Aircraft Maintenance & Electrical Safety

• Electrical shock, short circuits, loose connections, arcing, and battery service precautions are all key aspects of aircraft electricity safety.
• Electrical power must be removed from components during maintenance.
• Switches should be turned OFF, and circuit breakers should be OPENED.
• Circuit breakers should be "collared".

Placards

• Placards communicate important information.
• Instruction - used to communicate DO NOT OPERATE.
• Reason - Identifies why the power relay is being replaced.
• Identification - Includes NAME, Employee number, or crew.
• Includes the Time and Date.
• Always remove placards when the job is completed.
• Do not remove someone else's placards unless instructed.

Tools

• Tools should be selected to prevent them from causing short circuits.
• Rubber tools do not provide 100% protection.

Grounding Aircraft

• Always connect the grounding wire to the ground post first, then attach the grounding wire to the airframe.

Connecting Ground Power

• Ensure all controls, gear handles, and flaps are in agreement with surface positions.
• Connecting ground power requires training.

Working with Circuits

• Circuits must be turned off before working on them.
• Never touch circuits with metal or bare hands.
• A Digital Multimeter (DMM) must be in SERIES to check current.

Short Circuits

• The aircraft structure of the airframe acts as ground.
• A short circuit occurs if a live electrical contact or wire touches the airframe.
• Short circuits lead to excessive current and heat, resulting in system failure or fire.
• Metal objects left aboard aircraft contribute to short circuits.

Loose Connections

• Loose connections can lead to chafing and arcing.
• They can also lead to open and short circuits.

Battery Servicing

• Remove the negative terminal first, then remove the positive terminal when servicing a battery.

Neutralize Spilled Electrolyte

• For Lead Acid batteries, Sulphuric Acid is the electrolyte, neutralized with baking soda.
• For Nicad batteries, Potassium Hydroxide is the electrolyte, neutralized with Boric Acid.

Thermal Runaway

• Thermal runaway occurs in Nicad Batteries when cell resistances become unstable due to temperature.
• Decreased cell resistance allows more current to flow, further lowering resistance and increasing current.
• This continues until the battery is severely damaged or explodes.

Thermal Runaway Symptoms

• Decrease in cell voltage
• Increase in cell temperature
• Increase in current draw
• "Begging of charge cycle": if recharging, 1.5V appears immediately, soak in distilled water.
• End of charge cycle: occurs when not paying attention or leaving charger on.

Block Diagrams

• Block Diagrams" do not show components, only "stages" of a circuit.
• Block Diagrams are useful for understanding principles of operation.

Pictorial Diagrams

• Pictorial Diagrams" show components as they physically appear.
• Pictorial diagrams are helpful in the assembly process.
• Useful for locating components on a board.

Schematic Diagrams

• Schematic diagrams represent components by icons and symbols and are used for testing and troubleshooting circuits.

Troubleshooting

• Understand how the system is supposed to work.
• Understand how it works currently.
• Understand the reported failures or problems from normal operation.
• Determine the reasons that caused the issues.
• Avoid the "shotgun approach," which involves randomly replacing components.

Lead Acid Batteries

• Voltaic cells are packages made of dissimilar metals and an electrolyte.
• Negative Plate: Surplus of electrons.
• Positive Plate: Deficit of electrons.
• Batteries convert chemical energy into electrical energy.

Types of Cells

• Alkaline: used in standard flashlights
• Zinc-Mercury: used in watches
• Lithium-ion: used in laptops, digital cameras
• Nickel Metal Hydride: used in laptops
• Lead-Acid: used in cars and aircraft
• Nickel Cadmium: used in aircraft and appliances

Primary / Dry Cells

• Primary (Dry) cells are sealed with a "paste-like" electrolyte and are disposable.

Secondary / Wet Cells

• Secondary (Wet) cells use a liquid electrolyte and can be recharged.
• Two or more cells connected electrically form a battery.

Battery Values

• The type of material used in making the cell determines the voltage of the cell.
• The amount of active material determines the current.
• Battery capacity is the amount of current that can be delivered over a given time.
• Capacity is measured in Ampere Hours.
• Ampere Hour Rating: Ability of a battery to deliver a given amount of current over time (e.g., 40 AH = 40 A for 1 hour).

Internal Resistance

• As a battery discharges, internal resistance increases.
• More voltage is dropped across the battery, resulting in less voltage across the load.
• Open Circuit Voltage (OCV) indicates a higher reading than when a load is connected.

Aircraft Lead Acid Battery Voltage

• Typically 12 V or 24 V
• There are 6 or 12 cells
• Each cell has an OCV of 2.1 V
• The actual battery voltage is 12.6 and 25.2
• Used as Main battery/"Ships Battery”/“Storage battery".
• The 2 most common types are NiCad and Lead Acid batteries.

Lead Acid Construction

• Construction includes positive and negative plates insulated by separators.
• Each plate has a framework "grid" and active material held in the grid.
• 90% Lead and 10% Antimony is added to strengthen the grid.
• Active material is applied to the grid as a paste.

Negative Plates

• Spongy Lead is applied to the grid to form the negative plates, which are gray.

Positive Plates

• Lead Peroxide is applied to the grid to form the positive plates, which are reddish brown.

Plate Groups

• Positive plates make up a positive plate group.
• Negative plates make up a negative plate group.

Cell Assembly

• Plate groups are intermeshed with rubber or fiber separators to prevent touching.
• Plates are immersed in an electrolyte of 70%, 30% sulfuric acid.
• Each cell has one more negative plate than positive plates which reduces warping and increases chemical interaction.
• Cells are electrically connected in series.
• Space at the bottom of the case allows “active material" that falls off the plates to collect without shorting out the cells.

Chemical Action During Discharge

• Positive plates are Lead Peroxide.
• Negative plates are spongy Lead.
• Electrolyte is made of sulfuric acid and water.
• When the battery delivers current, a chemical change occurs.
• During discharge, the electrolyte breaks up into Hydrogen ions (positive ions).
• Sulfate ions are negative ions.
• Sulfate ions combine with spongy lead plates (negative plates).

Continued Discharge

• Converts active material into lead sulfate.
• Delivers extra electrons to the negative plates.
• Hydrogen ions combine with lead peroxide plates (positive plates).
• Hydrogen combines with oxygen in peroxide water, forming lead peroxide into lead.
• Lead combines with sulfates to form lead sulfate.
• A high percentage of water in the electrolyte and all plates convert to lead sulfate.
• Electrolyte diluted by water and plates covered with lead sulfate means no chemical reaction is is discharged.
• Remaining in a discharged state hardens the lead sulfate hardening which is termed “sulfated".
• A sulfated battery cannot be recharged.
• When a battery is charged, sulfate ions are driven back into the electrolyte to form sulfuric acid.
• This returns the plates into og composition of lead peroxide and sponge lead.

Safety Precautions

• Wear all Personal Protective Equipment (PPE) at all times.
• Neutralize spilled electrolyte with baking soda.
• Flush the area with water.
• Do not wear metal while servicing.

Lead Acid Battery Servicing

• Remove the ground wire first, and connect it last.
• Recharging generates oxygen and hydrogen.
• Vent caps allow gas to escape, but should be removed during charging.
• Use Positive (red) and negative (black) polarity when charging a lead acid battery.
• Adjust the charger to proper current and voltage.
• Add proper electrolyte to each cell.
• Add distilled water if required.
• Check and record the Specific Gravity (S.G.) of each cell.
• Turn on the charger.
• The state of charge can be determined by checking the specific gravity of the electrolyte with a hydrometer.
• S.G. is the density of the electrolyte compared to the density of water.

Specific Gravity of Lead Acid Batteries

• The S.G. of the electrolyte decreases as the battery discharges.
• During charging, sulfates are driven from the plates back into the electrolyte.
• Since sulfuric acid is denser than water, S.G. increases during charging.
• A lead acid cell should read between 1.275-1.300 when fully charged.
• The battery should be charged until cells are gassing and S.G. has remained the same for over an hour.

Cold Weather Operations

• All batteries perform better in warm conditions.
• Keep cold batteries fully charged to prevent freezing.
• A cold battery will read higher S.G.
• A hot battery will read lower than actual S.G
• The electrolyte temperature must be considered when measuring S.G.

NiCad Batteries (Nickel Cadmium Battery)

• Plates manufactured by sintering (heating at high temperature in a reducing atmosphere)
• Pure carbonyl nickel powder around a woven wire screen.
• Thin and highly porous at 85%.
• Imprinted with nickel salts for positive plates and Stadium salts for negative plates.
• Electrochemically active forms of these are nickel (oxy)hydroxide and cadmium hydroxide.

Electrolyte

• Has a specific gravity of 1.3 (1.24-1.32).
• Consists of 30% potassium hydroxide (KOH) and 70% distilled water.
• Used to transport ions between positive and negative plates.

NiCad Chemistry

• Negative plates are metallic cadmium (Cd).
• Positive plates are nickel oxyhydroxide (NiOOH).
• Electrolyte is potassium hydroxide (KOH).
• During discharge, electrons move from positive to negative plates through the electrolyte, and the process is reversed during charging.

NiCad Chemistry Equation

• Discharge: 2NiO(OH) + Cd + 2H2O
• Charge: 2Ni(OH)2 + Cd(OH)2
• Positive Plate and Negative Plate.

Voltage Characteristics of NiCad

• NiCad cells have a voltage of approximately 1.2 Volts.
• Maintains full voltage until almost completely discharged.
• Specific gravity should be between 1.24 and 1.32.
• An Open Circuit Voltage (OCV) of 1.28V is consistent with all vented Nicad cells.
• A Closed Circuit Voltage (CCV) should be between 1.2 and 1.25V.

NiCad Advantages

• Higher power to a weight ratio.
• Capable of providing very high currents due to low internal resistance.
• CCV remains constant throughout the discharge cycle.

NiCad Battery State of Charge

• Electrolyte does not significantly change electrolyte during charge/discharge processes.
• Hydrometer measurements cannot determine the state of charge.
• Only determined by putting a known amount of charge into it.
• Nicads are charged to 140% of their Ampere hour rating.
• A 50 AH battery would have 70 AH of current.
• Charge the battery at its 5 hour rate for 7 hours.

Nicad Battery Overcharging Risks

• Electrolyte will decompose and hydrogen will be released at negative plates as well as oxygen at positive plates leading to risk of explosion.
• A cellophane barrier prevents O2 from reaching the negative plates which prevents excessive heat by chemical reaction and thermal runaway.

Nicad Servicing

• Protective equipment at all times.
• Neutralize electrolyte with Boric Acid.
• Flush area with water if electrolyte spills.
• No metal should be worn.
• Nicad Thermal Runaway can occur at the beginning and end of the charging process.

Thermal Runaway Symptoms

• Decrease in cell voltage
• Cells become warm
• Decrease in resistance
• Increase in current draw
• Shut off the charger if suspected.
• Leave the room immediately.
• Notify a professor.

NiCad Record Keeping

• Maintain a battery log to make note of cell charges, hot cells, and cells that require water.
• Record cell voltages hourly.
• Ask a professor if unsure.

NiCad Servicing Procedures

• Cell vents do not need to be removed for charging.
• Add water immediately after charging.
• Never adjust electrolyte level unless the voltage is unusually high, with distilled water ONLY.
• Deep cycle the battery which may be tested, recharged, and returned to service.
• Step 1: Charge the battery, routing it to the shop.

Key steps in the process

• A capacity check is performed.
• If battery passes it is recharged.
• If the battery fails, a deep cycle is performed (fully discharged/recharged).
• A Battery that passed is tagged as "serviceable," and paperwork is completed.
• Use 140% of AH rating is delivered into the battery and connect the battery to the charger and place on “main charge”.
• Add 5-10cc of distilled water to any cells reading more than 1.5V and Watch for thermal runaway.
• Make sure all cells measure between 1.55V to 1.75V.

When complete

• Turn off the charger
• Disconnect the battery
• Check the cell for leaks and if the leakage current is >100ma, the cell must be recycled.
• Step 2 -capacity test, connect the battery to the discharge cycle.
• If the low voltage light illuminates or ANY cells drop >1.0 V, the battery has failed the capacity test.
• If it passes, allow it to recover for at least 1 hour (followed by step 4).
• Step 3 - Deep cycling is only performed when the battery fails. Place the analyzer selector witch to the manual discharge and Monitor cell voltages as the battery discharges.If a If a cell drops >0.5 V, short that cell and a landing light may be used to expedite the discharge process is completed.
• After all the cells are shorted, leave the battery to rest for at least 3 hours and remove the shorting strips.
• After the battery is fully discharged, wash the battery with a water and non-metallic brush.
• If the battery failed the "leakage current", disassemble them and clean them (wash and reassemble) and dry them with compressed air.
• The battery is now ready for charging.

Charging

• Step 4 - Perform a final charge.
• After passing the capacity check or deep cycle, allow to recover it one hour.
• After a delay, set to main charge to complete the charge and monitor for a thermal runaway.
• Step 5 - Complete the paperwork when fully recharged and place on the serviceable rack, and ensure the log sheets are completed.
• Since the state of charge of a Nicad cannot be determined, it's essential that all paperwork is completed, preventing the repetition of previously performed work.
• When Mixing Nicad and lead acid, one requires electrolytes capable of neutralizing each other, even the fumes.
• They aren't serviced in the same area and should not share tools.
• Venting & sump jars provide an airflow over the battery.
• They provide cooling and removes gasses, and sump jars containing Boric acid or baking soda neutralize discharge . They are checked daily.

Week 2B: Electrical Load Analysis

• All aircraft require an electrical load analysis to be performed.
• ELA provides indication showing if the electrical power generation system can handle all the electrical loads in the aircraft.
• Aircraft from manufacturers will have ELA from design, certification, and testing phase of the aircraft development & manufacturer.
• Changes to electrical loads have to be analyzed to determine if the existing power generation system can support the changes later in life.
• The total continuous load should not exceed 80% the output load of the generator or alternator.
• If there is a net increase in electrical load, further analysis is required and must list both the continuous and intermittent electrical loads on the aircraft.

Normal operation processes

• Identify what phase of flight the loads used in
• Identify whether a load is used in a specific or normal phase of flight
• Identify a calibrated ammeter on a feeder line and switch on loads to record current.

Alternator & Generator Components

• Generators, alternators, starter generators and external power (batteries) are the main electrical components
• Electrical power is produced by electromagnetic induction, converting mechanical energy into electrical energy.
• Principle 1: When relative motion between conductor and a magnetic field happens, CURRENT FLOW is induced in the conductor.
• Principle 2: When current flows through a conductor, generated a magnetic field is created around the conductor.
• Generator: Utilizes multiple conductors (called ARMATURE) that rotate within a magnetic field
• Alternator: Rotating magnetic field which causes current to be induced into a stationary conductor (called a STATOR).

Generators and Alternators

• Aircraft use mechanical power from the engine to rotate the generator's ARMATURE and the alternators FIELD.

AC Generator

• Pole piece
• Armature
• Slip ring -Brush Means and rotating armature of delivering a current to the load.
• Stationary magnetic field will deliver an alternating current to the load with slip rings and brushes.
• Alternating current basically turns on and off many times a second.
• Because DC power is not AC, a Split ring or commutator is used in DC generators instead of a slip ring.
• Communications convert alternating current from the armature to a pulsing direct current at the brushes.

AC vs DC generators

• Difference depends on whether slip rings or a commutator ring is used to deliver the armature current to the brushes.
• Practical generators are an electromagnetic field using output which allows a change of output voltage with this “Field winding"
• The construction includes Armature, Field Frame and Brush Assemblies, and then the End Frames. The wire coils are wound around a laminated soft-iron core which generates a current as it rotates.
• Windings connected to a commutator ring which delivers output current to load.

Field Frame

• Constructed with Iron or steel.
• Field windings are attached to the inner wall of the field frame, are rectangular.
• Small generators can have 2 or 4 poles, but larger ones may have more.
• Brushes are located at commutator end of the generator

Components

• Carbon brushes are spring loaded to ensure constant surface contact and constant friction which means they are regularly checked.

Neutral plane

• Brushes make contact with commutator segments at the point where no current is being induced in the windings when in the position known as the NEUTRAL PLANE

Armature reaction

• Current alters main generator field causing compensating windings or interpoles to be necessary to maintain this.
• Alternators have a rotating magnetic field and a stationary output winding (Stator)
• A stationing magnetic field and a rotting armature (windings) will output ac

Alternators cont.

• Automotive are similar producing current to be used to charge batteries, a DC output used to power the systems
• High Output alternators provide AC for aviation (PORTION may be rectified for DC), on the side below the cockpit.

Week 3B: AC Power Generator

• AC vs AC Alternator
• AC Generator has Fixed magnetic field
• Fixed Magnetic field, Rotor Magnetically field

Components

• Alternator has 1 or 2 pairs of magnetic field, 2 coils = 1 complete cycle for each rotation, 4 coils is 2 cycles for each rotation.

AC frequency control

• Some system needs Stable Freq, controlled by some system needs stable frequency, Frequency output is proportional to rotations speed using Constant CSD and state solid
• Constant speed drive (CSD) is used to maintain the 400 hz.Hydromechanical (CSD) and solid state are used to maintain constant AC.
• CSD is an output driven by engine gearbox, but maintains output even if Engine Flucuates Hydro Trans.
• Variable RPM with Axial gear set, and sight glass. The IDG (integrated drive generator) is the constant speed drive and alternator.
• A VSCF or brushless alternator is commonly used to deliver power.
• VSCF is a brushless alternator driven by a gerbox.
• Small/light aircraft provide using smaller, lighter alternators.

Alternators

• Large aircraft uses shaft driven alternators with higher output and referred to as "Generators".

Week 4: Components

• Generator output depends on the rate at which Magnetic lines of flux are cut.
• 3 Factors- rotation,armature, lines.

Manual regulation

• Adjustment to field controls outputs of generator. Output increase
• Vibrators- As output increases, field strength increases.

Carbon pile Voltage Regulator

• Field limited by carbon pile voltage, less current=output decreases.
• Solid State-
• As voltage change bias transistors, increasing or decreasing output
• Enables buses to be powered, voltage equalized to current bus. Once equal can be added to same source.

Reverse Current Relays

• The reverse relays acts as a prevent in the reverse or lower state output

Current limiters

• Current Limiters Prevent current
• Generator Controls units
• GCU is hot ground- open the fault
• Current Transfer are used to sense the voltage and other electrical characteristics

Load & transfer shedding

• Processes to drop the non-Essentials to transfer. -Flashing The field
• Starting the engine and the residual field helps the process to work

Week 5A: Power Conservation

• Invert DC to AC source like emergency AC sources
• in the large aircraft.
• 26 VAC in small plane

Types

• Static and rotary like DC input motor and generators
• Transformers increase AC current
• Theory of operation when corresponding coil is set on the current.
• Rectifiers converts AC to DC

M1 Aircraft

• Electronic bus acts school with load circuits that collects and distributes powerful electricity Electrical bus

Week 5B: M2 Aircraft / B737 Series aircraft

• 3 Types- split, parallel, and split Parallel Busses

Components cont.

• Generator one bus, for protection, parallel all is for the same source.
• The bus generator is designed to trip or reset
• Like a switch or relay.
• Used source bus for failure, B737-200 own CSD system.

B737 MAX Busses

• Max has Both engines drive 90 KVA- APU
• Output include DC power with 26nicad.
• Power used in the ENTIRE AC. The power used in the AC is