Generals Written – Basic Electricity (AMT / A&P)

Overview

Aircraft electrical systems power essential equipment such as lighting, ignition, avionics, pumps, and warning systems. As an A&P mechanic, you must understand the relationship between voltage, current, and resistance to troubleshoot safely and accurately. Electrical faults can cause intermittent failures, component damage, overheating, and fire hazards. This lesson focuses on the core electrical fundamentals most commonly tested in the General Written portion and used in everyday maintenance.

Core Concepts

Voltage, Current, and Resistance

Voltage is electrical pressure. It is the force that pushes electrons through a conductor and is measured in volts (V).

Current is electron flow. It is the rate at which electrons move through a conductor and is measured in amperes (A).

Resistance is opposition to current flow. It restricts electron movement and is measured in ohms (Ω).

Ohm’s Law

Ohm’s Law connects these three values:

V = I × R

If voltage stays constant and resistance increases, current decreases. If resistance decreases, current increases. This cause-and-effect relationship is foundational for diagnosing faults such as voltage drop, weak motors, dim lights, and overheating connections.

Power

Electrical power is the rate at which electrical energy is used. It is measured in watts (W) and calculated by:

P = V × I

Higher current increases power and heat. Excess heat can damage insulation, terminals, and components. In maintenance, heat is a warning sign that current is excessive or resistance is abnormal at a connection.

Series and Parallel Circuits

In a series circuit, current has only one path. The same current flows through each component. Total resistance increases as more resistance is added.

In a parallel circuit, there are multiple paths for current. Voltage is the same across each branch, and current divides. Total resistance is lower than any single branch resistance. Recognizing circuit type helps you predict what will change when a component fails.

Voltage Drop and Heat

As current flows through resistance, some voltage is lost across that resistance. This is normal across designed loads, but excessive resistance in wires, connectors, crimps, switches, or relays causes unwanted voltage drop and heat.

Common causes of excessive resistance include corrosion, loose terminals, damaged conductors, poor crimps, and incorrect wire gauge. Excessive resistance can create hot spots that lead to intermittent faults and insulation damage.

Circuit Protection

Fuses and circuit breakers protect wiring from excessive current. They are designed to open the circuit before wiring overheats. Replacing a fuse with a higher rating or bypassing a breaker defeats protection and increases fire risk. When a fuse or breaker opens repeatedly, correct maintenance practice is to find the cause (short circuit, overload, faulty component), not to increase the rating.

Maintenance Application

When troubleshooting electrical issues, think in three steps:

• Pressure: Is correct voltage present at the source?
• Flow: Is current reasonable for the load (not excessive, not near-zero)?
• Restriction: Is resistance abnormal due to corrosion, looseness, or damage?

If voltage is correct at the power source but low at the component, suspect voltage drop caused by high resistance between source and load. Measure voltage at key points to isolate the loss. Always remove power before measuring resistance, and follow safe practices when working around energized circuits.

Examiner Perspective

An examiner may ask you to:

• Define voltage, current, and resistance.
• State Ohm’s Law and explain what happens to current when resistance increases.
• Explain what voltage drop indicates during troubleshooting.
• Describe why excessive resistance can cause overheating.
• Explain the purpose of circuit breakers and fuses.
• Identify basic differences between series and parallel circuits.

Common Mistakes

• Confusing voltage (push) with current (flow).
• Applying Ohm’s Law without checking units.
• Ignoring loose/corroded connections as a source of voltage drop.
• Measuring resistance with power applied.
• Replacing fuses/breakers with incorrect ratings instead of correcting the fault.

Memory Anchor

• Voltage pushes.
• Current flows.
• Resistance restricts.
• Ohm’s Law: V = I × R.
• Power: P = V × I.

After completing the quiz, continue to the Quiz Review lesson.