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Lightning

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Category: Weather Weather
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Tag(s) Weather Phenomena

Definition

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.

Description

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.

Map showing the uneven distribution of lightning strikes across the globe
Map showing the uneven distribution of lightning strikes across the globe. Image courtesy of NASA (NSSTC Lightning Team)

Effects

  • 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. see separate article on Lightning Strike Risk to Engines.

Defences

  • 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.

Typical Scenarios

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.

Solutions

  • 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.

Related Articles

Further Reading

Accident & Incident Reports Including Lightning as a Factor

  • D228, vicinity Bodø Norway, 2003 (On 4 December 2003, the crew of a Dornier 228 approaching Bodø lost control of their aircraft after a lightning strike which temporarily blinded both pilots and damaged the aircraft such that the elevator was uncontrollable. After regaining partial pitch control using pitch trim, a second attempt at a landing resulted in a semi-controlled crash which seriously injured both pilots and damaged the aircraft beyond repair. The Investigation concluded that the energy in the lightning had probably exceeded certification resilience requirements and that up to 30% of the bonding wiring in the tail may have been defective before lightning struck.)
  • AS3B, en-route, northern North Sea UK, 2008 (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.)
  • SB20, vicinity Sumburgh, UK 2014 (On 15 December 2014, a 'MAYDAY' was declared by a Saab 2000 after a lightning strike at night. The aircraft then climbed to 4000 feet before descending rapidly at up to 9500 fpm and reaching 330 KIAS - 80 knots above Vmo - until EGPWS 'SINK RATE' and 'PULL UP' Warnings were annunciated and a recovery climb was commenced. The minimum altitude reached was 1100 feet. Initial investigation has concluded that the lightning strike caused only minor damage and notes that crew flight control inputs following it suggested that they believed the AP had disconnected when actually it had not.)
  • B752, Girona Spain, 1999 (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.)
  • E145, vicinity Manchester UK, 2001 (On 25 September 2001, an Embraer 145 in descent to Manchester sustained a low power lightning strike which was followed, within a few seconds, by the left engine stopping without failure annunciation. A successful single engine landing followed. The Investigation concluded that the cause of failure of the FADEC-controlled AE3007 engine (which has no surge recovery logic) was the aero-thermal effects of the strike to which all aircraft with relatively small diameter fuselages and close mounted engines are vulnerable. It was considered that there was a risk of simultaneous double engine flameout in such circumstances which was impossible to quantify.)