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Lightning

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Revision as of 10:17, 24 December 2012 by Editor1 (talk | contribs)
Article Information
Category: Weather Weather
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Tag(s) Weather Phenomena

Definition

An atmospheric discharge of electricity.

Description

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 in Volcanic Ash clouds formed in the immediate vicinity of eruptionso 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.

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 to cause engine shutdown on both FADEC and non-FADEC engines with close-spaced engine pairs.

Defences

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 deck crew notice a discrepancy between the standby compass and the flight management system. After landing a small hole, a few milimetres in diameter, is found in the radome.

Solutions

  • 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.

Related Articles

Accident & Incident Reports Including Lightning as a Factor

  • 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.)
  • SU95, Moscow Sheremetyevo Russia, 2019 (On 5 May 2019, a Sukhoi RRJ-95B making a manually-flown return to Moscow Sheremetyevo after a lightning strike caused a major electrical systems failure soon after departure made a mismanaged landing which featured a sequence of three hard bounces of increasing severity. The third of these occurred with the landing gear already collapsed and structural damage and a consequential fuel-fed fire followed as the aircraft veered off the runway at speed. The subsequent evacuation was only partly successful and 41 of the 73 occupants were killed and 3 sustained serious injury. An Interim Report has been published.)
  • 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.)
  • 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.)

... further results