Static Discharge Wicks

I believe the antennas you are asking about are the static discharge wicks that most airplanes carry on their wingtips. Examples of these wicks can be seen in the following pictures of a Boeing 737.

Static discharge wicks on the wingtip of a Boeing 737
Static discharge wicks on the wingtip of a Boeing 737

The purpose of static discharge wicks is to dissipate the static electricity that builds up on planes in flight. Most of us are familiar with static electricity in our daily lives. Common examples include the static cling in fabrics after being removed from a clothes dryer and the shock you may feel after rubbing your feet on thick carpeting or after stepping out of a car. Perhaps the most powerful example of static electricity is a bolt of lightning.

Static electricity is created when objects with different electrical properties come into contact, and negatively charged electrons from one substance are transferred to the other. This electron imbalance causes the object losing electrons to become positively charged while the other becomes negatively charged. The transfer of electrons is often aided by friction when two objects rub together, such as rubbing a balloon against your hair that causes hair to stand up. The same effect occurs on an aircraft due to the friction against the plane as it moves through the air. This friction strips electrons from the atmosphere and causes them to build up on the skin of the aircraft.

The creation of static charges is largely dependent on the ability of the two substances to conduct electricity. Metals are good conductors and allow electrons to flow freely through them. Other materials like rubber and air tend to be good insulators since they conduct electricity poorly. However, the ability of a substance to conduct electricity can also vary significantly with conditions. Air, for example, is typically a much better insulator at higher altitudes where aircraft fly because the atmospheric conditions here are much less humid than at low altitudes. As a result, a static charge that builds up on an aircraft at high altitude tends to become much larger because the air acts as an insulator preventing the excess electrons from returning back to the air.

This electrical charge can eventually become so great that the excess electrons begin to ionize local air particles and create a corona around parts of the plane. This corona causes electrons to discharge back into the surrounding air and dissipates the charge on the aircraft. Unfortunately, the coronal discharge most often forms on surfaces like communication and navigation antennas that transmit and receive radio waves. This ionized corona causes interference that reduces the effectiveness of communication gear or blocks reception entirely. NASA research has indicated that the interference typically occurs over a range of frequencies between 10 kHz and 350 MHz where most radios and other communication gear operate. An extreme example of ionizing interference occurs during re-entry when a spacecraft's high speed causes such intense friction against the atmosphere that the surrounding air is turned into ionized plasma and communications are lost for several minutes.

Close up of a pair of static discharge wicks
Close up of a pair of static discharge wicks

The static charges that build up on an aircraft during flight tend to accumulate near sharp edges like the trailing edges of wings and tail surfaces. The purpose of static wicks is to provide a conductive path for these excess electrons to flow or "leak" from the aircraft back into the atmosphere. This transfer of electrons reduces the charge on the plane's skin and structure.

The wicks are composed of hundreds of individual carbon fibers wrapped into a cylinder around three to eight inches (7.6 to 20.3 cm) long and about the diameter of a soda straw. Each fiber ends in a sharp point to create a strong gradient in the local electrical field. This gradient attracts the static charge and encourages the electrons to flow off the aircraft and back into the atmosphere. Instead of an ionized corona building up on vital communication antennas, the electrical charges find these wicks more attractive.

Static discharge wicks also provide other important safety benefits. In the event of a lightning strike, a plane is designed to conduct the excess electricity through its skin and structure to the wicks to be safely discharged back into the atmosphere. Though this massive electrical charge often burns or melts the wicks, these devices are simple and easy to replace. Wicks are also relatively fragile and easy to damage, so pilots and ground crew routinely inspect them in case replacement is needed. Because of their importance, regulatory agencies like the Federal Aviation Administration (FAA) require static wicks aboard all civil aircraft.

Static discharge wicks along the wing of an Airbus A320
Static discharge wicks along the wing of an Airbus A320

Airbus aircraft, like the A320 pictured above, are usually equipped with static wicks on the flap track fairings along the wingspan in addition to those at the wingtip. These wicks provide additional paths for excess electrons to escape the aircraft structure and return to the atmosphere. Wicks are also a common sight on aircraft tail surfaces like the elevator and rudder.
- answer by Joe Yoon, 26 June 2005

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