Static dischargers: What they do and how they do it

Passengers might have seen them, pilots verify their presence during the pre-flight check: the static dischargers. Those simplistic looking devices faithfully perform their function during every flight and yet their working principle all too often remains obscure. This article is aimed at changing that. While it does involve some math in the middle-section, it is worth sticking with it to the end, where you will find real-world operational examples!

One point for starters: these devices are not intended for lightning protection!

Figure 1: Static dischargers on transport category aircraft, credit T. Niermann CC BY-SA 3.0 [1]

Introduction

During flight, aircraft accumulate a significant amount of electric charge for reasons we will soon discover. This charge will inevitably create an electric field, which in turn will reach breakdown values if not accounted for. This will cause uncontrolled discharging with its associated side-effects. This phenomenon was discovered well before World War II and was usually linked to precipitation, hence the name precipitation static or p-static [2]. For pilots and maintenance personnel, it is beneficial to have a conceptual understanding of how static dischargers work, in order to better anticipate operational issues and optimize troubleshooting.

Why would an aircraft accumulate charge in flight?

• Triboelectric (frictional) charging

  Whenever dissimilar materials encounter friction, triboelectric charging will occur [3] [4]. Depending on the material pair, the effect is more, or less pronounced. A famous example of this is the combination of a rubber rod and cat fur. During flight, an aircraft comes into contact with a variety of particles, reaching from the typical gases found in the atmosphere to more macroscopic particles such as dust, rain or snow. As these particles impact the airframe and leave, a net charge is left on the surface, usually negative.

• Engine exhaust charging

  This is especially relevant for jet aircraft during flight at lower altitudes: the engine exhaust has been shown to be slightly positively charged due to combustion physics [3]. In other words, an equal and opposite charge will accumulate on the aircraft, further contributing to the negative charge of the aircraft.

• Exogenous charging

  This effect refers to aircraft transitioning between differently charged air masses (therefore being subject to an electric field) and thus acquiring a non-uniform charge distribution [3].

Figure 2: Charging mechanism summary, based on [3]

Exogenous charging is more intricate than the two other mechanisms, since in exogenous charging the aircraft does not acquire a net charge the way it does in triboelectric or exhaust charging. Instead, the ambient electric field — the field between two oppositely charged cloud layers, acts on the free electrons in the conducting airframe and drives them to redistribute. Negative charge accumulates on the side facing the positive cloud; positive charge (electron depletion) accumulates on the side facing the negative cloud. The total charge on the aircraft remains unchanged. This is classical electrostatic induction on a conductor.

Why this still matters

Although the net charge is unchanged, the local surface charge density σ is affected. Since the electric field outside a conductor is proportional to the surface charge density (as we will see later) the field at a wing tip or fin tip pointing into a strong cloud field can exceed the corona discharge threshold even with no net charge at all on the aircraft.

More importantly, when the aircraft flies out of the cloud field region, the induced charge redistribution must reverse. The induced charges were held in place by the external field; when that field disappears, they relax back to uniform distribution. This relaxation happens by corona discharge at the extremities — the induced surface charge density was highest there, the local field was highest there, and the discharge relieves it. This produces a burst of electromagnetic noise that is indistinguishable in character from triboelectric p-static.

The secret of sharp edges: why a smaller radius means a stronger electric field

The following derivation is for the more mathematically inclined and is based on MIT lecture 6 of module 8.02, given by Prof. Walter Lewin [5]. It deals with a theoretical setup of two conductive spheres which are connected by a conductive wire (think aircraft and static discharger tip). They have the same potential. A given amount of charge is placed on them, that distributes proportional to the radius of the respective sphere. So, 5 times greater radius leads to 5 times more charge. However: the surface grows by a factor of 25, if the radius is increased 5 times. This means that the larger sphere will have a surface charge density of only one fifth of the small sphere.

Gauss` law (one of the fundamental Maxwell equations) stipulates that it is the surface charge density that governs the E-field on the surface of a conductor.

Figure 3: Electric field on the surface of two connected spheres, based on [5]

That leads to the following observation: for an equipotential surface, the surface charge density will increase in areas of small radii and so will the E-field. This principle lies at the core of static discharger design.

The Trichel pulse – the physics behind corona discharge

When electrons accumulate near the sharp edges, the surrounding air is ionized (row one in figure below). As soon as the discharge threshold is reached, a so-called Townsend avalanche is fired (time frame 0 below). The fast electrons produced in the avalanche attach to oxygen molecules just outside the active zone, forming a slow-moving O₂⁻ space-charge cloud that partially shields the tip field. When the shielded field drops below the ionisation threshold, the avalanche self-quenches — a built-in negative feedback that prevents the discharge from transitioning into an arc. The cloud then disperses by diffusion or airflow, the tip field recovers, and the next pulse fires, repeating the cycle at frequencies between 10 kHz and 1 MHz.

Figure 4: Trichel pulse sequence filmed by ICCD camera [6] (CC-BY 4.0)

To proof the point that ion clouds exist and can have a macroscopic effect, let us look at the ionic wind: This is one of the lesser-known electrohydrodynamic phenomena. When ions are expelled from a high-potential sharp edge, they interact with the surrounding air and generate a visible flow as shown below:

Figure 5: Ionic wind [7], credit: Zátonyi Sándor CC BY-SA 3.0

Electro-magnetic emissions caused by corona discharges

As described above, large-radius objects achieve higher potentials before breakdown occurs. Also, the current pulse (known as Trichel pulse) is much higher for large radii. The key here is charge acceleration. A large amount of charge displaced in a short time (large acceleration) will create a lot of electromagnetic radiation. Furthermore, a pulse-sequence in the time domain creates a series of harmonics in the frequency domain, leading to relatively powerful interference (blue graphs below).

Figure 6: Blunt object Trichel pulses vs. micro Trichel pulses from static dischargers, based on [3]

Static dischargers containing lots of carbon wicks with small radii, generate a much more random pattern of lower amplitude Trichel pulses, leading to a more noise-like spectrum with lower power levels (green above). Additionally, static dischargers are connected to the airframe through a carbon-loaded resistive element (several ten to hundreds of MΩ), limiting the discharge current and thus reducing the magnitude of RF emissions.

Placement on the wing: why outboard rather than inboard?

There are three main reasons for this [4]:

1. Coupling to antennas is minimised outboard

   Corona noise sources at the wing tip, elevator tip, and rudder tip couple far less to fuselage-mounted antennas than sources located inboard. The coupled noise power falls off rapidly with separation (roughly as the inverse square of distance in the far field). Putting the dischargers as far outboard as possible — wing tips, aileron and elevator trailing edges, maximises the physical separation from the antennas and minimises the noise coupled into the communication and navigation systems. A discharger placed near the fuselage would be radiating corona pulses essentially adjacent to the antenna (front door coupling).

   
   

2. The electric field is naturally highest at outboard extremities

   The aircraft is not a sphere — it is an elongated body with thin projecting surfaces. The electric field concentrates more strongly at the tips of the wings and the trailing edges of control surfaces, for exactly the same reason the field is strongest on the smaller sphere above: small local radius of curvature. These are the points where uncontrolled corona would naturally break out first if dischargers were not present. Placing the dischargers precisely at those high-field points means the controlled corona from the wick tip occurs before the field anywhere else on the aircraft reaches its own discharge threshold.

   
   

3. Outboard tips are aerodynamically downstream of the antennas

   Most communication and navigation antennas sit on the fuselage, which are upstream or abeam of the wing and elevator trailing edges in the airflow sense. Corona produces an ionic wind and a space-charge cloud that drifts downstream. Locating the dischargers at the outboard trailing edges means the charge cloud is carried away from the antenna locations by the airflow rather than across them (refer to ionic wind).

What about the effect of altitude on static discharger performance?

There are really two effects here [2]:

1. Lower air density

   A decrease in air density lowers the corona onset threshold. In other words, it takes a lower potential to initiate corona discharge. Once triggered, it works more efficiently as described in point 2.

2. Higher TAS

   Recall from the Trichel cycle: after each pulse fires, the O₂⁻ ion cloud accumulates just outside the tip and shields the field, quenching the discharge. The next pulse cannot fire until that cloud clears. In still air, clearing happens slowly by diffusion alone. The time between pulses — the relaxation time — is set by how long the cloud takes to disperse. In flight, the external airstream sweeps the ion cloud away from the tip much faster than diffusion. The field recovers sooner, the next pulse fires sooner, and the repetition frequency rises. The consequence is that average discharge current rises with TAS at the same applied voltage.

   This is also why discharger performance is usually tested in wind tunnels rather than static bench tests — static measurements understate the in-flight discharge current significantly.

There is a second beneficial effect at high TAS: The space-charge cloud from the tip extends a finite distance downwind before dispersing. This cloud can suppress the corona on a neighbouring discharger if they are too close — the so-called DC correlation distance, typically around 30 cm between two dischargers on the same trailing edge. At higher airspeed, the cloud is swept away faster and its lateral extent is reduced, so the mutual suppression between adjacent dischargers decreases.

Real-world operational examples of static noise

Apart from missing or defect static dischargers, there are several other typical sources of static noise. Finding them can be quite tricky. A few real-world examples are described below:

The broken lightning diverter [2]

Aircraft radomes typically have metal lightning diverters embedded in the composite material that are quite visible from the outside. Their purpose is to prevent lightning damage to the radome itself. An interesting effect can occur if one of them is damaged, even without a lightning strike: An aircraft experienced problems with VOR reception and VHF communications. A close inspection revealed cracks in one of the lightning diverters. This led to static charge accumulating on the isolated portion of the diverter and eventually to spark discharges on the remaining diverter, causing significant interference.

The freshly painted wing inspection panel [2]

A general aviation aircraft began to encounter strong interference on the VHF communication systems, always after being airborne for some time. It was discovered that a wing-inspection panel had recently been re-painted. The organization performing the task unfortunately failed to ensure proper bonding of the newly painted panel. The result was significant uncontrolled spark discharges after some flight time making VHF communication difficult.

The damaged bonding strap [2]

Airlines operating the DC-8 encountered significant static interference after some years of operation. The design of the DC-8 vertical fin made use of a bonding strap to connect the static dischargers which were mounted on a fibre-glass structure to the metal part of the vertical fin. It turns out that bonding strap did not meet the expected lifetime and started failing pre-maturely, leading to spark discharge.

That is it, hopefully this article could enhance your understanding of static dischargers, even if you already knew the basics. So, next time someone tells you that these devices are for “lightning protection”, you can share the insights…

References:

[1]	T. Niermann, «Winglet and static dischargers of an Airbus A319-132 of Germanwings CC BY-SA 3.0».
[2]	J. E. Nanevicz, «Static charging and its effect on avionic systems,» IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATABILITY, Bd. 24, Nr. 2, 1982.
[3]	AGARD, «AGARD lectures series 110: Atmospheric electricity - Aircraft interaction,» NATO, 1980.
[4]	J. E. Nanevicz und R. L. Tanner, «AFCRL Technical Report 73: Precipitation charging and corona-generated interference in aircraft».
[5]	W. Lewin, «MIT OCW 8.02 Lecture 6,» [Online]. Available: https://www.youtube.com/watch?v=ww0XJUqFHXU&list=PLyQSN7X0ro2314mKyUiOILaOC2hk6Pc3j&index=7. [Zugriff am 2026].
[6]	Y. e. a. Zhang, «Trichel pulse in various gases and the key factor for its formation (CC BY 4.0),» nature, 2017.
[7]	S. Zátonyi, «Electric wind (series) CC BY-SA 3.0».