Lightning Strikes

Within storm clouds, thermal convection causes collisions between ice particles resulting in the transfer of electrons between them. This process leads to the accumulation of electrical charges inside the cloud. When the electrical tension between these charged areas reaches a critical point, it overcomes the insulating properties of the air that separates them, resulting in a lightning discharge. (fig.1). Lightning can happen ① between cloud and ground, ② inside a cloud, or ③ between two clouds. Earth experiences an average of approximately 44 lightning strikes every second.

(fig.1) Typical types of lightning

A lightning bolt initiates with a column of ionized air (fig.2) that generally ① starts from the negative part of the cloud and moves towards the positively charged area. This column is called a ‘leader’. As it nears the positively charged area, ② secondary leaders develop from it. ③ When the two leaders come into contact with each other, an electrical current flows to neutralize the opposite electrical charges accumulated in the two areas.

There are often several successive discharges in a single lightning bolt. The discharge current can reach 200 000 A and the temperature inside the lightning channel can reach 30 000 °C. Cloud-to-ground lightning bolts are usually the most powerful.

The average worldwide frequency is 3 lightning strikes per square km per year (strike/km²/yr). However, there is a strong disparity of this value depending on the location on the planet. Based on a National Aeronautics and Space Administration (NASA) study, the regions more prone to lightning strikes are Central Africa and South America with more than 20 strikes/km²/yr and more than 70 strikes/km²/yr in Central Africa, reaching 158 strikes/km²/yr in Congo. In comparison, oceanic areas have a frequency of less than 1 strike/km²/yr reaching less than 0.1 strike/km²/yr in the polar regions and South Pacific area (fig.3).

(fig.3) Worldwide lightning distribution from 1995 to 2003 with color range indicating the average annual number of lightning flashes per square kilometre. Credits: National Oceanic and Atmospheric Administration (NOAA) Science and National Aeronautics and Space Administration (NASA).

Using the 3 strikes/km²/yr average value and an average aircraft surface of 300 m², an aircraft should theoretically be struck by lightning once every 1 000 years. The operational reality is that an in-service aircraft will be struck by lightning on average once a year, or every 3 000 flight hours. This is 1 000 times the projected value above.

This difference can be explained by the fact that an aircraft will tend to attract lightning in flight when it is in the proximity of a storm’s high electrical field (fig.4).

If a lightning leader initiates close to an aircraft ①, some lightning leaders will also initiate from the aircraft extremities (e.g. nose cone, wing tips, vertical tailplane) toward the lightning leader.②

If one aircraft leader joins with the lightning leader, the aircraft becomes part of the lightning channel ③, and continuing leaders initiate from the other extremities of the aircraft toward the positively charged area (ground) ④.

When one of these leaders nears this positively charged area, ⑤ new leaders initiate from it and ⑥ create lightning when joining the main leader.

(fig.4) How lightning strikes an aircraft.

All large aircraft must be designed and certified to withstand lightning strikes without sustaining significant damage to their structure or effects on their systems that would adversely affect safety for the remainder of the flight. This includes protection of the airframe structure against the direct effects of lightning, and the protection of the electronic circuitry versus lightning current induced effects.

The aircraft structure is designed to conduct the electrical current induced by a lightning strike. For composite fuselages or components, integrated conductive metallic foils and metallic strips are used to ensure this.

All components of the aircraft structure (metallic or composite) must be bonded together with bonding leads or with fasteners to ensure electrical continuity (fig.5). This will enable the lightning current to travel through the aircraft structure without creating significant damage.

Direct effects of a lightning strike are normally limited to the damage caused by the lightning strike at its initial point of contact. The observable effects of a lightning strike include:

• Metallic components: Burns, pitting, holes, and melt marks on the aircraft skin or structure, structural deformation, heat damage, and paint discoloration.
• Composite components: Paint discoloration, skin punctures, fiber damage (including fiber tufts or fiber delamination), loss or damage of copper foil mesh and metallic strips.

As the aircraft moves during the strike, and due to the pulsating discharge of the electrical current, the lightning attachment points (on entry or exit) can move along the surface of the aircraft creating the so-called multiple “swept strokes” (up to 20 strokes) (fig.6). They can also remain fixed on the rearmost parts of the aircraft (known as a “hang-on” attachment point), which is a single attachment point sustaining several discharges.

The level of damage at the attachment points depends on the intensity of the lightning strike.

(fig.6) Example of “swept strokes” and “hang-on” attachment points shown on an A320 aircraft with the initial entry point of the lightning current on the nose cone and an exit point on the left side of the horizontal tailplane.

Lightning attachment zones are identified depending on the probability and type of lightning attachment on the aircraft structure (fig.7).

Electromagnetic fields related to a lightning strike can cause unwanted transient voltages and currents in the aircraft wiring and its systems. As required by the regulations, aircraft must be designed so that there is no perturbation of a critical or essential system in the case of a lightning strike that could temporarily or permanently affect its operation. The level of protection given to a particular system depends on the likelihood of the system being affected by lightning and the impact that a loss of this system would have on the safety of the flight.

• Protection from the indirect effects of lightning strikes is ensured by:
• System redundancy
• Physical and electrical segregation of the redundant systems
• Electromagnetic protection on the electrical harness where required, using differential transmission lines, shielding and over-shielding, and specific routing rules
• Electrical isolation or use of lightning surge arrestors specified inside equipment ports depending on their potential exposure to lightning strike effects
• Management of corrupted data by system software.

Most of the reported lightning strike events on aircraft usually occur in flight between 5 000 ft and 15 000 ft, or when the aircraft is on the ground.

Flight crews should take advantage of all available means to avoid lightning conditions such as weather forecasts, use of the onboard weather radar, and ATC guidance.

Certain weather radars are equipped with a lightning prediction function that provides additional indications to the flight crew of areas within a storm where an aircraft may be more prone to lightning strikes (fig.8).

In the case of a storm with lightning activity when the aircraft is parked or stored outside for maintenance activities, the maintenance and ground servicing personnel should apply specific safety precautions to limit the consequences of a potential lightning strike.

Grounding (earthing) the aircraft reduces the risk of injury to personnel and risk of damage to the aircraft in the case of a lightning strike. If the aircraft is not grounded, the lightning current can exit from any point of the aircraft structure. This is normally close to the landing gears where it can cause significant damage and a risk of serious injury. Any ground servicing equipment (e.g. platforms, access stairs, cargo loaders, ground service carts, cargo loaders, and pushback vehicles) that may be in contact with an aircraft that is not grounded when it is struck by lightning, may also be damaged.

A grounding cable with less than 500 mOhm of resistance and with a minimum cross section of 22 mm2 (0.034 in2) must be attached to one of the aircraft grounding points.

Maintenance or ground servicing operations should be stopped pending the end of the storm and lightning conditions. Even if the aircraft is grounded, the resulting shockwave created by a lightning strike can cause injuries to ground personnel. Anyone working in the vicinity of the aircraft should not touch metal parts equipment or any other item connected to the aircraft.

Disconnection of external equipment (e.g. external power supply, air conditioning carts, and other ground servicing vehicles) prevents damage in the case of a lightning strike.

In lightning conditions, ground operators should disconnect or remove their headsets and communicate with the crew in the cockpit using standard hand signals.

When available, the operators must review and follow the local airport or airline policy and procedures for managing safety when there is lightning and storms.

When lightning strikes an aircraft, a specific process must be applied to detect any damage caused by the strike, evaluate the damage, and perform the necessary repair before returning the aircraft back into service.

In the case of a lightning strike, the flight crew must make a logbook entry to inform maintenance personnel of the event, so that they can perform the appropriate inspection and necessary repairs. The flight crew should provide as much information as possible such as the landing gear position when the strike happened, a description of any system malfunction during or after the strike, and a list of the ECAM (EICAS for A220) alerts that may have been triggered.

The maintenance personnel should gather all the information provided by the flight crew about the event and print a Post Flight Report (PFR) to analyze any effect on the aircraft systems.

It is possible to choose between several types of post lightning strike inspection (except for A220 and A300 aircraft). This enables flexibility depending on airline operations, time constraints, human resources, availability of ground support equipment, etc.

The standard inspection after a lightning strike is divided into several phases. The first phase being initial damage detection consisting of a thorough inspection of the entire surface of the aircraft and testing certain systems. The following phases are additional checks to be performed further to damage detection.

The Quick Release Inspection (QRI) allows the operator to perform a reduced inspection to release the aircraft in a shorter time frame, and postpone the full standard inspection for a limited number of flight cycles when time and logistics permit. The QRI is only available for A300-600, A310, A320 family, A330, A340, A350, and A380 aircraft.

The QRI consists of inspecting the areas that are most prone to lightning strikes and testing certain systems. If no damage is found during the QRI, the aircraft can be dispatched for up to 50 or 200 Flight Cycles (FC) depending on the aircraft type. A full standard lightning strike inspection must be performed on the aircraft before the end of this grace period. If a new lightning strike occurs during the grace period, a new QRI must be performed, but the grace period applicable before a full inspection remains the 50 or 200 FC from the initial lightning strike event.

This alternative was added in the A320 family AMM to enable further flexibility and allows a maximum of 2 FC (ferry or revenue flight) to return the aircraft to an airfield with sufficient manpower and logistics to perform a standard or quick release inspection. However, this procedure is not applicable if one of the following conditions occurred:

• The flight crew reported a lightning flash with the sound of detonation
• The flight crew decided to divert the flight after a lightning strike event in flight
• There were Injuries to passengers and/or crew members caused by the lightning strike event.

The 1 flight back inspection consists of:

• A visual Inspection of the Air Data/Inertial Reference System (ADIRS) probes and sensors (at touching distance)
• A visual inspection of the flight control surfaces (from a 3m platform or passenger entry stairs)
• A functional check of the aileron and elevator servo controls
• A functional test of the flight control surfaces.

The 1-flight-back inspection was made available for A350 aircraft in the 01-NOV-2023 revision and is under study for A330, A340, and A380 aircraft.

Damage due to lightning strikes should be repaired using an approved repair as per local authority regulations. Airbus recommends using the SRM or ASR (for A350) or ASRP (for A220) to evaluate and repair the damage. SRM/ASR/ASRP repairs are certified to be capable of withstanding additional lightning strikes.

When the lightning strike damage is outside of the SRM/ASR/ASRP limits, it is necessary to obtain repair instructions either from Airbus via a Repair and Design Approval Form (RDAF) or a Repair Engineering Order (REO) for A220 aircraft, or via an instruction provided by an EASA PART 21 approved organization.

To obtain the Airbus RDAF/REO, the operator or repair organization should prepare a damage assessment as required in SRM/ASR/ASRP.

Airbus analyzes any reported lightning strike events in order to further increase knowledge in this domain. Operators and repair organizations are encouraged to report lightning strike occurrences to Airbus even if no damage is found on the aircraft after the inspection. Operators can also participate in the forums and working groups organized by Airbus. For example, Airbus hosts a Lightning Strikes Expert Forum in order to exchange experience about managing lightning strikes with all operators. An SRM working group (except A220) also meets regularly since 2014 to exchange experience about structural repair topics.