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Special Conditions: Boeing Model 747-8/-8F Airplanes, Structural Design Requirements for Four-Post Main Landing Gear System

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Federal Aviation Administration (FAA), DOT.


Final special conditions.


These special conditions are issued for the Boeing Model 747-8/-8F airplane. This airplane will have novel or unusual design features associated with a four-post main landing gear system. The applicable airworthiness regulations do not contain adequate or appropriate safety standards for this design feature. These special conditions contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards.


Effective Date: November 9, 2009.

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Mark Freisthler, FAA, Airframe & Cabin Safety Branch, ANM-115, Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue, SW., Renton, Washington 98057-3356; telephone (425) 227-1119; facsimile (425) 227-1149.

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On November 4, 2005, The Boeing Company, P.O. Box 3707, Seattle, WA 98124, applied for an amendment to Type Certificate Number A20WE to include the new Model 747-8 passenger airplane and the new Model 747-8F freighter airplane. The Model 747-8 and the Model 747-8F are derivatives of the 747-400 and the 747-400F, respectively. Both the Model 747-8 and the Model 747-8F are four-engine jet transport airplanes that will have a maximum takeoff weight of 970,000 pounds and new General Electric GEnx-2B67 engines. The Model 747-8 will have two flight crew and the capacity to carry 660 passengers. The Model 747-8F will have two flight crew and a zero passenger capacity, although Boeing has submitted a petition for exemption to allow the carriage of supernumeraries.

Type Certification Basis

Under the provisions of 14 CFR 21.101, Boeing must show that the Model 747-8 and 747-8F (hereafter referred as 747-8/-8F) meet the applicable provisions of part 25, as amended by Amendments 25-1 through 25-117, except for earlier amendments as agreed upon by the FAA. These regulations will be incorporated into Type Certificate No. A20WE after type certification approval of the 747-8/-8F.

In addition, the certification basis includes other regulations, special conditions and exemptions that are not relevant to these special conditions. Type Certificate No. A20WE will be updated to include a complete description of the certification basis for these model airplanes.

If the Administrator finds that the applicable airworthiness regulations (i.e., 14 CFR part 25) do not contain adequate or appropriate safety standards for the 747-8/-8F because of a novel or Start Printed Page 51760unusual design feature, special conditions are prescribed under the provisions of § 21.16.

In addition to the applicable airworthiness regulations and special conditions, the 747-8/-8F must comply with the fuel vent and exhaust emission requirements of 14 CFR part 34 and the noise certification requirements of 14 CFR part 36.

Special conditions, as defined in 14 CFR 11.19, are issued under § 11.38, and become part of the type certification basis under § 21.101.

Special conditions are initially applicable to the model for which they are issued. Should the type certificate for that model be amended later to include any other model that incorporates the same or similar novel or unusual design feature, or should any other model already included on the same type certificate be modified to incorporate the same or similar novel or unusual design feature, the special conditions would also apply to the other model under § 21.101.

Novel or Unusual Design Features

The Boeing Model 747-8/-8F airplane will incorporate the following novel or unusual design features: A four-post main landing gear system with two wing main landing gears and two body main landing gears.


The Boeing Model 747-8/-8F airplane will retain the landing gear arrangement which is unique to the 747 family of airplanes. The conventional arrangement for the main landing gear of transport category airplanes is two-underwing posts. The 747 was the first to introduce a four-post main landing gear arrangement, two underwing posts supplemented by two body posts. This arrangement was adopted to accommodate the then unprecedented increased weight and size of the Model 747 airplane.

Existing regulations are written to address the conventional landing gear configuration commonly found on transport category airplanes. This being the case, they are not appropriate to address the unique features of the Boeing 747 design. The increased number of posts alters the load distribution between the gear units during landing and ground handling conditions addressed by the regulations. This arrangement also loads the airframe differently than conventional landing gear designs. The FAA determined that, while the general conditions addressed by §§ 25.473 and 25.479 through 25.485 were still applicable, specific details contained in these regulations may not be directly relatable to the four-post arrangement.

In 1968 the FAA issued Special Condition A-4 to address the ground load requirements for the main landing gear system for Boeing Model 747-100 series airplanes. That special condition provided clarification on the applicability of §§ 25.473 and 25.479 through 25.485 to the Model 747 airplane. In 1971 Special Condition A-4 was amended to address Boeing Model 747 airplanes with the landing gear load evener system deleted or made inoperable.

The FAA has determined that Special Condition A-4 is applicable to the 747-8/-8F series airplanes, provided that all the applicable part 25 regulations cited in Special Condition A-4 (recorded as an enclosure to FAA Letter WE-120/8110 (CT3488WE-D) to the Boeing Company, dated May 12, 1971) are upgraded to the latest amendment level (i.e., 25-117). Furthermore, as several of these regulations have been updated or consolidated, and acceptable methods of compliance have been described for some of these regulations via advisory circular (AC), new special conditions are needed to clarify the applicable requirements. By updating these special conditions, we are ensuring that the Boeing design provides an equivalent level of safety to conventional landing gear meeting these regulations.

Discussion of Comments

Notice of proposed special conditions No. 25-09-05-SC for the Boeing Model 747-8/-8F airplanes was published in the Federal Register on April 27, 2009 (74 FR 19023). Airbus, an original equipment manufacturer, provided several specific comments.

Airbus suggested that certain special conditions be revised to follow the intent of rule changes and guidance recommended by the Aviation Rulemaking Advisory Committee (ARAC). This committee, comprised of representatives from industry and the regulatory authorities, developed recommendations that would revise many of the ground load requirements in 14 CFR 25.

While the FAA agrees with the ARAC recommendations, those recommendations have not yet been adopted into 14 CFR 25. Therefore, evaluation of the ARAC recommendations is not required per § 21.101, Designation of applicable regulations (the Changed Product Rule). Furthermore, the FAA does not believe that there is any safety concern that requires application of the ARAC recommendations. On past programs, the FAA has only applied parts of the ARAC recommendations, either by equivalent safety findings or by special conditions, and only when requested by an applicant. Even in those cases, the updated requirements have not been used in their entirety.

As previously indicated, the proposed special condition was derived from the original 747 special condition and was updated to the latest amendment level. The service history of the 747 landing gear design indicates that the original special condition provides adequate design requirements. Furthermore, the landing gear design has not been significantly changed for the 747-8/-8F. Finally, the proposed special condition is an improvement on the original as it takes into account numerous updates to the rules and guidance material it references.

Airbus proposed the following specific changes:

1. The special conditions require consideration of the effects of runway crown for ground handling conditions, as defined in § 25.511(b)(4). Airbus suggested that these effects should also be considered for landing conditions as specified in the ARAC recommendations.

FAA Response: While the ARAC recommendations represent an improvement in design standards, we do not believe consideration of runway crown is necessary. Furthermore, the FAA has not mandated this requirement on any other program.

2. The special conditions include the following: “The level landing criteria of § 25.479 are directly applicable. The four main landing gear units must be assumed to contact the ground with the airplane longitudinal axis in a horizontal attitude.” Airbus suggested that the second sentence is in conflict with the first, and that the conditions specified in the ARAC recommendations should be used.

FAA Response: The second sentence is intended to clarify how § 25.479 is applied to the 747 design, which is not envisaged by the current requirement. The FAA has not mandated the level landing requirement from the ARAC recommendations on any other program.

3. The special conditions include: “The criteria of § 25.495 (ground turning loads) are directly applicable.” Airbus suggests it is not correct to state that these criteria are directly applicable and that it would be more appropriate to apply the rational analysis described in the ARAC recommendations.

FAA Response: We believe that application of this requirement can be directly applied, as has been done on previous 747 designs, and that such application is sufficiently conservative.Start Printed Page 51761

4. With regard to the shock absorption test requirements of § 25.723, Airbus suggested that the test conditions be expanded as specified in the ARAC recommendations.

FAA Response: We do not believe that using the latest ARAC recommendations is necessary, nor has this requirement been mandated on any other program to date.


As discussed above, these special conditions are applicable to the Boeing Model 747-8/-8F airplanes. Should Boeing apply at a later date for a change to the type certificate to include another model incorporating the same novel or unusual design features, these special conditions would apply to that model as well under the provisions of § 21.101.


This action affects only certain novel or unusual design features of the Boeing Model 747-8/-8F airplanes. It is not a rule of general applicability.

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List of Subjects in 14 CFR Part 25

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The authority citation for these special conditions is as follows:

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

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The Special Conditions

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Accordingly, pursuant to the authority delegated to me by the Administrator, the following special conditions are issued as part of the type certification basis for the Boeing Model 747-8/-8F airplanes.

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The requirements of §§ 25.471, 25.473, and 25.479 through 25.485 apply as follows:

1. General. The general design criteria of § 25.471 are directly applicable. The basic landing gear dimensional data must be expanded to include the additional main landing gear units.

2. Ground Load Conditions and Assumptions. The criteria specified in § 25.473 are applicable for the design landing conditions except as noted in paragraph 6 of these special conditions.

3. Landing Gear Arrangement. The multiple oleo main landing gear configuration does not meet the “conventional arrangement” criterion of § 25.477, with respect to the application of paragraphs 4 through 7 of this special condition. Nevertheless, the landing impact design conditions must meet the intent of §§ 25.473 through 25.485.

4. Level Landing Conditions. The level landing criteria of § 25.479 are directly applicable. The four main landing gear units must be assumed to contact the ground with the airplane longitudinal axis in a horizontal attitude.

5. Tail-Down Landing Conditions. The airplane must be assumed to contact the ground in any tail down attitude between level and the maximum tail down attitude allowing clearance with the ground of each part of the airplane other than the main landing gear wheels. The airplane forward velocity component must be the most critical value from VL1 to 1.25 VL2 where VL1 and VL2 are defined in § 25.481. Each main landing gear unit must be designed for its most critical combination of vertical load and drag load. All other criteria in § 25.481, not superseded by the above criteria shall be directly applicable. The distribution of loads between the gear units for the effects of critical combinations of spin-up and spring-back loadings on the main landing gear units must be considered for the gear units and their supporting structure.

6. One-Wheel Landing Conditions. Unless the airplane and landing gears are designed for equivalent or more critical conditions, the airplane will be assumed to land in a level pitch attitude at design landing weight with a descent velocity of 7 fps at the maximum roll angle attainable within the geometric limitations of the airplane with the contact velocities and gear landing conditions of §§ 25.479(a), (c) and (d).


This condition need not be coupled with either a 6 fps landing at maximum take off weight or a 12 fps reserve energy drop test.

7. Side Load Conditions. On the main landing gear units, side loads of 80% of the vertical reaction (on one side) acting inward and 60% of the vertical reaction (on the other side) acting outward must be combined with one-half of the maximum vertical ground reactions obtained in the level landing, tail-down landing, or rolled attitude landing conditions. These loads shall be assumed applied at the ground contact point and to be resisted by the inertia of the airplane. Drag loads may be assumed to be zero.

8. Rebound Landing Condition. The criteria of § 25.487 are directly applicable.

9. Ground Handling Conditions. The criteria of § 25.489 are directly applicable. The effects of runway crown as defined in § 25.511(b)(4) shall be considered in distributing the loads to the individual main landing gear units. The ground reactions must be distributed to the individual landing gear units in a rational or conservative manner, accounting for airframe flexibility and shock strut and tire stiffness.

10. Take-Off Run. The criteria of § 25.491 are directly applicable. Compliance may be shown in accordance with Advisory Circular (AC) 25.491-1.

11. Braked Roll Conditions. The criteria of §§ 25.493(b), (c), and (d) shall be directly applicable. The formula in § 25.493(e) is not applicable to the B747 due to the 4-post gear arrangement.

12. Turning. The criteria of § 25.495 are directly applicable.

13. Nose-Wheel Yaw. The criteria of § 25.499 are directly applicable. The criteria are interpreted to apply braking to all main landing gear wheels on one side of the airplane centerline.

14. Pivoting. The criteria of § 25.503 are applied individually to each wing main landing gear unit. In addition, all main landing gear units must be designed for the scrubbing and/or torsion loads induced by pivoting about the most critical point consistent with the available main gear braking on one side of the airplane and the available thrust and torque on the airplane. Maximum static engine thrust must be considered only on the engines on the opposite side of the airplane centerline from the pivot point.

15. Reversed Braking. The criteria of § 25.507 are directly applicable, except that the phrase “three point” is expanded to include “five point.”

16. Towing Loads. The criteria of § 25.509 are directly applicable.

17. Fatigue Evaluation of Landing Gear. The criteria of § 25.573 at Amendment 25-0 are directly applicable to main landing gear units.

18. Shock Absorption Tests. The criteria of § 25.723 are directly applicable. Compliance may be shown in accordance with AC 25.723-1.

19. Substantiation of the design criteria must include a dynamic taxi and landing analysis.

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Issued in Renton, Washington, on September 29, 2009.

Ali Bahrami,

Manager, Transport Airplane Directorate, Aircraft Certification Service.

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[FR Doc. E9-24339 Filed 10-7-09; 8:45 am]