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An atmospheric discharge of electricity.
Lightning occurs as a result of a build up of static charges within a Cumulonimbus (Cb) cloud, often associated with the vertical movement and collision of ice particles (Hail), which result in a negative charge at the base of the cloud and a positive charge at the top of the cloud. Beneath the cloud, a "shadow" positive charge is created on the ground and, as the charge builds, eventually a circuit is created and discharges takes place between the cloud and the ground, or between the cloud and another cloud. An aircraft passing close to an area of charge can initiate a discharge and this may occur some distance from a Thunderstorm.
Lightning strikes on aircraft commonly occur within 5,000 feet of the freezing level.
Lightning is accompanied by a brilliant flash of light and often by the smell of burning, as well as noise. A lightning strike can be very distressing to passengers (and crew!) but physical damage to an aircraft is rare and is not likely to threaten the safety of the aircraft. Damage is usually confined to aerials, compasses, avionics, and the burning of small holes in the fuselage. Of greater concern is the potential for the transient airflow disturbance associated with lightning to cause engine shutdown on both Full Authority Digital Engine Control (FADEC) and non-FADEC engines with close-spaced engine pairs.
Lightning also occurs within Volcanic Ash clouds, also because of the vertical movement and collision between particles within the cloud which generates static charge.
- Aircraft Damage. Structural damage to aircraft from Lightning strikes is rare and even more rarely of a nature that threatens the safety of the aircraft. Nevertheless, there have been many incidents of lightning strikes leaving puncture holes in the radomes and tail fins of aircraft (entry and exit holes) and damage to control mechanisms and surfaces (see Further Reading).
- Crew Incapacitation. Momentary blindness from the lightning flash, especially at night, is not uncommon.
- Interference with Avionics. A lightning strike can effect avionics systems, particularly compasses.
- Engine Shutdown. Transient airflow disturbance associated with lightning to cause engine shutdown on both Full Authority Digital Engine Control (FADEC) and non-FADEC engines with close-spaced engine pairs.
- Avoidance. Standard advice to pilots is to remain 20 nautical miles displaced from any Cumulonimbus (Cb) cloud. The dangers from Turbulence, Low Level Wind Shear, and In-Flight Icing associated with Cumulonimbus clouds are far greater than the threat of Lightning.
An aircraft flying in the vicinity of a cumulonimbus cloud is hit by lightning. Cabin crew see a football sized ball of fire move slowly down the length of the cabin before dissappearing. The flight deck crew notice a discrepancy between the standby compass and the flight management system. After landing a small hole, a few millimetres in diameter, is found in the radome.
- If flying close to cumulonimbus clouds, or lightning is seen close to the aircraft, then review manufacturers guidelines for action to be taken in the event of a lightning strike. If the aircraft is equipped with gyro-magnetic compasses, consider selecting one of the compasses to gyro while there is a risk of lightning.