Definition

Lightning protection is a critical element of aircraft certification, and national regulators set standards for such protection. One example is the U.S. Federal Aviation Administration's (FAA) 14 CFR Part 25. The European Union Aviation Safety Agency (EASA) sets similar requirements.

Background

The first known aircraft accident attributed to lightning took place in 1929. Eight people died when a Transcontinental Air Transport Ford Tri-Motor crashed near Mt. Taylor, New Mexico, in the United States. In 1945, a U.S. military Curtis C-46D transport aircraft en route from Dallas, Texas, to Jackson, Mississippi, suffered a lightning strike at 3,000 feet and crashed into a wooded area. In 1963, a Pan American World Airways Boeing 707 crashed while in a holding pattern as it waited to land at Philadelphia, Pennsylvania, killing 81 people. As a result, the U.S. government mandated lightning discharge wicks on all commercial jetliners.

In 1976, an Iranian military Boeing 747 crashed in Spain following a lightning strike. The accident, which killed 17 people, took place when fuel vapors ignited. Such accidents led to stricter standards for lightning protection and for the maintenance of lightning protection systems.

Description

 Researchers estimate that on average, any given transport aircraft gets hit by lightning once in every 1,000 flight hours. For commercial airliners, that equates roughly to once every year. Aircraft attract lightning when flying anywhere near a thunderstorm's electrical field. Most of time, lightning encounters inflict little damage, thanks to rigorous design standards for aircraft and their various components.

To the casual observer, a lightning flash represents a single event. But aeronautical engineers must consider dynamics such as multiple lightning strokes within a single flash, and multiple bursts of current. (See FAA Advisory Circular 20-136B, Appendix 2 for definitions.)

These lightning phenomena can harm aircraft in many ways. Hazards include electrical damage, structural damage, fuel ignition, and the flash blinding of crew members. In addition to the blast and burning effects of a lightning strike, voltage and current spikes injected into wiring and plumbing can cause equipment to fail.

No system can stop lightning from hitting an aircraft in the first place. The design goal is to create paths for the electrical current to enter and exit the airframe with minimal damage. Shielding and protective methods are usually divided into three categories:

• Airframe and structure
• Fuel systems
• Avionics

Design of most lightning protection systems begins with the Faraday cage concept. A Faraday cage, first invented by English scientist Michael Faraday in 1836, uses a cage of conductive material that protects whatever it contains. The cage distributes the electrical charge faster than the charge can penetrate. An aircraft's aluminum skin performs that function up to a point. Aircraft constructed with composite materials need a mesh of conductive material installed to serve the same function.

Broad certification requirements typically state that an aircraft must have the ability to withstand lightning without catastrophic results. More specific requirements address components ranging from fuel tanks, fuel pumps, radios, instruments, flight controls, and flight control computers. Protective devices such as bonding and grounding wires, lightning dissipators, and flame-suppression foam help reduce the effects of lightning. The purpose of bonding is to join all structural components of an aircraft to form a complete circuit, which allows current to pass through without arcing.

Braided metal sheathing is sometimes used to protect avionics. 

Additionally, some aircraft have systems that use nitrogen or other inert gases to displace fuel vapor in the ullage of fuel tanks, thereby reducing the risk of a fuel vapor explosion. Additionally, wire mesh can be installed at locations where a lightning arc is likely to attach to the aircraft, minimizing potential damage.

Certification Steps

The various steps in certification against lightning damage normally involve:

• Determining lightning strike zones on the aircraft
• Identifying possible ignition sources
• Identifying critical airframe sections and components
• Establishing protection criteria
• Testing and verifying the effectiveness of protective designs 

Testing and verification usually involves simulating lightning strikes by means of waveform generators. These tests are conducted on full-size aircraft models.

The FAA's Advisory Circular 20-136B, Aircraft Electrical and Electronic System Lightning Protection, provides manufacturers with specific guidance. The document states:

Redundancy alone cannot protect against lightning because the lightning-generated electromagnetic fields, conducted currents and induced currents in the aircraft can simultaneously induce transients in all electrical wiring on an aircraft.

The document also says some systems will have different requirements applied for different phases of flight. For example, an automatic flight control system failure may be catastrophic if the failure occurs when the system is used for autoland. However, an autopilot failure during cruise may be considered hazardous but not catastrophic.

With regard to the inspection and maintenance of lighting protection systems, the advisory circular says:

Avoid using devices or features that may degrade with time because of corrosion, fretting, flexing cycles, or other causes. Alternatively, identify when to inspect or replace these devices.

Depending on the component, the inspections that examine these items range from daily flight crew preflight checks, to more thorough two-week airworthiness inspections, to heavy maintenance inspections conducted in a hangar. 

Emerging Technologies

Hybrid or electrically powered aircraft will present special issues with regard to lightning shielding. Manufacturers and regulators will also need to understand the vulnerabilities of composite materials and conductive polymer coatings. Aircraft skin, whatever its material, must be robust enough keep a lightning strike from burning through and reaching fuel tanks and other critical components.

Unmanned aircraft need the same lightning protection as manned aircraft, and the same challenges apply. 

Researchers at the Japanese telecommunications company Nippon Telegraph and Telephone Corporation (NTT) announced in early 2025 that they were developing a drone designed to induce lightning strikes. They hope to use such drones to draw lightning away from sensitive equipment and communities during thunderstorms.