance material in FAA AC 145-4 for the retreading
of aircraft tyres, through the release of Airworthiness Directive
AD/WHE/3 Amendment 1. The Australian retread facility responsible
for the retreading of the occurrence tyres had a process
specification approved by CASA that corresponded to AC 145-4.
Retread qualification requirements
For the speed rating
6 of the occurrence tyres, AC 145-4 required that the
retread process specification included overpressure and dynamometer
testing
7. Although not clearly stated in AC 145-4, the FAA and
CASA have accepted that the retread process for a tyre of a
particular size, speed rating and load rating shall be qualified by
overpressure and dynamometer testing to R1 (first retread) level.
To qualify for higher R-levels, the retreader need only perform a
set of ply adhesion tests to check the strength of the rubber
bond.
For bias-ply tyres
8, the FAA and CASA have also accepted that the
overpressure and dynamic testing need only be carried out on one
brand representing the size, speed rating and load rating, as 'Many
years of evolution have resulted in a nylon bias tire having a
casing structure that is essentially the same, irrespective of the
manufacturer'. The tyres that were the subject of this
investigation had not come from the brand of tyre that had been
used in the qualification testing for this tyre range. The ATSB has
had no reports of the observed type of failure in the brand used to
qualify the retread process for the H44.5x16.5-21 28 PR tyre on the
737-800 series aircraft.
The retread facility noted some small differences to the profile
between the subject brand and the other brands approved for use on
737 series aircraft.
Retread process
During retreading, before any physical work was carried out, the
casing was subjected to a non-destructive inspection (NDI) process
to determine if it was suitable for retread. The NDI process
consisted of visual, shearography, and casing leak
inspections.
The visual inspection was a check by a trained technician to
determine if there was any obvious damage in the surface of the
tyre casing. The technician assessed any damage found to determine
if it was within the limits allowed for the tyre, or if it was
suitable for repair.
The shearography was a computer based inspection system that
compared the tyre under atmospheric pressure and a reduced pressure
to determine if there were any separations within the ply
structure. Tyres with separations exceeding preset limitations were
rejected and destroyed. At the time the occurrence tyres were
retreaded, only a shoulder-to-shoulder (crown area) shearography
inspection was carried out. However, radial ply tyres
9 required a bead-to-bead (which included sidewalls)
shearography inspection.
The final check was a casing leak check. Pressurised air was
injected into the casing plies and soapy water sprayed over the
tyre. Any leaks found were either accepted, or repaired if they
were within the allowable limits, or the tyre was rejected and
destroyed.
After successful completion of all tests, the facility carried out
the physical retreading process. A visual and dimensional check
completed the retread process. Since the facility had been
operating the shearography equipment (early 2004), they have
included an additional, post-retread, shearography.
A review of the process documentation for each of the tyres
examined found that all applicable inspections had been certified
as having been completed before the tyre was released for
service.
The area of the sidewall where the fatigue occurred was not
subject to the pre or post-retread shearography inspection on any
of the tyres examined, nor was it required by their approved
process specification.
Aircraft tyre maintenance
A review of the tyre pressure maintenance practices found that
once fitted to the aircraft, the tyres were subject to a condition
and inflation pressure check as part of the daily inspection.
However, except for a layover in excess of 4 hours, there was no
requirement to examine the tyres in detail on a normal turn-around
pre-flight check. In that instance, only a condition check was
required.
The operator with the five failures had a fleet of 45 737-700
and -800 series aircraft, including low, medium and high gross
weight variants of each model. Each of the variants in the fleet
(five in total) had their own nominal pressure requirements of 190,
200 or 205 psig
10 and were all listed on the same daily check Task Card.
The Boeing Maintenance Manual inflation pressure check required
that 'all tyres on the same gear are inflated to the selected
nominal service pressure +/-5 psig'. The operator's Task Card did
not present this range, but did contain the statement 'NOTE: If any
tyre is below the nominal pressure by more than 5%, review the
Aircraft Maintenance Manual for further requirements.' The Aircraft
Maintenance Manual required that if the tyre pressure was 5% to 10%
below the nominal service pressure, the tyre was to be reinflated
to the nominal service pressure then checked again 24 hours later.
If the tyre pressure was again more than 5% below the nominal
service pressure, it was to be removed. If the tyre was found at
any stage to be more than 10% below the nominal service pressure,
it was to be removed from the aircraft immediately.
A review of a small sample of the operator's Maintenance
Discrepancy Reports indicated that some of the engineers that
checked tyre pressures had reinflated the tyres to 200 psig (the
lower limit for the aircraft model), rather than the nominal
service pressure of 205 psig. On one occasion, a tyre was found to
be 180 psig, which is 12% below the nominal service pressure of 205
psig, but was reinflated to 200 psig and rechecked 24 hours later.
The operator provided a revision of the Task Card that was reported
to be applicable at the time of these Maintenance Discrepancy
Reports that indicated that the nominal service pressure for the
tyres was 200 psig. A revision of the task card in 2004, prior to
the tyre failures, increased the nominal service pressure to 205
psig.
The operator carried out a survey of tyre pressures on a portion
of their fleet over an eleven day period and found that about 1% of
tyres were 5% or more below the nominal service pressure. Of these
low pressure events, about one third were recorded in accordance
with the operator's procedures.
Although both operators selected the same nominal main landing
gear tyre pressure (in accordance with the Boeing maintenance
instructions), the operator with the low number of failures had
opted to avoid the lower end of the normal service pressure range
and utilised the upper end of the pressure range (205 to 210 psig)
for inflation pressure servicing.
Factors affecting tyre fatigue life
The heat generated by operation of aircraft tyres is a
well-documented factor affecting tyre life. The heat in the tyres
may be from an external source, such as the brakes, however the
heat generated from flexing of the tyres is generally considered
the primary source of temperature rise in aircraft tyres.
Studies carried out by various tyre manufacturers (presented in
their tyre care and maintenance documentation) show that the
effects of taxi speed, taxi distance and inflation pressure on the
internally generated heat can be quite dramatic. The general
effects are:
- Increased taxi speed leads to increased tyre temperature
- Increased taxi distance leads to increased tyre
temperature - Decreased inflation pressure leads to increased tyre
temperature.
The most significant increase in temperature was identified in
the tyre bead/sidewall area.
The FAA provided guidance on the effects of turns on tyre
temperature in AC 20-97B. The guidance to operators in this AC
is:
Aircraft tires generate internal heating during normal
operations. Under high aircraft loads (particularly under
sideloading conditions), heat build-up is accelerated by excessive
taxi speed and/or excessive taxi distances. Tire integrity and
reliability may be compromised when a combination of these
conditions occur.
Studies by tyre manufacturers have found that the performance of
a tyre casing reduced by more than 50% when run continuously
under-inflated by only 5%.
- The casing is the body of the tyre
consisting of the nylon structural cords embedded in a rubber
compound. - H44.5x16.5-21 defines the size of
the tyre. In this case, the tyre has an outer diameter of 44.5
inches, a width of 16.5 inches and fits on a 21-inch wheel rim. PR
is the ply rating and is an indicator of the load capability of the
tyre. - The inner ply turn-ups are the tail
ends of the plies after they have wrapped around the bead and are
located in the mid-sidewall region. - Tubeless aircraft tyres are designed
with small vent holes in the outer sidewall. These vents release
air pressure from the plies that has leaked or diffused through the
inner liner. - The dynamometer test is a dynamic
test that cycles a tyre through a range of operational speeds and
loads to simulate a series of take-off and taxi cycles. - Because the tyres are designed to a
performance specification for the tyre as a stand-alone item
(TSO-C62d) and not to a particular aircraft, they are approved to a
maximum speed and load. These limits are referred to as the 'speed
rating' and 'load rating'. - AC 145-4 had additional
qualification requirements for tyres with a speed rating above 160
mph. - Bias-ply tyres are also known as
cross ply tyres. They are constructed from layers of rubber-coated
nylon ply cords that extend around the beads and are oriented at
alternating angles to the tread centreline. - Radial ply tyres are constructed
from nylon and rubber cords, however, the cords are oriented
differently to bias ply tyres. A set of cords run from one bead,
across the crown and to the opposite bead. Another set of cords lie
around the circumference of the tyre under the tread area. - The indicator reading, in pounds
per square inch, showing the amount by which the tyre pressure
exceeds atmospheric pressure.
ANALYSIS
Tyre failure mechanism and detection during maintenance
and retread
Each of the tyres examined sustained a casing break-up event as
a result of fatigue damage in the sidewall of the tyre. These
failures occurred at a similar retread level (R4 and R5), but
occurred well before the tread was due for replacement.
Aircraft tyres of this type had several operational measures to
prevent premature failure. These included non-destructive
inspection (NDI) during the retread process, inflation pressure
maintenance, and aircraft weight and speed limitations.
Examination of the failed tyres found that the fatigue damage
was quite extensive and was considered to be possibly present at
the last retread. Because the shoulder-to-shoulder NDI used by the
tyre retread facility was limited to the crown (tread area) of the
tyre, it could not detect damage in the sidewall unless the extent
of the damage breached the inner or outer surface of the tyre
resulting in a leak. The NDI technique utilised by the retread
facility was physically capable of inspecting the sidewall of the
tyres, but neither the Civil Aviation Safety Authority (CASA) nor
the Federal Aviation Administration (FAA) required sidewall
inspection for bias-ply tyres.
Diffusion of the air through the inner liner of tubeless
aircraft tyres resulted in the slow loss of pressure. Thus, it is
vitally important to regularly check and maintain the tyre
inflation pressures. The normal practice was a daily check on cold
loaded tyres. This not only ensured that the tyres were operated in
the correct pressure range, but also allowed maintenance engineers
to determine if a tyre was leaking at an abnormal rate.
An excessive leakage rate is a strong indication of imminent
tyre failure and so it is critical that it is identified as soon as
possible. The survey conducted by the operator indicated that only
about one third of the low-pressure events were being recorded by
the engineers. For those events that were not recorded, the only
way to identify a 'leaker' (a tyre with pressure leakage in excess
of the normal limits) was for a maintenance engineer to remember
that the same tyre on the same aircraft was topped-up on the
previous day. As the operator had 45 aircraft, each with six tyres
(four main and two nose) and operated from different ports around
Australia, the detection of a 'leaker' was very unlikely.
The maintenance Task Card used by the operator during the daily
check listed procedures for all of the 737 variants in their fleet.
This was particularly evident for the tyre inflation checks where
there were three possible nominal inflation pressures. This Task
Card did not directly indicate the inflation pressure range for
each aircraft variant, but contained a note to refer to the
Aircraft Maintenance Manual (AMM) if the pressure was below nominal
by more than 5%. Although the abbreviated data on the Task Card
provided the correct nominal pressure, it left the appropriate
inflation pressure range open to some level of interpretation and
possible confusion, as the limit for the inflation pressure in the
AMM was the nominal ±5 psig, but the special maintenance procedures
started at 5% (i.e. 10 psig) below the nominal pressure.
Until 2004, when the task card had the tyre nominal service
pressure increased from 200 psig to 205 psig, the tyres had been
operated at or below the lower limit of the normal service pressure
range for extended periods. Under-inflation is a well known factor
in the development of fatigue in the sidewall of tyres resulting in
reduced life performance.
The examination of the incident flight data did not show any
single event that would have resulted in the failure. However, a
review of the taxi speed data from two aircraft that had tyre
failures, indicated that they were occasionally turned at speeds
approaching, or exceeding, that recommended by the aircraft
manufacturer. Considering the examination of the failed tyres and
the analysis presented above, the damage to the tyres was not due
to a single event, but an accumulation of damage during normal
operations. The effect of turning at taxi speeds above that
recommended by the manufacturer would further reduce the life
expectancy of the tyre casing.
Certification of tyres and qualification for
retread
The basic design standard for the tyres (FAA TSO-C62d) required
requalification for a tread design change. This applied to each
manufacturer. If a manufacturer changed their basic tread design,
such that it was the same as a retread package, they would be
required to retest to show compliance with the TSO. However, the
practice for retread qualification accepted by both CASA and the
FAA, required only one brand of tyre to be fully tested
(overpressure and dynamometer). This testing may verify the retread
package, but does not capture the effect that the tread design,
which may be different to the original tread design on the tyre,
has on the performance of the casing of the non-tested brands.
Because the tyres were originally approved to Technical Standard
Order TSO-C62d and had lasted for more than three retread lives, it
was clear that there was no immediate weakness inherent in the
original tyre design. As the aircraft type was fitted with several
tyre brands, but only developed similar fatigue flaws in one brand,
it appears that there were performance differences between the
brands that the retread qualification testing of a single brand at
R1 could not identify. There was no apparent compliance
substantiation for the untested brands to the original design
standard (i.e. TSO-C62), other than historical acceptance of
similarities in the construction of bias-ply tyres.
SIGNIFICANT FACTORS
- Flexing of the tyre sidewalls in operation resulted in fatigue
damage in the inner casing plies. - The fatigue damage was possibly present at the last retread
process, but was not detected by the non-destructive inspection
used in the process. - The fatigue in the plies allowed cracks to develop and grow in
the inner liner, allowing high-pressure air to leak into the
casing. - Maintenance crews did not detect an increase in the rate of
pressure loss in the days leading up to the tyre failure. - The pressure leakage into the casing increased to the point
that exceeded the capacity of the in-built venting and, combined
with operational loads, resulted in the failure of the tyre
structure.
SAFETY ACTION
Australian Transport Safety Bureau
On 23 December 2004 the Australian Transport Safety Bureau
issued safety recommendation R20040093. That recommendation
stated:
The Australian Transport Safety Bureau recommends that
Australian operators of Boeing 737-800 series aircraft review the
practice of fitting retread tyres of R4 (fourth retread) or above,
until their serviceability limitations can be identified.
In response to the safety recommendation, the ATSB was advised
that the following actions were taken:
- The operator of VH-VOH limited their in-service retread level
to R3 on the 737-800 series fleet. - The other operator with 737-800 series aircraft performed a
visual inspection of all in-service tyres and instigated a more
stringent inspection of the tyres by maintenance and flight crews
as part of the pre-flight checks. - The retread facility limited the maximum retread level for the
H44.5x16.5-21 28 PR tyre to R3.
Aircraft manufacturer
The aircraft manufacturer noted a worldwide increase in 737 tyre
failures, particularly in the 737NG (600, 700, 800, 900 and BBJ
series). As a result, they conducted a study of the failures and
released a Flight Team Digest in January 2005. Although their study
found that no particular root cause for the failures was apparent,
they were able to make some recommendations to improve the
operation of the tyres. These recommendations included:
Inflate tires to the high end of the allowable ranges shown in
AMM [Aircraft Maintenance Manual] 12-15-51. This reduces the
sidewall deflection of the tire and therefore reduces the heat
generated by the tire.
It is important to check inflation pressures frequently.
If a tire is identified as a leaker (as evidenced by two
successive pressure checks where pressure is more than 5% low), it
should be removed from the airplane immediately. Note that
tires which are leaking gas through their inner-liner pose an
imminent threat of a tread loss.Operators should require that their retreaders perform the more
complete bead-to-bead NDT checks (holography/shearography) on their
tires as opposed to just checking the crown area. This appears to
be especially important on 737NG tire sizes and is most critical
when the tires reach high retread levels.
On 28 June 2005, the Boeing Commercial Airplane Company released
Service Letter 737-SL-32-128-A 'Tire Retreading Recommendations' to
737-600/-700/-800/-900/BBJ operators. This Service Letter provided
guidance on retreading NDI, retread limits and frequent and
thorough pressure checks.
Operator of VH-VOH
On 1 June 2005, the aircraft operator released a General
Engineering Notice (GEN) regarding tyre pressure checking and
discrepancy recording. This GEN was associated with a change to the
daily Task Card and provided more detailed guidance on the pressure
maintenance of the tyres in the fleet and actions to be taken when
a tyre pressure is found to be low. The GEN and daily Task Card
included the following improvements:
- Tyres are to be maintained in the upper end of the pressure
range (i.e. nominal pressure +5/-0 psi). - Any tyre requiring a pressure top-up during the daily check is
to be recorded in the Flight Technical Log (with pre- and
post-inflation pressures) and the pressure re-checked on the next
daily check. - The allowable pressure range (+5/-0 psi) has been listed with
the nominal pressure for each aircraft variant in the fleet. - Expanded maintenance actions from the AMM for under-inflated
tyres. - A note to ensure that the gauge used to check pressures is
within calibration prior to use.
Tyre retread facility
On 9 September 2005, the Australian tyre retread facility
included in their CASA approved Process Specification, bead-to-bead
shearography inspections on all 737NG main gear tyres of retread
level R3, and above.
United States Federal Aviation
Administration
On 9 March 2006, the United States Federal Aviation
Administration advised the Australian Transport Safety Bureau that
they are currently reviewing and revising Technical Standard Order
TSO-C62 and Advisory Circular AC 145-4. The changes to AC 145-4 are
to include the recommendation for a reliability program and the
increased use of shearography at increasing retread levels.