AFS-15-2430-E

Exemption No. 13337

UNITED STATES OF AMERICA

DEPARTMENT OF TRANSPORTATION

FEDERAL AVIATION ADMINISTRATION

WASHINGTON, DC 20591

 

In the matter of the petition of 

 

SOUTHWEST AIRLINES Regulatory Docket No. FAA-2015-2368

CORPORATION 

 

for an exemption from § 121.629(b) 

of Title 14, Code of 

Federal Regulations 

 

GRANT OF EXEMPTION

 By letter dated June 5, 2015, Mr. Alan Kasher, Managing Director/Director of Operations, 

Regulatory Programs and Compliance, Southwest Airlines Corporation, 2702 Love Field 

Drive, P.O. Box 36611, TOP-1DO, Dallas, TX 753235, petitioned the Federal Aviation 

Administration (FAA) on behalf of Southwest Airlines for an exemption from § 121.629(b) of 

Title 14, Code of Federal Regulations (14 CFR). The proposed exemption, if granted, would 

exempt Southwest Airlines from the requirement that “no person may take off an aircraft 

when frost, ice, or snow is adhering to the wings,” if Boeing, the original equipment 

manufacturer (OEM) of the B-737-700, -800 Next Generation (NG) airplanes, has determined 

a safe zone within which cold soaked fuel frost (CSFF) is permissible; visibly marked the 

zone on the wing; and placed an appropriate limitation in the FAA-approved Airplane Flight 

Manual (AFM). 

The petitioner requests relief from the following regulation:

Section 121.629(b) states that no person may take off an aircraft when frost, ice, or snow is 

adhering to the wings, control surfaces, propellers, engine inlets, or other critical surfaces of 

the aircraft or when the takeoff would not be in compliance with paragraph (c) of this section. 

Takeoffs with frost under the wing in the area of the fuel tanks may be authorized by the 

Administrator.
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The petitioner supports its request with the following information:

The petioner supports its request by explaining how CSFF is formed. In general, airframe 

icing is known to be a hazard to the safe operation of aircraft, especially on the wing and other 

parts of an aircraft that exist to produce lift. Hence, the regulation, § 121.629(b), which states 

that wings, control surfaces, propellers, engine inlets, or other critical surfaces must be clean 

from forst, ice or snow, is a mitigation for this known hazard. In the case of CSFF, wing 

surface texture changes can disrupt laminar airflow on the wing causing degradation of lift 

and increased stall speed, adversely affecting aircraft performance. Ice on the wing may form 

in a particular set of conditions that may include near or sub-freezing temperatures in 

combination with precipitation and/or high humidity (especially fog). Additionally, frost may 

form in clear, meteorological conditions on wing areas with underlying cold soaked fuel 

caused by cruise at high altitude followed by landing in a humid climate, which is a condition 

known as CSFF. 

The petitioner states that the relief requested by Southwest Airlines is precedented by 

engineering and aerodynamic analysis performed in conjunction with the National 

Aeronautics and Aerospace Administration (NASA) and by Boeing, the OEM for the 

B737-700 and -800 NG series aircraft. This CSFF analysis determined that lift impairment is 

negligible, given the controlling factors set-forth by the OEM’s miscellaneous limitations and

FAA-approved AFM. Given these results, the expense and environmental impact of applying 

de-icing fluid in the CSFF condition is not warranted. The petitioner states the relief 

requested presents no decrease in safety and is in the interest of the flying public. The 

petitioner explains that the appropriate procedures and limitations will be documented in the 

FAA approved Boeing B-737-700, -800 NG AFM. The relief requested is limited to those 

aircraft for which the analysis was performed by Boeing and approved by the FAA. (This 

analysis includes the ER model as specified by Boeing.)

The petitioner states that Boeing has applied painted lines defining an area on the upper 

surface of the wings on certain B737NG/ER aircraft, where with certain other limitations,

CSFF may exist with negligible impairment of performance. The petitioner states that all 

Southwest B737-700 and -800 NG series aircraft have the lines defining the permissible area 

of CSFF painted on the upper wing surfaces. The petitioner states that Boeing has issued 

specific “Miscellaneous Limitations” regarding CSFF in its FAA-approved AFM that set forth 

conditions under which takeoff is not permitted. The extracted Boeing-developed CSFF 

miscellaneous limitations are as follows:

MISCELLANEOUS LIMITATIONS

COLD SOAKED FUEL FROST (CSFF) 

Takeoff with cold soaked fuel frost on the wing tank upper surfaces is not permitted if 

any of the following conditions are present:
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1. The ambient air temperature is below +4 degree celsius (C), +39 degree 

fahrenheit (F).

2. The tank fuel temperature is below -16 degree C, +3 degree F.

3. There is cold soaked fuel frost on the wing tank upper surfaces that is 

beyond the lines defining the permissible cold soaked fuel frost area.

4. There is precipitation or visible moisture (rain, snow, drizzle, or fog with 

less than 1 mile visibility).

Based on the data and analysis presented by Boeing, the petitioner has concluded that if the 

above listed limiting conditions are not present, operations with CSFF on the wing tank upper 

surfaces may be safely conducted. 

The petitioner states that FAA approval of the amended AFM and the Southwest Airlines

training program provide the basis upon which operations with CSFF is permissible and 

should be allowed pursuant to § 121.629(b) which provides that “takeoffs with frost under the 

wing in the area of the fuel tanks may be authorized by the Administrator.”

The petitioner states that the amended AFM limitation and training program in conjunction 

with Boeing’s technical data analysis and procedures assures that operations are only 

permitted in certain instances of CSFF. The petition highlights that operations not 

specifically covered within the Boeing procedures, FAA-approved AFM, and training 

program would not be authorized by this exemption and § 121.629(b) would apply.

A summary of the petition was published in the Federal Register on April 29, 2015, 80 FR 

23848.1 Six sets of public comments were submitted to the docket. Representatives of the 

airline industry, including Airlines for American, Delta Air Lines, Southwest Airlines, United 

Airlines, and Alaska Airlines, supported a grant of exemption on the basis that the conditions 

tests by Boeing and included in the FAA-approved AFM demonstrate that safety of flight 

would not be negatively affected. The Air Line Pilots Association supported a grant of 

exemption but emphasized that the Captain retains the authority to make the final decision on 

operating in CSFF conditions.

The FAA's analysis is as follows:

The purpose of § 121.629(b) is to ensure the safest possible operation in the presence of ice, 

frost and snow. Therefore, the regulations require a clean wing but state that take-offs with 

frost under the wing in the area of the fuel tanks may be authorized by the Administrator. 

 1 The FAA requested public comment on this proposal in response to Airlines for America’s petition for 

exemption from § 121.629(b), Docket No. FAA-2015-0555. That petition was withdrawn by the petitioner 

before FAA action.
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Although this petition for exemption is requested by Southwest Airlines, the aircraft 

manufacturer, Boeing, provided the FAA with the associated conditions and limitations for 

operations with CSFF. Boeing approached the FAA in 2003 providing data and performance 

analysis which it believed established the basis for a grant of exemption from the 

requirements of § 121.629(b). The FAA allowed industry to apply the Boeing procedure for a 

trial period of 6 months. While no incidents related to the existence of CSFF occurred during 

this period, the FAA, as an operations policy decision, did not extend the approval. 

In 2011, Boeing approached the FAA requesting reconsideration. The FAA formed a team to 

study the Boeing proposal. Led by the subject matter expert on airplane icing, the team 

consisted of the Chief Scientific Technical Advisor for Icing for the FAA, the Chief Aircraft

Aerodynamcist for NASA, the Head of Aircraft Icing Research at the FAA Technical Center, 

and the Chief Ground De-icing Scientist for the FAA Technical Center. After a three-year 

study and in-depth analysis to validate the Boeing presentation, including consultation with 

Transport Canada (which had previously granted this type of exemption), the team 

recommended acceptance of the Boeing proposal.

After careful review, FAA Aircraft Certfication (AIR) in coordination with Flight Standards 

(AFS) validated the data, procedures, and limitations set forth within the Boeing 737-700, -

800, and ER performance analysis of CSFF. The CSFF analysis determined that lift 

impairment is negligible, given the controlling factors set-forth by the OEM’s miscellaneous 

limitations and FAA-approved AFM.

The FAA also determined there would be specific training required for all flight and ground 

personnel whose job function could involve the recognition of CSFF. This training would 

include: 1) recognition of CSFF; 2) the conditions that cause CSFF; 3)recognition and 

differentiation of the airplanes for which this exemption would or would not apply; 4) the 

limiting upper wing surface CSFF area; and 5) the Boeing B-737-700, -800 NG AFM CSFF 

limitations. Additionally, in line with a comment submitted by the Air Line Pilots 

Association, the FAA notes that the Captain shall retain final authority for the safe 

accomplishment of the flight and may determine a stricter standard with respect to CSFF and 

flight conditions.

The FAA has determined that allowing Southwest Airlines to dispatch and takeoff B737-700 

and -800 NG aircraft with CSFF within the limitations specified in the FAA-approved Boeing 

AFM accompanied by the required training of associated personnel would not adversely 

affect safety. In addition, the FAA finds that a grant would be in the public interest because it 

would decrease environmental impact of using deicing fluids and would increase operational 

efficiency and effectiveness by allowing specified aircraft as defined by the Boeing analysis

and written in the FAA-approved AFM limitations to dispatch and takeoff without having to 

remove CSFF from the delineated upper wing surfaces.

To apply the revised B737NG AFM limitation for CSFF, Southwest Airlines must have the 

permissible black lines defining the boundaries of the CSFF area on the upper wing in a 

manner which is clear and discernable by flightcrew and ground personnel.
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In consideration of Boeing’s aerodynamic analysis, the FAA finds that the Boeing AFM 

limitation revision concerning CSFF on upper wing surfaces with respect to defined 737 

aircraft models and types when combined with FAA-approved procedures contained in the 

associated Southwest Airlines manuals and the Southwest Airlines training program, and 

subject to the discretion of the pilot-in-command provides a level of safety equivalent to the 

requirements of § 121.629(b).

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. §§ 106(f), 40113, and 44701, 

delegated to me by the Administrator, Southwest Airlines is granted an exemption from 

14 CFR § 121.629(b) that allows Southwest to take off an aircraft when frost, ice, or snow is 

adhereing to the wings, subject to the conditions and limitations described below. 

Conditions and Limitations

1. The dispatch and takeoff with CSFF on upper wing surfaces only applies to 

Southwest Airlines Boeing 737-700, -800 NG aircraft as specified in the Boeing

engineering and aeronautical analysis.

2. Southwest Airlines will use and adhere to the revised Boeing AFM miscellaneous 

limitations for CSFF in all instances concerning CSFF.

3. This exemption does not relieve Southwest Airlines from any other limitation or 

requirements with regards to deicing and anti-icing operations.

4. This exemption is provided and effective under the following conditions:

(a) The appropriate Southwest Airlines operations manual contains the following 

limitations and conditions from the FAA approved B-737-700, -800 NG AFM:

Takeoff with cold soaked fuel frost on the wing tank upper surfaces is not 

permitted if any of the following conditions are present:

(1) The ambient air temperature is below +4 degree C, +39 degree fahrenheit.

(2) The tank fuel temperature is below -16 degree C, +3 degree F.

(3) There is cold soaked fuel frost on the wing tank upper surfaces that is 

 beyond the lines defining the permissible cold soaked fuel frost area.

(4) There is precipitation or visible moisture (rain, snow, drizzle, or fog with

 less than 1 mile visibility).
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(b) FAA has approved Southwest Airlines CSFF pilot training program and any 

assigned pilots must have been trained prior to applying CSFF exemption

standards.

(c) Southwest Airlines must provide ground and operational training to all pilots 

and ground personnel to ensure each is able to clearly discern the applicability 

of the CSFF exemption requirements, to include identifying which Southwest 

Airlines aircraft have the painted CSFF upper wing areas. The training must 

include recognition of those airplanes approved for CSFF exemption.

(d) The pilot-in-command retains final authority for takeoff of a flight with CSFF.

This exemption terminates on November 30, 2017, unless sooner superseded or rescinded.

Issued in Washington, DC, on October 23, 2015. .

/s/ 

John Barbagallo

Deputy Director, Flight Standards Service