Chapter 8 Electrical, starting, and ignition systems

How can the remaining service life of starter-generator brushes be determined?

By visually inspecting the amount of wear groove remaining on the brushes.

What components of the starter-generator require periodic inspection?

Both commutator and brushes should be inspected for wear beyond operational limits.

Which windings in starter-generators receive current for operation in the start mode?

Both field and armature receive current for operation in the start mode.

What are possible sources of low pressure compressed air for starting jet transport aircraft?

A ground power unit (GPU), an on-board auxiliary power unit (APU), or compressed bleed air from an already running engine.

When should ignition ideally occur?

A specific number of degrees before the piston reaches top dead center on the compression stroke.

Why are dual magnetos used in the engine ignition system?

To improve combustion efficiency and provide redundancy for safety.

Why is magneto timing important?

The spark plug in a cylinder has to fire at a specific time for the engine to run properly.

What function does a magneto perform?

It produces a high voltage that forces a spark to arc across a spark plug gap.

Name the components of a high tension magneto system.

A permanent multipole rotating magnet, soft iron core, and pole shoes.

What are the three main circuits of a high tension magneto system?

Magnetic, primary, and secondary.

What is an E-gap angle?

It is a point a few degrees beyond the neutral position of a rotating magnet where maximum magnetic field stress exists.

What reduces arcing in the points of a magneto?

A capacitor.

How does a magneto produce the high voltage required to fire a spark plug?

When the magnetic rotor is in the E-gap position, the primary points open, causing a high rate of flux change and inducing a pulse of high voltage in the secondary coil.

What is the P-lead and how does it function?

The P-lead connects the ignition switch to the primary circuit of the magneto, grounding the magnetos when the switch is off.

What is a magneto timing light used for?

It is used for both internal magneto timing and magneto-to-engine timing, indicating when the magneto points open.

Why are turbine engine igniters generally not susceptible to carbon fouling?

Because the high energy sparks they produce clean off any deposits on the firing end.

What is the function of a duty cycle in a turbine engine ignition system?

It allows the ignition system to operate for a given amount of time, followed by a cooling down period.

What are the three main components of a turbine engine ignition system?

Ignition exciters, high tension leads, and igniters.

What does an ammeter/loadmeter indicate when connected to the battery positive lead?

It indicates whether the battery is charging or discharging.

What is done to ensure that each generator shares the load in a multiple generator system?

The generators are paralleled.

Where would you find generator rating and performance data?

On the data plate attached to the generator.

How is the voltage of a DC generator controlled?

By varying the field current strength.

What is the most common type of alternator used in most aircraft AC systems?

The three phase alternator.

How does a voltage regulator control the voltage of an alternator?

By regulating the voltage output of the DC exciter.

What does the speed of rotation and number of poles of an alternator determine?

The frequency of the alternator output.

What are some methods used to maintain 400 Hertz alternator output frequency?

Constant speed drives, integrated drive generators, and variable-speed constant frequency (VSCF) power systems.

Name the major parts of a DC motor.

The armature, field, brushes, and frame assembly.

Name the components of a direct cranking electric starter system.

An electric motor, reduction gears, and an automatic engaging and disengaging mechanism.

Name three types of DC motors.

Series, shunt, and compound.

What type of DC motor is commonly used for a reciprocating engine starter and why?

A series wound motor is commonly used because it has a high starting torque under heavy load conditions.

What are the operating modes of a turbine engine starter generator?

It operates first as a starter, then as a generator once the engine is running.

What is a possible indication when a starter drags?

A dirty or worn starter commutator.

Where would wire with high temperature insulation material be used?

When they run close to high temperature areas such as exhaust stacks or heating ducts.

What is the size standard for electrical wire used in US manufactured aircraft?

The American Wire Gauge (AWG).

How are wire sizes represented?

By a numbered gauge size, with smaller wires represented by larger numbers.

What is the maximum slack allowed between the supports of a single wire or bundle installation?

Not over ½ inch.

Why are wires bonded on powerplant installations?

To provide a current return path for electrical accessories and to prevent static discharge.