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CFR Final
Rule
Federal
Register Information
[Federal Register: February 26, 1993 (Volume 58, Number 37)]
[Page 11778]
Header
Information
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 26147; Amendment No. 25-78]
RIN 2120-AD87
Use of Nitrogen or Other Inert Gas for Tire inflation in Lieu of
Air
Preamble
Information
AGENCY: Federal Aviation Administration, DOT
ACTION: Final Rule
SUMMARY: This amendment to the Federal Aviation Regulations
(FAR) requires that an inert gas, such as nitrogen, be used in
lieu of air, for inflation of tires on certain transport
category airplanes. This action is prompted by at least three
cases in which the oxygen in air-filled tires combined with
volatile gases given off by a severely overheated tire and
exploded upon reaching auto-ignition temperature. The use of an
inert gas for tire inflation will eliminate the possibility of a
tire explosion.
EFFECTIVE DATE: This rule becomes effective March 29, 1993.
FOR FURTHER INFORMATION CONTACT: Robert C. McCracken, Flight
Test and Systems Branch, ANM-111, Transport Airplane
Directorate, Aircraft Certification Service, 1601 Lind Avenue
Southwest, Renton, Washington 98055-4056: telephone (206)
227-2118.
SUPPLEMENTARY INFORMATION:
Background
This amendment is based on Notice of Proposed Rulemaking (NPRM)
No. 90-7, which was published in the Federal Register on March
5,1990 (55 FR 7876), and a correction notice published March 21,
1990 (55 FR 10487). Notice 90-7 proposed to require that an
inert gas, such as nitrogen, be used in lieu of air, for
inflation of tires on certain transport category airplanes.
The airworthiness standards for airplane tires are contained in
Sec. 25.733 of the FAR. This section describes the loads and
speed ratings required of each tire, and requires that the tires
be shown to be suitable for their intended use. In addition,
each tire may be manufactured under technical standard order (TSO),
which means that the tire has passed a series of rigorous
dynamometer tests and is produced in accordance with an approved
manufacturing process and quality control system. Airplane tires
are designed for strength and durability and, since the advent
of turbojet transport airplanes, they have had a satisfactory
service history. Despite this emphasis on strength and
durability for airplane tires, there have been instances of tire
failures in the wheel well during flight on transport category
airplanes. The great majority of these have been classified as
tire bursts, with only a few having been identified as tire
explosions. This is an important distinction, which is relevant
to this rulemaking action.
A tire burst, as referred to in Sec. 25.729(f), is a sudden,
sometimes violent, venting of the pressure from within a tire,
usually associated with a flaw in the tire, foreign object
damage, or tire overheat/overload. The FAA assumes that tire
bursts will occasionally occur, given the severe operating
environment of airplane tires, and the fact that certain tire
damage may go undetected until tire failure. With this in mind,
equipment installed in wheel wells is evaluated at the time of
certification to determine its ability to withstand the effects
of a bursting tire. Analyses and laboratory tests are performed
to identify critical areas, and design changes are often made to
ensure that a single tire burst will not cause loss of critical
functions.
A tire explosion is a completely different phenomenon. It
results from the auto-ignition and explosion of a mixture of
explosive vapors released from the inner-liner of a severely
overheated or abused tire, and any oxygen that may be present
inside the tire. A tire explosion in the wheel well is an
unlikely event, since it is the result of a combination and
several related events: A brake must be severely overheated due
to some brake system failure; the wheel thermal fuse plugs,
because of their orientation when the landing gear are
retracted, must fail to respond quickly enough to the overheated
condition; the overheated and possibly damaged tire must hold
together long enough to allow the gas mixture in the tire to
reach auto-ignition temperature; and there must be sufficient
oxygen inside the tire to support an explosion.
It is impossible to design a thermal fuse plug that would be
effective in a tire explosion. Most thermal fuse plugs in use
today are hollow bolts installed in the wheel rim. The cavity in
the bolt is filled with a material that melts at a precisely
defined temperature and is ejected from the hollow bolt by tire
pressure, allowing the tire to deflate. Because the pressure and
temperature rise inside the tire would be nearly instantaneous
following ignition, the melting of the fuse plug, which is
basically a mechanical process, would not have time to take
place. Also, the cross sectional area of a series of fuse plugs
sufficient to safely vent the energy of an explosion would be so
large that it could seriously compromise the structural
integrity of the wheel. It is more logical to prevent a tire
explosion than to attempt to deal with it after it happens. A
tire explosion can be prevented by the use of an inert gas such
as nitrogen for tire inflation.
Laboratory tests conducted in 1973 show a definite relationship
between the quantity of oxygen in a tire and the gas mixture's
auto-ignition temperature. Test data indicate that at nitrogen
concentrations between 80 percent and 90 percent (the atmosphere
contains approximately 80 percent nitrogen and 20 percent
oxygen), ignition of inner tire liner samples occurred in a test
chamber with temperatures varying from 478 °F to 518 °F.
Nitrogen concentrations between 90 percent and 95 percent raised
the auto-ignition temperatures to a range of 520 °F to 531 °F.
At nitrogen concentrations greater than 95 percent, there was no
pressure increase in the test chamber, even at chamber
temperature of 670 °F, indicating that there was no ignition.
Based on these tests, it was concluded that any concentration of
oxygen in a tire in excess of 5 percent of the total gas will
support a reaction. At a concentration above 10 percent, this
reaction is an abrupt auto-ignition. At concentrations from 5
percent to 10 percent, this reaction is assumed to be a low
level auto-ignition, based on measurement of test chamber
pressure and temperature.
If a tire contains at least 95 percent nitrogen or other gases
shown to be inert, and is involved in a severe overheat
situation as described above, the atmosphere inside the tire
would prevent auto-ignition, or at least delay it long enough
either for the fuse plugs to react and release tire pressure, or
for the tire itself to fail from overheat, resulting in the less
severe tire burst.
Since the hazard associated with a tire explosion in the wheel
well during flight exists on large transport airplanes using
tires inflated with air, Sec. 25.733 is amended to require that
tires mounted on braked wheels be inflated with dry nitrogen, or
other gases shown to be inert, such that the gas mixture does
not contain oxygen in excess of 5 percent by volume. As other
means may be available to prevent tire explosions, this
amendment requires the use of an inert gas for tire inflation
unless the tire liner material will not produce volatile gases
when heated or means are provided to prevent tire temperatures
from reaching unsafe levels.
The Federal Aviation Administration (FAA) recognizes that
nitrogen may not always be available at some airports, and that
the prohibition against the use of air to refill a low tire may
cause some inconveniences. As indicated by the testing described
above, nitrogen in the tire may be diluted with oxygen to a 95
present concentration without compromising safety. Any
maintenance procedure developed by an operator that would assure
that any tire refill using air would not allow the nitrogen
concentration to drop below 95 percent would be an acceptable
method of compliance with the rule. For example, a manufacturer
has published in the maintenance manuals of its models, a chart
which explains a repetitive air refill procedure for a residual
tire nitrogen contest of 90 percent. While this chart would not
be usable for concentration of 95 percent, it shows that similar
procedures for a minimum nitrogen concentration of 95 percent
for a range of tire sizes and pressures could be easily
developed.
This new rule applies to large transport category airplanes with
a maximum certificated takeoff weight greater than 75,000
pounds. A review of service difficulty reports has revealed that
tire explosions, as opposed to tire bursts, tend to occur on the
larger, heavier airplanes. While the mechanism of a tire
explosion is not fully understood, it is clear that sufficient
energy to raise the air in a tire to auto-ignition temperatures
must be provided by an overheated brake. Larger airplanes
generally have higher takeoff and landing speeds and, at the
higher gross weights, this provides for more kinetic energy to
be absorbed by the brakes as heat. In addition, the volume of
gas present in the larger tires installed on larger, heavier
airplanes contain more combustible gas than would be found on
the smaller airplanes, which would result in a more damaging
explosion. Finally, the FAA has no records of adverse service
history on smaller transport category airplanes that would
suggest that the use of air for tire inflation constitutes a
hazard on these airplanes.
In addition, this amendment adds the requirement to use an inert
gas for tire inflation for braked wheels only, since there is no
source of excessive heat present on unbraked wheels.
In response to the unsafe condition associated with tire
explosions, the FAA issued an airworthiness directive (AD) in
April 1987 which requires the servicing of tires on certain
large transport category airplanes with nitrogen in lieu of air.
The AD requires the installation of a placard, either in the
wheel well or on or near each landing gear strut incorporating
braked wheels, and in a location so as to be easily seen and
readable by a person performing routine tire servicing. The
placard is to read "Inflate Tires With Nitrogen Only."
Alternatively, the operator is to incorporate into the
FAA-approved maintenance program procedures to ensure that the
gas mixture will not exceed 5% oxygen by volume. The FAA
believes that an appropriate placard installed on the airplane
and the addition of a suitable limitation on the type
certificate data sheet would be an acceptable means of
compliance with this new rule. It would also be appropriate to
include information regarding servicing tires with an inert gas
in the manufacturer's maintenance manuals under Instructions for
Continued Airworthiness.
Discussion of Comments
Comments were received from two foreign government agencies,
four trade organizations, and one company manufacturing landing
gear components. While all of the commenters support the
proposal, two parties offer comments suggesting changes of a
clarifying nature.
One commenter suggests that control of the inflation media
should apply to any aircraft with a maximum certificated takeoff
weight (MCTW) of more than 5,700 kilograms (12,560 pounds) that
has retractable undercarriage containing braked wheels, and not
be limited to aircraft of more than 75,000 pounds MCTW. No
information was presented by the commenter to support this
suggestion, and the FAA has no information to support the
suggestion. As noted in the preamble of Notice 90-7, the FAA has
no records showing adverse service history on smaller transport
category airplanes that would suggest that the use of the air
for tire inflation constitutes a hazard. This is due to the
higher energy that must be absorbed by the brakes, wheels, and
tires with the higher weights and landing speeds associated with
larger airplanes. Limiting the applicability to airplanes with
"retractable undercarriages" is not considered appropriate.
There will, in all likelihood, be no transport category
airplanes with takeoff weights above 75,000 pounds certificated
with fixed landing gear. Nevertheless, the hazard associated
with tire explosions would exist if such an airplane design were
proposed. The rule is therefore adopted as proposed in that
regard.
This commenter also notes that, while nitrogen is the most
likely inert gas that would be used to inflate tires, it would
be necessary to ensure that any other "gases shown to be inert"
are approved by the manufacturer as not being detrimental to the
tire. The commenter suggests changing the rule to refer to other
suitable gases shown to be inert, and approved by the tire
manufacturer. The FAA shares the concern expressed by the
commenter; however the wording in the rule as proposed in Notice
90-7 contains the phrase "nitrogen or other gases shown to be
inert." This requires that the applicant manufacturer
demonstrate to the FAA that any "other gas" is suitable for use.
The term "approved by the manufacturer" is inappropriate since
only the FAA or persons authorized to act on behalf of the FAA
have the authority to approve the use of another gas. The rule
is therefore issued as proposed in this regard.
This commenter also expresses concern regarding servicing of
tires at remote locations where dry nitrogen is not currently
available, and expresses the assumption that there is an
operational counterpart to the proposed change in part 25 of the
FAR. The commenter notes that some European countries have a
maintenance practice allowing servicing of tires at remote sites
where dry nitrogen is not available by either ascertaining that
the oxygen content stays below 5 percent or requiring that the
tire be purged and inflated with dry nitrogen within 15 flight
hours after servicing with air. This practice is also allowable
under airworthiness directives issued by the FAA addressing
existing airplanes. The FAA infers that the commenter is
suggesting a change to the final rule to allow servicing with
air at remove sites under the above maintenance practice. The
FAA does not concur with this suggestion. The cost of supplying
dry nitrogen servicing equipment to airports that are not now
equipped to service large transport category airplane tires in
accordance with this rule was investigated, as were the benefits
that would accrue from promulgation of the rule and, as was
noted in the preamble to Notice 90-7, it was determined that the
ratio of cost versus benefits was favorable. A tire explosion
could occur with catastrophic consequences within the 15 allowed
hours following servicing a tire with air. Further, this rule
does not affect airplanes now flying and the FAA is confident
that suitable service facilities can be supplied where needed
prior to new airplanes certificated in accordance with this rule
becoming operational.
One commenter recommends revising the new Sec. 25.733(e) to
state "For an airplane with a maximum certificated takeoff
weight of more than 75,000 pounds, means shall be provided for
tires mounted on braked wheels to preclude tire explosions." The
commenter states that "The proposed regulation is a design
dictate which precludes alternate designs, more economical
solutions, or any advancement in the state of the art beyond the
1980's," and notice that the new rule should "not preclude the
search for tire inner liners which do not release explosive
vapor. It should not preclude solutions which prevent tire
temperatures from reaching the temperatures at which outgassing
occurs since this would also preclude tire structural damage
from overheat." The FAA concurs with the intent of the
commenter's suggestion. There is currently no known means to
protect transport category airplanes from the hazards associated
with tire explosions except through mandatory use of an inert
gas for tire inflation. Nevertheless, if a means is found in the
future to preclude the hazard, either through the use of
different tire materials or via design changes that can be shown
to be effective in limiting tire temperatures to safe levels,
that means would be acceptable. Section 25.733(e) is changed to
reflect such alternative methods of compliance.
This same commenter goes on to state that the proposal "imposes
a constraint on design which does not achieve the intended
purpose -to ensure that tires are always filled with inert gases
during their entire service life." The commenter states that the
regulation should address the servicing and maintenance of the
aircraft. The FAA infers from this comment that the commenter
desires specific wording in the new rule to control maintenance
procedures. The FAA does not concur with this comment. The
operating rules require that all civil aircraft be operated in
accordance with operating limitations specified in the approved
Airplane Flight Manual, markings, and placards, or as otherwise
prescribed by the certificating authority of the country of
registry. Therefore, placing wording to control maintenance
procedures in part 23 as suggested by the commenter would be
duplicative.
Regulatory Evaluation Summary
This section summarizes the full regulatory evaluation prepared
by the FAA that provides more detailed estimates of the economic
consequences of this regulatory action. This summary and the
full evaluation quantify, to the extend practicable, estimated
costs to the private sector, consumers, Federal, State and local
governments, as well as anticipated benefits.
Executive Order 12291, dated February 17, 1981, directs Federal
agencies to promulgate new regulations or modify existing
regulations only if potential benefits to society for each
regulatory change outweigh potential costs. The order also
requires the preparation of a Regulatory Impact Analysis of all
"major" rules except those responding to emergency situations or
other narrowly defined exigencies. A "major" rule is one that is
likely to have an annual impact on the economy of $100 million
or more, a major increase in consumer costs, or a significant
adverse effect on competition.
The FAA has determined that this rule is not "major" as defined
in the executive order; therefore, a full Regulatory Impact
Analysis, which includes the identification and evaluation of
cost-reducing alternatives to this rule, has not been prepared.
Instead, the agency has prepared a more concise document, termed
a regulatory evaluation, that analyzes only this rule without
identifying alternatives. In addition to a summary of the
regulatory evaluation, this section also contains the Regulatory
Flexibility Determination required by the Regulatory Flexibility
Act and an International Trade Impact Analysis. If more detailed
economic information is desired, the reader may refer to the
full regulatory evaluation contained in the docket.
None of the comments on the NPRM for this amendment addressed
the evaluation of costs and benefits, and, accordingly, no
resulting changes have been made to the evaluation.
Economic Evaluation
It is likely, for reasons of efficiency and practicality, that
since air carriers are currently using inert gas, and inert gas
tire inflation equipment and procedures on their existing
fleets, they will voluntarily employ the same equipment and
procedures on airplanes with new type certificates. Since this
rule mandates what is already an existing industry practice, as
required by airworthiness directive, actual costs and benefits
attributable to the rule are expected to be negligible.
Notwithstanding, the FAA has analyzed potential costs by
assuming, as a worst-case scenario, that 5 percent of the
affected, newly-certificated airplanes would use compressed air
to service their tires in the absence of this rule. The
evaluation further assumes that nitrogen inflation equipment
could be required at as many as 20 destinations not previously
served by U.S. operated, large transport category airplanes.
Technically, none of the four new airplane models that are
currently scheduled for delivery between 1992 and 2001 will be
subject to the rule since their certification bases were
established at the time of application. Therefore, this
evaluation considers the succeeding 10-year period, 2002 to
2011, and assumes that the same numbers of new-model airplanes
will be delivered during that period as are currently forecast
for the 1992 to 2001 period.
The following additional factors were used in determining the
potential costs of this rule.
1.
Bottled nitrogen will cost an average of $0.98 per 100 cubic
feet.
2.
Each tire will require approximately 155 cubic feet of nitrogen
over its useful life.
3.
Nitrogen inflation equipment will cost $780 per unit for bottle
carts, hoses and fittings.
Under these assumptions, the present-value equivalence of $1,300
in operating costs and $5,468 in equipment costs could be
expended over the period from 2002 to 2011 as a result of this
rule.
The primary benefit of the rule is the elimination of any
remaining possibility of tire explosions caused by tire
inflation with compressed air. The FAA is unable to predict the
probability that a tire auto-ignition accident would occur in the
absence of this rule. However, the discounted present value of
averting an accident similar to the 1973 crash at Dulles would
equal $190,000. If the probability of averting a single
non-catastrophic accident is only 3.6 percent ($6,768/$190,000),
this rule will prove to be cost beneficial. Accordingly,
the FAA believes that the potential
benefits of this rule will exceed the potential costs.
Regulatory Flexibility Determination
The Regulatory Flexibility Act of 1980 (RFA) was enacted by
Congress to ensure that small entities are not unnecessarily or
disproportionately burdened by Governmental regulations. The RFA
requires a Regulatory Flexibility Analysis if a rule has a
significant economic impact, either detrimental or beneficial,
on a substantial number of small entities. FAA Order 2100.14A,
Regulatory Flexibility Criteria and Guidance, establishes
threshold cost values and small entity size standards for
complying with RFA review requirements in FAA rulemaking
actions. Based on these factors, the FAA has determined that
this rule will not have a significant economic impact on a
substantial number of small entities.
International Trade Impact Analysis
The provisions of this rule will have little or no impact on
trade for both U.S. firms doing business in foreign countries
and foreign firms doing business in the United States. In the
United States, foreign manufacturers will have to meet the U.S.
requirements, and thus would gain no competitive advantage. In
foreign countries, U.S. manufacturers are not bound by Part 25
requirements and could, therefore, implement the provisions of
this rule solely on the basis of competitive considerations.
Federalism Implication
The regulations adopted herein will not have substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.
Therefore, in accordance with Executive Order 12612 it is
determined that this final rule will not have sufficient
federalism implications to warrant the preparation of a
Federalism Assessment.
Conclusion
Because the requirement to use inert gas in lieu of air for tire
inflation is not expected to result in a substantial cost, the
FAA has determined that final rule is not major as defined in
Executive Order 12291. Because this is an issue which has not
prompted a great deal of public concern , this final rule is not
considered to be significant as defined in Department of
Transportation Regulatory Policies and Procedures (44 FR 11034,
February 26, 1979). In addition, since there are no small
entities affected by this rulemaking, it is certified, under the
criteria of the Regulatory Flexibility Act, that this final
rule, at promulgation, will not have a significant economic
impact, positive or negative, on a substantial number of small
entities. A copy of the final regulatory evaluation prepared for
this project may be examined in the public docket or obtained
from the person identified under the caption FOR FURTHER
INFORMATION CONTACT.
List of Subjects in 14 CFR Part 25
Air transportation, Aircraft Aviation safety, Safety.
Regulatory
Information
Adoption of the Amendments: The Amendment
Accordingly, Part 25 of the Federal Aviation Regulations (FAR)
(14 CFR Part 25) is amended as follows:
Part 25 - Airworthiness Standards: Transport Category Airplanes
1. The authority citation for part 25 is revised to read as
follows:
Authority: 49 U.S.C. app. 1344, 1354(a), 1355, 1421, 1423, 1424,
1425, 1428,1429, 1430; 49 U.S.C. 108(g).
2. By amending Sec. 25.733 by adding a new paragraph (e) to read
as follows:
Sec. 25.733 Tires
* * * * *
(e) For an airplane with a maximum certificated takeoff weight
of more than 75,000 pounds, tires mounted on braked wheels must
be inflated with dry nitrogen or other gases shown to be inert
so that the gas mixture in the tire does not contain oxygen in
excess of 5 percent by volume, unless it can be shown that the
tire liner material will not produce a volatile gas when heated
or that means are provided to prevent tire temperatures from
reaching unsafe levels.
Footer
Information
Issued in Washington, DC, on February 22, 1993.
Joseph Del Balze,
Acting Administrator.
[FR Doc. 93-4506 Filed 2-25-93; 8:45 am]
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