Most of the Questions in the Electrics part of the AGK ATPL Exam are based on the information presented below, it is in no way an exhaustive list, but students should have a good grasp of all the principles that the information below is based.
Students should consider the following list, a minimum level of understanding prior to taking the exam.
The most common units you will need to remember for electrics are the following:
- Work – Joules
- Electrical Resistance – Ohm
- Electrical Current – Ampere
- Electrical Power – Watt
- Frequency – Hertz
There are full 2/4 engine standard electrical system diagrams, that ask questions where you identify connection paths to restore power following certain failures.
It will require you to know the normal operation of the system and the abnormal system requirements to restore certain important parts after failure, of 1 or more generators, or TRUs or Bus Bars. (Too involved for this Blog – requires study)
An AC generator is driven by a drive unit, also known as a Constant Speed Generator Drive System, CSDU or CSD, in a Stable Frequency System.
The main function of this system is to take the varying rotational motion input, from the engine gearbox, which is then converted into hydraulic pressure, which drives a motor, into a stable rotational motion, which drives a generator with an AC stable output.
The output of the hydraulic pump at the units input, produces the pressure which is fed to a hydraulic motor, whose speed is dictated by the angle of a swash plate of the hydraulic pump.
The angle of the swash plate will either limit or increase the oil flow in the hydraulic system.
The angle of the swash plate is controlled by the speed governor, which in turn is controlled by the load controller, tied to the frequency of the AC generator.
As the pressures increases or decreases then the hydraulic motor tries to speed up or slow down. The hydraulic motor drives an AC three phase (commonly) Generator.
The load controller senses the output frequency of the Generator and will adjust the swash plate accordingly to increase or decrease the pressure and so the torque output to the generator.
In most cases the 2 units (generator and Drive) are integrated into 1 unit called an IDG, Integrated Drive Generator.
Frequency wild Generators (direct drive from engine gearbox) can only feed a limited number of electrical devices/systems on an aircraft, and not suitable without additional components for general use.
Transformer Rectifier Units (TRU’s) consists of a unit that combines a transformer and a rectifier.
Therefore it converts Alternating Current (AC) into Direct Current (DC) whilst at the same time transforming an input voltage into another output voltage.
These units are used in situations where AC distribution system needs to supply a DC system.
Batteries work with DC, therefore they require the rectifier to convert from AC to DC, and the transformer to provide the correct voltage for battery charging.
Transformer – A transformer will alter the voltage from what the AC system supplies to a different AC voltage which is required (higher or lower). Transformer only works with AC – There is no such thing as a DC transformer.
Current limiters are large fast acting fuses, used to protect bus bars.
Static inverters are used to convert DC into AC. Normally when a total electrical failure has occurred, and the emergency battery is the only electrical supply left on the aircraft.
Generally, you have volt and ampere indicators in a cockpit (e.g. 737 overhead panel).
Older aircraft have load meters, given in kW, kVA and (kVAR – with the use of a switch).
Ohm’s law states that the current passing through a conductor between two points is proportional to the voltage across the two points.
The current passing through a resistor between two points is proportional to the voltage across those two points.
V = I x R
Therefore we can say that:
V ∝ I
Or in other words the voltage is proportional to the current!
Power in Watts, can calculated by the formulas:
P = V x I (no resistance can be calculated in the question)
P = I2 x R (no voltage can be calculated in the question)
P = V2
R (no current can be calculated in the question)
The circuit breaker combines the function of a fuse and a switch in one unit.
They consist of an automatic thermo-sensitive tripping device and a manually or electrically operated switch.
Besides providing protection to any AC or DC circuit, they can be used to switch a circuit on and off under certain circumstances.
Circuit breakers are special because even after the circuit breaker has tripped, the circuit can be closed again, thus allowing to restore the circuit and potentially power important systems.
The circuit breaker is resettable.
The most common circuit breakers are the bimetallic and magnetic CBs.
The bi-metallic CB consists of two strips of metal bonded together.
The current flowing through the bonded metals will heat them and make them expand and bend.
If the flowing current is too high, the strip will bend so much that it will move a latch tripping the circuit breaker and break the circuit.
Because the functioning of the bi-metallic CB relays on the time it will take to heat it up there are times that it needs to be more responsive.
That’s exactly what a magnetic CB is correcting.
An abnormal increase of the circuit’s current will act on an electromagnet in the circuit breaker and trip it immediately.
Logic gates provide functions that are used in programming and is the hardware in digital circuits.
An integrated circuit may have several logic gate (now can be billions) circuits, each one can have several inputs, but only one single output. You need to recognise the symbols and know the truth tables.
The most common logic gates are the following:
AND gate – All or Nothing gate. Both inputs must be 1 to generate a 1.
OR gate – Any or All gate. If at least one input is 1, the output will be 1.
The output will be 0 only if both (or all) inputs are 0.
NAND gate (AND gate + NOT gate) = inverted AND output, symbolized by the small circle at the output of the gate.
NOR gate (OR gate + NOT gate) = inverted OR output, symbolized again by the small circle at the output of the OR.
NOT gate (invert gate), used to reverse the state of the input.
If input = 0, then output = 1. Only ever has 1 input and 1 output.
These pointers should be considered as a starting point for further revision prior to the AGK exam.