Electrical Starting and Ignition Systems Flashcards
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
Between field and armature windings in starter generators, which ordinarily receive current for operation in the start mode?
Generally both field and armature receive current for operation in the start mode
What are possible sources of low pressure compressed air used for starting jet transport aircraft equipped with air turbine starters?
A ground power unit (gpu), an on-board apu or compressed bleed air from an already started engine
When should ignition ideally occur?
A specific number of degrees, as determined by the manufacturer, before the piston reaches top dead center on the compression stroke
Why are dual magnetos used in engine ignition systems?
To improve combustion efficiency and provide redundancy for safety
Why is magneto timing so important?
For an engine to run properly, the spark plug in the cylinder has to fire at a specific time. To produce the required energy for the spark at required time, the magneto must be in the e-gap position. The breaker points must start to open and the distributor must be electrically aligned with a particular cylinder
What function does a magneto perform?
It produces a high voltages that forces a spark to arc across a spark plug gap
Name the components of a high tension magneto system
A permanent, multi-pole, rotating magnet, a soft iron core and pole shoes
Name 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 neutral position of a rotating magnet where maximum magnetic field stress exists
What reduces arcing in the points and also aids the collapsing of the magnetic field in a magneto?
A capacitor
How does a magneto produce the high voltage required to fire a spark plug?
In the rotation cycle when the magnetic rotor is in the e-gap position, the primary rotors open which interrupts the currents flow in the primary circuit causing a higher rate of flux change in the court and introducing 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. When the ignition switch is turned off, the p lead on both magnetos are grounded. When the ignition switch is turned to the left position, the right magneto is grounded so that only the left magneto operates and vice-versa
What is a magneto timing light used for and what does it indicate?
It is used for both internal magneto timing and magneto to engine timing and indicates the exact instant 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 relation to a turbine engine ignition system?
A duty cycle allows the operation of the ignition system for a given amount of time and is the followed by a minimum specified cooling down period
What are the three main components of a turbine engine ignition system?
Ignition exciters, high tension leads and igniters
When the engine is operating, what does an ammeter/load meter indicate when connected to one: battery positive lead and two: generator output lead?
When connected to the battery positive lead, the ammeter indicates whether the battery is charging or discharging. When connected to the generator output lead, it indicates the currents produced by the generator or electrical system load
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 of the methods used to maintain 400 hertz alternator output frequency on large turbojet or turbofan engines?
Constant speed drives, integrated drive generators and variable speed constant frequency 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 high starting torque under heavy load conditions
What are the operating modes of a turbine engine starter generator?
It first operates as a starter, then as a generator once the engine is running
What is the 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 the us manufactured aircraft?
American wire gauge
How are wire sizes represented?
By numbered gauge sizes. Smaller wires are represented by larger numbers. The smallest wire normally used in aircraft is 22 gauge
What is the maximum slack allowed between the supports of a single wire or bundle installation?
Not over 1/2”
Why are wires bonded on power-plant installations?
To provide a current return path for electrical accessories and to prevent static discharge
What happens when the start switch for a turbine engine with a starter-generator is placed in the START position?
a. Current flows into the starter relay which closes and actuates the ignition exciter.
b. Current flows through the series motor coils of the starter-generator and rotates the engine until it starts.
What is the main advantage of a magneto ignition system over a battery ignition system for an aircraft reciprocating engine?
A magneto has its own source of electrical energy and is not dependent upon the battery
What is an all-weather spark plug?
A shielded spark plug that has a recess in the shielding in which a resilient grommet on the ignition lead forms a watertight seal.
What is meant by the reach of a spark plug?
The length of the threads on the spark plug that screw into the cylinder head
What is the difference between a hot spark plug and a cold spark plug?
A hot spark plug has a long path for the heat to travel between the nose core insulator and the spark plug shell. In a cold spark plug, the heat has a shorter distance to travel, and the spark plug operates cooler than a hot spark plug.
What is the advantage of fine-wire spark plugs over massive electrode spark plugs?
Fine-wire spark plugs have a firing end that is more open than that of a massive electrode spark plug. The open firing end allows the gasses that contain lead to be purged from the spark plug so they will not form solid lead contaminates
Why is it important that the spark plugs be kept in numbered holes in a tray when they are removed from an engine?
Spark plugs tell a good deal about the internal condition of the cylinders from which they were taken. By knowing the cylinder from which each spark plug came, the mechanic can take the proper action when a spark plug indicates such conditions as detonation or overheating
Why is it important that a torque wrench always be used when installing spark plugs on an aircraft engine?
If the spark plugs are not put in tight enough, there is a possibility of a poor seal; if they are put in too tight, there is danger of cracking the insulation or damaging the threads on the spark plug or the cylinder
What kind of gauge should be used to measure the electrode gap in aircraft spark plugs?
A round wire gauge
What is a shower of sparks ignition system?
An ignition system that uses an induction vibrator to send pulsating DC into a set of retard breaker points on one of the magnetos. This provides a hot and retarded spark for starting the engine
What is an impulse coupling, and how does it work?
An impulse coupling is a spring-loaded coupling between a magneto shaft and the drive gear inside the engine. When the engine is rotated for starting, the impulse coupling locks the magnet so it cannot turn. The spring in the coupling winds up as the crankshaft continues to turn, and when the piston is near top center, the coupling releases and spins the magnet, producing a hot and retarded spark.
What are the three distinct circuits in a magneto system?
The magnetic circuit, the primary electrical circuit, and the secondary electrical circuit.
What are the advantages of a solid-state magneto?
No mechanical parts that wear, higher energy spark at low speeds, and no recurring inspection requirements
What is the purpose of a FADEC system?
FADEC (full-authority digital electronic control) is a digital electronic fuel control for a gas turbine engine that is functioning during all engine operations, hence full authority. It includes the EEC (electronic engine control) and functions with the flight management computer. FADEC schedules the fuel to the nozzles in such a way that prevents overshooting power changes and over-temperature conditions. FADEC furnishes information to the EICAS (engine indicating and crew alerting system).
What kind of electric starting system is used on many of the smaller turbine engines?
A starter generator
Why are air-turbine starters superior to electric starters for large turbine engines?
They are light weight for the torque they produce
Where does an air-turbine starter get its air for starting the engines on a jet transport airplane?
From an APU, GPU, or from a running engine
What prevents too high an air pressure from overspeeding an air-turbine starter?
The air shutoff and regulating valve
Where does an air-turbine starter get its lubricating oil?
It has a self-contained lubrication system with the oil held in the starter housing
What device in an air-turbine starter warns an aviation mechanic if there are any metal chips or particles in the oil?
Magnetic chip detectors warn of any metal contamination in the oil.
What is the function of the capacitor in a magneto?
The capacitor minimizes arcing at the breaker points, and it speeds up the collapse of the primary current as the breaker points open.
What happens in a magneto ignition system when the ignition switch is placed in the Off position?
The primary circuit is connected to the ground
What is checked when a magneto is internally timed?
Internally timing a magneto consists of adjusting the breaker points so they will open at the instant the rotating magnet is at the E-gap position, and the distributor rotor is in position to direct the high voltage to cylinder number one
In what position should the ignition switch be placed when using a timing light on the magnetos?
In the Both position
What is the E-gap in magneto timing?
The E-gap angle is the position of the rotating magnet when the primary current flowing in the magneto coil is the greatest. The breaker points open when the rotating magnet is in its E-gap position.
What malfunction in the ignition system would cause an aircraft reciprocating engine to continue to run after the ignition switch is placed in the Off position?
The ignition switch is not grounding the primary circuit.
What type ignition system is used in most turbine engines?
High-intensity, intermittent-duty, capacitor discharge ignition systems
How many igniters are used with most turbine engines?
2
What are two types of ignition systems used in turbine engines?
High-voltage systems and low-voltage systems.
With which type of turbine-engine ignition system is a glow plug igniter used?
A low-voltage system.
How is the strength of the magnet in a magneto checked?
The magneto is put on a test stand and rotated at a specified speed. The breaker points are held open and the primary current is measured. The strength of the magnet determines the amount of primary circuit
In what position is the magnet in a magneto when the greatest change in flux density in the coil core takes place?
It is a few degrees beyond its neutral position. When it is in this position, the breaker points open and the primary current is interrupted. The flux change in the coil core is the greatest.
In what position is the magnet in a magneto when the breaker points begin to open?
In it’s E-gap position, just a few degrees beyond its neutral position.
