01. Introduction
Lightning is probably the least understood phenomenon encountered by flight crews. Even the most experienced pilot will find his hands tightening on the controls when an unexpected flash of lightning illuminates his night-time cockpit with daylight brilliance. Other electrical disturbances, like St. Elmo’s Fire and precipitation static, while less spectacular, also alert the crew to the electrified air they’re flying through.
This article will explain the nature and cause of lightning, and some of its variations. It will show why lightning strikes an aircraft, and where one can look for the strike. It will mention some of the ways lighting can affect operations, and more important, what one can do about it.
02. Nature and Cause of Lightning
The invention of the high-speed camera was one of the greatest break-throughs in understanding the nature of lightning. Using this camera, scientists found that a lightning stroke from a cloud to the ground is not simply a great spark of electricity suddenly leaping across the gap between two electrical charge centers. Instead, that sudden flash is a complex sequence of events.
Photographs show that the first visible evidence of a lightning flash is in the form of a “stepped ladder”, a faintly luminous trail of ionized gas which reaches downward from the cloud in distinct spurts or “steps” about 150 ft long. The ladder twists, turns and divides as it encounters vague “walls” of higher resistance in the atmosphere. As this ladder carries the cloud’s charge center nearer and nearer to the ground (or other electrical charge center), it violently speeds up ionization in the surrounding air. This causes luminous ribbons, similar to the stepped ladder, to grow from the ground toward the cloud. These are called positive streamers. Once the streamers contact the stepped ladder, a conductive path is established between the two charge centers. This entire process usually takes only a few thousandths of a second.
When the ionized path is completed, there is a tremendous surge of electrons down the path, creating the brilliant flash and explosive sound associated with the lightning bolt. This rush of electrons quickly drains the charge from one portion of the cloud. Now a new stepped ladder may form from a nearby charge area to the depleted area and, if the original ionized path is still intact, another surge of electrons will occur.
Three strokes per lightning bolt is average, but as many as 14 have occurred along the path established by the stepped ladder. It is this multiple flash process which creates the impression of flickering in large electrical storms.
There is considerable variation in the distances between charge centers, but the distance is expressed in miles – not feet or thousands of feet – vast dimensions when compared to even the largest aircraft. While one does not intentionally fly inside thunderclouds, one frequently can intercept lightning strokes between nearby clouds.
03. Lightning Variations
The short discussion on the nature and cause of lightning wasn’t intended to make the reader an expert on the subject, but to give him a basic understanding of the mechanism of lightning. The cloud-to-ground example is used for illustration. But there are several variations between nearby clouds. (See illustration.)
Cloud to cloud flashes
These flashes make up the majority of all lightning strokes, and are by far the most troublesome to aircraft. They involve negative and positive charge centers that flash from cloud to cloud, cloud to upper air, or flash within the cloud. And there are comparatively rare strokes that wander for many miles before they dissipate in the air without reaching a target.
Lightning flashes which do not touch the earth are particularly prevalent in arid regions where the bases of the thunderclouds are much higher than they are in moist or cold areas. Lightning would rather flash from one cloud to another than travel the greater distance to the ground.
Bolts from the blue
This rare phenomenon is also found in arid regions where cloud bases are high. The name comes from the observer seeing a lightning bolt in locally clear skies, while the cloud that spawned the bolt is out of view many miles from the observer. These rarely hit the ground, but rather dissipate in the air, sometimes up to 40 miles from their source.
Sheet lightning
This term describes flashes within a cloud that are generally hidden from view, so all that can be seen is a diffused general illumination which lights up portions of the cloud. Often sheet lightning will light up the whole cloud as two oppositely charged regions at the top and the base of the cloud come together and neutralize one another.
Ribbon lightning
This term is applied to a flash which appears to be several strokes closely paralleling each other. Actually this is the result of the ionized path of a multiple stroke flash being blown sideways by the wind. An example would be a six-stroke lightning flash of a half-second total duration. Each of the strokes would be displaced about four feet in a 30-mile wind.
Glow discharge from cloud tops
Probably several pilots have seen the tops of clouds glow as they flew over a thunderstorm area. This is similar to St. Elmo’s Fire, where positively charged upper regions of the thundercloud tend to draw in electrons from the upper air.
Thunder
This is an explosion that occurs along the length of a lightning channel. The flood of electrons heats the air in its path so suddenly (raising the temperature of the channel at about 15,000°C almost instantaneously) that the air expands in all directions faster than the speed of sound. The rumble heard is due to the distance between the listener and the various parts of the lightning channel.
Ozone
This faintly bluish gas with its characteristic pungent odor is often smelled in the vicinity of a lightning strike. It is caused by the ionization of oxygen molecules. Ozone is unstable and readily reverts to normal oxygen.
Ball lightning
This phenomenon is the most unusual encountered, and usually leaves startled observers in its wake. Reports agree that the balls are luminous, but descriptions of them include every size and color. Fireballs may disappear quietly, or with a loud bang. If one is touched by it, one may be burned. The smell of Ozone is sometimes present. The lightning ball can last several seconds or several minutes. During that time it may float across the floor, glide down a tree or a ship’s mast, and appear to negotiate its way around obstacles.
There is an authenticated report of a lightning ball originating on the flight deck of a C-121 after a lightning strike. It proceeded down the aisle between the seats, and then disappeared in the rear of the cabin.
All these variations have one thing in common. They all involve at least one cloud charge center, pinpointing the cloud as the prime instigator of all lightning flashes.
04. Why Are Aircraft Struck?
The best answer to that question is that the aircraft is struck only when it happens to be in, or very near (within one wing span of), the natural path of a lightning bolt. But there are other theories.
The most prevalent is that an airplane is a better conductor than the surrounding air, and it shortens the distance between cloud charge centers, and triggers the discharge stroke. On this subject Newman and Robb of The Lightning and Transient Research Institute, say, “Little can be done in the immediate vicinity of an aircraft to control whether or not it is struck by lightning, as the aircraft, which is relatively small, can have little effect in determining the overall stroke path, which may extend several miles”.
Still another theory is that the airplane becomes charged by friction or passage through a charged portion of the atmosphere. Then, as it approaches another charge center, it attracts a stroke to itself, or discharges itself to the cloud center. However, the charge the aircraft is capable of carrying is so small compared to the cloud, that it could hardly be considered an important factor. About the only thing that can be said for this theory is that the airplane’s charge might conceivably be useful to an already existing stepped ladder.
The common opinion is that airplanes which fly near thunderstorms will be struck only if they happen to pass near the natural stroke path at the time of its occurrence.
So, if one flies near a thunderstorm one can expect lightning strikes.
When lightning does strike an aircraft, damage is usually slight. Injury to occupants is quite rare, and there are no known reports of anyone being killed by lightning in a metal airplane. In many cases, crews did not know they had been struck until damage was discovered after landing.
If the aircraft were a perfect, uninterrupted metal shell with no conductors, such as antennas leading inside, it is unlikely a dangerous current would ever be transmitted inside. Damage would be limited to pitting and an occasional puncture of the skin.
One factor which determines the extent of lightning damage – the conducting quality of the aircraft – is controllable, but only as far as design and function will allow. The usual approach to the problem is to determine the points which are most likely to be struck by lightning. Generally, these points are the nose, tail section and wing tips.
Aircraft manufacturers can have metal scale models of new designs subjected to artificial lightning from several points. Once the probable points of lightning strikes are known, detailed studies are made to determine the best protection.
However, aircraft cannot be made perfect metal shells. The extremities are fitted with non-metal radomes; antennas project beyond the surfaces and offer pointed projections which are vulnerable because of their shape, their extreme location, and their attachment to the shell. As might be expected, these are the components most often damaged by strikes.
Radomes
Non-conductive shells such as radomes present design problems and get particular attention since they receive many lightning strikes. Their susceptibility stems principally from their non-conductivity and their location. Primarily antenna shields, their streamlining is an important factor in aircraft flight characteristics. If a radome is damaged, debris carried aft by the slip stream can end up in an intake doing additional damage.
If it seems odd that a non-conductive radome should be struck at all, one should remember that the radar dish and other components under the radome send out electrical streamers, which might attract an approaching stepped ladder. The radome is heated intensely by a lightning stroke and though damage can be limited to pitting or small punctures, heat expansion of the material can cause large holes or even destruction of the radome.
Since relocating the radome is usually impractical the only alternative is to divert any lightning strokes to the skin by a chosen path via a conductor built into the radome. There are two types of diverters – the permanent and the consumable. The consumable are usually in the form of braided wire or thin metallic strips cemented to the radome and bonded to the metal skin. Not designed to survive a lightning strike, they divert the strike to an area where damage is less likely. Obviously, if the radome suffers a second strike, part of the protection will be missing, but the protective zones are made to overlap.
An example of permanent protection is found on the Navy P-3 anti-submarine aircraft. The long housing on the aft of the fuselage contains a faraday cage that spreads the current and protects the aircraft.
Radar as an attractor of strikes
There has been much speculation as to whether or not radar can attract a lightning strike. Scientists tell that if the power of to-day’s radar should be increased by a factor of 10, the possibility might exist. This is particularly true in the 400,000-foot range, since the problem increases with altitude. Ionization which attracts lightning discharges could be significant with very high power radars, high HF-antenna voltages, and high altitudes.
Antennas
An antenna can be instrumental in leading lightning into the cabin where it may damage equipment and expose personnel to hazardous voltage. To combat this problem, lightning arresters have been developed. These usually include a spark gap between antenna and aircraft structure, a blocking condenser, and a static leak resistor, all encased in metal or glass enclosures. They are located in the antenna lead-in adjacent to the aircraft skin.
They are not intended to protect the external antenna, but simply provide a weak spot in the antenna system which will break down and carry off the destructive peak voltage. Not all antennas have arrestors, only those that are likely targets due to location and shape.
Control surfaces
Rudders and elevators are prime lightning targets because of their location, and are susceptible to damage because they are necessarily hinged and made of light gage material. Bonding jumpers are provided between the main structure and the movable portions to provide paths of low resistance in parallel with the hinges so that a large portion of the current is safely bridged.
Static dischargers
These are commonly fixed to the trailing edge of a control surface, and aid in dispersing friction charges which build up on the skin of the aircraft. They consist usually of metal rods of a specific form, coated with a conductive material. After a lightning strike, they should be inspected and replaced as necessary. An interesting note is that aircraft skin friction can generate moderately high voltages of about 100,000 volts and 0.0005 amperes. However, these values hardly compare with a “natural” lightning discharge of 10 to 100 million volts and 100,000 amperes.
05. Effects on Air Operations
Lightning strikes and associated electrical phenomena are generally considered more of a nuisance than a hazard to air operations, especially when compared to turbulence and icing, which are the primary dangers from thunderstorms. Only rarely is lightning damage serious.
Precipitation static is very annoying, and its interference with electrical gear can cause serious navigation and communication difficulties. This electrical problem increases with size and speed of the aircraft.
St. Elmo’s fire is visible evidence of electrical discharge at a relatively slow rate, and does not pose a problem. In fact, it serves to alert crew that the environment is electrified, and that lightning strikes may occur.
There has been considerable discussion if a lightning strike could cause an explosion in aircraft fuel tanks. Lightning, it is supposed, could strike the fuel tank vents, ignite the vapours, and then flame back to the tank. This has yet to be proved, however, but it is considered possible under very extreme circumstances.
While lightning has damaged aircraft, by itself it seldom hinders air operations. However, the other weather phenomena associated with lightning, if severe enough, can slow one down considerably.
06. What Can Flight Crew Do?
Lightning strikes can be quite troublesome to flight and maintenance crews alike. Ferrous metals may become magnetized, and compass readings can be rendered unreliable.
Being aware going into an area where lightning is possible, one should make sure that the gyro-compass is accurate. Gyros are not so prone to malfunction as a magnetic compass if a strike occurs, and should provide an accurate heading.
Secondary currents are often induced by near passage of a lightning bolt, and these can actuate electrical circuits inadvertently. Cases have been reported in which passenger emergency oxygen masks have dropped from their containers, tip tanks have been jettisoned, and armament pylons have been dropped. Another item to check after a lightning strike is each electrically discharged fire extinguisher.
Secondary currents sometimes work through audio circuits to produce deafening crash static in the headset. Many pilots like to unseat the earphones a little for self-protection.
There have been many instances of crews being temporarily blinded by lightning strikes while flying at night, and such blindness can persist for several minutes. One precaution is to turn up the cockpit lights to full bright, or turn on the white thunderstorm lights and put on dark glasses. Keep eyes on the instruments when lightning is in the air. Of course the crew shouldn’t restrict their vision this way, but if one pilot does, the loss in observation will be well repaid by the protection from temporary blindness. Remember too, that a lightning strike can disengage the auto-pilot and if that happens unnoticed, one may find himself in other than straight and level flight.
The golden rule to avoid lightning is not to fly in or near thunderstorms. Avoiding lightning then goes hand-in-hand with avoiding turbulence, icing and hail. If one does find himself in the middle of a thunderstorm, it may help to know these facts:
- aircraft are more likely to be struck below 20,000 feet;
- most strikes occur in the 5000 to 15,000-foot level. Strike probability almost certainly coincides with the altitude range of the cumulo nimbus cloud which is commonly found between 3000 and 30,000 feet, occasionally higher.
Another method of avoiding strikes near thunderstorms is to keep a watchful eye on the ambient temperature. About 80% of the strikes occur in the -10°C to +10°C temperature range. A good precaution is to avoid flight at or near the freezing level. Also remember, temperatures inside and outside clouds can vary a great deal.
Summing up it can be noted that since the day Benjamin Franklin flew a kite in a thunderstorm and got his first feel for electricity, man has learned a great deal about lightning and associated phenomena. Man has learned how to take precautions against the hazard. But man is a long way from controlling it. These facts dictate that, as in all air operations, one must use his training, good judgement and common sense to fly safely.