Advisory 

Circular

 

Subject: PROTECTION OF AIRCRAFT 

FUEL SYSTEMS AGAINST FUEL 

VAPOR IGNITION CAUSED BY 

LIGHTNING 

Date: 6/5/06

Initiated by: AIR-100

AC No: 20-53B 

1. PURPOSE. 

a. This advisory circular (AC) will show you how to gain Federal Aviation Administration 

(FAA) approval of your compliance with Title 14, Code of Federal Regulations (14 CFR) §§ 

23.954, 25.954, 27.954, and 29.954, Fuel system lightning protection. We recommend how you 

can protect the aircraft’s fuel system from lightning strikes that may ignite fuel vapors. 

b. In establishing compliance to 14 CFR § 25.981, Fuel tank ignition prevention, this AC

can also be used to supplement the guidance in AC 25.981-1, Fuel Tank Ignition Source 

Prevention Guidelines, for ignition prevention in regards to lightning.

c. This AC is not mandatory and does not constitute a regulation. It describes an acceptable 

means, but is not the only means, to show you how to gain certification for fuel system lightning 

protection. However, if you use the means described in this AC, you must follow it in all 

important respects. 

2. SCOPE. 

a. You can apply this AC to both conventional fuel systems and those using advanced 

composite structures or other new technologies. This AC gives you both information on, and 

references to, lightning effects and verification methods. 

b. We address ignition hazards caused by direct effects on main and auxiliary fuel tanks, 

fuel tank components and plumbing, and the indirect effects (upset or damage) on analog or 

digital electronic and electrical systems that could lead to ignition hazards on wires or equipment 

such as fuel quantity probes located in a fuel tank cavity containing fuel vapor. 

NOTE: For information on how lightning affects electric and 

electronic systems, see the current version of AC 20-136, 

Protection of Aircraft Electrical / Electronic Systems Against the 

Indirect Effects of Lightning. 

c. You can also apply this AC when modifying fuel systems, such as adding tanks or other 

fuel system components. Since externally mounted tanks are often in direct lightning strike 
6/5/06 AC 20-53B 

zones, those tanks may be especially vulnerable to lightning hazards and must be adequately 

protected. 

 

3. CANCELLATION. AC 20-53A, Protection of Aircraft Fuel Systems Against Fuel Vapor 

Ignition Due to Lightning, dated April 12, 1985, is cancelled. 

4. DEFINITIONS. 

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 opposite charge. 

d. Direct effect: physical damage caused by lightning attaching to the system’s hardware 

or components. Examples are arcing, sparking, or puncturing in the fuel tank skin. 

e. Indirect effects: electrical transients caused by lightning strikes. 

f. Lightning flash: the total lightning event consisting of one or more lightning strokes, 

plus intermediate or continuing currents. 

g. Lightning strike: attachment of the lightning flash to the aircraft. 

h. Lightning strike zones: aircraft surface areas and structures that are susceptible to 

lightning attachment, dwell time, and current conduction. 

i. Lightning stroke: current surge occurring when lightning makes contact with the 

ground or another charge center. 

j. Streamering: branch-like ionized path caused by a direct lightning stroke or lightning 

strokes that are imminent. 

k. Upset: permanent or momentary degradation to the fuel system’s performance. 

5. LIGHTNING EFFECTS ON FUEL SYSTEMS. 

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 seemingly 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 system 

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 materials, and electrically insulating materials, all of which react differently when 

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6/5/06 AC 20-53B 

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. Electrically insulating 

materials, however, offer little or no electric or magnetic field shielding. Because of this, 

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

Lightning-induced arcing, sparking, or corona may be sufficient 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 directly ignite fuel vapors. 

 

6. FUEL SYSTEM LIGHTNING PROTECTION 

a. To protect fuel systems from lightning, you should do one, or a combination, of the 

following: 

(1) Rid the fuel tank and fuel system of ignition sources. This is the preferred 

approach. To do this, you must specifically design for ignition source prevention, 

because thousands of amperes of current are conducted, and a spark of only about twotenths of a millijoule released inside a fuel tank can start a fire or explosion. 

(2) Reduce fuel tank flammability to ensure the fuel tank atmosphere 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. 

NOTE: FAA reports DOT/FAA/CT-83/3, Users 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 protection, consider the following factors: 

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

(2) Streamering or direct lightning strikes may ignite flammable 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 

sparking inside the fuel tank, and could ignite flammable vapors. 

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

fuel tank structures. 

(6) Lightning 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 

flammable vapors. 

(7) Lightning currents flowing in the airframe produce voltage differences between 

adjacent parts or structure. The lightning electromagnetic fields can induce transient voltage and 

current in the electrical wiring and components of the fuel system. 

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6/5/06 AC 20-53B 

(8) Strike attachment may weaken adhesives/structural bonds or mechanical 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 require 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 conductivity affects both metallic and non-metallic structures such as 

rubber or fiberglass. Also, indirect effects in composite structures, such as lightning-induced 

voltages in electrical wiring in the fuel system and other electrically-conducting parts, may be 

higher and have different waveforms than 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 vapors. Protect all the following parts from lightning strikes when designing the 

aircraft: 

• Vent outlets, 

• Metal fittings inside fuel tanks, 

• Fuel filler caps and access doors, 

• Drain plugs, 

• Tank skins, 

• Fuel transfer lines inside and outside the tanks, 

• Electrical bonding jumpers between components in a tank, 

• Mechanical fasteners inside tanks, and 

• Electrical and electronic fuel system components and wiring. 

7. STEPS TO SHOWING COMPLIANCE. 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. Where designs consist of unique characteristics or 

materials, we may impose special conditions, issue papers, or other regulations to show 

compliance with the certification requirements. 

b. Determine the Lightning Strike Zones. 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 Lightning Environment. Identify the particular aircraft components 

that will be in each lightning strike zone, and determine whether certain components should be 

located elsewhere. 

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6/5/06 AC 20-53B 

NOTE: AC 20-155, SAE Documents to Support Aircraft 

Lightning Protection Certification, offers methods you can use in 

determining lightning strike zones and the aircraft lightning 

environment. 

d. Identify Possible Ignition Sources. Identify systems and components that might ignite 

fuel vapor. 

e. Set Protection Criteria. Set lightning protection pass/fail criteria for items you are 

evaluating.

f. Verify Protection Adequacy. 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. When your 

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

(1) Describe, in your test plans: 

• The production or test articles, 

• Production or test article drawings as required, 

• Installation procedures for the product, 

• Applicable lightning zones, 

• Test voltage or current waveforms used for your lightning simulations, 

• Spark detection methods, 

• Pass/fail criteria, and 

• Schedules and locations of proposed tests. 

(2) Get FAA acceptance of your test plan. 

(3) Get FAA conformity of the test articles, and installation conformity of applicable 

portions of the test setup. 

(4) Schedule dates for FAA to witness tests. 

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

8. MAINTAINING FUEL SYSTEM LIGHTNING PROTECTION. 

a. Some fuel systems or equipment require dedicated protection devices or specific 

techniques to protect them. For these 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 those types of devices, you must specify in the instructions for continued airworthiness 

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6/5/06 AC 20-53B 

manual, procedures to prevent these protection items from remaining on the aircraft in a 

degraded condition, or how to prevent their accidental removal during 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 those types of devices, you must 

publish their replacement cycles in your instructions for continued airworthiness manual. When 

using devices such as surge protectors to protect against direct and indirect effects, 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. 

9. RELATED DOCUMENTS AND HOW TO GET THEM. 

a. 14 CFR. Order copies of the 14 CFR sections referenced in this AC from the 

Superintendent of Documents, Government Printing Office, Washington, D.C. 20402-9325. 

Telephone 202-512-1800; fax 202-512-2250. You can also get copies from the Government 

Printing Office (GPO) electronic CFR Internet website at www.gpoaccess.gov/cfr/. 

b. FAA ACs. Order copies of AC 20-136, AC 20-155, and AC 25.981-1 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 can also get copies from our Regulatory and Guidance library (RGL) at 

www.airweb.faa.gov/rgl. On the RGL website, select “Advisory Circulars,” then select “By 

Number.”

c. FAA Reports. Order DOT/FAA/CT-83/3, Users 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. Telephone 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 website, select 

“Search Library Catalog,” then input the report number. 

Susan J. M. Cabler 

Acting Manager, Aircraft Engineering Division

 

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