On average, an aircraft is struck by lightning once every 104 hours of flight (Ref. 5.1.3-4). It is not yet completely understood, why lightning strikes aircraft (Ref. 5.1.3-4). The prevailing opinion is that lightning only strikes when the aircraft happens to be in or near a storm. The probability of a lightning strike at different altitudes is shown in Fig. "Lightning distribution". Lightning strikes also occur at altitudes above 12,000 meters, although most take place below about 6,000 meters. Lightning strikes are evidently especially common at temperatures around 0 °C, i.e. in the usual ice formation zone. Lightning strikes can also occur in clouds that are not part of a storm system (Ref. 5.1.3-2). In rare cases lightning strikes were recorded although there were no clouds in the area. The reference literature even states that “the probability of lightning striking an aircraft increases with fewer storms in the area. Lightning strikes generally occur in clouds that neither meteorologists or pilots consider to be storm clouds.” Lightning strikes are a fairly frequent occurrence. Aircraft with jet engines are evidently more susceptible to lightning strikes than those with piston engines (Ref. 5.1.3-3). The explanation for this is usually given as the hot exhaust gas jet, which functions as a „charge carrier “and is dragged behind the aircraft in the form of an ionized gas bubble. The high flight speeds and the not endlessly short recombination time of the fairly inert charge carrier during cooling result in what is essentially a conductive gas bubble, the volume and dimensions of which are many times greater than those of the aircraft (at least with fighter aircraft with small fuselages and high-performance engines). The ionized gas bubble combined with the aircraft itself present a considerable capacity which disturbs the electric field and, in some situations can cause lightning strikes to occur when the aircraft enters storm zones.
Due to the variety of their mission types and complexity of their systems, helicopters may be even more at risk of lightning strikes than winged aircraft (Ref. 5.1.3-4).
Lightning-related damage mechanisms (Ref. 5.1.3-1)
Electric and magnetic mechanisms:
Because lightning is made up of a sequence of short electric impulses, it creates a pulsing magnetic field around the conductor. This magnetic field can induce currents of 100 volt in neighboring insulated and protected electric conductors.
The magnetic fields magnetize the ferromagnetic engine parts in contact with the air flow, as well as all neighboring parts. An engine`s roller bearings may become magnetized, which can be detected and verified through magnetic measurements (Figure "Bearing after lightning strike").
If lightning strikes in front of the engine inlet, it creates a plasma cloud. If this cloud is ingested by the engine, it can cause temporary power losses or extinguish the combustion chamber flame in extreme cases (Ref. 5.1.3-12, Example "")).
When lightning strikes, the transition resistance creates brief extreme temperatures in some areas that are above the melting points of the engine materials, but these are usually limited to a relatively small section of the engine. Arc-over at poor mass connections also results in extreme temperatures. If the current enters relatively small cross-sections such as wire strands, it can heat these to temperatures great enough to cause considerable damage to organic materials (synthetics) in the vicinity.
Inside hollow parts (e.g. flow cones) an arc-over can temporarily increase the temperature of the air and cause it to expand explosively, mechanically overloading the parts.
The magnetic forces, especially at the entrance and exit points, can plastically deform filigreed parts. Trials showed that materials reinforced with electrically conductive fibers (e.g. C-fibers, boron fibers) can be explosively destroyed by the current.
Engine damage due to lightning strikes
The notion that the nacelle of a completely metal aircraft acts as a Faraday cage and provides sufficient protection from lightning strikes is refuted in Ref. 5.1.3-3, at least as far as military fighter aircraft are concerned. Experience has shown that there is a sufficient connection between the inside of the fuselage and the air flowing outside the aircraft for damaging flows to occur. There is insufficient shielding due to the many openings, windows, and valves.
These problems will most likely become more pronounced due to the increased use of fiber-reinforced nacelle coverings, even though efforts are being made (for example, installing electroconductive wire webbing) to minimize this risk.
Recognizing a lightning strike
Not all damage caused by a lightning strike makes itself known through the spontaneous failure of an engine part or a malfunction that is reported to the pilot. Other damage can include bearing damage, for example. Therefore, it is important to determine whether a potentially damaging lightning strike occurred.
Lightning is usually seen or heard by the crew. Lightning strikes are often preceded by visual signs such as St. Elmo's fire or discharges. However, it is also possible that a lightning strike occurs unnoticed by the flight crew. Therefore, it is the task of the maintenance crew to conduct appropriate inspections. Typical signs of a lightning strike are impact marks such as burned or melted holes, melted pittings, and flaking and/or elongated burn marks on organic paints and protective coatings. These signs can be expected in certain preferred areas of the aircraft. Additionally, certain parts may be malfunctioning. If there are indications of a lightning strike which may have damaged the engine (see the maintenance guidelines, for example), the engine should be closely inspected and perhaps even disassembled. Extra attention should be given to the main bearings.
Preferred nacelle areas for lightning strikes and how to inspect them
Preferred engine areas for lightning strikes:
Figure "Lightning distribution" (Ref. 5.1.3-5): This diagram is a simplified summary of five studies from the USA, Europe, and the USSR between 1950 and 1975. The results are different from those of a NASA study from the early 80s:
The median lightning strike height was 8.7 km at 32°C. Lightning strikes were most frequent at around -40°C and not at 0°C!
In storm areas, lightning strikes were observed at all temperatures and altitudes. These were usually areas with relatively light turbulence and slight rainfall.
Figure "Typical strike positions": This picture shows the damages, described in the literature, in the area of aeroengines of aircrafts and helicopters. The black arrows (“1”, “3”, “5”) point at the concerned aeroengine, the bolts at the striking point.
Direct effect at the aeroengine: To these belong the deterioration of bearings and gears (Fig. "Lightning damage") from electric arcs by circuit continuity („1” Fig. "Traces" and Fig. "Bearing after lightning strike"). The damage at the below shown helicopter (“6”) can be assigned Example "".
A further very important direct influence is the triggering of a surge in the compressor (Fig. "Typical strike positions", Example "Plasma" and Example "Lightning along the fuselage", Ill. 22.214.171.124-11 and Ill. 126.96.36.199-13) with the flame out of the aeroengine (“7”). Similar effects can be observed during ingestion of hot gas (recirculation, Ill. 188.8.131.52-11 and Ill. 184.108.40.206-12).
Additionally the suspicion exists, that the pressure wave around a lightning channel, caused by the heating of the air, contributes to a surge. This effect can be found at fighter aircrafts if canons or missiles are fired near the intake of the aeroengine (Ill. 220.127.116.11-10 and Ill. 18.104.22.168-11).
Indirect influence of the aeroengine: Fragments of the fuselage/structure from the struck radom “4” get into the aeroengine “5”. The here produced foreign object damage leads to a drop-out of the aeroengine, caused mechanical or by the disturbed flow. By the explosion of an outer tank e.g., tiptank (“2”,“3”) from a fighter airplane, as well dangerous fragments as also an intense pressure wave can develop. Both are able to flame out the aeroengine (Example "No electronic components II").
Figure "Flame out danger" (Ref. 5.1.3-14): An analysis of 40 cases with a lightning strike at 'business jets' with aeroengines at the rear of the fuselage, since 1970 produced interesting tendencies and connections. These airplanes distinguish themself through the feature, that its aeroengines and with this its airintakes are positioned relatively near the fuselage (sketch below, Example "Lightning along the fuselage").
In 20 cases it came to a flame out of aeroengines, which however in most cases could be restarted during flight. In one case both aeroengines flamed out in about 12000 (!) meters hight (example 15.1.3-6).
In a newer case such an aircraft suffered a lightning strike in the night, which caused both aeroengines to flame out. Because of a fast drop of the battery power a restart was no more possible. In this case the aeroengines, different to the Example "Lightning along the fuselage", had no electronic control unit (FADEC).
Also fighter aircrafts are endangered by the extinction of an aeroengine by the effect of hot gas (Figure "Flame out danger"). Here the air intakes are positioned at the sides of the fuselage (sketch above).
In an other statistic of 14 incidents at airliners of different configurations, also a special sensitivity of small airplanes with aeroengines, mounted at the rear of the fuselage, have been determined. There have been also aeroengine drop-outs at big three engined airliners, two engined airplanes and turboprop airplanes.
The mechanism of a lightning strike, which causes the flame out of an aeroengine is explained as follows:
Usually the lightning strikes at an end of the fuselage and then passes along the sides of the fuselage to the other end to exit. Around this path of current the air will be heated up to a plasma condition (aero-thermal effects). Are the aeroengine intakes near the fuselage, as shown at the displayed types, the heated air can be ingested (Example "Plasma" and Example "Lightning along the fuselage"). Such a disturbance of the intake air can trigger a surge in the compressor (Ill. 22.214.171.124-12). The aeroengine can quench because of lack of air/oxygen in the combustion chamber. The reduced amount of air is in the position to trigger overheating of the hot parts during the same or even increasing fuel supply (control unit responds at drop of the rotation speed). Obviously certain aeroengine types with FADEC are especially prone because an unsuitable reaction at the symptoms.
Has an airplane an own to be tilted air stream powered generator (Ram Air Turbine =RAT), this can at least guarantee the electric supply during drop-out of aeroengines, to improve the chance of a controlled emergency landing.
Excerpt: “An actual (lightning) strike occurred during a test flight…about a mile from a thunderstorm. Lightning struck the nose and exited the tail. The nose dome was split open and weather radar antenna destroyed. Several flight test instrumentation transducers also were destroyed. There was no indication that the engines even knew it happened..(the manufacturer assured) that both engines could not be inadvertently shut down simultaneously in flight due to shorted wiring or a lightning strike. Measures to insure that engine controls are isolated from lightning hazards include:
Each electronic engine control consists of two independent computers mounted on a single box on the engine fan housing. The two computers, known as channels, are powered by independent permanent magnet alternators.
Each channel can sense all relevant engine parameters separately and feed such information as fuel flow back to the cockpit.”
Comment: Obviously damaging consequences of a lightning strike can be avoided also at modern airliners with electronic control units of the aeroengines.
If this is also guaranteed during operation influences like corrosion or wear of the cabling (Ill. 19.2.1-1.2) will show the experience.
It seems to be important, that no plasma (hot air) gets into the intake of the aeroengines and triggers a source in the compressor (Ill. 126.96.36.199-12). In the present case, different to the Example "Plasma", this can be ruled out, because the entrance and the exit of the lightning are located sufficient far from the intakes of the aeroengines.
Figure "Lightning damage": Lightning usually strikes the propeller of propeller engines. The lightning then travels through the propeller shaft into the reduction gears (usually planetary gears) and through the housing into the engine suspension. The lightning can take different paths, damaging various components (Figure "Traces"). However, in all cases the charge travels through a shaft bearing (1). The charge may travel directly into the nose housing or enter the housing via the teeth of the planetary gears (6, 7, 8). In this case, the highly sensitive friction bearings (2,4) of the planetary gears are affected.
The charge may also travel from the planetary gears into the sun gear and be directed into the low-pressure shaft. In order to avoid these dangerous electricity transfers and the corresponding risk of parts melting, critical areas should be protected against electrical transmission wherever possible. This can be accomplished through the use of ground cables on non-rotating parts.
Example "Bearing failure by lightning" (Ref. 5.1.3-7):
Excerpt: “…In helicopters, the sharp edges and corners (that are prone to lightning strikes) are mainly on the rotors. In the case in question, the lightning entered and exited through the rotors, and the effects were not considered serious. After the damaged parts were replaced, the helicopter was again cleared for flight.
However, after several flight hours the rotor head had to be replaced due to increasing vibrations. Also, the hydraulic servo-jacks began to leak. Moving parts began to fail one after another, especially various bearings.
This bizarre failure sequence lead to an inspection of the entire machine. It was discovered, that as the current traveled between moving and fixed parts (roller bearings, pistons, tracks, etc.), it left craters of various sizes /Fig. "Bearing after lightning strike"). The strength of the current was evident from the completely magnetized balls in the wobble plate. The craters greatly increased friction and wear, leading to bearing temperatures that were high enough to tarnish the metal. If this unusual wear had not been noticed immediately, total engine failure could have occurred after only a few flight hours.
Even though the damage was seen as minor on the surface, serious damage had occurred. If helicopters are subject to even a small lightning strike, the following flight hours should be monitored very closely. Oil analyses, conducted in short intervals, can help detect bearing damage early.”
Comments: Even though the engine in this case was seemingly unaffected, with helicopters one cannot be certain. In this case, several oil analyses should be conducted at regular intervals in order to detect any metal wear products early (pittings in the bearing tracks and the faces of transmission gear teeth, Fig. "Traces").
Example "No electronic components I" (Ref. 5.1.3-8):
Excerpt: “…after a long-distance navigation flight…during approach the aircraft was struck by lightning, the engine failed and the aircraft crashed on undeveloped land.”
Comments: Concerned are aeroengine failures of an elder single engine fighter airplane. This had not yet electronic components, especially no electronic engine control units. Cases, at which a lightning lead to the failing of an aeroengine are relatively seldom documented.
In the first case, it is unclear whether the lightning strike caused mechanical damage, a compressor stall, or the failure of electrical devices that control the jet nozzles.
In the second case, one can assume that the explosion of the tip tank either caused fragments to enter the engine and mechanically damage the compressor, or whether the pressure wave caused the compressor surge (Figure "Typical strike positions").
Figure "Traces" and Figure "Bearing after lightning strike": The top diagram shows the areas of a propeller planetary gearing that are typically susceptible to damage, when the charge from a lightning strike passes through them (electric continuity).
The bottom diagrams show the typical state of a bearing (5) after a high-energy electrical charge (lightning) has passed through. There are signs of melting on the seat surface of the inner ring as well as spot-shaped melted areas (initial fusings) on the bearing track (not shown). The alignment of the initial fusings at the circumference normally does not correspond to the distances of the rolling elements. The intervals arise from a repeated electric arc formation (Figure "Bearing after lightning strike"). This develops, if the passing of the lightning current from the rolling element to the race is interrupted by a hydrodynamic formed oil film. Collapses this lubrication film in the electric arc, a direct contact of the rolling elements with the race occurs and the electric arc quenches. Then the lubrication film again builds up and the process repeats.
Measurements of the magnetic field strength in the area where the charge passed have a characteristic pattern. If a compass is guided along the circumference, a reversal of poles can be detected (magnetic needle turns 180°). This should be a fairly simple method of verifying, that a strong electric charge passed through a roller bearing, even if there are no obvious melted areas.
Figure "Influence of angle of attack": Electronic devices such as boosters or digital control systems can malfunction and/or sustain permanent damage through overvoltage caused by induced currents or partial currents of the lightning.
One example is the control system for the jet nozzle adjuster (“A”, “B”) in an older fighter aircraft type (Example "No electronic components I"), the jet nozzle of which tended to suddenly open during flight through clouds under certain bad weather conditions. This malfunction was found to be due to moisture entering into the poorly sealed device („A“, Fig. "Leaky electronics casing", Example "Leaky electronics casing", volume 5 Ill. 19.2.1-4), as well as the influence of external electric currents (lightning strikes and charges (Example "Plasma"). Without an afterburner, this resulted in a serious thrust loss and acute danger of crashing (Example "Open jet nozzle I" and Example "Open jet nozzle II").
Excerpt “1” (Ref. 5.1.3-10): ”….during approach… in a storm…crashed into an open field with an open jet nozzle.“
Comments: This case involved a single-jet fighter aircraft. There have been many similar incidents recorded with this aircraft type. The open jet nozzle causes a serious thrust loss (see Fig. "Environmental risks for electronics"). It is not clear, to what degree the electrical malfunction was caused by water incursion or by foreign electrical currents.
Excerpt (Ref. 5.1.3-12): „Most important: When a lightning strike occurs ahead of a turbine engine the resulting Plasma can cause short term power loss or flameouts. It has appeared to a… corporate Jet. The igniters on a jet engine are supposed in the or near thunderstorms to effect relights if plasma, rain or turbulence has flamed out the engine. Apollo Twelve lost all power for seven seconds after a lightning strike that occurred soon after lift off.”
Comment: The plasma of a lightning can obviously already cause the flame out of an aeroengine without hitting itself or influencing it directly with the electric effect. Primarily effective is the hot ingested air (Ref. 5.1.3-14, Fig. "Typical strike positions"). It triggers a surge in the compressor (volume 3 Ill. 188.8.131.52-12, Example "Lightning along the fuselage").
Excerpt: „…The corporate jet, which was acquired…. two years ago, was cruising 33 000 ft when it encountered the thunderstorm and was subsequently struck by lightning, damaging its electrical system. Both engines also failed….The pilots were killed, but their two passengers survived the crash… According to business aviation accident analyst … there has never before been a fatal accident in a corporate aircraft attributed to a lightning strike…“
Comment: A lightning strike at this high altitude seems unusual and shows that lightning strikes can occur above the zone at which weather usually occurs.
Example "Lightning along the fuselage" (Ref. 5.1.3-14): In This case of a business jet an aeroengine dropped out during a lightning strike at the fuselage while descending for landing. 5 up to 10 seconds after the lightning strike, the left aeroengine showed a high turbine temperature. The rotation speed decelerated fast to idle. After this the aeroengine was shut down by the electronic control unit (FADEC). The landing with only one aeroengine succeeded without problems.
As cause a surge in the compressor of the left aeroengine, triggered by the influence of disturbed flow/high temperature (aero-thermal effects) by the lightning was declared (Fig. "Typical strike positions", Fig. "Flame out danger" and Example "Plasma").
Comment: Just at similar airplane types with aeroengines at the rear fuselage, a proneness for a surge after a lightning strike at the fuselage is observed. This is explained with the path of the lightning along the fuselage side (Figure "Typical strike positions"). Not seldom both aeroengines quench. A further problem is the shut down by the FADEC.
Figure "Causes for electronic damage": The increased use of electronic parts in control systems and electrical aggregates directs interest towards influences that are damaging to these parts. This diagram shows a composition:
Electrical currents: Electrical currents can be created through direct contact with (lightning, see Example "Plasma" and Example "High altitude lightning strike") current-carrying engine parts or indirectly through induced currents due to the fluctuating character of lightning currents. This type of currents of around 100V have been measured, which as overvoltage can destroy electronic parts. Even direct currents with relatively low voltages can destroy semiconductors (transistors), depending on the direction of the current (polarity). A further possibility is the erasing and/or destruction of memory chips.
Stronger lightning currents can also damage electric auxiliary components such as generators.
The same is true for creeping currents caused by insufficient insulation and/or insufficient bonding from other current-carrying aggregates such as starter-generators.
Electrical and electromagnetic fields: A case has become known in which the steering gear of a fighter jet flying near transmission towers was strongly affected and caused it to crash. On the other hand, it seems less likely that engine control systems could be affected in this way.
Extreme temperatures: Cooling systems protect electronic systems from extreme temperatures that could damage parts such as processors or memory chips. In some cases fuel is used as a coolant.
Vibrations and accelerations: Special suspensions systems on the engine protect auxiliary components, especially electronic devices, from external acceleration forces that are normal during operation.
In special cases such as containment (released blade fragments) and/or extreme rotor imbalances, overloads and dynamic fatigue fractures can occur.
Moisture and corrosion: If the housings of electronic devices are insufficiently sealed, atmospheric moisture (flight through rain) or condensation (especially during standstill on the ground) can result in water entering the engine. This process might be aided by air pressure changes at various altitudes or through temperature changes in the housings (change in internal pressure). Moisture, which combines with atmospheric impurities (salts in ocean atmosphere) to form a conductive electrolyte, can temporarily impair the function of engine parts (Fig. "Causes for electronic damage"). Over longer periods, corrosion can occur on the conductor paths of the circuit boards or on the soldered connections, for example. It is also possible, that electroconductive corrosion products form and are deposited in other areas.
Damaging media: The suitability of all auxiliary components and materials must be directly or indirectly verified during the certification process; i.e. there must be no unallowable effects from external media. The use of unsuitable auxiliary materials such as aggressive cleaning solutions or of part materials that do not meet the specified requirements can lead to aggressive chemical reactions. Auxiliary materials can have a direct effect if the device fails due to damage or an indirect effect if, for example, a housing seal fails due to the chemical reaction and moisture enters.
5.1.3-1 G. Dietrich, “Die Gefährdung der Flugsicherheit durch Blitzschlag”, TIZL 12 (1976) Volume 3, pages 129-141.
5.1.3-2 W. Hartmann, “Zuverlässigkeit und Systemsicherheit”, periodical “Industrielle Organisation”, 39 (1970) Nr. 2 page 79-81.
5.1.3-3 K. Hoffmann, “Blitzschlag in Flugzeugen”, periodical “Wehrtechnik”, Volume 1/74.
5.1.3-4 J. Marshall, “Lightning Strikes to Aircraft”, periodical “Aviation Mechanics Journal”, April 1989, No 4 pages 32-34.
5.1.3-5 J.K. Bogard, “The Hazard of Hightning”, periodical “The International Journal of Aviation Safety”, September 1984, pages 124-130.
5.1.3-6 “757 Engine Controls Survive Lightning Strike”, periodical “Aviation Week & Space Technology”, August 20, 1984, page 22.
5.1.3-7 “Potz Blitz”, periodical “Flug Revue”, 2/1986 page 71.
5.1.3-8 G.Fischbach, “916 Deutsche Starfighter, ihre Bau- und Lebensgeschichten”, page 348.
5.1.3-9 G.Fischbach, “916 Deutsche Starfighter, ihre Bau- und Lebensgeschichten”, page 449.
5.1.3-10 G.Fischbach, “916 Deutsche Starfighter, ihre Bau- und Lebensgeschichten”, page 229.
5.1.3-11 G.Fischbach, “916 Deutsche Starfighter, ihre Bau- und Lebensgeschichten”, page 256.
5.1.3-12 W. McCormick, M.P. Papadakis, “Aircraft Accident Reconstruction and Litigation”, Lawyers & Judges Publishing Company Inc. ISBN 0-913857-678-19996, page 163
5.1.3-13 L. Burton Ricksk, “Lightning Strike Downs Sabreliner”, newspaper article
5.1.3-14 Civil Aviation Authority United Kingdom, „Aeronautical Information Circular AIC 29/2004”, 29 April.