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The Electrical System
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Contents
Owner Maintenance Item; Electrical System Failure; Electrical System; ...Alternators; ...Generator; ...Ammeter; Alternator Failure checklist; ...Voltage Regulator; ...Electrical Failure; Electrical Failure Checklist; Electrical Emergency; ...Position Lighting; ...Starters; ...Precipitation Static; Halogen Lights; Lights;

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 an 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.

Alternators
Alternator failure is the most frequent cause of electrical system failure. If you note alternator failure, shut down as much of the system as you can. My choice would be to leave only the transponder operating. A popped circuit breaker will indicate that an overload has occurred. Reset one time only. Resetting the breaker must be done with a very firm push. It is reset when the low-voltage light goes out. Know where this breaker is and what it feels like when set.

Belt or gear driven. Gear driven alternators can contaminate engine oil on failure as in C-150. Proper installation is important. Life 700 hours +. A belt driven alternator must have the belt at the proper tension. This means about 1/2" flex in the belt when pushed with a finger. Tension can also be checked by giving  90-degree turn to belt.  Too tight will burn out the bearings of the alternator. Too loose will cause the belt to slip under load.

Modern aircraft have electric power that is generated by an alternator as alternating current. AC current is a current that changes direction in cycles as does house current. DC current goes in one direction from minus to plus as in a battery. On electrical failure, AC-powered instruments will hold the last reading created, while DC instruments will read zero on power failure.

An alternator requires a "priming" voltage in the field coil before it can begin to produce electricity. For this reason, starting an aircraft with a totally dead battery will not enable it to produce electricity for the radios and lights unless it is an older aircraft with a generator. A quick battery charge of only a few minutes may be sufficient to activate the alternator field coil.

Once the alternator begins producing voltage the battery's voltage is no longer needed. Instead the alternator re-charges the battery and keeps it charged. Associated with the charging process of the battery and the production of DC current is a rectifier. This device has internal parts that chop off the lower half of the AC cycle and lets the top half proceed. This top half is filtered to take out the ripple of power surges until it can be used by DC instruments.

Most instruments are DC powered. By turning off your electricity, you will find that those instruments that remain working are independently powered by vacuum, pressure, or thermister. Thus, losing the electrical system in VFR is not an emergency except at night and during actual IFR flight. Your turn-coordinator, radios, and electric HIS will cease functioning. The vacuum powered AI and DG will function as will the pressure operated airspeed indicator, VSI and altimeter.

A relatively common failure is related to the electrical system. There are cigarette-lighter plug-ins available that will keep track of your voltage. When the alternator falls off line you must follow the POH procedures since the process may be both type and aircraft specific. Re-set should only be done once. An alternator may work normally until it is subjected to a full load. At maximum load a failed diode may cause total failure. Turning everything on prior to shut down so see what happens could make an alternator check of this condition.

Generator
Older aircraft may be equipped with a generator to provide electrical power to the systems when the engine in running. The generator does not need battery power to begin functioning as does the alternator. This is its only advantage. The generator will provide insufficient electricity during low power engine operations such as pattern work. The battery must provide the difference and will become discharged in an hour of such operations. The increased electrical requirements of modern aircraft often exceed the generating capacity of the generator, hence the alternator.

Ammeter
There are two types, the charge/discharge and the load. The charge -discharge will remain centered so long as the system output can meet the system demand. Beyond this point the needle will indicate a discharge and use of battery power. The load type of ammeter will begin near zero and rise as more electrical load is put online. The voltage warning light will indicate if the load requirement is beyond the alternator's ability to produce. The load meter reflects the actual electrical load as it is turned on. A load ammeter at zero or discharge is saying that you are using battery power The ammeter is an essential element of any pilot's instrument scan. It should be a part of the pre-takeoff, prelanding, and checkpoint checklists.

Alternator Failure Checklist 
1. Low voltage light
2. Loadmeter to zero
3. Ammeter to discharge
4. Popped circuit breaker

Sequence of action
1. Master off
2. Minimum electrical load
3. Check/reset alternator breaker
4. Reset and if it pops again do not reset.
5. Master on

You should know that radios use .5 amp when receiving and 5 amps when transmitting (Ten times as much power to transmit. Nav radios and electric gyros use l amp. Transponders use about 2 amps. You can get an idea of usage by the ammeter reading according to type. As the electrical load increases the alternator tension belt becomes ever more important. Under load a loose alternator belt will begin to slip and the alternator will fail to produce the required electricity. Suspect a loose belt if you get unusual ammeter indications. You can tell you have a voltage problem if you can't hear the transmitter relay.

Voltage Regulator
In the circuit with the alternator is a voltage regulator. It adjusts the field coil electrical flow to provide an output commensurate with the load applied.

If the voltage regulator malfunctions it may activate a high/low voltage light. This is most likely to occur at low power settings. If a circuit breaker "pops", reset only once. If an over-voltage occurs it is important that the alternator be turned off by switch or by circuit breaker. The circuit breaker may be included as part of the master switch. Shut down (except the engine) and start over with the alternator. If the warning light comes on, shut down the alternator. Run off the battery. After the alternator cools down you might try again.

Electrical Failure
There are several forms of electrical failure. The safest form is the blowing of a fuse or the popping of a circuit breaker. Never make more than one replacement or reset before getting on the ground. These protective circuits don't always function. It is technically illegal to jump start an aircraft since the electrical system comes under the airworthiness requirement.

The next likely electrical problem will first appear as a burning odor followed by smoke, an annunciator light coming on, or a failure of some sort. Wiring and wiring connections are the most likely causes of failure. Your preflight should note any loose wiring in the cockpit. and when they don't fire is not far behind. Shutting off the master switch is the emergency first step.

2. Get to VFR conditions 
3. Get on the ground
4. Know where you are
5. Don't navigate on limited power

Electrical Failure Checklist
Master switch off
All switches off
Try battery switch (ammeter indication)
Try alternator switch (Voltage light)
Try one radio and light at a time to isolate problem
Run on minimum equipment
In the event of smoke, shut everything off and get on the ground. 
Run through failure sequence only as second choice. 
If you have a split master switch, you might use just the battery side and the transponder to 7600. Don't reset circuit breakers or replace fuses more than once.

Electrical Emergency
1. Reset master switch
2. Turn off master switch
3. Fire
    Turn off electrical
    Extinguish fire
    Vent cabin-close vents and land
4. Turn off all electrical items and try to obtain voltage
    Check electrical items one at a time.
    Land.
5. Conserve battery power for landing
Electrical Failure Follow-up
You still have:
---notice of an electrical failure
---A PLAN
---conserve battery
---go to closest VFR
---your compass
---VSI
---A south compass heading is your best no-gyro heading
---your air powered heading indicator.
---any handheld devices
---flashlights and charts.
---knowledge of systems
---master battery relay closed above 9 or 17 volts with 14 and 28 volt systems
---knowledge of circuit breakers and fuses
You do not have:
---Pitot heat or deicing that is electrical
---Autopilot
----Turn coordinator
----IVSI (instant vertical speed indicator)
----electric HIS
---hard-wired gps or moving map
---hard-wired radios and navs.
---electric flaps or gear
---master battery relay closed below 9 or 17 volts with 14 and 28 volt systems

Position Lighting
Wingtip navigation lights can be seen in a 110-degree arc from front to sides and vertically for 180-degrees. The rear light need not be on the tail but must be visible 70-degrees to either side of center. The use of low-powered pulsed landing lights as recognition aids is a $200 self-defense system. Such lights must not affect pilot vision or position lights. Wing tip landing lights give greater longevity but make some taxiing more difficult. Part 91 flight at night does not require landing lights. (Good PTS Oral question.) Aircraft switches are on when up or in, off then down or out.
(
Another good PTS Oral question)

The position lights are angled so that they have non-intersecting arcs. At no time can three position lights be seen at the same time except from above and below. Two lights can be seen from head-on, 110-degrees to the left rear, and 110degrees to the right rear.

For collision avoidance if you see a red light give way. If you see a green light expect traffic to yield but be prepared if he doesn't. If you see a white light break right to clear traffic you are passing. If you see red and green you are head-on to traffic, break right.

All lights, position, strobe, and landing lights lose their intensity over time. Periodic replacement is a good idea.

Starters
A student should have knowledge as to the starting mechanism. A key switch activates the starter motor by closing a relay. The initial spins of the starter usually (Not C-150) uses a Bendix spring to engage the starter gear with the engine flywheel gear as with automobiles. Once the engine starts the gear disengages. Occasionally the lack of lubrication or battery power will prevent the gears from engaging. Get help if after four tries the gears do not function.

Starters usually die from overheating while starting hard to start (cold) engines. The Bendix will stick if dirty and fail to retract. Since it remains engaged with the flywheel the Bendix drive will soon fail. Lack of use affects the Bendix, brushes and rusts the gears. These defects require more battery current to operate the starter and cause overheating. Should be part of engine TBO (time before overhaul) cycle.

Precipitation Static
As an aircraft flies through air a static charge can be created on the aircraft. Static dissipation wires are often placed on aircraft trailing edges to remove this static. When such static exists, the radios may make an undesirable noise.

Halogen Lights
Halogen gas allows recycling of the tungsten filament inside the hot quarts thus prolonging the life of the filament.
The higher temperatures also produce a light that looks closer to blue.

Lights
I am wondering about the anti-collision system because FAR 91.209(b) says, No person may:
(b) Operate an aircraft that is equipped with an anti-collision light system, unless it has lighted anti-collision lights.
However, the anti-collision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off. Basic rule is if you have them, use them.

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