CAA NZ AME Electrical cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In DC circuits these three forms are interchangeable, so choose the one whose two quantities you already know.

Series circuit: R_total = R1 + R2 + ...; current is common to all elements; voltages add. Parallel circuit: 1/R_total = 1/R1 + 1/R2 + ...; voltage is common; branch currents add.

Kirchhoff's laws: current law - sum of currents into a node equals sum out; voltage law - algebraic sum of EMFs and drops around any closed loop is zero.

Capacitance in farads: C = Q/V; energy stored = ½CV². Capacitors block DC and pass AC; capacitive reactance X_C = 1/(2πfC), falling as frequency rises.

Inductance: energy stored = ½LI²; inductive reactance X_L = 2πfL, rising with frequency. Inductors oppose change in current, so current lags voltage in an inductive circuit.

Impedance Z = √(R² + (X_L − X_C)²); phase angle θ = arctan((X_L − X_C)/R). At resonance X_L = X_C, Z = R and the circuit is purely resistive.

AC values: RMS = peak/√2 ≈ 0.707 × peak; average of a full sine is zero; form factor = RMS/average = 1.11 for a sine wave.

Transformer relationship: V1/V2 = N1/N2 = I2/I1. Power in equals power out for an ideal transformer; a step-down in voltage is a step-up in current.

Three-phase: line voltage = √3 × phase voltage in star (wye) connection; line current = phase current. In delta, line current = √3 × phase current and line voltage = phase voltage.

A relay is an electromagnetically operated switch: coil current creates flux that pulls an armature, closing or opening contacts. A contactor is the same principle scaled for heavy current loads.

A solenoid converts coil current into linear mechanical force on a plunger; it is a single-acting actuator, not a switching device with separate load contacts.

DC motor rotation reverses if either the field polarity or the armature polarity is reversed, but not both. Series-wound motors must never be run unloaded - overspeed results.

A generator's output frequency depends on poles and speed: f = (poles × rpm)/120. Voltage depends on field strength and speed, controlled by the voltage regulator.

A starter-generator is a single machine that cranks the engine as a motor, then generates once the engine runs; the changeover is made by external contactor logic, not by the machine itself.

Diodes conduct from anode to cathode above forward threshold; a freewheel/suppression diode across a coil clamps the inductive spike when coil current is interrupted.

Thermal circuit breakers trip on current-squared heating and are ambient-temperature sensitive; magnetic types respond to instantaneous overcurrent and are less affected by temperature.

A voltage regulator senses generator output and modulates field current; overvoltage protection protects the bus, undervoltage protection protects loads from abnormal supply.

Typical AC aircraft system: 115 V AC, 400 Hz, three-phase. Higher frequency reduces transformer and motor iron weight; 400 Hz is the long-standing transport standard.

Typical DC system: 28 V DC, provided by transformer-rectifier units on AC aircraft or by engine-driven DC generators on smaller types. 14 V systems exist on light aircraft.

Bus architecture is hierarchical: main/generator buses feed essential buses, which feed battery/battery-direct buses. Essential and emergency buses remain powered after loss of normal generation.

External power receptacles include a polarity/voltage sensing circuit that prevents closure of the external power contactor if polarity is wrong or voltage is out of limits.

A generator drive (constant-speed drive or integrated drive generator) holds frequency constant as engine speed varies; a drive disconnect is provided for a failed drive.

Split-bus and bus-tie contactors allow one generator to feed both main buses; the tie closes automatically on loss of one generator only if the remaining source has capacity.

Battery state: a lead-acid or NiCad battery supplies emergency power, absorbs bus transients, and provides APU starting. NiCad batteries require electrolyte level checks and can suffer thermal runaway.

Ground power must be connected and verified before switching the external power switch; the aircraft battery is normally disconnected from the external power path to prevent paralleling of dissimilar sources.

Wire selection is by current rating, voltage drop and temperature rise; derating accounts for bundle size and ambient temperature. Voltage drop is calculated as V_drop = 2 × L × I × R_per_unit_length for a two-wire run.

Aluminium wire requires special terminals and anti-corrosion treatment; copper is standard. Never splice aluminium to copper without an approved connector - galvanic corrosion results.

Bonding resistance limits: a primary bond (structural return path) is typically required to be below 0.0025 ohm (2.5 milliohms); secondary bonds for equipment grounding are held to a lower-current but still low-resistance limit.

Insulation resistance testing uses a megohmmeter at a specified test voltage; the reading must be taken after the circuit is isolated and discharged, and compared to the maintenance manual minimum.

Continuity testing proves a low-resistance path; bonding testing proves the resistance between two structural points is within limits. Both require a calibrated meter and a clean contact point.

Shielded wire terminates the shield at one end to avoid ground loops; the shield is grounded to the connector backshell or a dedicated ground point per the wiring diagram.

Wire identification uses a marking code giving circuit function, wire number and segment; the same code appears on the wiring diagram and the physical wire.

Arcing and loose terminals are found by torque checks, pull tests and thermal inspection; a loose terminal shows heat discolouration, not necessarily a continuity failure on a low-current meter.

CAA NZ rules: Part 43 covers maintenance; Part 66 covers licensing of aircraft maintenance engineers; Part 145 covers maintenance organisations. Know which Part governs which activity.

A maintenance release certifies that the maintenance has been carried out and the aircraft is airworthy; it is issued by a person holding the appropriate licence and rating.

Aircraft maintenance engineer licences carry category and type ratings; work outside the holder's rating requires certification by someone appropriately rated.

Continuing airworthiness is the operator's responsibility, supported by the maintenance programme approved under the applicable rule; the programme defines scheduled tasks and intervals.

Defect recording and rectification: defects are entered in the technical log or maintenance record, and deferral is only permitted where the approved documents allow it with defined limitations.

Form 1 / authorised release certificate accompanies parts and certifies airworthiness status; a part without acceptable release documentation is not eligible for installation.

Occurrence reporting: certain defects and incidents must be reported to the CAA; the requirement is defined by rule, and the report is separate from the maintenance record entry.

Approved data hierarchy: the maintenance manual and its revisions, service bulletins, airworthiness directives, and approved design data. Work is performed to the current revision - check the revision status before starting.

Airworthiness directives are mandatory; service bulletins are usually recommended unless made mandatory by an AD or the operator's programme.

A task card or work pack translates manual instructions into a recordable step sequence; sign-off occurs at each step, not retrospectively at the end of the task.

Human factors: the 'dirty dozen' includes complacency, fatigue, pressure, distraction, lack of communication, lack of resources, lack of teamwork, lack of knowledge, norms, stress, assertiveness and awareness.

Error defences: independent inspection, dual sign-off on critical tasks, tool control and shadow boards, and a clean handover at shift change.

Shift handover must be verbal and written: what is complete, what is in work, what is unserviceable, and what tooling or parts are staged.

Aircraft documentation must be completed in permanent ink, legible, dated and signed; corrections are made by a single line through the error with initials, never by overwriting or erasing.

Fatigue degrades attention and error detection; regulated duty limits exist because fatigue is a recognised maintenance error precursor, not a personal failing.

A voltmeter is connected in parallel with the component; an ammeter is connected in series. Connecting an ammeter across a supply will short the circuit and blow its fuse.

A clamp meter measures current by sensing the magnetic field around a conductor; it measures one conductor at a time, so a two-wire cable read gives the difference, not the load current.

Insulation testers apply a high DC voltage; always isolate, discharge capacitors, and disconnect sensitive electronics before testing to avoid damage.

Lockout/tagout: isolate the source, prove dead with a meter, and tag the isolation point before working on a circuit that could be energised remotely.

Static discharge damages electronic components; use a grounded wrist strap and conductive packaging when handling line-replaceable units containing circuit cards.

A meter's burden voltage and internal resistance affect readings; a low-impedance meter loads the circuit, so use a high-impedance meter for electronic measurements.

Thermal imaging locates high-resistance joints and overloaded conductors by temperature rise; it is a condition-monitoring tool, not a substitute for a resistance measurement.

No output from a generator: check drive condition, field supply, voltage regulator and output contactor in that order before condemning the generator.

Overvoltage trip on the bus: suspect the voltage regulator or a lost sensing lead; do not reset repeatedly without finding the cause.

A bus that will not power from either generator or external power points to a contactor or wiring fault, not to both sources failing simultaneously.

Intermittent faults: check connector pin tension, chafing points and bonding straps; vibration-related faults often clear when the aircraft is on the ground.

Load analysis: total connected load must not exceed generator capacity; adding avionics without a load analysis can cause nuisance trips.

Battery that will not hold charge: check electrolyte, specific gravity, charging voltage and parasitic drain before replacing the battery.

A tripped circuit breaker should not be reset more than once without investigation; a repeated trip indicates a genuine fault, not a nuisance.

Minimum bend radius and support spacing are specified in the maintenance manual; exceeding them stresses conductors and insulation.

Wires are routed with adequate clearance from hot surfaces, fuel lines and moving parts; use standoffs and grommets at penetrations.

Connector assembly: correct insertion/removal tool, correct contact position, and a pull test on each contact before closing the shell.

Terminal lugs are crimped with the specified tool and die; a solder joint is not a substitute for a crimp unless the manual permits it.

Wire splices are permitted only where the approved data allows, using approved splice types, with the correct insulation system.

Cable ties are not over-tightened; over-tensioning crushes insulation and can cause chafing failures later.

Marking and identification must remain legible after installation; heat-shrink labels are preferred where the environment degrades ink.

Maintenance records must be retained for the period specified by rule; the operator holds the record set and must produce it on request.

A certificate of airworthiness remains valid only while the aircraft is maintained per the approved programme and all ADs are complied with.

Major modifications require approved design data and a supplementary type certificate or equivalent approval before installation.

The maintenance control manual describes how the operator controls maintenance; it is approved and must reflect actual practice.

Licence privileges are defined by category; a person may only certify work they are rated and current for.

Release to service after maintenance requires the specified certification, and any deferred defect must be recorded with its limitations.

Error types: slip (right intention, wrong execution), lapse (forgotten step), mistake (wrong intention), violation (deliberate deviation). Each needs a different defence.

Situational awareness degrades under high workload; verbalising the next step and using checklists restores it.

Communication: use standard phraseology and read-back for critical instructions; ambiguous handovers are a known error source.

Complacency is highest on repetitive, familiar tasks; independent inspection is the designed defence against it.

Stress narrows attention; time pressure increases shortcut-taking. Both are managed by planning and by stopping when the plan fails.

Just culture distinguishes error from recklessness; reporting is encouraged so that systemic defences can be improved.

Relay test: verify coil resistance, then operate the coil and confirm contact continuity and insulation between open contacts.

Diode test: forward drop about 0.6–0.7 V for silicon; a shorted or open diode reads near zero or infinite in both directions.

Motor test: check winding resistance balance between phases and insulation to case; an unbalanced reading indicates a shorted turn.

Transformer test: continuity of each winding, insulation between windings, and turns ratio by voltage ratio.

Bonding test: use a four-wire or calibrated low-resistance meter; a two-wire meter includes lead resistance and can mask a marginal bond.

Insulation resistance: record the value and test voltage; a falling trend across checks is more significant than a single pass.

Fuses protect against overcurrent and are sized to the wire's current rating, not to the load's nominal current.

Circuit breakers are resettable and are chosen so the wire is protected; the breaker rating must not exceed the wire's capability.

Thermal breakers are ambient-sensitive; in a hot bay they trip at lower current, so location matters.

Current limiters and fuses in parallel feeds must be replaced as a set where the manual requires matching characteristics.

Reverse-current protection prevents a failed generator from motoring from the bus; a reverse-current relay or diode performs this function.

Differential protection compares current in and out of a feeder; a difference indicates a fault to ground within the protected zone.