Electrical Instruments & Switches

This is a brief run down. There is generally not much in the exam about ny particular detail on these, but you must know them.

Master switch – turns the power to every electrical component on and off (note most engines will run even with the master switch turned off) Check the Pilot Operating Handbook (POH).

master

Ignition Switch – controls whether the engine runs, starts or which ignition it runs on

electrical

Voltmeter – reads the voltage of the battery. Normal voltage is above 12V usually up around 13.5, but check your POH. Very low voltage could indicate a battery failure or generator failure.

Ammeter – shows the battery charge and discharge. They come in various types. They should indicate a positive charge once the engine is running. A negative charge would indicate a generator failure. A very high positive can indicate battery failure.

The answer to ammeter faults are not in many documents, you need to learn and remember it.

If it is high and steady it is a weak, dying or dead battery. If it is high and climbing it is a failed regulator.CASA wants you to recognize you have a failed battery with a high centre-zero ammeter and;recognize you have a alternator with a low centre-zero ammeter

It would be unlikely for CASA to question you on a failed regulator, but it is something you ought to know as it can cook the battery and cause serious in flight safety issues.

Circuit Breakers – are resettable fuses. The break a circuit when the circuit becomes too hot, ie when something draws too much power. Their job is to break the circuit instead of destroying expensive avionics and instruments. They can be reset, but a circuit that keeps tripping has a problem.

circuit

Fuses – contain a piece of metal that melts when the current is too much. This acts like a circuit breaker , however the fuse needs to be replaced.

fuses

Batteries – battery stores the power. It charges during flight from the generator on the engine. Long periods with no flight the battery will need some external form of charging.

0819-0020

Above a typical modern RAA aircraft battery. 12V and approx 8,000 – 15,000 mAh

Please note warnings on lithium batteries from RAA

https://members.raa.asn.au/storage/safety-alert-lithium-batteries.pdf

lithium-batteries

Faulty Battery – When the battery is faulty and will not receive charge the zero-ammeter will indicate a higher than normal charge rate.

General Aviation(GA) Batteries

Most GA batteries are 12 volt and much bigger, as they often crank bigger engines and have more instruments.

The alternator/generator in most GA aircraft put out around 13.65+ volts, so the main system is called a 14V electrical system, even though the battery is only 12 volts.  In some cases they have 2 batteries and 24V and the system is a 28 volt system.

Bus Bar Voltage and System Voltage

Note CASA, Boeing, Airbus, Cessna, Bechecraft, Cirrus, Diamond and I call them 28VDC Systems. The only thing in the system 24v is the battery. The BUS BAR which runs everything is running at 28VDC.

Do not use assumed knowledge from the auto industries is correct.

High Voltage Indicator

In flight a sustained over voltage situation is likely to ruin a battery, fry electronics and may present a fire or fumes or smoke hazard.
However, initially upon startup of the engine, you may notice a temporarily higher voltage while the alternator recharges the battery from the start.
This should not last long, unless your battery is particularly low in voltage.

****Squat Switch

In aircraft with retractable landing gear, a squat switch is installed to prevent downlocks activating when weight is on the wheels. The default position for a faulty switch is to always allow the gear to go down. The gear goes up or down and the up locks and down locks are mechanical. The gear can now only be retracted when the squat switch is activated and allows the down locks to move. The squat switch is activated when weight comes off the gear. To activate, means to be operative. You do not want down locks to be able to operate when weight is on the gear.

Squat Switch