Static Discharge Wicks, SDW
When an aircraft flies with reasonable high speeds the friction with which the air flows over the skin causes electrons to strip from the air molecules and build up on the aircraft skin. The amount of buildup depends on the ability of the two materials to conduct electricity and also on the exact atmospheric conditions and altitude the aircraft flies at.
The effect from this electron buildup is that radio reception becomes difficult or even impossible due to a high static noise level. This happens for VHF COM and NAV frequencies but it will also have some effect on the higher UHF frequencies where the transponder listens to radar interrogations and DME talks to groundstations.
Special devices are used to remove this unwanted electrical charge, and they are called static discharge wicks (SDW) or trailing discharger. It is therefore important that the pilot checks these wicks during pre-flight and these wicks are the subject here.
The above mentioned static charges accumulate near edges of the aircraft: like trailing edges of wings, flaps, ailerons, elevator and rudder. To remove this electrical charge, manufacturers install static discharge wicks (see image) on these trailing edges and they are connected with the aircraft aluminum to conduct and remove the charge back to the air.
Without these wicks the charge can become so large that excess electrons start to ionize air molecules and form a corona around sharp objects on the aircraft, e.g. antenna or wing tips. And this is what causes the interference and reduces the effectiveness of the radio equipment.
Example: the same happens during re-entry of the Space Shuttle into the Earth's atmosphere. Due to the high speeds of the craft the surrounding air ionizes and turns into a plasma and when that happens: communications are lost until the speed is sufficiently lowered and the plasma has disappeared.
These SDW's are constructed with thousands of strands made from low resistance, conductive carbon fiber inside a flexible enclosure and extending about 10 to 20 cm from the trailing edges.
Each of those fibres should end in a sharp point which helps returning the electrons to the air thus releasing the electrical charge.
For these wicks to work properly, all the parts of the aircraft need to be on the same electrical charge or level. This is done by bonding strips connecting all separate components to provide a clear low resistance path for the electrons to flow along.
These strips consist of braided low resistance wiring or even an adhesive tape with a conductive medium impregnated. You might see these bonding strips externally near hinge points of elevators, rudder, flaps etc etc. See image to the right.
These bonding cables must be installed against clear metal, i.e. before the aircraft or parts are painted (paint is an isolator). This will ensure a good connection (lowest resistance) and conduction for the electrical charge.
Another advantage of bonding is that all static charge is equal along the fuselage of the aircraft and therefore no arcing between parts can occur. Without proper bonding the static charge tends to accumulate near sharp objects like COM and NAV antenna causing previously mentioned interference.
During the walk around by the pilot he/she should check the exposed binding strips and discharge wicks for integrity, security and condition. Missing or damaged wicks could be an indication of a previous lightning strike on the aircraft and should this be reported to the maintenance crew and/or have them replaced. Should you embark on a VFR flight then missing a SWD may not be any problem.