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Lightning is an atmospheric discharge of electricity. A lightning strike can be very distressing to passengers and crew but damage to an aircraft in flight which is sufficient to compromise the safety of the aircraft is rare.
Lightning occurs as a result of a build up of static charges within a Cumulonimbus 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 significant physical damage to an aircraft is rare and the safety of an aircraft in flight is not usually affected. 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 FADEC and non-FADEC engines with close-spaced engine pairs.
Lightning may also occur in Volcanic Ash clouds formed in the immediate vicinity of eruptions because the vertical movement and collision between solid particles within the cloud generates static charges.
The Following map shows the uneven distribution of lightning strikes across the globe. The data is from space-based sensors.
- 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 may cause engine shutdown on both FADEC and non-FADEC engines on aircraft with close-spaced engine pairs.
- Avoidance. Standard advice to pilots is to remain 20 nautical miles displaced from any Cumulonimbus cloud. The dangers from Turbulence, Wind Shear, and 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 crew notice a discrepancy between the standby compass and the flight management system. After landing a small entry hole, a few milimetres in diameter, is found in the radome and an exit hole elsewhere.
- If flying in the vicinity of 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, it may be recommended that one of the compasses is selected to gyro while there is a risk of lightning.
Accident & Incident Reports Including Lightning as a Factor
- B752, Girona Spain, 1999 (WX RE HF) (On 14th September 1999, a Britannia Airways Boeing 757 crash landed and departed the runway after a continued unstabilised approach in bad weather to Girona airport, Spain.)
- D228, Bodo Norway, 2003 (WX LOC) (On 4th December 2003, a Dornier-228 approaching Bodo, Norway, was struck by Lightning and suffered damage to the elevator control. The crew were temporarily blinded and momentarily lost control of the aircraft but managed to crash land just short of the runway threshold.)
- AS3B, en route, northern North Sea UK, 2008 (LOC WX AW HF) (On 22 February 2008, a Eurocopter AS332 L2 Super Puma flying from an offshore oil platform to Aberdeen was struck by lightning. There was no apparent consequence and so, although this event required a landing as soon as possible, the commander decided to continue the remaining 165nm to the planned destination which was achieved uneventfully. Main rotor blade damage including some beyond repairable limits was subsequently discovered. The Investigation noted evidence indicating that this helicopter type had a relatively high propensity to sustain lightning strikes but noted that, despite the risk of damage, there was currently no adverse safety trend.)