U.S. Department
of Transportation

Federal Aviation

Administration 

# Advisory Circular

Subject: Protection of Aircraft Fuel Systems Against Vapor Ignition Caused by Lightning Date: XX/XX/XX
Initiated By: AIR

- 1 0 0 AC No: 20

- 53C

1. P urpose

.

a. This advisory circular (AC) provides applicants with guidance for obtaining Federal Aviation Administration (FAA) approval of your compliance pursuant to Title 14, Code of Federal Regulations (14 CFR) 23.954, 27.954, and 29.954, Fuel System L ightning P rotection

. This AC recommends how to protect the aircraft’s fuel system from lightning strikes that may ignite fuel vapors.

b. For g uidance on compliance pursuant to § 25.954, see AC 25.954

- X

, Transport Airplane Fuel System Lightning Protection , dated (INSERT EFFECTIVE DATE).

c

. This AC is not mandatory and doe s not constitute a regulation. This AC describes an acceptable means, but not the only means, to obtain certification for fuel system lightning protection. However, if you use the means described in this AC, you must follow it in all important respects. The term “must” is used to indicate mandatory requirements when following the guidance in this AC. The term “should” is used when following the guidance a s recommended

, but not required to comply with this AC.

2. Audience. This AC affects persons and orga nizations who are seeking certification for fuel system lightning protection pursuant to 14 CFR 23.954, 27.954 and 29.954.

3. Changes. The purpose of this revision is to reference AC 25.954

- 1 for guidance on compliance pursuant to § 25.954.

4. Cancellation. T his AC cancels, as of its effective date, AC 20

- 53B, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition caused by Lightning , dated June 5, 2006.

5. Effective Date. This AC is effective (INSERT EFFECTIVE DATE).

6. Related Publications.

a. 14 CFR. Order copies of the 14 CFR sections referenced in this AC from the Superintendent of Documents, Government Printing Office, Washington, DC 20402

- 9325. Telephone (202) 512

- 1800; fax (202) 512

- 2250. You can also get copies from the Government Printing Offi ce’s (GPO) CFR website at www.access.gpo.gov/cfr/

.mm/dd/yy AC 20

- 53C

2 b. FAA ACs. Order copies of AC 20

- 136, AC 20

- 155, and AC 25.954

- 1 from the U.S. Department of Transportation, Subsequent Distribution Office, M

- 30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20795. Telephone 301

- 322

- 5377; fax 301

- 386

- 5394. You c an also get copies from our Regulatory and Guidance library (RGL)

at http://rgl.faa.gov . On the RGL website, select “Advisory Circulars” and then select “By

Number.”

c. FAA Reports. Order DOT/FAA/CT

- 83/3, User’s Manual for AC 20

- 53A Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning, and DOT/FAA/CT

- 89/22, Aircraft Lightning Protection Handbook, from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. Te lephone (703) 605

- 6000; fax (703) 605

- 6900,

website www.ntis.gov . You can also get copies from the FAA’s William J. Hughes Technical Center Reference and Research Library at http://actlibrary.tc.faa.gov/. On the websit e, select “Search Library Catalog” and then input the report number.

7. Scope.

a. You can apply this AC to both conventional fuel systems and those using advanced composite structu res or other new technologies. This AC provides information on, and

references t o, lightning effects and verification methods.

b. This AC add resses ignition hazards caused by lightning

- related arcing, sparking, puncture, and induced transients on main and auxiliary fuel tanks, fuel tank components and plumbing

. This AC also add resses analog or digital electronic and electrical systems which

could lead to fuel vapor ignition in a fuel tank.

Note: For information on how lightning affects electric and electronic systems, see the current version of AC 20

- 136, Aircraft Electrical and

Elect ronic Systems Lightning Protection.

c. You can also apply this AC when modifying fuel systems, for example when adding tanks or other fuel system components. Since externally mounted tanks are often in lightning

attachment zones, those tanks may be especia lly vulnerable to lightning hazards and must be adequately protected.

8. D efinition

.

a. Attachment point: The point where the lightning flash contacts the aircraft.

b. Corona: Luminous discharge caused by an electrical potential difference between the

aircraft and the surrounding atmosphere.

c. Charge center: Area of high potential of electrical charge.

d. Lightning flash: The total lightning event consisting of one or more lightning strokes, plus intermediate or continuing currents.

e. Lightning strike: Attachment of the lightning flash to the aircraft.mm/dd/yy AC 20

- 53C

3 f. Lightning strike zones: Aircraft surface areas and structures that are susceptible to lightning attachment, dwell time, and current conduction.

g. Lightning stroke: Current surge occurring when lightning makes contact with the ground or another charge center.

h. Streamer: Branch

- like ionized path caused by a lightning str i ke or lightning str i kes

that are imminent.

9. L ightning E ffects on F uel S ystems.

a. Lightning may strike aircraft flying in or near thunderstorms, and nearby lightning flashes may produce corona and streamer formations on the aircraft. Lightning can be a hazard to aircraft fuel systems if those systems are not properly designed, built, and maintained.

b. The effects of lightning on aircraft can range from seemin gly insignificant sparking at fasteners or joints to severe damage. If sparking occurs in a fuel vapor environment, it may ignite the fuel vapor, damaging the aircraft.

c. All or some of the lightning current may be conducted through fuel tanks or fuel syste m components. It is important to determine where the current flows through the aircraft to allow for adequate design measures to protect the fuel system.

d. Aircraft skin and fuel tanks are constructed of metals, low electrical conductivity composite materia ls, and electrically insulating materials, all of which react differently when subjected to a lightning strike. Metals and carbon fiber composite materials (CFC) offer a high degree of electric field shielding and some magnetic field shielding. Electrica lly insulating materials, however, offer little or no electric or magnetic field shielding. For this reason

, lightning does not have to come into direct contact with fuel systems to pose a hazard. Lightning

- induced arcing, sparking, or corona may be suffi cient to ignite fuel.

e. Damage from lightning strikes to non

- conducting materials, such as fiberglass and

aramid

- reinforced composites, can be more severe than damage to metal surfaces. A lightning strike can penetrate non

- conducting materials easier and ig nite fuel vapors.

10. F uel S ystem L ightning P rotection

.

a. To protect fuel systems from lightning, you should perform one, or a combination of, the following steps :

(1) Rid the fuel tank and fuel system of ignition sources. This is the preferred approach. To achieve this, you must specifically design for ignition source prevention, as thousands of amperes of current are conducted, and a spark of only a bout two

- tenths of a millijoule

, released inside a fuel tank

, can ignite fuel vapor ; or (2) Reduce fuel tank flammability to ensure the fuel tank atmosph ere will not support combustion; or (3) Design the fuel tank so that fuel tank pressures do not exceed structural design limits if fuel ignition occurs.mm/dd/yy AC 20

- 53C

4 Note: FAA reports DOT/FAA/CT

- 83/3, User ’ s Manual for AC 20

- 53A

, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Due to Lightning , and DOT/FAA/CT

- 89/22, Aircraft Lightning Protection Handbook, provide detailed fuel system lightning protection techniques.

b. When designing the fuel system lightning protect ion, consider the following factors:

(1) Flammable fuel vapor may exist in any part of the fuel system;

(2) Streamers or lightning strikes may ignite f lammable vapors in vent outlets;

(3) Streamers or corona can contain enough energy to serve as an ignition source;

(4) Strike attachment to poorly conducting parts may contain enough energy to induce sparks and arc inside the fuel tank, and could ignite fuel vapors;

(5) Strike attachments may puncture the skin, heat fuel tank skins, or cause arcing in fuel tank structures;

(6) Lig htning currents flowing in the internal components of the fuel system (such as fuel and vent lines, conduits, or internal structural parts) may produce electrical sparks and ignite

fuel vapors;

(7) Lightning currents flowing in the airframe produce voltage dif ferences between adjacent parts or structure. The lightning electromagnetic fields can induce transient voltage and current in the electrical wiring an d components of the fuel system; and

(8) S trike attachment may weaken adhesives/structural bonds or mechanic al fasteners enough to affect the integrity of the fuel tank.

c

. Composite materials such as CFC have lower electrical conductivity than aluminum. When used to construct fuel systems, CFC often requires design features different from those required on metallic structures to provide an equivalent level of protection. Using adhesive bonding in the construction of CFC to build fuel systems may decrease lightning current conductivity

. Decreased conductivit y affects both metallic and non

- metallic structures such as rubber or fiberglass. I n composite structures, lightning

- induced voltage and current in electrical wiring in the fuel system and other electrically

- conducting parts may be higher and have

differe nt waveforms than in conventional aluminum airframes.

d

. Parts of the fuel system are typically found throughout much of an aircraft, and occupy

considerable volume. These parts include the fuel tanks themselves, and other areas that may contain fuel vapo rs. Protect all the following parts from lightning strikes when designing the

aircraft:

(1) Vent outlets,

(2) Metal fittings inside fuel tanks,

(3) Fuel filler caps and access doors,

(4) Drain plugs,mm/dd/yy AC 20

- 53C

5 (5) Tank skins,

(6) Fuel transfer lines inside and outside the tanks,

(7) Electric al bonding jumpers between components in a tank,

(8) Mechanical fasteners inside tanks, and

(9) Electrical and electronic fuel system components and wiring.

11. S teps to Demonstrate C ompliance

. You may demonstrate compliance with the applicable certification requirements by following the steps below:

a. Create a certification plan. Describe the fuel system lightning protection approach and analytical procedures or qualification tests planned to show the protection effectiveness of your proposed aircraft fuel lightning protection. For designs consist ing of unique characteristics or materials, the FAA may impose special conditions, issue papers, or other regulations to show compliance with the certification requirements.

b. Determine the l ightning s trike z ones. Determine the aircraft surfaces, or zones, where lightning strike attachment will likely occur to your particular design, and the portions of the airframe through which currents may flow between these attachment points.

c. Determine the l ightning e nvironment. Identify the particular aircraft components that will be in each lightning strike zone, and determine whether certain components should be

located elsewhere.

Note: The current revision of AC 20

- 155, Industry Documents to Support Aircraft Ligh tning Protection Certification , offers methods you can use in determining lightning strike zones and the aircraft lightning environment.

d. Identify p ossible i gnition s ources

. Identify systems and components that might ignite fuel vapor.

e. Set p rotection c riteria

. Set lightning protection pass/fail criteria for items you are evaluating.

f. Verify p rotection a dequacy

. To verify the adequacy of the protection designs, perform simulated lightning tests, perform analysis, or compare the protection design with previously proven protection designs. When analyzing protection designs, you should also identify margins to account for assumptions made in the analytical techniques. Compliance is verified by determining that the fuel system can tolerate th e applicable lightning environments. If the acceptance criteria are not met, modifications will be made to the design

. When your intended means of compliance includes tests, you should do the following:

(1) In your test plans, describe:

a. The production or test articles,

b. Production or test article drawings as required,

c. Installation procedures for the product,

d. Applicable lightning zones,mm/dd/yy AC 20

- 53C

6 e. Test voltage or current waveforms used for your lightning simulations,

f. Spark detection methods,

g. Pass/fail criteria, and

h. Schedules and locations of proposed tests.

(2) Obtain FAA acceptance of your test plan.

(3) Obtain FAA conformity of the test articles and installation conformity of applicable portions of the test setup.

(4) Schedule dates for the FAA to witness tests , as coordinated

. (5) Submit test reports describing all results for FAA review and approval.

12. M aintaining F uel S ystem L ightning P rotection

.

a. Some fuel systems or equipment require dedicated protection devices or specific techniques to protect them. For t hese fuel systems and equipment, you should define requirements for maintaining and monitoring the lightning protection devices or techniques to ensure their integrity remains intact. You should also define the aircraft dispatch requirements, limitations, or both when a protection device has degraded.

b. Some fuel system components may be protected through the use of shield and connector electrical bonding, sealing materials, grounding jumpers, and structural foil shield liners. When using the se types of dev ices, you must specify in the instructions for continued airworthiness

manual the procedures you will use to prevent these protection items from remaining on the aircraft in a degraded condition, or describe how you will prevent their accidental removal

du ring normal aircraft maintenance. Degradation or removal of these items could cause the system to lose its designed protection integrity.

c. Where possible, do not use devices susceptible to corrosion, fretting, flexing cycles, or

other life

- limiting design features. However, when using the se types of devices, you must publish their replacement cycles in your instructions for continued airwort hiness manual. When using devices such as surge protectors, you must identify the number of lightning strikes each device can encounter before it must be replaced.

d. Define the inspection techniques and intervals necessary to ensure the continued integrity of the lightning protection features. Examples of inspection techniques include built

- in test equipment, resistance measurements, or continuity checks of the entire system.

13. Where to Find This AC. You may find this AC

at http://www.faa.gov/regulations_policies/advisory_circulars/

.mm/dd/yy AC 20

- 53C

7 If you have any suggestions for improvements or changes, you may use the template provided at

the end of this AC

.

David W. Hempe Manager, Desi gn, Manufacturing, &

Airworthiness Division

Aircraft Certification Servicemm/dd/yy AC 20

- 53C Appendix 1

A1 Appendix 1 . Advisory Circular Feedback Information

If you have comments or recommendations for improving this advisory circular (AC), or suggestions for new items or subjects to be added, or if you find an error, you may let us know about by using this page as a template and 1) emailing it to 9

- AWA

- AVS

- AIR500

- Coord@faa.gov or 2) faxing it to the attention of the AIR Directives Management Officer at 202

- 267

- 398 3.

Subject : (insert AC number and title) Date: (insert date)

Comment/Recommendation/Error: (Please fill out all that apply)

An error has been noted:

Paragraph ____________________

Page ______

Type of error (check all that app ly): Editorial:

----- Procedural

-----

Conceptual____

Description/Comments:______________________________________________

_________________________________________________________________

_________________________________________________________________

Recommend paragraph ______ on page ______ be changed as follows:

(attach separate sheets if necessary)

_________________________________________________________________

In a future change to this advisory circular, please include coverage on the following subject: (briefly describe what you want added attaching separate sheets if necessary)

________________________________________________________________

Name: ______________

____________