UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

RENTON, WASHINGTON 98057-3356 

In the matter of the petition of 

Exemption No. 15762 

The Boeing Company Regulatory Docket No. FAA-2015-6811 

for an exemption from§ 25.341(a) of 

Title 14, Code of Federal Regulations 

GRANT OF EXEMPTION 

By letter dated November 19, 2015, Mr. David Horn, ODA Lead Administrator, The Boeing 

Company, 6001 S. Air Depot, Oklahoma City, Oklahoma, 73135, petitioned the Federal Aviation 

Administration (FAA) for an exemption from the requirements of§ 25.341 (a) of Title 14, Code 

of Federal Regulations (14 CFR). This exemption, if granted, would exempt the aerial refueling 

boom and local attachment structure of 767-2C tanker airplanes that have been modified by 

supplemental type certificate (STC) for military operations from the tuned discrete gust criteria of 

§ 25.341(a). 

The petitioner requests relief from the following regulations: 

Section 25.341- Gust and Turbulence Loads 

(a)- Discrete Gust Design Criteria 

The petitioner supports its request with the following information: 

This section quotes the relevant information from the petitioner's request, with minor edits for 

clarity. The petitioner quoted the equations and text of§ 25.341(a) in the request; however, we 

are omitting that information from this document for brevity. The complete petition is available 

at the Department of Transportation's Federal Docket Management System, on the Internet at 

http://regul ations.gov, in Docket No. F AA-20 15-6811 . 
Description of Issue 

Application of 14 CFR 25.341(a), Tuned Discrete Gust (TDG) to the STC-modified 

767-2C aerial refueling boom (ARB) would result in significantly increased loads, 

causing impractical redesign of the boom and local attachment structure. 

Aerial refueling aircraft were not considered when tuned discrete gust criteria were 

originally derived by the Joint Aviation Authority. Tuned discrete gust criteria were 

intended for conventional transport aircraft and used conventional transport aircraft 

accelerations to back out gust velocities using the static gust loads formula. In addition, 

aerial refueling aircraft were not evaluated for the load effects or economic impact of the 

tuned discrete criteria, as was done with conventional transport aircraft. Tuned discrete 

gust criteria have not been used for prior boom design on the KC-10A or KC-135. 

Prior to Amendment 25-86, static gust was the discrete gust requirement in 

14 CFR 25.341. The replacement of static gust with tuned discrete gust in 

14 CFR 25.341(a) at Amendment 25-86 was to provide for harmonization of the discrete 

gust requirements with the Joint Aviation Requirements (JARs). Tuned discrete gust was 

incorporated into the Code of Federal Regulations so that the requirements would be 

common and applicants would not have to comply with multiple discrete gust criteria. 

The change from static gust to tuned discrete gust was for harmonization and was not 

done with the goal of increasing safety as stated in the preamble to Amendment 25-86, 

"The objective was to achieve common requirements for the certification of transport 

airplanes without a substantive change in the level of the safety provided by the 

regulations." 

Tuned discrete gust simulates the aircraft being hit with an extreme single discrete gust, 

with gust velocities backed out from the static gust formula using aircraft center of 

gravity (CG) accelerations that occur once in 70,000 flight hours. This gust velocity is 

then coupled with the worst possible gust frequency. Loads using this type of criteria are 

more significantly affected for boom-type structure than for conventional transport 

aircraft structure (wing, fuselage, and empennage). Because TDG criteria derives gust 

velocities based on aircraft CG accelerations, the accelerations for structure far away from 

the CG, which create the majority of the load, have large variations that are analysisdriven and not validated by fleet data. In addition, the boom structure's constant 

spanwise mass and stiffness properties cause loads to be more affected by tip 

accelerations, thus increasing the loads significantly higher than past successful design 

practice. Conventional transport aircraft structure do not have this effect as the 

distributed mass and stiffness is significantly lower at the tip and increases towards the 

root, thus reducing the load effect from the high tip accelerations. 

As tuned discrete gust criteria were not created for or evaluated on military aircraft and 

the load effects of this criteria are significantly higher for boom-type structure, this 

criteria is not appropriate for an aerial refueling boom, which is unique military structure. 

Structural Capability 

The STC-modified 767-2C boom baseline is the KC-10A. The KC-10A and STCmodified 767-2C booms are structurally similar. They have the same cross-sectional area 

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and the same basic materials, with the exception of some modem alloys that were 

implemented on the STC-modified 767-2C. All modem alloys were selected to have 

equivalent or better properties than the original alloys. The critical frequencies of the first 

natural bending modes of the two booms are very similar. 

The expected usage and gust environment of the STC-modified 767-2C and KC-10A 

booms are very similar. The KC-1 OA fleet has an extensive service history with over 34 

years and 1.5 million flight hours (59 aircraft). In that time, there have been no incidents 

of recorded boom damage due to turbulence. 

The STC-modified 767-2C boom and local attachment structure is designed to both the 

static gust requirements used for the KC-1 OA, in addition to USAF -approved military 

gust requirements. The STC-modified 767-2C boom design provides the same or better 

structural capability as the KC-10A boom. Thus, the STC-modified 767-2C boom design 

provides sufficient and appropriate structural capability needed for USAF operations. 

Statement of Public Interest 

Application of 14 CFR 25.341(a) at Amendment 25-86 would cause significant 

modification to the STC-modified 767-2C boom that is utilized only for military missions 

and has no civil utility. Additionally, the redesign of the boom and local attachment 

structure would be impractical and provide no net benefit in the configuration and no 

commensurate increase in safety. 

The STC-modified 767-2C will be converted to the KC-46A prior to delivery to the 

USAF. lfthe relief is not granted, it could lead to delays in the delivery ofthe KC-46A to 

the USAF at a critical time of need. It remains in the public's interest to grant Boeing an 

exemption to 14 CFR 25.341(a) for the STC-modified 767-2C aerial refueling boom and 

local attachment structure. 

Statement of No Adverse Effect on Safety 

Existing aircraft with installed booms such as the KC-10A have a long safety history. 

The KC-1 OA was certified in 1981 and the aircraft fleet has 1.5 million flight hours with 

no gust-related damage affecting the boom. The STC-modified 767-2C and KC-10A 

booms are structurally very similar. The STC-modified 767-2C boom and local 

attachment structure is designed to both the KC-1 OA static gust criteria as well as the 

USAF-approved military gust criteria. Thus, applying the later gust criteria of 

§ 25.341 (a) for tuned discrete gust does not add additional safety. 

Conclusion 

The Boeing position is that tuned discrete gust criteria in§ 25.341(a) were derived for 

conventional transport aircraft and are not appropriate for the STC-modified 767-2C 

aerial refueling boom as the boom is unique military structure. The STC-modified 767-

2C boom's similarity to the KC-10A boom, the safe history of the KC-lOA boom, in 

addition to the fact that the STC-modified 767-2C boom and local attachment structure is 

designed to both the KC-1 OA static gust requirements and the USAF -approved military 

gust requirements, provides justification for an exemption from § 25.341 (a). Based on 

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the rationale above, Boeing hereby requests FAA issuance of an exemption from the 

requirements of§ 25.341(a), Amendment 25-86, for the boom and its local attachment 

structure for the STC-modified 767-2C Model airplane, FAA project No. ST00279MC-T. 

Federal Register publication 

Although the petitioner requested that action on its petition not be delayed for publication in the 

Federal Register, the FAA found that the petition, if granted, would set a precedent. Therefore, 

to allow an opportunity for the public to comment on the petition, we published a summary of it 

in the Federal Register on January 19,2016 (81 FR 2941). No comments were received. 

The FAA's analysis 

This airplane is intended for military aerial refueling without any applicability to commercial 

operations. The design of the refueling boom for the modified Model 767-2C airplane is 

substantially similar to the one used for the Boeing KC-1 OA, which has a long and successful 

service history. The applicant assures the FAA that the boom will comply with USAF-approved 

gust requirements, and these static gust requirements are at least as stringent as that used for the 

KC-1 OA. Since the modified 767-2C airplane will perform the same missions as the KC-1 OA, 

this exemption would not adversely impact public safety. It will enhance the United States 

military by replacing older, less efficient tanker airplanes and ensure consistency within the 

current USAF fleet. For this reason, we agree that granting this exemption is in the public 

interest. 

The FAA's decision 

In consideration of the foregoing, I find that a grant of exemption is in the public interest. 

Therefore, pursuant to the authority contained in 49 U.S.C. 40113 and 44701 delegated to me by 

the Administrator, I grant The Boeing Company an exemption from 14 CFR 25.341(a), to the 

extent necessary to install the aerial refueling boom and local attachment structure on STCmodified 767-2C military tanker airplanes. Operations with the aerial refueling boom installed 

are limited to military use only, as allowed by FAA Order 8110.101A. 

Michael Kaszycki 

Acting Manager, Transport Airplane Directorate 

Aircraft Certification Service 

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