Every time current flows through a wire, a magnetic field forms around that wire. And every time that magnetic field changes, it induces a voltage in the very conductor that created it. This circular relationship — current creating a field, the field creating a voltage that fights the current — is the essence of inductance. For an aviation maintenance technician, understanding inductance is not academic trivia. Inductors appear in aircraft ignition systems, voltage regulators, radio filters, and motor circuits. The FAA General written test expects you to explain what inductance does, why it does it, and how to predict its effects in real circuits.
This article covers self-inductance, back-EMF (electromotive force), the factors that control how much inductance a coil has, how inductors behave in series and parallel, the time constant of an inductive circuit, and practical aircraft applications.
The Physics of Self-Induction
When current flows through a conductor, it creates a concentric magnetic field around that conductor. If the current is steady, the field is steady and nothing unusual happens. But the moment current changes — either increasing or decreasing — the magnetic field changes too. A changing magnetic field, as Faraday's law tells us, induces a voltage in any nearby conductor. When the conductor causing the changing field is the same conductor that feels the induced voltage, the effect is called self-induction.
The voltage produced by self-induction is called the back-EMF (also written counter-EMF or CEMF). The direction of this back-EMF always opposes the change that produced it — a principle stated by Lenz's Law. If current is rising, the back-EMF acts like a voltage source pushing against that rise. If current is falling, the back-EMF reverses polarity and tries to keep the current flowing. In practical terms, an inductor fights any change in current, whether up or down, acting like an electrical version of inertia.
The unit of inductance is the henry (H), named after American physicist Joseph Henry. One henry is defined as the inductance of a circuit in which a current change of one ampere per second produces a back-EMF of exactly one volt. Aircraft circuits typically deal with smaller values — millihenries (mH) or microhenries (µH) — but the henry is the base unit used in all formulas.
What Controls the Amount of Inductance
A straight piece of wire has a tiny amount of self-inductance, but to get useful inductance engineers wind wire into a coil, called an inductor or choke. The coil concentrates the magnetic field and multiplies the self-induction effect. Four factors determine how much inductance a coil produces:
- Number of turns (N): More turns means more inductance. The relationship is actually proportional to the square of the number of turns, so doubling the turns quadruples the inductance. This makes the number of turns the most powerful design lever available.
- Core material (permeability, µ): A ferromagnetic core — iron, silicon steel, or ferrite — concentrates the magnetic flux far more efficiently than air. Placing such a core inside the coil dramatically increases inductance. Air-core inductors are used at radio frequencies where core losses would be prohibitive.
- Cross-sectional area of the core: A larger core cross-section allows more magnetic flux for a given number of ampere-turns, so inductance increases with core area.
- Length of the coil: Spreading the same number of turns over a longer coil reduces the inductance, because the magnetic field density inside the coil decreases. A shorter, tighter winding produces higher inductance.
These four factors are combined in the standard inductor formula, but for the AMT General test the qualitative relationship — more turns, higher permeability, larger area, shorter length all increase inductance — is what you must know cold.
Back-EMF in Detail
The magnitude of the back-EMF depends on two things: the inductance of the coil (in henries) and the rate of change of current (in amperes per second). The relationship is expressed as: VL = L × (ΔI / Δt). A coil with 2 H of inductance experiencing a current change of 3 A in 1 second produces a back-EMF of 6 volts. Notice that the faster the current changes, the larger the back-EMF. This is why inductors are especially effective at blocking high-frequency AC signals — rapid oscillations demand rapid current changes, which generate large back-EMFs that severely limit current flow.
Back-EMF has a very practical safety implication in aircraft maintenance. When a highly inductive load — such as a relay coil or a motor field winding — is suddenly de-energized by opening a switch, the collapsing magnetic field tries to maintain current flow. With nowhere for that current to go, the back-EMF can spike to hundreds of volts in a fraction of a millisecond, easily puncturing the insulation of nearby wiring or destroying solid-state components. Technicians address this with a suppression diode (flyback diode) or a resistor-capacitor snubber wired across the inductive load to provide a safe discharge path.
Inductors in Series and Parallel
When inductors are connected in series with no mutual coupling between them, the total inductance simply adds together, exactly like resistors in series: LT = L1 + L2 + L3 + ...
When inductors are placed in parallel (again assuming no mutual coupling), the total inductance is found using the reciprocal formula, just like resistors in parallel: 1/LT = 1/L1 + 1/L2 + ... The total parallel inductance is always less than the smallest individual inductor in the group.
The assumption of no mutual coupling is important. If two coils are physically close, the magnetic field of one can link with the other — a phenomenon called mutual inductance. Mutual inductance can either add to or subtract from total inductance depending on whether the fields aid or oppose each other. Transformers are deliberately designed to maximize mutual inductance, while aircraft wiring harnesses route inductive components apart to minimize unwanted coupling.
The RL Time Constant
When a DC voltage is suddenly applied to a circuit containing resistance (R) and inductance (L) in series, the current does not jump instantly to its Ohm's Law value. The back-EMF of the inductor slows the current rise. The time required for current to reach approximately 63.2% of its final steady-state value is called the time constant (τ, tau), and it equals: τ = L / R, where L is in henries and R is in ohms, giving τ in seconds.
After one time constant the current has reached 63.2% of final value. After two time constants, about 86.5%. After five time constants, the current is considered to have fully reached its steady-state value (over 99%). The same exponential curve applies when the circuit is de-energized: current falls to 36.8% of its original value after one time constant. A large inductance and small resistance produce a long time constant — the circuit takes a long time to respond to switching. A small inductance and large resistance produce a short time constant.
Why Inductance Matters for Aircraft Technicians
Inductance is not just a classroom concept. You will encounter it in several aircraft systems:
- Ignition systems: The aircraft magneto ignition coil is an inductor. When the breaker points open, the rapid collapse of the primary magnetic field induces a very high voltage in the secondary winding — typically 20,000 volts or more — to fire the spark plug. Back-EMF is the entire operating principle of the ignition coil.
- Electrical noise suppression: RF chokes (inductors in series with a wire) block high-frequency interference from reaching avionics while passing low-frequency DC and AC power. Inductors and capacitors work together in filters throughout aircraft radio and navigation equipment.
- Relay and solenoid coils: Landing gear actuators, fuel shutoff valves, and starter contactors all use inductive coils. Understanding back-EMF spikes helps technicians install proper suppression components and diagnose avionics interference problems.
- AC motors: Aircraft AC motors present an inductive reactance to alternating current in addition to their resistance. The total opposition to AC current in an inductive circuit, called inductive reactance (XL), equals 2π × frequency × inductance, and is measured in ohms. Higher frequency means higher inductive reactance and lower current for the same applied voltage.
Key Numbers and Rules
- Unit of inductance: the henry (H); 1 H produces 1 V back-EMF when current changes at 1 A/s.
- Back-EMF formula: VL = L × (ΔI / Δt).
- RL time constant: τ = L / R seconds; current reaches ~63.2% of final value in one time constant.
- Five time constants to reach full steady-state current (or full decay).
- Series inductors (no coupling): LT = L1 + L2 + ...
- Parallel inductors (no coupling): reciprocal formula; total is always less than smallest individual value.
- Inductance increases with: more turns (squared relationship), higher permeability core, larger core area, shorter coil length.
- Inductive reactance: XL = 2πfL, measured in ohms, increases with frequency.
Common Test Traps
- Direction of back-EMF confusion: The FAA test may ask whether back-EMF aids or opposes the applied voltage. It always opposes the change — it opposes rising current and also opposes falling current. It does not simply oppose the applied voltage at all times.
- Turns-squared relationship: Many students assume doubling the turns only doubles the inductance. In fact, inductance is proportional to N², so doubling turns quadruples inductance. The FAA test exploits this misconception.
- Series vs. parallel confusion: Students sometimes apply the reciprocal formula to series inductors (where it does not apply). Series inductors add directly; only parallel inductors require the reciprocal formula.
- Time constant percentage: The value 63.2% (one time constant) and the five-time-constant rule are both exam favorites. Do not confuse the time constant with the time to reach full current — that takes five time constants, not one.
- Back-EMF spike on de-energization: The test may describe a relay being switched off and ask what happens to voltage across the coil. The back-EMF spike can far exceed the supply voltage, potentially damaging adjacent components — a key reason suppression diodes are required in many aircraft circuit designs.