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System Description
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WHITE MESSAGE Require immediate crew attention. These messages are accompanied by flashing master warning, triple chime attenson with or without voice message or dedicated tone. Require immediate crew awareness and future crew action may be required in the form of alternate system selection or performance limitation. These message are accompanied by flashing master caution and a single chime. Indicate safe or normal system operation which require crew awareness, over and above the dark cockpit philosophy. Indicate minor failures or reduction in systems capability, which require no crew action.

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Overview

The function of static discharging is to let electrostatic charges bleed gradually away from the aircraft to prevent radio interference.

Electrostatic charges on the aircraft are caused by flight through precipitation, electrical fields, and engine-produced ionization. Electrostatic charges cause radio interference on the communication and navigation systems. To prevent radio interference, static dischargers (graphite-coated rods) are installed at particular locations on the airframe.

Flight through precipitation, electrical fields, and engine produced ionization, cause electrostatic charges to collect on the aircraft. When the aircraft is in flight these charges increase. When a given potential is reached, the electrostatic charge leaks off the aircraft and causes radio frequency noise.

The radio interference caused by this phenomenon can stop reception on the communication and navigation systems. Radio interference is caused by different electrostatic sources that include precipitation and moisture ionization, radio noise caused by improperly bonded metallic surfaces, and radio noise caused over insulated surfaces.

Electrostatic electricity usually discharges at the wing and tail ends. The static discharge system is installed on the aircraft to keep concentration of electrostatic electricity to a minimum. The static discharge system is a group of 24 static discharger assemblies. Each static discharger assembly has a discharger element installed on a mounting base. The mounting base is attached to the aircraft structure with screws. The static dischargers are installed at particular locations in the wing and empennage zones.

12/22/15

Static Discharger

Each static discharger assembly has the two parts that follow:

  • Discharger
  • Mounting Base

There are four different types of dischargers. Also, there are two types of materials used for the mounting bases (titanium or aluminum).

The different dischargers and mounting bases are installed on the aircraft as follows:

  • Type 2-11SC-1:

    • Two dischargers on the rudder, with aluminum mounting bases
    • Six dischargers on the winglets, with titanium mounting bases.

  • Type 2-12SC-1:

    • Two dischargers on the tail-cone, with titanium mounting bases.

  • Type 2-17SC-1:

    • Six dischargers on the elevator, with aluminum mounting bases
    • Six dischargers on the wings, with aluminum mounting bases.

  • Type 2-18SC-1

    • Two dischargers on the vertical stabilizer, with titanium bases.

The discharger is a resistive graphite-coated rod of aligned fibers tapered towards the tip. This design gives flexibility and graded resistance, which gives noise reduction quality. The discharger also has a heat-shrunk sheath for more protection.

The discharger tip is a small brush that is made up of 700 nichrome wires to give high-current capacity.

The mounting base is part of the aircraft structure. You can replace the discharger without the removal of the mounting base. The discharger mounting ends attach to the mounting bases with internal thread, external thread, and/or screws. The dischargers on the vertical stabilizer and tail-cone make an angle with their applicable mounting base. In other locations, the discharger and the mounting base are not angled.

The Static dischargers are electrically bonded to the aircraft structure and sealed.

300_2361_001

300_2361_002

Operation

Radio interference caused on communication system receivers and navigation system receivers is caused by different phenomena that include streaming noise, arcing noise, and corona noise. Streaming noise is caused on insulated surfaces such as radomes, fiberglass winglets and other fiberglass panels positioned on frontal impact areas of the aircraft. As particles strike, they deposit electrons on the insulated surfaces.

As more particles reach this isolated pool, the voltage increases until it reaches the discharge point. When the pool of charge flashes over the surface of the insulated material, it generates radio frequency noise.

This phenomenon is also observed over painted metal surfaces. Paint forms an insulating material over the aluminum skin, and because of this condition, charges that collect on the surface give streamers to a rivet head or screw fastener.

Streaming noise can be solved by coating the non-conductive surface with a high-resistance paint. Such paint bleeds charged particles quietly off the aircraft fuselage.

Radio interference can also be generated by arcing noise. This interference is caused by the improperly bonded metallic objects. This area condition causes charges to collect until arcing occurs between the structures. This arcing can produce radio noise that extends to 1,000 Mhz.

Installation of ground straps between the structures prevents the arcing noise problem. Problems related to ground straps condition can be identified with electrical bonding tests.

Corona noise occurs when the aircraft collects sufficient charge to ionize air around the wingtips, vertical stabilizer, horizontal stabilizer, and other aircraft protrusions. As current bleeds off trailing edges, it gives radio frequencies that sound like loud hissing on the radio receivers. The same effect can also cause antennas to bleed off the static charges. When the bleeding-off occurs through the antenna (corona discharge), it can appear like a strong signal to the receiver. The squelch circuits then cause a large decrease in the receiver sensitivity and effectively stop radio reception.

Static dischargers are installed at particular points on the aircraft (where there is little or no interference from the static into the radio receiver antennas). Static dischargers bleed off the static charge quietly and prevent corona discharge.

10/15/20

Component Location Index

Component Location Index
IDENT DESCRIPTION LOCATION IPC REF
      - STATIC DISCHARGER TYPE 2-11SC-1 ZONE(S) 342 23-61-01
      - STATIC DISCHARGER TYPE 2-11SC-1 (LH) ZONE(S) 540 23-61-01
      - STATIC DISCHARGER TYPE 2-11SC-1 (RH) ZONE(S) 640 23-61-01
      - STATIC DISCHARGER TYPE 2-12SC-1 ZONE(S) 320 23-61-01
      - STATIC DISCHARGER TYPE 2-17SC-1 (LH) ZONE(S) 551 23-61-01
      - STATIC DISCHARGER TYPE 2-17SC-1 (RH) ZONE(S) 651 23-61-01
      - STATIC DISCHARGER TYPE 2-17SC-1 (LH) ZONE(S) 352 23-61-01
      - STATIC DISCHARGER TYPE 2-17SC-1 (RH) ZONE(S) 362 23-61-01
      - STATIC DISCHARGER TYPE 2-18SC-1 (LH) ZONE(S) 346 23-61-01
      - STATIC DISCHARGER TYPE 2-18SC-1 (RH) ZONE(S) 346 23-61-01

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