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Lightning Protection of Fuel Systems
Transport Category Airplanes
§25.954 vs. §25.981(a)(3)
Eng. Maurício Veloso
mauricio.veloso@anac.gov.br
+55 12 3797-2385
Summary
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Background
Timeline
Regulations
Policies
References
Background
• Lightning
Zone 1 – First Return Stroke
Zone 2 – Swept Stroke
Zone 3 – Unlikely Attach. (Conduction)
Zone 2
Zone 3
Zone 2
Zone 1
Background
• Fuel Tanks
• Structures
• Systems
Timeline
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Sep. 1962 – CAR 4b. 628 Lightning Strike Protection
Dec. 1964 – 14CFR Part 25 -> §25.1015 Lightning Strike Protection
Sep. 1967 – §25.954 Fuel System Lightning Protection (identified as §25.981
on NPRM) and §25.981 Fuel Tank Temperature
Apr. 1970 – §25.581 Lightning Protection
Apr. 1994 – §25.1316 System Lightning Protection
Jul. 1996 – TWA-800 accident
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Oct. 1999 – NPRM for §25.981 Fuel Tank Ignition Prevention
Ago. 2000 – TWA-800 NTSB final report
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May 2001 – §25.981 effective (Amdt. 102)
May 2009 – FAA Policy memo ANM-112-08-002
Nov. 2009 – Start Large Airplane Fuel System Lightning Protection ARC
May 2011 – ARC Finish
Regulations
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§25.954
§25.901(c)
§25.1309
§25.981(a)(3)
§25.954
§ 25.954 Fuel system lightning protection.
The fuel system must be designed and arranged to prevent the ignition of fuel
vapor within the system by—
(a) Direct lightning strikes to areas having a high probability of stroke
attachment;
(b) Swept lightning strokes to areas where swept strokes are highly probable;
and
(c) Corona and streamering at fuel vent outlets.
[Amdt. 25–14, 32 FR 11629, Aug. 11, 1967]
§25.954
• Before Amendment 25-102 to part 25, § 25.954 was the
regulation applied to lightning protection of fuel tanks.
• As it was normally applied, that regulation only required
prevention of ignition of vapors in the tank with no
consideration of:
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expected design failures,
aging,
wear,
or maintenance errors for airplane structure.
§25.901(c)
Subpart E – Powerplant
General
§ 25.901 Installation.
...
(c) For each powerplant and auxiliary power unit installation, it must be
established that no single failure or malfunction or probable combination
of failures will jeopardize the safe operation of the airplane except that
the failure of structural elements need not be considered if the probability
of such failure is extremely remote.
§25.1309
§ 25.1309 Equipment, systems, and installations.
(b) The airplane systems and associated components, considered separately
and in relation to other systems, must be designed so that—
(1) The occurrence of any failure condition which would prevent the
continued safe flight and landing of the airplane is extremely improbable,
and
…
(d) Compliance with the requirements of paragraph (b) of this section must
be shown by analysis, and where necessary, by appropriate ground, flight,
or simulator tests. The analysis must consider—
(2) The probability of multiple failures and undetected failures.
§25.901 and §25.1309
• Existing composite and aluminum structural technology can
generally provide the ability to withstand single faults and still
prevent ignition sources in the event of lightning attachment
for most areas of the design.
• However, systems with potentially catastrophic failure modes
typically meet requirements such as §§ 25.901 and 25.1309
through a design architecture that can withstand multiple
independent failures without a catastrophic effect.
• As it applies to fuel tank lightning protection for basic airplane
structure, compliance with § 25.981(a)(3) would typically
need a design with three highly reliable, independent, and
redundant protective features to prevent ignition sources.
§25.981(a)(3)
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25.981(a): “ No ignition source may be present at each point in the fuel
tank or fuel tank system where catastrophic failure could occur due to
ignition of fuel or vapors. This must be shown by:
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“(3) Demonstrating that an ignition source could not result from each
single failure, from each single failure in combination with each latent
failure condition not shown to be extremely remote, and from all
combinations of failures not shown to be extremely improbable. The
effects of manufacturing variability, aging, wear, corrosion, and likely
damage must be considered.”
AC 25.981-1C
(1) Section 25.981(a)(3) defines three failure scenarios that must be
addressed in order to show compliance with the rule:
(a) Each single failure, regardless of the probability of occurrence of the
failure, must not cause an ignition source.
(b) Each single failure, regardless of the probability of occurrence, in
combination with any latent failure condition not shown to be at least
extremely remote (i.e., not shown to be extremely remote or extremely
improbable), must not cause an ignition source.
(c) All combinations of failures not shown to be extremely improbable must
not cause an ignition source.
AC 25.981-1C
• For compliance with §25.981(a)(3):
• “fuel tank vapor should be assumed to be flammable
at any time (probability of flammability = 1).”
• “applicants should assume that a lightning
attachment could occur at any time (probability of
lightning = 1).”
Fuel Tank Ignition Sources
§25.981(a)(3)
Lightning Protection
Structure
Compliance
demonstrated
Impractical
Petition for
Exemption
Policy Memo
Prob.Light.= PL
Prob. Flamab.=PF
Assess. Prob. Ignition Event
Systems
Compliance
Practical
Compliance
- Prob. Lightning = 1
- Prob. Flammability = 1
- Prob. of Ignition Source
comply with §25981(a)(3)
Exemption
Petition for
Exemption
Fault
Tolerant
Design
Non Fault
Tolerant
Design
Demonstrate
F.T. Impratical
Demonstrate two independent,
effective, and reliable lightning
protection features
Demonstrate
Ignition of Fuel Vapor
Extremely Improbable
Applicable Regulations
• § 25.954 Fuel system lightning protection.
• § 25.981 Fuel tank ignition prevention.
• § 21.50 Instructions for continued airworthiness and
manufacturer’s maintenance manuals having airworthiness
limitations sections.
• § 25.901 Installation. (Powerplant)
• § 25.1301 Function and installation. (Equipment)
• § 25.1309 Equipment, systems, and installations.
• § 25.1316 System lightning protection.
• § 25.1353 Electrical equipment and installations.
• § 25.1529 Instructions for continued airworthiness.
• Appendix H to part 25–Instructions for Continued Airworthiness
Applicable Policies
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AC 25.981-1C - Fuel Tank Ignition Source Prevention Guidelines
FAA Policy Memo ANM-112-08-002 - Policy on Issuance of Special Conditions
and Exemptions Related to Lightning Protection of Fuel Tank Structure
AC 25-8 Auxiliary Fuel System Installations
AC 20-53B Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition
Caused by Lightning
AC 20-136A Protection of Aircraft Electrical/Electronic Systems against the
Indirect Effects of Lightning
AC 20-155 SAE Documents to Support Aircraft Lightning Protection
Certification
AC 25-16 Electrical Fault and Fire Prevention and Protection
AC 25-19 Certification Maintenance Requirements
AC 25.981-2A Fuel Tank Flammability Reduction Means
AC 25.1701-1 Certification of Electrical Wiring Interconnection Systems on
Transport Category Airplanes
Applicable Guidance
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ARP 5412A - Aircraft Lightning Environment and related test waveforms
ARP 5414A - Aircraft Lightning Zoning
ARP 5416 - Aircraft Lightning Test Methods
ARP 4761 - Aerospace Recommended Practice, Guidelines and Methods
for Conducting the Safety Assessment Process on Civil Airborne Systems
and Equipment.
Obrigado.
Dúvidas e Discussão até 9:30.
Eng. Maurício Veloso
mauricio.veloso@anac.gov.br
+55 12 3797-2385
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