Lightning by itself is usually not considered to be a force that can crash airplanes. The last confirmed US crash that was concluded to be caused by lightning was PAN AM Flight 214 in 1967, which crashed due to a lightning strike that hit one of the fuel tanks, causing an in-flight explosion. So with that in mind, what are the causes and effects of triggered lightning?
Since that accident, tools to deal with lightning have significantly improved within aviation, and it is no longer considered a massive threat to most aircraft.
However, lightning can still cause damage, disrupt systems, cause engine flame-outs, and in the case of helicopters cause catastrophic damage if the main or tail rotor gets critically damaged.
So what exactly happens when lightning hits an aircraft, and what is the phenoma where lighting is actually caused by the airframe rather than the atmosphere by itself?
This is called triggered lightning and does present a huge threat to, for instance, the UK offshore helicopter industry. It is not limited to just offshore though, let’s talk about it.
THE SOURCE OF LIGHTNING
So let’s talk about lightning by itself first. How does it work? It all starts when tiny droplets form inside a cloud. Strong updrafts blow these to the top of the cloud where they turn to ice or even grow into hail. If they are too heavy to be propelled up they will start falling again to a lower portion of the cloud, bumping in to other particles as they get thrown around the cloud.
During these collisions within the cloud, electrons are transferred to the moving hail, giving it a negative charge, while the other ice particles that have given this electron away gain positive charge. When the updrafts continue to carry the lighter ice particles (with a positive charge) upwards, the top of the cloud gains a positive charge.
The hail then continues to fall to the bottom part of the cloud, giving it a negative charge. This has now created two electrically charged areas that look like this:
If the attraction between the two areas is strong enough, the electricity will try to equalise the charge and make a path between the two, in the form of lightning.
This can happen within the cloud, or also between the cloud and the ground. The ground will become positively charged as the electrons near the ground are transferred to the bottom of the cloud.
Again, if the electric charge difference becomes too strong, lightning between the cloud and the ground will be the result. This is initiated by a charged particle called a ‘step leader’ or ‘leader’, and will carve the path for other particles to follow from one charge to another.
SO WHAT ABOUT TRIGGERED LIGHTNING?
So we know that ‘conventional’ lightning is associated with CB’s as an interaction between positively and negatively charged particles within a cloud or between a cloud and the ground. However, triggered lightning is where the aircraft itself acts as a trigger for lightning to discharge.
It has been discovered that both fixed wing and helicopters get a strong negative charge when flying through the air because of the static electricity buildup due to friction with the air.
Usually this type of charge is discharged back into the atmosphere or to the ground upon landing, but if the aircraft gets closer to the positively charged bottom half of a cloud as discussed above, the positive charge of the cloud and the negative charge of the aircraft will try to interact with each other to equalise the charge in the form of lightning.
The potential for triggered lightning is the highest when an aircraft flies close to:
- A positively charged part of a CB
- A positively charged anvil of a CB
- A rapid change of electrical charge within a cloud
Most triggered strikes are positively charge, and they tend to originate from portions of a cloud that are close to the 0°C isotherm.
This is because the rate of change from liquid to frozen is the highest here, which adds to the amount of electron separation and therefore electrical charge changes.
While, again, most lightning strikes do not result in fatalities, the effects can be severe, especially for helicopters. On the fixed-wing side, there was one fairly recent case where an Embraer 145 was passing FL70 during a descent into Manchester, where a lightning strike actually caused an engine flame-out.
After the PAN AM flight discussed above, a survey was issued covering over 40 aircraft lightning strike events. Out of the 40 instances, 20 had resulted in engine flame-outs. Even 1 fatal crashed was caused by a a dual engine flame-out, the report can be found here.
There is a correlations between dual flame-outs and the distance between the 2 engines, where narrow-bodied aircraft (and therefore helicopters) suffered more dual engine flame-outs during 1 strike, then wide body aircraft types. The CAA concluded here:
There is a potential hazard of a lightning strike affecting both intake airflows on narrow-bodied aircraft equipped with FADEC controlled, fuselage mounted engines. With the associated potential for a double engine flame-out.
Engines without FADEC run the risk of over-temperature conditions during lightning strikes, but since shutdown will be in full control of the pilots, lightning induced flame-outs are not likely.
On the helicopter side of things, the most relevant accident occurred in 1995 on Bristow flight 56C. An AS332L Super Puma was in flight from Aberdeen to the Brae Alpha Oil Production Platform, flying 16 maintenance engineers.
As it began its descent from 3000’, the helicopter was struck by lightning. Initially this caused severe vibrations followed by, a few minutes later, loss of tail rotor control, resulting in an immediate ditching in heavy seas.
The cause of the loss of tail rotor control was confirmed to be caused by lightning damage. Significant damage and mass loss occurred at the tail rotor as one of the carbon composite tail rotor blades got hit with a strike that exceeded its lightning protection capability, the report can be found here.
Another interesting accident report features a Sikorsky S-76A+ that suffered damage as a result of a lightning strike, the report can be found here.
Other reports where less sever than this, where it has been concluded lightning tends to enter and exits the helicopter fuselage via the main and tail rotor, but typically leaves damage in the electrical components, avionics and other flight sensors, with nav equipment magnetised and therefore unusable.
As time progressed, numerous organisations have become significantly more capable of forecasting triggered lightning. The UK Met Office has developed a model that can predict the timing and location of 80% of helicopter triggered lightning strikes and to this day is heavily relied upon by various helicopter operators.
According to the Met Office Model, Triggered lightning is most likely between October and April, with temperature bands between -2°C and +2°C, between 2000 and 3000’, with a freezing level above 750 ft.
To show how important the temperature band can be, have a look in this table provided by the Royal Meteorological Society:
For helicopter triggered lightning, the North Sea area is one of the few places in the world where the conditions are so enabling for substantial periods, together with sections of sea near Japan. Between the period of 1991 and 2010, 34 lightning strikes on offshore helicopters have been reported, which is just under 2 strikes every year.
To download the entire research paper, please find it here:
Triggered lightning is a bigger threat for helicopters than for fixed wing. While lightning can strike any aircraft, aircraft with engines closer to each other are more susceptible to a dual engine flame-out.
The North Sea area’s conditions enable the potential for triggered lightning a lot more than other areas in the world, making it even more important to use proper planning tools to assess the risk levels involved for flights, especially from October to April.
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