Recent experimental flights designed to determine probabilities and causes of aircraft lightning strikes have provided some new information related to a pilot's chances of receiving an in-flight encounter with lightning bolts. Although there have been several recent studies, the bulk of the new information comes from two research projects: a USAF/FAA study which involved the use of a Convair 580 specially instrumented transport aircraft which flew for 42 hours and experienced 21 lightning strikes; and from a NASA Storm Hazards Program, which involved the use of a specially instrumented F-106B aircraft which made 1,154 thunderstorm penetrations and received 637 lightning strikes. These studies showed:

1. The majority of strikes (greater than 90 percent) were triggered by the aircraft itself.

2. The probability of an aircraft triggering a lightning discharge in a thunderstorm increased with altitude.

3. The probability of a lightning strike to an aircraft flying in a thunderstorm increased from a minimum at the thunderstorm base to a maximum at the 36,000- to 40,000-foot level. The temperature at this level was from -40 degrees C to 45 degrees C. The strike rate encountered at these high altitudes was two strikes per minute of penetration time. At 18,000 feet, the frequency was one strike every 20 minutes. An average of only one aircraft strike every 3 hours was encountered when flying below active thunderstorms.

4. Lightning strikes at high altitudes generally resulted in greater total charge transfer than strikes at lower altitude; however, the low altitude strikes sometimes produced greater instantaneous discharge.

5. The entire surface of the aircraft may be susceptible to lightning attachment even though strikes are more probable to particular areas such as the aircraft extremities (nose, wingtips, tail) and composite surfaces.

6. During penetration of thunderstorms at low levels, lightning strikes were found to occur in areas of moderate or greater turbulence at the edge of and within large downdrafts. Conversely, lightning strikes experienced in the upper areas of thunderstorms and in the vicinity of decaying thunderstorms most frequently occurred under conditions of little turbulence or precipitation.

It should be remembered that prior to this research it was thought that an aircraft had to fly into the path of naturally occurring lightning to get struck, and the altitudes near the freezing level (0 degrees C) were considered the most probable location for this to happen. The research data, however, seem to conflict with previous statistics.

This does not mean that the old rules do not apply any longer. What it does mean is that we are learning more about the behavior of lightning and its effects on aircraft. Many of the old rules are still valid, and several new rules are being developed which we will be able to apply in the future.

Thunderstorms in Perspective

Actually thunderstorms and lightning are part of a global electric circuit. According to nature's plan to maintain an electric potential between the earth's surface and the ionosphere (called the "fair weather" electric potential), thunderstorms are necessary. They, in fact, play a key role in maintaining the earth's fair weather electric potential. The total number of thunderstorms occurring at any given time around the globe is approximately 2,000. These thunderstorms average about 100 lightning strikes per second. They act as an electric generator, maintaining the electric field. From this perspective, lightning within a thunderstorm cloud helps maintain the earth's electric potential.

Statistics show that commercial pilots experience an average of one lightning strike for every 3,000 flight hours, and the commercial airlines average one hit per aircraft per year. Air Force statistics show a somewhat lesser rate than civilians, but nonetheless the USAF in the past has averaged 51 lightning mishaps per year.

Aircraft Damage Caused by Lightning

Aircraft damage from lightning can be caused as a direct or indirect effect. Direct effects result when the lightning current attaches to and flows through the aircraft skin. Locations on the aircraft where lightning strikes occur experience extreme heating which causes burning and melting damage. Current flowing through the aircraft structure can result in isolated arcing or sparking and heating. If this occurs in a fuel tank, explosion and fire can result.

Indirect effects are caused by transient electrical pulses produced by the changing electric and magnetic fields due to the lightning current. Unless avionics and other systems are properly shielded, they are easily damaged by indirect lightning effects. It is also interesting to note that 57 percent of the mishaps attributed to lightning strikes to aircraft occur during the months of March through July.

Facts and Myths

Combining new research on lightning with the lessons of the past, we can learn the following:

It is true that some aircraft are less prone to lightning strikes. Size, shape, and speed are all aircraft-specific variables which determine an aircraft's susceptibility to a lightning strike. However, it is also true that all aircraft are susceptible to a lightning strike. It is also true that aircraft damage varies with aircraft type. Careful aircraft design can minimize lightning damage. However, all surfaces are susceptible to lightning strikes, and all unprotected systems can be affected.

It is true that some pilots are better at avoiding lightning strikes than others. The wider the berth given to thunderstorms, the better the chance of avoiding a lightning strike; however, the pilot who tries to pick his or her way between thunderstorm cells is asking for trouble.

The theory that if you avoid thunderstorms you will avoid all lightning strikes is false. Statistics show that many triggered strikes have occurred during flights that did not penetrate thunderstorms. Aircraft have triggered strikes in cirrus clouds downwind of previous thunderstorm activity, in cumulus clouds around the periphery of thunderstorms, and even in stratiform clouds and light rain showers not associated with thunderstorms.

The statement that if you are greater than 20 miles from radar-indicated precipitation, you are not susceptible to a lightning strike is also false. Aircraft have been struck by the proverbial "bolt from the blue" on more than one occasion. In fact, aircraft have been struck at distances out to 50 nautical miles from thunderstorms, particularly when cirrus clouds existed above or at their altitude, or when there were other developing showers nearby that had not yet reached maturity. Also, flying through precipitation, volcanic ash, or heavily polluted air can cause an aircraft to experience electrostatic discharge or triggered lightning. Usually these discharges cause only minor aircraft damage; however, there is always the chance for catastrophic damage if the discharge passes through the vaporized fuel-air mixture in the fuel tank.

The belief that lightning strikes to aircraft occur only near the freezing level and are always associated with turbulence and precipitation is false. Thunderstorm penetration studies show that lightning strikes can be encountered at all temperatures and altitudes. In fact, they are most likely to occur in the upper levels of mature or decaying storms near temperatures of -40 degrees C. In addition, the studies showed most strikes occurred in regions where turbulence intensities were light to negligible.

It is true that aircraft flying at altitudes above the freezing level are more likely to be involved with in-cloud or inter-cloud lightning flashes, and that aircraft flying at altitudes below the freezing level are more likely to be involved with a cloud-to-ground lightning event. It is also true that the more frequently a thunderstorm is flashing, the lower the probability of being struck by lightning if the aircraft flies into the storm. However, the greater the flash rate, the higher the potential for severe turbulence, heavy rain, and hail. Therefore, this information should in no way be interpreted as a reason to fly in or near any thunderstorm.

Some Rules to Fly By

  The most important thing is to stay clear of thunderstorms. Do not attempt to "pick your way through"; deviate around the area on the upwind (non-anvil) side if possible.

  The higher the aircraft altitude, the farther away from a thunderstorm you should fly. Lightning strikes have been known to occur in the clear air up to 50 miles downwind from the nearest thunderstorm.

  At low levels, avoid flying close to high surface features (ridge tops, towers, etc.), or between such features and an overhead thunderstorm.

  If you fly above the freezing level in or near thunderstorms, you can trigger an in-cloud or cloud-to-cloud discharge. If you fly below the freezing level, you could be involved with a cloud-to-ground lightning strike. Overall, if you must penetrate or fly close to a thunderstorm system, you can expect more strikes penetrating a thunderstorm area well above the freezing level.

  Lightning damage is usually worse for large total current transfers. At altitudes above the freezing level, you are more likely to experience longer-lasting lightning attachments made up of numerous small pulses and a large total current transfer. Below the freezing level, you are more likely to experience shorter lightning attachments with a few strong current pulses; however, the total current transfer is usually less than that above the freezing level.

  Electrical activity generated by a thunderstorm may exist even after the thunderstorm cell has decayed; therefore, avoid penetrating the cirrus decks that were recently associated with thunderstorms.