Lightning strikes to aircraft
Answer: C
Airliners are struck roughly once a year on average and are designed to conduct the current over the skin. Bonding and static discharge wicks are part of the standard design. Certification requires demonstrated lightning tolerance.
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Assertion (A): An aircraft usually suffers little damage from a lightning strike.
Reason (R): The conducting skin carries the current around the occupants and the systems.
Answer: D
Both are true and the reason gives the protection, and this is an application of the Faraday cage principle. Composite airframes require conducting mesh to achieve the same effect. Metal skins provide the shielding naturally.
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All of the following are elements of aircraft lightning protection EXCEPT
Answer: A
Correct answer: restricting flight to below the freezing level
Flying below the freezing level would neither avoid lightning - strikes occur there, and aircraft frequently trigger their own - nor be operationally possible. Protection instead accepts the strike and manages it: a continuous conducting path across the airframe so the current passes over the skin and out, conducting mesh in composite panels which are not naturally conductive, and static wicks to bleed off accumulated charge.
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A Faraday cage protects its contents by
Answer: B
Charge redistributes on the conducting surface and the interior field remains near zero. A car body acts in the same way, which is why a vehicle is a reasonably safe place in a storm. The shielding comes from the shell, not the tyres.
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State whether the following is true or false.
A car offers reasonable protection from lightning because of its metal body rather than its rubber tyres.
Answer: C
The tyres are irrelevant to a discharge that has already crossed kilometres of air. The conducting shell carries the current around the occupants. Rubber cannot insulate against such a discharge. The tyres play no part in the protection at all.
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The charge structure of a winter thunderstorm often differs from a summer one because
Answer: D
The lower freezing level brings the charging region down towards the surface. Winter storms in coastal Japan are noted for producing unusually energetic positive flashes. The low freezing level brings the charge centres down.
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Assertion (A): Winter thunderstorms can produce unusually powerful positive flashes.
Reason (R): Their charge centres lie low, bringing the upper positive region within reach of the ground.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason explains it. Winter thunderclouds are shallow and their charge centres sit low, so the upper positive region is close enough to the ground for a positive flash to reach it. Positive flashes carry far greater charge and continuing current than negative ones, which is why winter lightning in Japan and Scandinavia is disproportionately damaging to wind turbines and transmission lines.
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Corona discharge from vegetation during a storm
Answer: B
Correct answer: carries a current between the ground and the air
Corona from grass, trees and crops in the strong field beneath a storm passes a small current continuously from the ground into the air, and integrated over the area beneath a storm it forms a significant part of the electrical budget of the global circuit. It occurs before and during the storm rather than after a strike, and it does not prevent lightning - it merely bleeds off a fraction of the charge.
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Three of the following require supercooled liquid water. Which is the odd one out?
Answer: C
Correct answer: the fair weather conduction current
The fair weather conduction current flows in clear air far from any cloud and involves no water at all, supercooled or otherwise; it is carried by ions produced by cosmic rays and natural radioactivity. The other three all require supercooled liquid water: riming builds graupel by freezing droplets onto it, aircraft icing is the same process on an airframe, and non-inductive charging occurs only in its presence.
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Charge in a thundercloud is carried principally by
Answer: D
Ice crystals, graupel and droplets carry the charge and are transported by air motion and gravity. Free ion mobility is far too low to build the charge centres observed. Bulk transport of charged particles does the work.
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