Chapter

9

DC Electrics - Bonding and Screening

Bonding . . . . . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	. .	145
The Static Discharge System or Static Wicks							145
Discharge of Static on Touchdown							145
Screening . . . . . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .	.145
Questions . . . . . . . . . . . . . .	. . . . . .	. .	. .	. .	. .	. .	.146
Answers . . . . . . . . . . . . . .	. . . . . . .	. .	. .	. .	. .	. .	148

143

9		DC Electrics -Bonding and Screening

Screening and Bonding - Electrics DC 9

144

DC Electrics - Bonding and Screening		9

Bonding

An aircraft in flight will pick up, or become charged with, static electricity from the atmosphere. Bonding will prevent any part of the aircraft from building up a potential so great that it will create a spark and generate a fire risk.

Each piece of the metal structure of the aircraft, and each component on the aircraft, is joined to the other by flexible wire strips. All strips must be clean and free from any insulating coatings such as anodizing, paint, grease and oxides to prevent electrolytic corrosion occurring which would introduce resistance.

This process is called bonding, and it provides an easy path for the electrons from one part of the aircraft to another.

Bonding can also act as part of the earth return system in a unipole circuit and will also help to prevent radio interference due to static discharges.

The Static Discharge System or Static Wicks

The static discharge systems, or static wicks, are fitted to reduce static build-up on the airframe. They were originally made of cotton of about the thickness of a cigarette.

They are fitted to the trailing edge of the aircraft control surfaces, and the tips of wings, or stabilizers. Static electricity is dispersed from them into the atmosphere.

The free end of the wick becomes ‘teased’ (spread out) and a brush discharge action takes place. Modern wicks are like miniature barbed antenna, small wire brushes, or alternatively are straight metal wicks.

Discharge of Static on Touchdown

To ensure that no static electrical charge, with its possible fire risk, remains on the aircraft after landing, the main bond must be brought into instantaneous contact with the ground as the aircraft touches down.

This is achieved by fitting nose, tail or main wheel tyres which contain a high proportion of carbon in the rubber.

The tyre is in contact with the main bond via the wheel bearing and any static charge is dissipated to earth on touchdown.

Screening

Screening is designed to prevent radio interference by absorbing electrical energy.

Static electrical charges, produced by the operation of certain electrical equipment, create interference on radio circuits.

This interference is overcome by fitting interference suppressors in the cables connected to the source of interference, and by total enclosure of the cables in a continuous metal sheath.

Screening is required for ignition systems, DC generators and motors (commutator machines), slip ring machines operating at over 200 RPM and also for any electrical equipment operating by making and breaking a circuit at a frequency greater than 10 Hz.

DC Electrics - Bonding and Screening 9

145

9 Questions

Questions 9

Questions

1.Why are static wick dischargers fitted to aircraft?

a.To smooth the generator output

b.To prevent tyres bursting on landing

c.To minimize radio interference

d.To act as an earth return in a single pole electrical system

2.Bonding is used to protect the aircraft against fire from arcing of static electricity by:

a.providing an earth return

b.shortening the negative strips

c.maintaining different electrical potential throughout the structure

d.ensuring the same electrical potential of all metal components

3.Static electricity constitutes a fire hazard because:

a.metal components become very hot and ignite inflammable gases and materials

b.sparks occur due to differences of potential and could ignite inflammable gases and materials

c.of colour charged electrons

d.aircraft tyres become heavily charged and may burst on landing

4.Static electrical charges and currents in an aircraft structure are evened out by:

a.hardening

b.screening

c.bonding

d.anodizing

5.The electrical components of aircraft systems are screened to:

a.bond the circuit to reduce risk of fire

b.prevent them discharging

c.prevent short circuits in radio equipment

d.prevent them interfering with the function of radio equipment

6.Bonding is a method of:

a.heat screening

b.providing a positive reaction

c.ensuring that the different parts of the aircraft are maintained at a different potential

d.ensuring that the different parts of the aircraft are maintained at the same potential

7.The purpose of electrical bonding on an aircraft is:

a.to prevent compass malfunctioning and accumulation of local static charges

b.to reduce the anodizing effect

c.to isolate all components electrically and therefore make static potential constant

d.to provide a low resistance path for earth return circuits and safely dissipate local static charges and lightning strikes

146

Questions 9

Questions 9

147