U.S. Deparrment 

oflronsportomn 

Federal Aviotion 

Administration 

Advisory 

Circular 

s”bjecf’ PROTECTION OF AIRPLANE FUEL 

SYSTEMS AGAINST FUEL VAPOR 

IGNITION DUE TO LIGHTNING 

Ihk: 4/ 12185 

Initialed by: ACE-100 

AC No: 20-53~ 

ChPngC: 

1. PURPOSE. This Advisory Circular (AC) provides information and guidance concerning an acceptable means, but not the only means, of compliance with Parts 23 

or 25 of the Federal Aviation Regulations (FAR), applicable to preventing ignition of fuel vapors due to lightning. Accordingly, this material is neither mandatory nor regulatory in nature and does not constitute a regulation. In lieu of 

following this me:hod, the applicant may elect to establish an alternate method 

of compliance that is acceptable to the FAA for complying with the requirements 

of Section 23.954 and 25.954. 

2. SCOPE. This Advisory Circular provides guidance for a means of showing 

compliance with regulations for protection against lightning fuel vapor ignition 

hazards to airplane fuel systems of conventional design as well as for those 

involving advanced composite structures or other new technologies. The document 

incorporates information and references related to improvements in the state-ofthe-art, with respect to lightning effects and verification methods that have 

taken place since the previous version of this Advisory Circular was published. 

Ignition hazards addressed include those due to direct effects (on fuel tank 

structure/components and plumbing) as well as indirect effects on wires or circuits in a fuel vapor cavity (such as fuel quantity probes), Guidance in this 

document applies to fuel tanks and systems which are a part of the structure of 

an airplane, as well as externally mounted tanks on wing tips, fuselage, or other 

parts of the airplane. This guidance applies to systems included in the initial 

design as well as modifications, such as additional tanks or other fuel system 

components. Since externally mounted tanks are often located in direct lightning 

strike zones, they may be especially vulnerable to lightning hazards if not adequately protected. 

NOTE: This advisory circular does not address the indirect induced 

effects (upset or damage) on either analog or digital electronic or 

electrical systems, except as they relate to fuel ignition hazards. 

3. CANCELLATION. Advisory Circular No. 20-53, .effective October 6, 1967. 

4. RELATED FAR SECTIONS. Part 23, section 23.954, and Part 25, section 25.954. 

5. RELATED READING MATERIAL. A comprehensive discussion on this subject, with 

additional nonregulatory guidance information, is available in the following 

; document: 

-i 

Users Manual for AC20-53A, 

Fuel Vapor Ignition Due to Lightning,” 

“Protection of Airplane Fuel System Against 

Report Number DOT/FAA/CT-8313. This document is available to the public by order through the National Technical 

Information Service, Springfield, Virginia 22161. 
AC 20-538 4/12/85 

6. BACKGROUND. 

a. Airplanes flying in and around thunderstorms are often subjected to 

direct lightning strikes as well as to nearby lightning strikes which may produce 

corona and streamer formations on the airplane. 

b. Elements of the fuel system are typically spread throughout much of an 

airplane and occupy much of its volume. They include the fuel tanks themselves, 

as well as other areas that may contain fuel vapors, plus associated vents, 

transfer plumbing, electronic controls, and instrumentation. Careful attention 

to all of these elements is necessary for adequate protection. 

C. For purposes of design and provisions of lightning protection, it is 

assumed that the properties of the fuel used by civil airplanes, both piston and 

turbine engine powered, are such that a combustible mixture is present in the 

fuel tank at all times. Therefore, the combination of the flammable fuel/air 

ratio and an ignition source at the time of a lightning strike could produce a 

hazardous condition for the vehicle. To prevent this condition from occurring, a 

review and elimination of the possible ignition sources within the fuel tank/fuel 

system should be conducted. 

d. Assuming that flammable mixtures may exist in any part of the fuel 

system, some items and areas susceptible to fuel ignition include, but are not 

limited to, vent outlets, metal fittings inside fuel tanks, fuel filler caps and 

access doors, drain plugs, tank skins, fuel transfer lines inside and outside of 

the tanks, electrical bonding jumpers betveen components in a tank, mechanical 

fasteners inside of tanks, and electrical and electronic fuel system components 

and wiring. 

e. User’s manual contains further discussion and illustrations of hazardous 

areas and possible corrective measures. 

f. Protection of fuel systems from lightning should be accomplished by one 

of the following approaches: 

(1) Eliminating sources of ignition. 

(2) Ensuring that allowable tank pressure levels are not exceeded if 

ignition does occur and/or that the fuel tank atmosphere will not support combustion. 

g- The preferred approach is to prevent any direct or indirect source of 

ignition of the fuel by lightning. Accomplishment of this approach is quite 

challengin 

of G2xlO’ & 

because thousands of amperes of current are conducted, and a spark 

joule may be all the energy that need be released inside a fuel tank 

to initiate a fire or explosion. 

7. DISCUSSION. 

a. Key considerations of the fuel system/lightning phenomena are: 

2 
4/12/85 AC 20-53A 

(1) Flammable mixtures may exist in any part of the fuel system particularly in the tank and/or vent components. 

(2) Flammable vapors in vent outlets may be susceptible to ignition by 

either streamering or direct strokes. 

(3) Streamers or corona can contain sufficient energy to serve as an 

ignition source. 

(4) Strike attachment to poorly conductive parts may contain enough 

energy to cause sparking on the inside of the fuel tank which, in turn, 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 fuel system internal components, such 

as fuel and vent lines, conduits, or internal structural elements may produce 

electrical sparks capable of igniting flammable vapors. This possibility is of 

special concern when the tank structure is made of nonmetallic material. 

(7) Lightning currents flowing in the airframe can create voltage differences and electromagnetic fields which may induce transient voltages and 

currents in fuel system electrical wiring and hardware. 

(8) Strike attachment may cause deterioration of adhesives/structural 

bonding or fasteners to the extent that the tank integrity would, or could, be 

lost. 

b. The excellent lightning safety record of civil airplanes is attributed 

to the high electrical conductivity of the aluminum alloys used in airplane fuel 

tank construction and to designs which suppress interior sparking at severe 

lightning current levels. 

C. Composite materials, such as the carbon fiber composites (CFC), when 

used for fuel systems, present difficulties in providing equivalent protection 

because of their lover electrical conductivity. New construction techniques, 

such as adhesive bonding, may have limited conductivity for lightning current 

flow. Also, indirect effects, such as lightning induced voltages in fuel system 

electrical wiring and other conducting elements, may be more severe within composite structures than within conventional aluminum airframes. 

d. Airplane fuel system lightning interaction. 

(1) Lightning can be a hazard to airplane fuel systems if they are not 

properly designed. The protection of a properly designed system may be negated 

if it is not correctly fabricated and maintained. 
AC 20-53A 4/12/85 

(2) The effects of lightning on airplanes can range from severe obvious 

damage (such as tearing and bending of airplane skins resulting from high magnetic forces, shock waves, and blast effects caused by the high current, and 

melting of metal skins caused by the lower level longer duration currents of some 

lightning strikes) to seemingly insignificant sparking at fasteners or joints. 

However, if the sparking occurs in a fuel vapor space, ignition of the fuel vapor 

may result with unacceptable explosion damage. 

(3) All or a portion of the lightning current may be conducted through 

fuel tanks ‘or fuel system components. It is important to determine the current 

flow paths through the airplane for the many possible lightning attach points so 

that current entry into the fuel system can be safely accounted for by 

appropriate protective measures. 

(4) Metals, low electrical conductivity composite materials (e.g., carbon fiber reinforced composites), and electrical insulating materials (e.g., 

fiberglass and aramid reinforced composites) all behave differently when 

subjected to lightning. Yet each of these materials may be used in similar airplane applications (e.g., wing skins or fuel tanks). The metals offer a high 

degree of electrical shielding and some magnetic shielding, whereas the electrical insulating materials (dielectrics) offer almost no electrical or magnetic 

shielding. As a result of the latter properties, lightning does not have to come 

in direct contact with fuel systems to constitute a hazard. Lightning can induce 

arcing, sparking, or corona in fuel areas which may result in fuel ignition. 

(5) The damage when using totally nonconducting materials, such as the 

fiberglass and aramid (e.g., Kevlar) reinforced composites, can be considerably. 

more severe as the discharge can more easily penetrate into the interior and 

cause direct fuel vapor ignition. 

(6) Lightning strikes can result in sparking and arcing within fuel 

systems unless they are designed to be spark free. Flammable vapors can be 

ignited in metal and semi-conducting fuel tanks by arcing and in dielectric fuel 

tanks by magnetic and electrical field penetration which can cause sparking, 

arcing, streamer, or corona discharge. 

8. DEFINITIONS. See appendix 1 for List of Definitions. 

9. APPROACHES TO COMPLIANCE. In general, the steps below outline an effective 

method to show compliance: 

a. Determine the Lightning Strike Zones. Determine the airplane surfaces, or 

zones, where lightning strike attachment is likely to occur, and the portions of 

the airframe through which currents may flow between these attachment points. 

The lightning strike zone locations are defined in paragraph lob, and guidelines 

for locating them on particular airplanes are given in paragraph 10~. 

I 

\ 

b. Establish the Lightning Environment. Establish the component(s) of the 

total lightning event to be expected rn each lightning strike zone. They are the 

currents and voltages that should be considered and are described in paragraph 11. 
AC 20-53A 

C. Identify Possible Ignition Sources. Identify systems and/or components 

that might be ignition sources to fuel vapor. Ignition hazards may include 

structures as well as fuel system mechanical and electrical/electronic components associated with the fuel tanks. 

NOTE: In order to provide concurrence on the certification compliance, 

the above three sequential steps should be accomplished, revieved 

with the appropriate FAA personnel, and an agreement reached prior 

to test initiation to prevent certification delays. 

d. Establish Protection Criteria. Establish Lightning protection pass/fail 

criteria for those items to be evaluated. 

e. Verify Protection Adequacy. Verify the adequacy of the protection 

designs by similarity with previously proven installation designs, simulated 

Lightning tests, or acceptable analysis. When analysis is utilized, appropriate 

margins to account for uncertainties in the analytical techniques may be 

required. Developmental test data may be used for certification when properly 

documented and coordinated with the certification agency. See section titled 

“Comparison with Development Tests” of the Users Manual. 

NOTE : Except for standard design/installation items which have a history 

of acceptability, any new material, design, or unique installation 

should follow the additional guidelines provided herein to ensure 

certification compliance can be accomplished. 

f. Listed below are the steps to be followed as appropriate in assuring 

compliance with the regulations: 

(1) Generate a certification plan which describes the analytical procedures and/or the qualification tests to be utilized to demonstrate protection 

effectiveness. Test plans should describe the production or test article(s) 

to be utilized, test drawing(s) as required, the method of installation that 

simulates the production installation, the Lightning zone(s) applicable, the 

Lightning simulation method(s), test voltage or current waveforms to be used, 

spark detection methods, and the appropriate schedules and Location(s) of proposed 

test(s). 

(2) Obtain FAA concurrence that the certification plan is adequate. 

(3) Obtain FAA detail part conformity of the test articles and 

installation conformity of applicable portions of the test setup. 

(4) Schedule FAA witnessing of the test. 

(5) Submit a final test report describing all results and obtain FAA 

approval of the report. 

. 
AC 2D-53A 4/12/85 

10. LIGHTNING ATTACHMENT PHENOMENA. 

a. Swept-Stroke Phenomenon. 

(1) The lightning channel is somewhat stationary in air while it is 

transferring electrical charge. When an airplane is involved, the airplane 

becomes part of the channel. However, due to the speed of the airplane and the 

length of time that the lightning channel exists, the airplane can move relative 

to the lightning channel. When a forward extremity, such as a nose or wing 

mounted engine pod is an initial attachment point, the movement of the airplane 

through the lightning channel causes the channel to sweep back over the surface 

as illustrated in figure 1 of appendix 2, producing subsequent attachment 

points. This is known as the swept-stroke phenomenon. As the sweeping action 

occurs, the characteristics of the surface can cause the lightning channel to 

reattach and dwell at various surface locations for different periods of time, 

resulting in a series of discrete attachment points along the sweeping path. 

(2) The amount of damage produced at any point on the ai.rplane by a 

swept-stroke depends upon the type of material, the dwell time at that point, and 

the lightning currents which flow during the attachment. Both high peak current 

restrikes with intermediate current components and continuing currents may be 

experienced. Restrikes typically produce reattachment of the arc at a new point. 

(3) When the lightning channel has been swept back to one of the 

trailing edges, it may remain attached at the point for the remaining duration of 

the lightning event. An initial attachment point at a trailing edge, of course, 

would not be subjected to any swept-stroke action, and therefore, this attachment 

point will be subjected to all components of the lightning event. 

(4) The significance of the swept-stroke phenomenon is that portions of 

the vehicle that would not be targets for the initial attachment points of a 

lightning flash may also be involved in the lightning strike process as the 

lightning channel is swept backwards, although the channel may not remain 

attached at any single point for very long. On the other hand, strikes that 

reach trailing edges must be expected to remain attached there (hang-on) for the 

balance of their natural duration. 

b. Lightning Strike Zone Definitions. To account for each of the possibilities described in the foregoing paragraphs, the following zones have been 

defined: 

(1) Zone 1. 

1. Zone IA: Initial attachment point with low possibility of 

lightning arc channel hang-on. 

ii. Zone 1B: Initial attachment point with high possibility of 

lightning arc channel hang-on. 
4/12/85 AC ZO-53A 

(2) Zone 2. 

1. Zone 2A: A swept-stroke zone with low possibility of 

lightning arc channel hang-on. 

ii. Zone 2B: A swept-stroke zone with high possibility of 

lightning arc channel hang-on. 

(3) Zone 3. All of the vehicle areas other than those covered by 

Zone 1 and 2 regions. In Zone 3, there is a low possibility of any attachment of 

the lightning channel. Zone 3 areas may carry substantial amounts of electrical 

current, but only by conduction between some pair of attachment points. 

(4) The zone definitions are in basic agreement with the definitions of 

earlier versions of this Advisory Circular, except that the former Zones 1 and 2 

have been subdivided to account for low and high possibilities of the lightning 

arc channel hang-on (figures 2 h 3) shown in appendix 2. The locations of these 

zones on any airplane are dependent on the airplane’s geometry and operational 

factors, and often vary from one airplane to another. 

C. Location of Lightn.ing Strike Zones. With these definitions in mind, the 

locations of each zone on a particular airplane may be determined as follows: 

(1) Extremities such as the nose, wing and empennage tips, tail cone, 

wing-mounted nacelles, and other significant projections should be considered as 

within a direct strike zone because they are probable initial leader attachment 

points. Those that are forward extremities or leading edges should be in 

Zone lA, and extremities that are trailing edges should be in Zone 1B. Most of 

the time, the first return stroke will arrive shortly after the leader has 

attached to the airplane, so Zone 1A is limited to the immediate vicinity (i.e., 

approximately 18 inches (0.5~) aft) of the forward extremity. However, in rare 

cases the return stroke may arrive somewhat later, thereby exposing surfaces 

further aft to this environment. This possibility should be considered if the 

probability of a flight safety hazard due to a Zone 1A strike to an unprotected 

surface is high. 

(2) Where questions arise regarding the identification of initial 

attachment locations or where the airframe geometry is unlike conventional 

designs for which previous experience is available, scale model attachment point 

tests may be in order. Information on model testing can be found in the User’s 

Manual. 

(3) Surfaces directly aft of Zone IA should be considered as within 

Zone 2A. Generally, Zone 2A will extend the full length of the surface aft of 

Zone lA, such as the fuselage, nacelles, and portions of the wing surfaces. 

(4) Trailing edges of surfaces aft of Zone 2A should be considered 

Zone ZB, or Zone 1B if initial attachment to them can occur. 

edge of a surface is totally non-conductive 

If the trailing 

then Zone 2B (or IB) should be projected forward and/or inboard to the neares; conductive surface. 

7 
AC 20-53A 4/12/U 

(5) Surfaces approximately 18 inches (0.5m) to either side of initial- ’ 

or swept-attachment points established by steps (1) and (2) of paragriph c should 

also be considered as within the same zone, to account for small lateral movements of the sweeping channel and local scatter among attachment points. For 

example, the tip of a wing would normally be within Zone IA (except for its 

trailing edge, which would usually be in Zone 1B). To account for lateral motion 

of the channel and scatter, the top and bottom surfaces of the wing 18 inches 

(0.5m) inboard of the tip should also be considered as within the same zones. - 

(6) Surfaces of the vehicle for which there is a low possibility of 

direct contact with the lightning arc channel that are not within any of the 

above zones, but which lie between them, should be considered as within Bone 3. 

Zone 3 areas must carry substantial amounts of electrical energy. 

11. IJCHTNING ENVIRONMENT. For verification purposes, the natural lightning 

environment (which comprises a wide statistical range of current levels, duration, and number of strokes) is represented by current test Components A through 

E, and voltage Components A, B, and D (per figures 4, 5 and 6) shown in 

appendix 2. When testing or analysis are required, the following waveforms 

should be used. (Applications of waveforms and lightning zones are detailed in 

appendix 3. > 

a. Current Waveforms. There are four current components (A, B, C, and D) 

that are applied to determine direct effects. Current waveform E is used in 

tests to determine indirect effects. Components A, B, C, and D each simulate a 

different characteristic of the current in a natural lightning flash and are 

shown in figure 4 of appendix 2. They are applied individually or as a composite 

of two or more components together in one test. The tests in which these waveforms are applied are presented in appendix 3. 

(1) Component A - Initial High Peak Current. Component A has 6” seak 

amplitude of 200kA (210 percent) and an action integral (fi2dt) of 2x10 A s 

(220 percent) with a total time duration not exceeding 500 microseconds. This 

component may be unidirectional or oscillatory. For analysis purposes, a double 

exponential current waveform should be used. This waveform re 

stroke of 200,000 amperes peak at a peak rate of rise of 1x10 1P 

resents a return 

A/s. This waveform is defined mathematically by the double exponential expression shown below: 

-wt -pt 

i(t) = I,(C- El 

where 

IO 

- 223,000(A) 

-- 11,000 (s”) 

P= 460,000 (6-I) 

t = time(s) 
4/12/85 AC 20-538 

(2) Component B - Intermediate Current. Component B has an average 

amplitude of 2kA (210 percent) flowing for a maximum duration of 5 millisecond6 

unidirectional; e.g., rectangular, exponential, or linearly decaying. For analysis purposes, a double exponential current waveform should be used. This waveform is described mathematically by the double exponential. 

m act-pf 

i(t) = IO (E - e ) 

IO - 11300(A) 

=- 700 (6") 

p = 2000 (6") 

t = time (s) 

If the dwell time is more than 5ms, apply an average current of 

400A for the remaining dwell time. The dwell time shall have been determined 

previously through a swept-stroke attachment test or by analysis. If such determination has not been made, the dwell time shall be taken to be 50ms. 

(3) Component C - Continuing Current. Component C transfers a charge 

of 200 coulombs (220 percent) in a time of between 0.25 and 1 second. This 

implies current amplitudes of between 200 and 800 amps. The waveform shall be 

unidirectional: e.g., rectangular, exponential or linearly decaying. For analysis purposes, a square waveform of 200A for a period of 1 sec. should be utilized. 

(4) Component D - Restrike Current. Component D has a peak amplitude 

of lOOkA (~10 percent) and an action integral of 0.25x106A2 s (+20 percent). 

This component may be either unidirectional or oscillatory with-a total time 

duration not exceeding 500 microseconds. For analysis purposes a double esponential current waveform should be used. This waveform represents a re-strike of 

100,000 amperes peak at a peak rate-of-rise of 0.5~lO~~A/s. The waveform is 

defined mathematically by the double exponential expression shown below: 

-cxt - fjt 

i(t) = IO (E - c > 

IO = 130,000 (A) 

o( = 27,500 (s-5 

P = 415,000 (S-5 

t = time (6) 

(5) Current Waveform E - Fast Rate-of-Rise Stroke Test for Full Size 

Hardware. Current waveform E has a rate-of-rise of at least 25kA/p6 for at least 

0.5 microseconds, as shown in figure 4 of appendix 2. Current waveform E has a 

minimum amplitude of 50kA. Alternatively, 

a 25kA/Ps rate-of-rise for at least 0.5 

components A or D may be applied with 

microseconds and the direct and indirect 

effects evaluation conducted simultaneously, 

I 

9 
AC 20-53A 4112185 

. 1. Indirect effects measured as a result of this waveform must be 

extrapolated as follows. Induced voltages dependent upon resistive or diffusion 

flux should be extrapolated linearly to a peak current of 200 kA. 

ii. Induced voltages dependent upon aperture coupling should be 

extrapolated linearly to a peak rate-of-rise of 100 kA/us. 

b. Voltage Waveforms - Test. There are three voltage waveforms, “A,” “B,” 

and “D,” which represent the electric fields associated with a lightning strike. 

Voltage waveforms-“A” and “D” are used to test for possible dielectric puncture 

and other potential attachment points. Voltage waveform “B” is used to test for 

streamers. The tests in which these waveforms are applied are presented in 

appendix 3. 

(1) Voltage Waveform A - Basic Lightning Waveform, Waveform A has an 

average rate-of-rise of 1x106 volts per microsecond (+50 percent) until its 

increase is interrupted by puncture of, or flashover <cross, the object under 

test. At that time, the voltage collapses to zero. The rate of voltage collapse 

or the decay time of the voltage if breakdown does not occur (open circuit 

voltage of lightning voltage generator) is not specified. Voltage waveform A is 

shown in figure 5 of appendix 2. 

(2) Voltage Waveform B - Full Wave. Waveform B rises to crest in 1.2~s 

(+20 percent). Time-to-crest and decay time refer to the open circuit voltage of 

the lightning voltage generator, and assume that the waveform is not limited by 

puncture or flashover of the object under test. This waveform is shown in 

figure 5 of appendix 2. 

(3) Voltage Waveform D - Slow Front. The slow-fronted waveform has a 

rise time between 50 and 250 microseconds to allow time for streamers from the 

test object to develop. It should give a higher strike rate in tests to the low 

probability regions that might have been expected in flight. This waveform is 

shown in figure 6 of appendix 2. 

&XPH A. POSTECORVO 

Deputy Director, Office of Airworthiness 

10