106: Avionics / Electrical Fundamentals

106.1 Define the following acronyms:

a. AACS Aircraft approach control system
b. ACLS Automatic Carrier Landing System
c. ADF Automatic direction finding
d. AFCS Automatic flight control system
e. AOA Angle of attack
f. CAINS Carrier aircraft inertial navigational system
g. GPS Global positioning system
h. AHRS Attitude heading reference system
i. ICS Intercommunication system
j. VOR VHF omnidirectional range

106.2 State the purpose of a thermocouple.

A junction of two dissimilar metals that produce a voltage when heated.

106.3 State the purpose of the following test equipment:

a. Multimeter:

Used for troubleshooting to measure voltage, current, and resistance. The multimeter contains circuitry that allows it to be a voltmeter, an ammeter, or an ohmmeter. A multimeter is often called a volt-ohm-millimeter (VOM).

b. Megohmmeter:

A test instrument that applies high voltage to the component under test and measures the current leakage of the insulation. The megger consists of a portable hand-operated dc generator and an indicating meter. It measures resistances of many megohms.

c. Time domain reflector:

You will use the TDR test set to check and trouble shoot aircraft wiring, transmission lines and antenna systems for shorts, opens, bad couplings, etc. To do this, you will monitor TDR reflected waveforms. TDR’s operate on the same principal as radar; that is they send pulses of energy into a system to see what, if anything, is reflected. The amplitude of the reflected signal corresponds directly to the impedance of the discontinuity. (Discontinuity is any abnormal resistance or impedance that interferes with normal signal flow) You can find the distance to the discontinuity by measuring the time required for the pulse to travel down the line to the reflecting impedance and back to the monitoring oscilloscope.

106.4 State the difference between alternating current (ac) and direct current (dc).

Direct current (dc), that is, current which does not change direction. A coil rotating in a magnetic field actually generates a current, which regularly changes direction. This current is called ALTERNATING CURRENT or ac.

AC AND DC

Alternating current is current which constantly changes in amplitude, and which reverses direction at regular intervals. Direct current flows only in one direction and that the number of electrons flowing past a point in a circuit in one second determines the amplitude of current. If, for example, a coulomb of electrons moves past a point in a wire in one second and all of the electrons are moving in the same direction, the amplitude of direct current in the wire is one ampere. Similarly, if half a coulomb of electrons moves in one direction past a point in the wire in half a second, then reverses direction and moves past the same point in the opposite direction during the next half-second, a total of one coulomb of electrons passes the point in one second. The amplitude of the alternating current is one ampere.

DISADVANTAGES OF DC COMPARED TO AC

When commercial use of electricity became widespread in the United States, certain disadvantages in using direct current in the home became apparent. If a commercial direct-current system is used, the voltage must be generated at the level (amplitude or value) required by the load. To properly light a 240-volt lamp, for example, the dc generator must deliver 240 volts. If a 120-volt lamp is to be supplied power from the 240-volt generator, a resistor or another 120-volt lamp must be placed in series with the 120-volt lamp to drop the extra 120 volts. When the resistor is used to reduce the voltage, an amount of power equal to that consumed by the lamp is wasted. Another disadvantage of the direct-current system becomes evident when the direct current (I) from the generating station must be transmitted a long distance over wires to the consumer. When this happens, a large amount of power is lost due to the resistance (R) of the wire. The power loss is equal to I2R. However, this loss can be greatly reduced if the power is transmitted over the lines at a very high voltage level and a low current level. This is not a practical solution to the power loss in the dc system since the load would then have to be operated at a dangerously high voltage. Because of the disadvantages related to transmitting and using direct current, practically all modern commercial electric power companies generate and distribute alternating current (ac). Unlike direct voltages, alternating voltages can be stepped up or down in amplitude by a device called a TRANSFORMER. (The transformer will be explained later in this module.) Use of the transformer permits efficient transmission of electrical power over long-distance lines. At the electrical power station, the transformer output power is at high voltage and low current levels. At the consumer end of the transmission lines, transformer steps down the voltage to the value required by the load. Due to its inherent advantages and versatility, alternating current has replaced direct current in all but a few commercial power distribution systems.

106.5 State the purpose of a battery.

The battery is a reserve source of electrical power for select electrical systems. The battery converts chemical energy into electrical energy. Provides features such as engine starting, aircraft servicing, auxiliary and emergency power.

106.6 Explain the term static as it pertains to the Pitot Static system.

The pitot-static system in an aircraft includes some of the instruments that operate on the principle of the barometer. It consists of a pitot-static tube and three indicators, all connected with tubing that carries air. The three indicators are the altimeter, the airspeed indicator, and the rate-of-climb indicator. Each operates on air taken from outside the aircraft during flight. Static means stationary or not changing. The static port introduces outside air, at its normal outside atmospheric pressure, as though the aircraft were standing still in the air.