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. Minimumize 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 transmitt.. 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.
Continue To Next Page