Static electricity in aircraft:
As mentioned earlier, the effects of static electricity are of considerable importance in the design of aircraft and aircraft equipment. An aircraft in flight picks up static charges as it flies through rain, cloud, snow, dust and other particles in the atmosphere. This build-up of statics is referred to as precipitation static.
The amount of charge that builds up in any particular part of the aircraft depends on the atmospheric conditions to which it is subjected, and the material of which it is made. If two adjacent pieces of material are able to build up charges at different rates, a potential difference will exist between them. Eventually the potential difference will be sufficient to break down the insulation and current will jump as a spark between the 2 materials. This spark creates numerous problems; it damages the materials, it causes corrosion, it radiates radio frequencies that interfere with radio and navigation equipment and it could ignite fuel or oil vapour. In order to prevent this happening, it is essential that all of the aircraft structure and equipment is interconnected or bonded. Bonding allows small currents to continuously flow between materials and equipment, thereby preventing the build up of large static charges.
An aircraft often accumulates very high electric charges, not only from precipitation but also from the high velocity gases exiting the engine exhausts. When the charge is sufficiently large, it will start to dissipate into the surrounding atmosphere from any sharp or pointed parts of the aircraft, such as the trailing edges of aerofoil sections. The point at which this occurs is called the corona threshold. The corona discharge produces severe radio interference and needs to be controlled. This is achieved using special devices called wicks, that allow the charge to dissipate in a controlled manner from specific points on the aircraft so that it causes minimum interference.
The subject of static electricity can be considered amusing or annoying when one suffers from its effects. However, it must be taken very seriously by aircraft maintenance engineers. The following are a few points to consider.
• It essential to maintain the integrity of bonding when carrying out any maintenance work on aircraft.
• You can build up a charge on yourself as you move and work around the aircraft. Much of the equipment in modern aircraft is electronic, and can easily be destroyed by you discharging static through it.
• When an aircraft is refuelled, is the refuel vehicle at the same potential as the aircraft. If it isn't, then it could be possible for a spark to ignite fuel vapour as the fuel nozzle comes into close proximity with the aircraft. It is essential that the two vehicles are interconnected electrically before any hoses or fillers are opened.
• An aircraft in flight can have a potential several thousand volts higher than the ground. This charge is dissipated through the tyres or special straps on the undercarriage when the aircraft lands.
• When an aircraft is inside a hangar for maintenance it should be correctly grounded.