Final Exam Review

Potentialing and Looping Circuits

• Black = Circuit Status

  • Switches, Relays opened, closed

  • Faults

• Blue = Current Flow (Electron Flow Theory)

• Red = Positive Potential

• Green = Positive Potential

• Draw positive potential or negative potential to an open(switch or relay) and ON loads

Current and Amperage

• Must have a complete path from - to +

• Total amperage depends on total loads or resistance of the circuit

  • Parallel circuits: IT = I1 + I2 + I3

  • Series circuits: I is constant

3 Prime faults in electrical circuits

• Open

  • No continuous loop = no current flow

  • A break in the loop where there should not be

  • “OL“ reading on multimeter

• Short

  • Internal short

    • If a load has a specific resistance but indicates lower than normal resistance

    • Current flow increases

    • Circuit breaker will pop depending on how much of an internal short

  • Bypass short

    • Wires are touching another part of the circuit or are switched

  • Short to ground (STG)

    • Unintended path to ground (airframe)

    • No load or resistance but path is completed

    • Will have high current

    • Circuit breaker will pop depending on location

• Added resistance

  • More resistance = less amperage

  • Higher than normal indication on multimeter

    • Examples:

      • Light will be dimmer

      • Landing gear or a flap motor would be slower than normal

Crimping and Wire Stripping

• Use the correct size wire stripper for the size wire

• Strip only insulation, do not strip or nick any of the conductor strands

  • Reject if any strands are nicked or broken

• Strip the correct amount of insulation form the conductor

  • Amount of insulation depends on the terminal barrel that the wire is being crimped to

Inspection points for crimps

• Proper strip length

  • Too much insulation stripped can cause interference with installation hardware

  • Too little insulation stripped will not allow proper inspection after crimping

• No conductor should be showing at the end of the terminal

  • Exception: Daniels crimpers instructions specify that there should be some conductor visible at the end of the connector

• Two crimps are made with one crimping action

  • Barrel to insulation

  • Barrel to conductor

• Tug test: Slight pull to test strength of crimp to wire

  • Pull only: Do NOT bend or twist

Connectors

• Plugs and jacks

  • P1 mates to J1, P2 mates to J2, ect.

• Mating or demating connectors

  • Ensure power is OFF

  • Ensure proper alignment so as not to damage or bend any pins

  • Always use proper tools (if tools are required)

  • Do NOT pull on the wires

AC43.13

• Use this when the manufacturer has not provided data regarding the repair or inspection being preformed

• (Figured 11-2) Determining wire size

  • Wire size is based on the current carrying capability of the wire and the allowable voltage drop

• Circuit breakers and fuses protect the wiring, not the loads

  • Amperage rating for circuit breakers and fuses are determined by wire size

    • If circuit breaker amperage rating is too high it could lead to damaged wires

    • If circuit breaker amperage rating is too low the circuit will not operate without tripping the circuit breaker continuously

Wire ID

• Prefix letter: only used when there are two of the same type of circuits installed (ie 2 lighting circuits)

• Circuit function code: L2B20

  • L = Lighting

  • F = Flight instruments

  • X = AC power

  • P = Power

  • W = Warning

• Wire number: L2B20

  • This is the 2nd wire in the lighting system

• Segment letter: L2B20

• Wire size L2B20

  • 20 AWG

• Suffix (if used) L2B20N

  • N indicates the wire goes directly to ground or the negative bus

3 Things You Need for Electromagnetic Induction (Induced Voltage)

• Magnetic field (coil is energized)

  • Can change the field by the number of windings in the coil or increasing the current for a stronger magnetic field

• Conductor

• Relative motion

Alternators and Related Systems

• Rotor (Rotates)

  • Rotating electro-magnet

  • Strength of magnet is determined by the input current (field current)

  • Current travels through the brushes to the slip rings then to the rotor coil

• Stator (Stationary) - Conductor

  • Rotor rotates inside of stator windings

  • 3 sets of windings, 120° apart that produce AC current (3 windings = 3 phase)

  • Voltage is induced across the stator wires

  • 2 types: “Wye“ type and “Delta“ type

• Rectifier

  • Changes the 3 phase AC produced in the stator to DC

  • Output voltage is pulsating DC

  • Diodes in series and reverse bias between a power supply and a coil = rectification diode

  • Because the diodes are in reverse bias to the battery, they also prevent the stator coils from becoming a load to any DC source (ie battery or external power) when the Engine is not running

• Capacitor

  • Reduces the ripple of the pulsating DC

  • Also reduces radio noise (static)

Alternator Inspection

• Rotor

  • Test for open

    • One meter probe on each slip ring

    • Open rotor = OL or infinite reading on meter

  • Short (Internal)

    • One meter probe on each slip ring

    • Internally shorted rotor = lower resistance than normal specified by manufacturer

  • Short to ground (STG)

    • One meter probe on either slip ring and one on ground (center post)

    • Rotor that is STG = any other reading than OL or infinity on meter

    • OL = normal indication

• Stator

  • Testing for open

    • For “Wye“ stator: one meter probe on a phase or one meter probe on a phase and one on neutral (must tell all 3 phases)

    • For “Delta“ stator: disconnect one junction of stator wires and test at that point (one meter probe on each side)

      • This tests all 3 phases at the same time

    • Normal indication on meter = <1 Ohm (check specific manufacturer’s manual)

  • Testing for STG

    • One meter probe on any phase and one on ground (Frame)

    • Anything other than OL or infinity is an indication of STG

• Rectifier

  • Each diode must be tested in FWD bias and REV bias using the diode test function on mulit meter

  • 6 diodes test total 12 times

    • FWD Bias = 0.5V

    • REV Bias = OL

Voltage Regulator (Magic Circle)

• Change in RMP or change in load will change the output voltage of the alternator

  • Change of output voltage sensed by voltage regulator

  • Voltage regulator sense change and adjusts current going to the rotor (field current) as necessary

    • Increase field current to increase output voltage and vice versa

Alternator Schematic

• Ammeter #1 - alternator ammeter

  • Never will indicate left of zero

• Ammeter #2 - battery ammeter

  • Left of zero indication = battery is the source for the electrical system and is discharging

  • Right of zero = battery is load to the alternator and is charging

• External power relay

  • Power must be correct polarity for the external power relay to energize

• Hot bus

  • CTTO loads - can’t turn them off. Loads that are always on even when the battery or master switch is OFF

    • Clock

    • Door lights

• Diodes

  • Blocking diodes (D7) - prevents hot bus from powering main bus, but allows main bus to power hot bus

  • Polarization diode (D5, D8) - prevents opposite polarity form entering the system

  • Clipping diode or voltage spike suppression diode (D1, D6, D10) - prevents opposite polarity voltage spike form the collapsing magnetic field of a coil from entering the system

• Alternator run scenarios

  • Ammeter #1 Alternator ammeter (will never indicate left of 0)

  • Ammeter #2 Battery ammeter

    • Indicating left of 0 = battery discharging

    • Indicating right of = battery charging

A&P Privileges

• A&P cannot repair instruments

  • Repair of instruments must completed by an appropriately rated repairman or repair station

Pitot Static System

• Altimeter

  • Static

• Vertical Speed Indicator

  • Static

• Airspeed Indicator

  • Pitot & static

• Pitot Heat

  • Prevents ice from blocking the pitot tube

Vacuum Pumps

• Wet type pumps

  • Uses engine oil for lubrication

• Dry type pumps

  • Uses dust from carbon vanes for lubrication

Sensors, sensing & Switches

• Micro - switches

  • Takes only a small (micro) amount of movement to activate

• Synchros indicate movement and can indicate in an infinite number of positions (i.e. they are synchronized with the movement of something)

Temperature Sensing

• Thermocouples

  • EGT probes are typically chrome - Alumel combination because they can withstand the hottest temperatures

  • When a thermocouple fails, it typically indicates cooler than normal

  • CHT can be measured from either the side of the cylinder or at the spark plug

    • If using a spark plug gasket type CHT thermocouple, be sure to remove the original spark plus gasket

MAP (Manifold Absolute Pressure)

• When engine is not running, the MAP gauge should indicate ambient pressure (Approximately 30” HG)

• Normally aspirated engines typically red line around 29” HG

• Turbocharged engines typically red line above 30”HG

Types of Nav Radios

• GPS

  • Location (Position)

  • Provide Earth’s latitude and longitude

• ADF

  • Bearing (Direction)

  • Direction to ground station

• VHF Nav or VOR

  • Bearing (Direction)

  • For navigation to and from a ground station

• DME

  • Distance

  • Distance to ground position

Types of Transponder

• Mode A - Basic ID only

• Mode C - ID and altitude (MSL)

• Mode S - ID, altitude, and TCAS information

Antennas

• Marconi

  • ¼th wavelength

  • Need a ground plane

• Hertz Di - Pole

  • ½ wavelength

• Higher frequency means shorter antenna

Modulation

• Amplitude Modulation (AM)

  • Amplitude varies

  • Frequency does not change

• Frequency Modulation (FM)

  • Frequency varies

• Pulse Width Modulation (Digital)

  • Amplitude varies

  • Frequency does not change

Installation of equipment into aircraft and paperwork requirements

• Major alteration

  • Approved data used for FAA form 337

• Minor alteration

  • Equipment list

Notes

• A open switch cannot be a fault