Revised Structural Loads Requirements for Transport Category Airplanes

SUMMARY: This amendment revises the structural loads design

requirements of the Federal Aviation Regulations (FAR) for transport

category airplanes by incorporating changes developed in cooperation

with the Joint Aviation Authorities (JAA) of Europe and the Aviation

Rulemaking Advisory Committee (ARAC). This action makes some of the

requirements more rational and eliminates differences between current

U.S. and European requirements that impose unnecessary costs on

airplane manufacturers. These changes are intended to achieve common

airworthiness standards and language between the requirements of the

U.S. regulations and the Joint Aviation Requirements (JAR) of Europe

while maintaining at least the level of safety provided by the current

regulations and industry practices.

EFFECTIVE DATE: August 28, 1997.

FOR FURTHER INFORMATION CONTACT: James Haynes, Airframe and Propulsion

Branch, ANM-112, Transport Airplane Directorate, Aircraft Certification

Service, FAA, 1601 Lind Avenue, SW., Renton, WA 98055-4056; telephone

(206) 227-2131.

SUPPLEMENTARY INFORMATION:

Background

The manufacturing, marketing and certification of transport

airplanes is increasingly an international endeavor. In order for U.S.

manufacturers to export transport airplanes to other countries the

airplane must be designed to comply, not only with the U.S.

airworthiness requirements for transport airplanes (14 CFR part 25),

but also with the transport airworthiness requirements of the countries

to which the airplane is to be exported, unless the importing country

accepts the aircraft without findings of compliance with specified

regulations.

The European countries have developed a common airworthiness code

for transport category airplanes that is administered by the JAA. This

code is the result of a European effort to harmonize the various

airworthiness codes of the European countries and is called the Joint

Aviation Requirements (JAR)-25. It was developed in a format similar to

14 CFR part 25. Many other countries have airworthiness codes that are

aligned closely to part 25 or to JAR-25, or they use these codes

directly for their own certification purposes.

Although JAR-25 is very similar to part 25, there are differences

in methodologies and criteria that often result in the need to address

the same design objective with more than one kind of analysis or test

in order to satisfy both part 25 and JAR airworthiness codes. These

differences result in additional costs to the transport airplane

manufacturers and additional costs to the U.S. and foreign authorities

that must continue to monitor compliance with different airworthiness

codes.

In 1988, the FAA, in cooperation with the JAA and other

organizations representing the U.S. and European aerospace industries,

began a process to harmonize the airworthiness requirements of the

United States and the European authorities. The objective was to

achieve common requirements for the certification of transport category

airplanes without a substantive change in the level of safety provided

by the regulations and industry practices. Other airworthiness

authorities such as Transport Canada have also participated in this

process.

In 1992, the harmonization effort was undertaken by the Aviation

Rulemaking Advisory Committee (ARAC). A working group of industry and

government structural loads specialists of Europe, the United States,

and Canada was chartered by notice in the Federal Register (58 FR

13819, March 15, 1993) to harmonize the design loads sections of

Subpart C of part 25. The bulk of the harmonization tasks for Subpart C

were completed by the working group and recommendations were submitted

to FAA by letter dated February 2, 1995. The FAA concurred with the

recommendations and proposed them in Notice of Proposed Rulemaking

(NPRM) No. 95-14; which was published in the Federal Register on August

29, 1995 (60 FR 44998).

In establishing a design requirement for the nose gear, its

attaching structure and the forward fuselage structure, Sec. 25.499(e)

continues to require consideration of positioning the nose gear in any

steerable position. The term ``any'' is continued from the current

regulation. The term, and the requirements of the section, are

understood in the engineering and regulated communities to require

demonstration that the nose gear and associated structures will sustain

the applicable loads throughout the full range of nose gear positions.

Discussion of Comments

Comments were received from transport airplane manufacturers,

industry associations and foreign airworthiness authorities. All of the

commenters express support for the proposals in Notice No. 95-14

although a few make some recommendations for changes. One comment

believes the changes proposed for Sec. 25.415 could be a burden to some

applicants with airplanes that are derived from models that were

certified to earlier amendment levels of the FAR and JAR. To provide

relief for these derivative airplanes, the commenter proposes a change

to paragraph (b) of Sec. 25.415 which would allow the use of

``realistic'' aerodynamic hinge moment coefficients for control

surfaces in lieu of the prescribed coefficients of paragraph (b). The

FAA does not agree that there is likely to be a burden for derivative

airplanes since the proposed rule applies to new designs. In addition,

the design gust speed does not create an increased requirement over

existing design requirements. Part 24 and JAR-25 were identical in

using 88 feet per second (about 52 knots) in defining hinge moment for

ground gust conditions. However, JAR Sec. 25.519 prescribes a 65 knot

wind speed for ground gusts during jacking and tie-down, and

specifically requires application of those gusts to control surfaces.

As a result, aircraft designs already have to meet the 65 knot rather

than the 52 knot requirement. The ARAC recommends, with FAA and JAA

concurrence, that ground gusts on control surfaces be addressed in just

one section, Sec. 25.415, so Notice No. 95-14 proposes to revise this

section to achieve the same effect as the Sec. 25.519 of JAR-25 by

incorporating the 65-knot wind speed into Sec. 25.415. The net effect

is that there is no change in the ground gust speed requirement for

control surfaces over that already required by JAR-25.

Furthermore, the use of rational aerodynamic hinge moment

coefficients would necessitate a rational ground gust speed as well,

and the 65 knot design gust speed is not necessarily a rational design

speed for ground gusts. Jet blasts in airport operations and normal

storm conditions often exceed 65 knots but service history has shown

that the 65 knot design speed when combined with the conservative

prescribed hinge moments of paragraph (b) provides a satisfactory

design.

[[Page 40703]]

One commenter recommends that the formulation of the requirement

for hinge moments in Sec. 25.415 be changed to show the 65 knot wind

speed explicitly rather than embedding this value into the multiplying

constant. The FAA agrees that this has merit since the connection

between the 65 knot wind speed of Secs. 25.415 and 25.519 could

otherwise be missed in any future rulemaking actions. The rule is

adopted with a change to show the 65 knot wind speed explicitly in the

formula for control surface hinge moments.

One commenter points out that the proposed revision to paragraph

(a) of Sec. 25.481 references paragraphs 25.479(c)(1) and (2) for

vertical and drag load conditions and that these latter paragraphs, as

proposed, no longer specify those conditions. Notice 95-14 proposes to

express the substance of Sec. 25.479(c)(1) and (2) in more general

terms in Sec. 25.473(c). The commenter is correct. The rule is adopted

with a change to delete the incorrect references.

Regulatory Evaluation Summaries

Regulatory Evaluation, Regulatory Flexibility Determination, and Trade

Impact Assessment

Changes to Federal regulations must undergo several economic

analyses. First, Executive Order 12866 directs that each Federal agency

shall propose or adopt a regulation only upon a reasoned determination

that the benefits of the intended regulation justify its costs. Second,

the Regulatory Flexibility Act of 1980 requires agencies to analyze the

economic effect of regulatory changes on small entities. Third, the

Office of Management and Budget directs agencies to assess the effects

of regulatory changes on international trade. In conducting these

analyses, the FAA has determined that this rule:

(1) Will generate benefits that justify its costs and is not a

``significant regulatory action'' as defined in the Executive Order;

(2) is not significant as defined in DOT's Regulatory Policies and

Procedures; (3) will not have a significant impact on a substantial

number of small entities; and (4) will not constitute a barrier to

international trade. These analyses, available in the docket, are

summarized below.

Regulatory Evaluation Summary

Depending on airplane design, the rule could result in additional

compliance costs for some manufacturers. If manufacturers choose to

design to and justify a VD-VC magin of 0.05 Mach,

there will be an increase in analysis costs of approximately $145,000

per certification. The requirement in Sec. 25.473 to consider

structural flexibility in the analysis of landing loads and the

increase in the factor on the maximum static reaction on the nose gear

vertical force in Sec. 25.499 could add compliance costs, but the FAA

estimates that these will be negligible.

The rule will also result in cost savings. Revisions in the

conditions in which unchecked pitch maneuvers are investigated could

reduce certification costs by as much as $10,000 per certification. The

FAA estimates that the change in the speed margin between VB

and VC from a fixed margin to a margin variable with

altitude could result in substantial, though unquantified, cost savings

to some manufacturers. Manufacturers that design small transport

category airplanes with direct mechanical rudder control systems could

realize a savings as a result of the modification in the rudder control

force limit in Sec. 25.351. No comments were received on the costs or

cost savings resulting from these changes.

The primary benefit of the rule will be the cost savings associated

with harmonization of the FAR with the JAR. In order to sell airplanes

in a global marketplace, manufacturers usually certify their products

under the FAR and the JAR. The cost savings from reducing the resources

necessary to demonstrate compliance with non-harmonized design load

requirements will outweigh any incremental costs of the rule, resulting

in a net cost savings. These savings will be realized by U.S.

manufacturers that market airplanes in JAA countries as well as by

manufacturers in JAA countries that market airplanes in the U.S.

The change to Sec. 25.335(b)(2) in the minimum speed margin for

atmospheric conditions from 0.05 Mach to 0.07 Mach could produce safety

benefits. The increase in the margin between VD/

MD and VC/MC is more conservative and

will standardize training across international lines. Crews could

cross-train and cross-fly and this standardization will enhance safety

as well as result in more efficient training.

Regulatory Flexibility Determination

The Regulatory Flexibility Act of 1980 (RFA) was enacted by

Congress to ensure that small entities are not unnecessarily and

disproportionally burdened by Federal regulations. The RFA requires a

Regulatory Flexibility Analysis if a proposed or final rule would have

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 standards for complying with RFA review requirements

in FAA rulemaking actions. The Order defines ``small entities'' in

terms of size threshold, ``significant economic impact'' in terms of

annualized cost thresholds, and ``substantial number'' as a number

which is not less than eleven and which is more than one-third of the

small entities subject to the proposed or final rule.

Order 2100.14A specifies a size threshold for classification as a

small manufacturer as 75 or fewer employees. Since none of the

manufacturers affected by this rule has 75 or fewer employees and any

costs of the rule will be negligible, the rule will not have a

significant economic impact on a substantial number of small

manufacturers.

International Trade Impact Assessment

The rule will not constitute a barrier to international trade,

including the export of U.S. airplanes to foreign markets and the

import of foreign airplanes into the U.S. Because the rule will

harmonize with the JAR, it would, in fact, lessen restraints on trade.

Federalism Implications

The regulations amended herein do not have a 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. Thus, in

accordance with Executive Order 12612, it is determined that this rule

does not have sufficient federalism implications to warrant the

preparation of a Federalism Assessment.

International Compatibility

In keeping with U.S. obligations under the Convention on

International Civil Aviation, it is FAA policy to comply with

International Civil Aviation Organization (ICAO) standards and

recommended practices to the maximum extent practicable. The FAA has

determined that this rule does not conflict with any international

agreement of the United States.

Paperwork Reduction Act

In accordance with the Paperwork Reduction Act of 1980 (Pub. L. 96-

511), there are no requirements for information collection associated

with this rule.

[[Page 40704]]

Conclusion

Because these changes to the structural loads requirements do not

result in any substantial economic costs, the FAA has determined that

this rule will not be significant under Executive Order 12866. Because

there has not been significant public interest in this issue, the FAA

has determined that this action is not significant under DOT Regulatory

Policies and Procedures (44 FR 11034; February 25, 1979). In addition,

since there are no small entities affected by this rulemaking, the FAA

certifies that the rule will not have a significant economic impact,

positive or negative, on a substantial number of small entities under

the criteria of the Regulatory Flexibility Act, since none will be

affected. A copy of the regulatory evaluation prepared for this project

may be examined in the Rules 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.

The Amendments

Accordingly, the Federal Aviation Administration (FAA) amends 14

CFR part 25 of the Federal Aviation Regulations as follows:

PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES

1. The authority citation for part 25 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701-44702, 44704.

2. Section 25.331 is amended by revising the introductory text of

paragraph (c) and paragraph (c)(1) to read as follows:

Sec. 25.331 Symmetric maneuvering conditions.

* * * * *

(c) Pitch maneuver conditions. The conditions specified in

paragraphs (c)(1) and (2) of this section must be investigated. The

movement of the pitch control surfaces may be adjusted to take into

account limitations imposed by the maximum pilot effort specified by

Sec. 25.397(b), control system stops and any indirect effect imposed by

limitations in the output side of the control system (for example,

stalling torque or maximum rate obtainable by a power control system.)

(1) Maximum pitch control displacement at VA. The

airplane is assumed to be flying in steady level flight (point

A1, Sec. 25.333(b)) and the cockpit pitch control is

suddenly moved to obtain extreme nose up pitching acceleration. In

defining the tail load, the response of the airplane must be taken into

account. Airplane loads that occur subsequent to the time when normal

acceleration at the c.g. exceeds the positive limit maneuvering load

factor (at point A2 in Sec. 25.333(b)), or the resulting

tailplane normal load reaches its maximum, whichever occurs first, need

not be considered.

* * * * *

3. Section 25.335 is amended by revising paragraphs (a)(2) and

(b)(2) to read as follows:

Sec. 25.335 Design airspeeds.

* * * * *

(a) * * *

(2) Except as provided in Sec. 25.335(d)(2), VC may not

be less than VB + 1.32 U REF (with

UREF as specified in Sec. 25.341(a)(5)(i)). However

VC need not exceed the maximum speed in level flight at

maximum continuous power for the corresponding altitude.

* * * * *

(b) * * *

(2) The minimum speed margin must be enough to provide for

atmospheric variations (such as horizontal gusts, and penetration of

jet streams and cold fronts) and for instrument errors and airframe

production variations. These factors may be considered on a probability

basis. The margin at altitude where MC is limited by

compressibility effects must not less than 0.07M unless a lower margin

is determined using a rational analysis that includes the effects of

any automatic systems. In any case, the margin may not be reduced to

less than 0.05M.

* * * * *

4. Section 25.345 is amended by revising paragraph (d) to read as

follows:

Sec. 25.345 High lift devices.

* * * * *

(d) The airplane must be designed for a maneuvering load factor of

1.5 g at the maximum take-off weight with the wing-flaps and similar

high lift devices in the landing configurations.

5. Section 25.351 is revised to read as follows:

Sec. 25.351 Yaw maneuver conditions.

The airplane must be designed for loads resulting from the yaw

maneuver conditions specified in paragraphs (a) through (d) of this

section at speeds from VMC to VD. Unbalanced

aerodynamic moments about the center of gravity must be reacted in a

rational or conservative manner considering the airplane inertia

forces. In computing the tail loads the yawing velocity may be assumed

to be zero.

(a) With the airplane in unaccelerated flight at zero yaw, it is

assumed that the cockpit rudder control is suddenly displaced to

achieve the resulting rudder deflection, as limited by:

(1) The control system on control surface stops; or

(2) A limit pilot force of 300 pounds from VMC to

VA and 200 pounds from VC/MC to

VD/MD, with a linear variation between

VA and VC/MC.

(b) With the cockpit rudder control deflected so as always to

maintain the maximum rudder deflection available within the limitations

specified in paragraph (a) of this section, it is assumed that the

airplane yaws to the overswing sideslip angle.

(c) With the airplane yawed to the static equilibrium sideslip

angle, it is assumed that the cockpit rudder control is held so as to

achieve the maximum rudder deflection available within the limitations

specified in paragraph (a) of this section.

(d) With the airplane yawed to the static equilibrium sideslip

angle of paragraph (c) of this section, it is assumed that the cockpit

rudder control is suddenly returned to neutral.

6. Section 25.363 is amended by revising the heading and paragraph

(a) to read as follows:

Sec. 25.363 Side load on engine and auxiliary power unit mounts.

(a) Each engine and auxiliary power unit mount and its supporting

structure must be designed for a limit load factor in lateral

direction, for the side load on the engine and auxiliary power unit

mount, at least equal to the maximum load factor obtained in the yawing

conditions but not less than--

(1) 1.33; or

(2) One-third of the limit load factor for flight condition A as

prescribed in Sec. 25.333(b).

* * * * *

7. Section 25.371 is revised to read as follows:

Sec. 25.371 Gyroscopic loads.

The structure supporting any engine or auxiliary power unit must be

designed for the loads including the gyroscopic loads arising from the

conditions specified in Secs. 25.331, 25.341(a), 25.349, 25.351,

25.473, 25.479, and 25.481, with the engine or auxiliary power unit at

the maximum rpm appropriate to the condition. For the purposes of

compliance with this section, the pitch maneuver in Sec. 25.331(c)(1)

must be carried out until the positive limit maneuvering load factor

(point A2 in Sec. 25.333(b)) is reached.

[[Page 40705]]

8. Section 25.415 is amended by revising paragraph (a)(2) to read

as follows:

Sec. 25.415 Ground gust conditions.

(a) * * *

(2) The control system stops nearest the surfaces, the control

system locks, and the parts of the systems (if any) between these stops

and locks and the control surface horns, must be designed for limit

hinge moments H, in foot pounds, obtained from the formula,

H=.0034KV2cS, where--

V=65 (wind speed in knots)

K=limit hinge moment factor for ground gusts derived in paragraph

(b) of this section.

c=mean chord of the control surface aft of the hinge line (ft);

S=area of the control surface aft of the hinge line (sq ft);

* * * * *

9. Section 25.473 is revised to read as follows:

Sec. 25.473 Landing load conditions and assumptions.

(a) For the landing conditions specified in Sec. 25.479 to

Sec. 25.485 the airplane is assumed to contact the ground--

(1) In the attitudes defined in Sec. 25.479 and Sec. 25.481;

(2) With a limit descent velocity of 10 fps at the design landing

weight (the maximum weight for landing conditions at maximum descent

velocity); and

(3) With a limit descent velocity of 6 fps at the design take-off

weight (the maximum weight for landing conditions at a reduced descent

velocity).

(4) The prescribed descent velocities may be modified if it is

shown that the airplane has design features that make it impossible to

develop these velocities.

(b) Airplane lift, not exceeding airplane weight, may be assumed

unless the presence of systems or procedures significantly affects the

lift.

(c) The method of analysis of airplane and landing gear loads must

take into account at least the following elements:

(1) Landing gear dynamic characteristics.

(2) Spin-up and springback.

(3) Rigid body response.

(4) Structural dynamic response of the airframe, if significant.

(d) The limit inertia load factors corresponding to the required

limit descent velocities must be validated by tests as defined in

Sec. 25.723(a)

(e) The coefficient of friction between the tires and the ground

may be established by considering the effects of skidding velocity and

tire pressure. However, this coefficient of friction need not be more

than 0.8.

10. Section 25.479 is revised to read as follows:

Sec. 25.479 Level landing conditions.

(a) In the level attitude, the airplane is assumed to contact the

ground at forward velocity components, ranging from VL1 to

1.25 VL2 parallel to the ground under the conditions

prescribed in Sec. 25.473 with--

(1) VL1 equal to VS0 (TAS) at the appropriate

landing weight and in standard sea level conditions; and

(2) VL2 equal to VS0 (TAS) at the appropriate

landing weight and altitudes in a hot day temperature of 41 degrees F.

above standard.

(3) The effects of increased contact speed must be investigated if

approval of downwind landings exceeding 10 knots is requested.

(b) For the level landing attitude for airplanes with tail wheels,

the conditions specified in this section must be investigated with the

airplane horizontal reference line horizontal in accordance with Figure

2 of Appendix A of this part.

(c) For the level landing attitude for airplanes with nose wheels,

shown in Figure 2 of Appendix A of this part, the conditions specified

in this section must be investigated assuming the following attitudes:

(1) An attitude in which the main wheels are assumed to contact the

ground with the nose wheel just clear of the ground; and

(2) If reasonably attainable at the specified descent and forward

velocities, an attitude in which the nose and main wheels are assumed

to contact the ground simultaneously.

(d) In addition to the loading conditions prescribed in paragraph

(a) of this section, but with maximum vertical ground reactions

calculated from paragraph (a), the following apply:

(1) The landing gear and directly affected attaching structure must

be designed for the maximum vertical ground reaction combined with an

aft acting drag component of not less than 25% of this maximum vertical

ground reaction.

(2) The most severe combination of loads that are likely to arise

during a lateral drift landing must be taken into account. In absence

of a more rational analysis of this condition, the following must be

investigated:

(i) A vertical load equal to 75% of the maximum ground reaction of

Sec. 25.473 must be considered in combination with a drag and side load

of 40% and 35% respectively of that vertical load.

(ii) The shock absorber and tire deflections must be assumed to be

75% of the deflection corresponding to the maximum ground reaction of

Sec. 25.473(a)(2). This load case need not be considered in combination

with flat tires.

(3) The combination of vertical and drag components is considered

to be acting at the wheel axle centerline.

11. Section 25.481 is amended by revising paragraph (a)

introductory text and by designating the undesignated text following

paragraph (a)(2) as paragraph (a)(3) and revising it to read as

follows:

Sec. 25.481 Tail down landing conditions.

(a) In the tail-down attitude, the airplane is assumed to contact

the ground at forward velocity components, ranging from VL1

to VL2 parallel to the ground under the conditions

prescribed in Sec. 25.473 with--

(1) * * *

(2) * * *

(3) The combination of vertical and drag components is considered

to be acting at the main wheel axle centerline.

* * * * *

12. Section 25.483 is amended by revising the heading, introductory

text, and paragraph (a) to read as follows:

Sec. 25.483 One-gear landing conditions.

For the one-gear landing conditions, the airplane is assumed to be

in the level attitude and to contact the ground on one main landing

gear, in accordance with Figure 4 of Appendix A of this part. In this

attitude--

(a) The ground reactions must be the same as those obtained on that

side under Sec. 25.479(d)(1), and

* * * * *

13. Section 25.485 is amended by adding the introductory text to

read as follows:

Sec. 25.485 Side load conditions.

In addition to Sec. 25.479(d)(2) the following conditions must be

considered:

* * * * *

14. Section 25.491 is revised to read as follows:

Sec. 25.491 Taxi, takeoff and landing roll.

Within the range of appropriate ground speeds and approved weights,

the airplane structure and landing gear are assumed to be subjected to

loads not less than those obtained when the aircraft is operating over

the roughest ground that may reasonably be expected in normal

operation.

15. Section 25.499 is amended by revising the heading and paragraph

(e) to read as follows:

Sec. 25.499 Nose-wheel yaw and steering.

* * * * *

[[Page 40706]]

(e) With the airplane at design ramp weight, and the nose gear in

any steerable position, the combined application of full normal

steering torque and vertical force equal to 1.33 times the maximum

static reaction on the nose gear must be considered in designing the

nose gear, its attaching structure, and the forward fuselage structure.

16. Section 25.561 is amended by revising paragraph (c) to read as

follows:

Sec. 25.561 General.

* * * * *

(c) For equipment, cargo in the passenger compartments and any

other large masses, the following apply:

(1) Except as provided in paragraph (c)(2) of this section, these

items must be positioned so that if they break loose they will be

unlikely to:

(i) Cause direct injury to occupants;

(ii) Penetrate fuel tanks or lines or cause fire or explosion

hazard by damage to adjacent systems; or

(iii) Nullify any of the escape facilities provided for use after

an emergency landing.

(2) When such positioning is not practical (e.g. fuselage mounted

engines or auxiliary power units) each such item of mass shall be

restrained under all loads up to those specified in paragraph (b)(3) of

this section. The local attachments for these items should be designed

to withstand 1.33 times the specified loads if these items are subject

to severe wear and tear through frequent removal (e.g. quick change

interior items).

* * * * *

Issued in Washington D.C. on July 14, 1997.

Barry L. Valentine,

Acting Administrator.

[FR Doc. 97-19040 Filed 7-28-97; 8:45 am]

BILLING CODE 4910-13-M