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Owner Maintenance Items:
--The landing light is the only electrical part that FAR Part-43 authorizes owners or pilots to repair. Any repairs must be entered into the aircraft logbook.
-- Advisory circular 25-10 guidance for Installation of Miscellaneous Non-required Electrical Equipment

Electrical System Failure
If you understand your aircraft electrical system, you will be able to judge the seriousness of any electrical failure. For example, the engine has its own independent method of creating operating electrical power through its magnetos. The engine will continue to operate without a battery, alternator or other electrical failure in the system.

There are two forms of electrical power in an aircraft, aside from the magnetos for the engine. The battery is a storage cell of direct current (DC) electrical power whose main purpose is to start the aircraft engine and provide limited back-up power and operational life for all systems that require electrical power.

Batteries are life limited in time and power. Once the alternator fails, the battery will begin to lose power. Depending on the conditions of flight, you are better off to kill all unnecessary systems. Turn off the high drain electrical components such as landing lights, strobes, navigation lights, and cockpit lights if you have a flashlight. Do this day or night. Use only one radio and your transponder. Tell ATC of your problem and tell of your intentions and then shut down the radio. At night your intentions are to get down in VFR.

The life of an aircraft battery is about three years. After that it is best to limit your flying to VFR conditions. For IFR make sure you have a good battery for insurance. With a voltmeter on the panel you can, over a time, determine how good the battery is by noting how quickly the voltage recovers from an engine start. When a 14-volt battery reaches 11 volts you are looking at trouble.

Older aircraft, say built before 1960, are most likely to have generators. The generator's operation is directly related to the engine rpm. It will put out DC electricity once the engine is started and does not need a battery. The ability of the generator to produce electricity is low when the engine rpm is low and better when the rpm is higher. An airplane flying too long at low power will eventually drain the battery.

The alternator operates differently. It needs the battery to provide some current to the primary coil before it can produce electricity. Once the alternator gets going its electrical output is not limited to rpm. However, the alternator produces alternating current (AC) which can be converted to DC through the use of a voltage regulator.

The electrical system has protective devices for each electrical element. A fuse will burn out before the wiring burns; a circuit breaker will pop-out to warn you of an overload that could damage the wiring or device. A fuse from the required aircraft reserve supply can be used to replace a failed fuse.
Never replace a fuse more than once.   A circuit breaker can be safely re-set once, but not twice.

Required Knowledge for Basic Operation:
--What to shutdown for minimum operation in conditions
--How to reset alternator or other circuit breaker. Do not reset a hot circuit breaker.
--Normal draw on a circuit breaker is half the amperage of the breaker.
--Amp hour rating of battery (Assuming the battery is new or fully charged)
--Amp hour draw of various equipment Each electrical component has papers showing its electrical draw.
The FAA restricts the electrical draw to the maximum alternator capacity
.
--Know the kind of ammeter you have, there are two types, and how to read it.
The zero ammeter is centered when operating correctly. It only goes negative when discharging. Its
Normal charging condition is slightly to the right of center as it charges the battery.
The load ammeter gets higher and higher as electrical components draw on its capability.
I have found the minimum draw possible is just using the transponder.

Depending on several factors the importance of an electrical failure extends from zero to fatal. Avoid use of anything with a heater element, transmitter or motor. This will preserve battery power. When it happened to me, I advised ATC and asked that they call ahead to my destination to advise them that I would be making a NORDO arrival. I then shut down all electrical until it was time to get the ATIS.
--VFR and in radio contact with battery power sufficient to operate avionics
--Day IFR instantaneous failure
--IFR night instantaneous failure in turbulence

Electrical System
The ignition system is independent of the rest of the electrical system. The ignition system is traditionally totally redundant. Thus, any problem with the electrical system is not an emergency requiring immediate action from the pilot.

The master switch or one-half of a split-master is a safety device to keep the high battery amperage used in starting out of the cockpit. The master activates a battery solenoid and relay. This relay lets electricity go to the primary bus. The primary bus is a relatively heavy metal bar capable of carrying heavy current loads. Each electrical circuit is connected to the bus bar as a separate terminal. The master switch makes it possible to use battery power to start the plane, but has nothing to do with keeping it running. The magneto system that runs the engine is independent of the master switch and battery. With the magnetos on, just turning the prop as little as 1/8 turn can cause the engine to start.

In line with this circuit is a fuse or circuit breaker which is sized to the wire used. Aircraft wiring is specially coated and covered to prevent corrosion. Automotive wire is not suitable for aircraft. The purpose of a fuse of circuit breaker is to protect the wiring from catching fire. Co-incidentally equipment is protected. Some fuses are slow-blow. This allows the fuse to accept a momentary overload without blowing. Some breakers can be pulled off (gear) but every breaker can be reset. No breaker should be held closed. At most, reset a breaker only once before getting expert advice/maintenance.

If the battery relay is not activated because of low voltage in the battery, there is no way for the alternator field coil to develop the voltage needed to recharge the battery or run the electrical system. When the battery is low some of the alternator energy is used for recharging. This recharging battery-load is bad for the electrical system. Having a cigarette lighter voltmeter is desirable.

Aircraft use lead-acid batteries made of a series of such cells to get the desired voltage. To reduce weight the battery casings are made lighter and smaller for aircraft. A 12-volt battery has six lead-acid cells giving two volts output each. Positive plates of lead dioxide and spongy lead are kept separated from the negative spongy lead plates in a sulphuric acid/water electrolyte. Battery capacity is measured in ampere-hours. Over its three to five-year life the battery capacity will decrease. A charged battery has an unbalance of electrons between the plates. When an electrical circuit is completed the excess electrons from one plate flow through the electrolyte to the other. This flow creates lead sulfate, which will eventually result in a 'dead' battery. Aircraft batteries can be run down in just a few minutes at the high amperage required for starting. Capacity and charge determine endurance. A load test is used to determine capacity. The airworthiness of an aircraft battery is about two years. A dead or weak battery is unairworthy.

The density of its electrolyte determines the charge of a battery. This state is checked with a hydrometer that measures specific gravity as it is floated in the electrolyte. Hydrometer readings are taken before adding water. Electrolyte is normally 30% acid by volume. Never add water to a battery unless it is below the plates. Charging will cause the level to rise. A specific gravity reading below 1.24 is considered low for an aircraft because of high load requirements. Such a battery needs to be charged or replaced. Normal range of specific gravity is from 1.26 to 1.3. A three-amp charge rate will be best to preserve the battery and prevent overheating. A weak battery is unairworthy. A dead battery puts a great load on the electrical system. Such loads are the number one cause of electrical system failure. All battery maintenance should be recorded in the airframe logbook per FAR 43.9 as to work performed.

Either current or voltage methods can do aircraft battery charging. A battery must be removed from the aircraft for charging. Remove and attach the negative cable first and last. Quick chargers will reduce battery life expectancy. The constant current method requires more time and can result in overcharging if excess time is used. Vent caps, which allow the battery to breathe, should be loosened but not removed during charging. Avoid sparks, which may ignite the hydrogen and oxygen being vented during the charging process. The aircraft alternator/generator system has a constant voltage due to its voltage regulator. An exploding battery is a terrifying experience. The battery may be allowed to become warm during charging but not hot. Allow the battery to rest for several hours and give final specific gravity test before reinstallation.

An aircraft battery is kept in a metal box for electrical and mechanical shielding. Each battery cell is sealed in hard rubber with a non-spill vent cap on top. A lead weight seals the vent during inverted flight. The hydrogen gas vented from a battery is highly explosive if ignited. No smoking around batteries. The battery box has an exhaust tube through which battery gases are vented. In some installations, intake air and a drain sump neutralizes the gasses before venting. If a cable is loosely connected or corroded, or if there is a direct shorting of the battery terminals the current may burn off the terminal and start a fire.

Rapid discharge or charging ruins a battery and shortens its life. Heat and cold cause shortened life. Keeping fluid levels correct in the summer is important. Batteries have a life potential of thousands of charge and discharge cycles. Longer periods between starts extend life. A battery with a low charge can be permanently damaged. The greatest killer of batteries is over-charging. The colder the temperature the greater charge that can be applied.

A newer type nickel-cadium (NiCd) is much more expensive but require less maintenance and have a longer service life. Voltage of the NiCd is constant up to 90% of its discharge cycle. NiCds use potassium hydroxide electrolyte, which as a base instead of acid requires the use of special tools and techniques. NiCds must be bench checked with special equipment.

A still newer type called a recombinant gas (RG) type is designed so it can't leak, doesn't require a sealed box, has greater capacity and gives more power. It uses a glass mat to soak up the acid and hold it in suspension. The mats can be packed closely with more lead plates per cell. The RG battery has greater capacity and is heavier for its size. It can be safely shipped by UPS.

Battery maintenance can be done as part of pilot's authority under FAR 43.3(g). For removal always remove the negative lead first and replace it last. Do not charge a battery while it is on the aircraft. A charging battery will release gases that can be exploded by a spark. Do not add water prior to charging. Terminal corrosion can be prevented by coating with a terminal sealer after reconnecting.

The battery starts the engine, which is geared or belted to the alternator, which, with initial assistance of the battery, generates AC current. DC current is needed for most of the instruments and primary field coil of the alternator. The battery is the primary source of DC power until the alternator produces the AC current needed to be converted into DC. None of the electrical power provided by the alternator has anything to do with engine operation.

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