22 Engine Electrical & Instruments Practice Questions & Answers

1. 1. A shunt-wound DC generator on a horizontally-opposed engine produces no output after reinstallation, although its drive and wiring check good. The maintenance data calls for flashing the field. What does that procedure accomplish?

   • A.A momentary pulse of battery current through the armature reverses the polarity of the generator output.
   • B.A steady current through the armature laminations lets the machine build voltage with no field at all.
   • C.A momentary pulse of battery current through the field coils in the normal direction restores residual magnetism.✓ Answer
   • D.A heavy current drawn through the brushes burns accumulated carbon dust off the commutator bars.

   A self-excited shunt generator needs residual magnetism in the field frame to produce the small starting voltage that then builds its own field. Flashing the field pushes battery current through the field coils in their normal direction to rebuild that residual magnetism; sending it backward would reverse output polarity.

   Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, Glossary: flashing the fieldReport a problem with this question

2. 2. Which practice is correct when a glazed commutator or slip ring is cleaned during engine accessory maintenance?

   • A.Polish the surface with fine emery cloth, then blow the residue out of the machine.
   • B.Polish the surface with double-0 (00) sandpaper, then blow the residue out of the machine.✓ Answer
   • C.Dress the surface with a fine mill file until all discoloration is gone, then oil it lightly.
   • D.Undercut the copper bars below the mica, then wipe the surface with a solvent rag.

   Fine 00 sandpaper is used because its abrasive is non-conductive. Emery is a conductive abrasive whose particles embed between the commutator bars and short them, so emery cloth is never used on a commutator or slip ring.

   Source: FAA-H-8083-30, Aviation Maintenance Technician Handbook — General, DC generator maintenance (commutator and slip ring servicing)Report a problem with this question

3. 3. Why does the DC alternator system on a light aircraft engine require no reverse-current cutout relay?

   • A.Its voltage regulator opens the output lead whenever bus voltage exceeds alternator voltage.
   • B.Its rectifier diodes conduct in one direction only, so bus current cannot flow back into it.✓ Answer
   • C.Its stator is grounded through the case, so any reverse current returns to the bus instead.
   • D.Its rotating field collapses below cruise rpm, so bus current has no path back into it.

   An alternator's three-phase output passes through a solid-state rectifier bridge, and the diodes block current in the reverse direction. A DC generator has no such block, so it needs a reverse-current cutout to keep the battery from motoring it when output falls below bus voltage.

   Source: FAA-H-8083-31, Aviation Maintenance Technician Handbook — Airframe, aircraft electrical systems (alternator rectification)Report a problem with this question

4. 4. An engine-driven three-phase AC generator has 8 poles and is turned at 6,000 rpm by its drive. What output frequency does it produce, and what sets that frequency?

   • A.200 Hz, because frequency follows rotor speed and field strength.
   • B.800 Hz, because frequency follows pole count and phase count.
   • C.400 Hz, because only rotor speed and pole count set it.✓ Answer
   • D.400 Hz, because frequency follows the load and the output voltage.

   Frequency equals poles multiplied by rpm and divided by 120, so 8 x 6,000 / 120 = 400 Hz. Because only speed and pole count set frequency, a constant-speed drive is needed to hold 400 Hz while engine speed changes.

   Source: FAA-H-8083-31, Aviation Maintenance Technician Handbook — Airframe, AC generation (frequency = poles x rpm / 120)Report a problem with this question

5. 5. Which statement describes the construction and installation of a turbine engine starter-generator?

   • A.A compound generator with a jaw clutch that the crew re-engages before each start attempt.
   • B.A series motor with an added shunt field, engaged by a clutch that releases after start.
   • C.A brushless alternator with rotating rectifiers, driven through a constant-speed drive housing.
   • D.A shunt generator with an added heavy series field, permanently engaged to the gearbox.✓ Answer

   A starter-generator is essentially a shunt generator with a heavy series winding added so it can act as a series motor for cranking. It stays permanently engaged to the accessory drive gears and simply changes function after the engine is self-sustaining, which saves weight, space and spare parts.

   Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, starter-generator starting systemsReport a problem with this question

6. 6. In a starter-generator, which windings carry current while the unit is motoring the engine during start?

   • A.The series (C) field with the compensating and commutating windings, all in series.✓ Answer
   • B.The shunt field with the compensating and commutating windings, with the series field unused.
   • C.The armature and shunt field only, with compensating and commutating windings switched out.
   • D.The shunt field and the series field only, with the compensating winding switched out.

   During start the unit works as a series motor: the heavy series (C) field, the compensating winding and the commutating winding are all in series with the power source, and the shunt field is not used. In the generate mode the shunt field replaces the series field while the compensating and commutating windings stay in use.

   Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, starter-generator internal circuitReport a problem with this question

7. 7. How does a generator voltage regulator hold bus voltage steady as speed and load change, and what does the overvoltage protection do if the regulator fails?

   • A.It varies generator drive speed through the accessory gearbox; the overvoltage relay trips the battery off.
   • B.It varies armature current with a rheostat; the overvoltage relay grounds the armature to shed load.
   • C.It varies the connected bus load; the overvoltage relay closes the cutout to bleed voltage.
   • D.It varies shunt-field current; the overvoltage relay then opens the field circuit and drops the generator off line.✓ Answer

   Every regulator type, from vibrating point to solid state, controls output by changing the current in the shunt field, because output voltage follows field strength. Overvoltage protection is a separate function that removes field excitation and takes the generator off the bus so a runaway regulator cannot damage equipment.

   Source: FAA-H-8083-31, Aviation Maintenance Technician Handbook — Airframe, voltage regulation and overvoltage protectionReport a problem with this question

8. 8. Two engine-driven DC generators supply a common bus through a paralleling (equalizing) circuit. What is the purpose of that circuit?

   • A.It isolates the weaker generator so the stronger one carries the whole connected load alone.
   • B.It alternates the bus between the generators so each rests while the other supplies power.
   • C.It compares bus frequencies and adjusts each drive so both generators turn at the same speed.
   • D.It compares equalizer voltages and biases each field current so the load is shared.✓ Answer

   Paralleled generators must share the connected load equally or one will be overloaded while the other loafs. The equalizing circuit senses the difference between the two equalizer (interpole) voltages and raises or lowers each regulator's field current until the outputs match, which is also why the machines must be at the same voltage before being paralleled.

   Source: FAA-H-8083-31, Aviation Maintenance Technician Handbook — Airframe, paralleling generator systems (equalizing circuit)Report a problem with this question

9. 9. Why is a constant-speed drive installed between the engine accessory gearbox and an AC generator, and what is an integrated drive generator?

   • A.It limits generator rpm only above cruise power; an IDG is a generator driven straight off the engine shaft.
   • B.It raises generator rpm with engine speed so output rises with power; an IDG adds a gearbox to the generator.
   • C.It holds generator rpm constant so output frequency stays constant; an IDG combines both in one housing.✓ Answer
   • D.It holds generator rpm constant so output voltage stays constant; an IDG puts a rectifier and generator together.

   Alternator frequency depends on rotor speed, so a constant-speed drive is what keeps the output at a fixed frequency while engine speed varies; voltage is handled separately by the regulator. An integrated drive generator packages that constant-speed drive and a brushless three-phase generator in a single unit, with a disconnect that may be operated once in flight and reset only on the ground.

   Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, Glossary: integrated drive generator; FAA-H-8083-31, Airframe, constant-speed driveReport a problem with this question

10. 10. A 20-foot power feeder for an engine-driven accessory is installed using a smaller conductor (higher gauge number) than the wire chart calls for. What is the electrical and thermal result?

    • A.Voltage drop over the run rises and the wire runs hotter, because resistance rises as area falls.✓ Answer
    • B.Voltage drop rises but heating is unchanged, because heating depends only on the bus voltage.
    • C.Voltage drop is unchanged, but the breaker trips, because breaker rating is set by the wire gauge.
    • D.Voltage drop over the run falls and the wire runs cooler, because current falls as resistance rises.

    Resistance rises as conductor area falls, so the same load current produces a larger IR drop along the run and more I-squared-R heating in the wire. That is why wire size must satisfy both the current-carrying capacity and the allowable voltage drop for the length of the run.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, wire size selection for current capacity and allowable voltage dropReport a problem with this question

11. 11. An engine electrical harness must run through the same nacelle area as a fuel line. Which installation practice is correct?

    • A.Route the wire level with or above the fuel line, keeping 6 inches of separation where practicable.✓ Answer
    • B.Route the wire below the fuel line so that any leaking fuel drips clear of the bundle.
    • C.Route the wire in contact with the fuel line and clamp both together as a single supported unit.
    • D.Route the wire below the fuel line inside the same conduit so the conduit contains chafing.

    Wiring is kept level with or above flammable fluid and oxygen lines so that a leak or condensation drips away from the wiring rather than onto it, and 6 inches of separation is preferred. Wire is never routed closer than half an inch to a plumbing line, and where separation is small, back-to-back clamps are used to hold rigid spacing.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, wiring routed near flammable fluid linesReport a problem with this question

12. 12. Which statement about circuit protection in an engine electrical installation is correct?

    • A.Resettable breakers must be trip-free; automatic-reset breakers are not used.✓ Answer
    • B.A current limiter protects the wire ahead of a fuse and must open faster than that fuse.
    • C.Protective devices go at the load end so the run stays energized when the device opens.
    • D.Breakers may be held closed on a faulted circuit as long as the load is monitored in flight.

    A trip-free breaker opens on a fault no matter what position the control is held in, so a fault cannot be held closed; automatic-reset breakers are not acceptable as aircraft circuit protection because they would re-apply power to a faulted circuit. Protection is located as close as practicable to the power source bus, and its time-current characteristic must open before the conductor is damaged.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, circuit protection devices (trip-free requirement)Report a problem with this question

13. 13. The manifold pressure gauge of a stopped, unsupercharged horizontally-opposed engine reads 29.8 in Hg on a day when field barometric pressure is 29.8 in Hg. What does this indicate?

    • A.The gauge is unserviceable, because ambient pressure never reaches it.
    • B.The gauge is functioning normally, because it indicates absolute pressure in the manifold.✓ Answer
    • C.The gauge is stuck, because a serviceable gauge indicates zero at rest.
    • D.The gauge over-reads, because a serviceable gauge shows about 15 in Hg at rest.

    Manifold pressure is absolute pressure, not differential, so with the engine stopped the manifold is simply open to the atmosphere and a serviceable gauge reads field barometric pressure. Comparing the static reading with the reported altimeter setting is the standard functional check of the instrument.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, manifold pressure indication (absolute pressure)Report a problem with this question

14. 14. Which thermocouple materials are used for turbine engine exhaust gas temperature, and which for reciprocating engine cylinder head temperature?

    • A.Iron and constantan for EGT; chromel and alumel or copper and nickel for CHT.
    • B.Chromel and alumel for EGT; iron or copper with constantan for CHT.✓ Answer
    • C.Platinum and rhodium for EGT; chromel and alumel or iron and constantan for CHT.
    • D.Copper and constantan for EGT; chromel and alumel or iron and nickel for CHT.

    Turbine exhaust and turbine inlet temperature thermocouples are chromel-alumel because that pair withstands the much higher gas temperatures, while cylinder head thermocouples are iron-constantan or copper-constantan. A thermocouple needs no ship's power: it generates a millivoltage proportional to the temperature difference between its hot and cold junctions.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, thermocouple temperature indicating systemsReport a problem with this question

15. 15. A turbine EGT thermocouple harness is longer than the run requires. What must the technician do with the excess length?

    • A.Cut the excess and install a trim resistor matched to the lead removed.
    • B.Coil and secure the excess length, because total loop resistance is calibrated into the system.✓ Answer
    • C.Splice in a copper extension so the two harness legs stay the same length.
    • D.Cut the excess and re-terminate, because lead length cannot affect a millivolt signal.

    A thermocouple indicator is a millivoltmeter calibrated for a specific total external loop resistance, which is why that resistance is usually marked on the harness. Shortening, lengthening or substituting leads changes the loop resistance and therefore the indicated temperature, so excess length is coiled and secured rather than cut.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, thermocouple lead resistance and harness lengthReport a problem with this question

16. 16. Several thermocouples are spaced around the exhaust duct of a turbofan near the turbine exit. How are they connected, and what does the flight deck gauge display?

    • A.Connected in parallel, so the indicator displays the average probe output.✓ Answer
    • B.Connected through a selector, so the indicator displays one probe at a time.
    • C.Connected in series, so the indicator displays the total of all the probe outputs.
    • D.Connected in parallel, so the indicator displays the highest single probe output.

    Exhaust gas temperature is not uniform around the duct, so several thermocouples are spaced circumferentially and wired in parallel; the indicator therefore shows the average of their outputs, which represents the whole gas stream. The circuit can be checked without running the engine by measuring thermocouple and circuit resistance and by continuity and insulation checks.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, turbine exhaust gas temperature indicating systemReport a problem with this question

17. 17. A turbine engine tachometer indicates 96 percent N2. What does that indication represent?

    • A.96 percent of the maximum rated exhaust gas temperature.
    • B.96 percent of the maximum rated rpm of that engine's high-pressure spool.✓ Answer
    • C.96 percent of the maximum rated thrust available.
    • D.9,600 rpm at the high-pressure compressor shaft.

    Turbine tachometers are calibrated in percent of maximum rated rpm rather than actual rpm so that engines of different sizes can be operated and compared with the same numbers. Each spool has its own indicator, N1 for the fan or low-pressure rotor and N2 for the high-pressure rotor.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, turbine engine tachometers (percent rpm)Report a problem with this question

18. 18. On a turbine engine, where is the fuel flow transmitter installed, and in what units does the flight deck indicator read?

    • A.Between the aircraft tank and the engine-driven pump inlet, reading gallons per hour.
    • B.Between the fuel control unit and the manifold nozzles, reading gallons per hour.
    • C.Between the boost pump and the firewall shutoff valve, reading gallons per minute.
    • D.Between the engine-driven pump and the fuel control unit, reading pounds per hour.✓ Answer

    The transmitter is mounted in the engine accessory section and measures the flow delivered between the engine-driven fuel pump and the fuel control unit, using an impeller whose speed rises with flow to raise the electrical signal. Turbine indicators read pounds per hour because turbine aircraft performance is computed from fuel weight rather than volume.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, turbine fuel flow indicating systemsReport a problem with this question

19. 19. What does the torquemeter of a free-turbine turboprop measure, and how is that measurement obtained?

    • A.Fuel energy to the burner, sensed as a pressure drop across the fuel control unit.
    • B.Exhaust jet thrust at the tailpipe, sensed as a pressure drop across the turbine.
    • C.Propeller blade angle at the hub, sensed as an oil pressure taken from the governor pump.
    • D.Shaft torque at the propeller gearing, sensed as an oil pressure set by that torque.✓ Answer

    In a typical torquemeter a helical gear moves axially as shaft torque changes and positions a valve that meters engine oil pressure in proportion to the torque delivered to the propeller shaft; a transducer converts that pressure into the flight deck indication. Because torque is used to set power, the system must be calibrated at the intervals the maintenance data specifies.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, torquemeter indicating systemsReport a problem with this question

20. 20. Engine pressure ratio is used to set takeoff power on a turbofan. Which pressures form the ratio, and why is EPR not used on a turboprop?

    • A.Turbine discharge total over engine inlet total; a turboprop makes its power as shaft torque.✓ Answer
    • B.Compressor discharge static over ambient static; a turboprop has no discharge tap available.
    • C.Engine inlet total over turbine discharge total; a turboprop reverses the ratio for shaft power.
    • D.Burner can pressure over turbine discharge pressure; a turboprop burns at constant pressure.

    EPR is turbine discharge total pressure divided by engine inlet total pressure, and it indicates the thrust a turbofan is producing. A turboprop gets only a small share of its output from the exhaust jet, so shaft torque, not EPR, is the meaningful power indication; a blocked or leaking inlet or discharge sensing line gives a false EPR.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, engine pressure ratio (EPR) indicationReport a problem with this question

21. 21. An engine instrument dial carries a green arc, a yellow arc, a red arc, and a red radial line. Which statement about these markings is correct?

    • A.The red arc is a limit not to be exceeded; the red line marks a range that may be crossed.
    • B.Both red markings forbid all operation; the yellow arc is the normal continuous range.
    • C.The red arc is a restricted range that may be passed through; the red line is a limit.✓ Answer
    • D.The green arc marks the caution range; the yellow arc marks the normal operating range.

    A red radial line is a maximum or minimum limit beyond which a dangerous condition exists, while a red arc marks a restricted range, usually a vibration range, that may be passed through but not operated in; green is the normal range and yellow the caution range. The values come from the aircraft specification or type certificate data sheet and manufacturer's data, and when the marks are on the cover glass the mechanic must confirm that the white slippage mark still lines up.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, engine instrument range markings; values from the aircraft type certificate data sheetReport a problem with this question

22. 22. On an integrated engine indicating and crew alerting display, a red, an amber, and a green annunciation are shown. What does that color convention convey?

    • A.Red means a failed lamp, amber means normal status, and green cautions the crew.
    • B.Red warns of a condition needing immediate action, amber cautions, and green is advisory.✓ Answer
    • C.Red shows a system off, amber shows it running, and green marks a caution range.
    • D.Red and amber are both advisory, and green warns of a condition needing action.

    The annunciator convention is red for a warning that requires immediate action, amber for a caution requiring awareness and later action, and green or white for advisory or normal status. Pressing the test switch proves only the lamp or display element, not the sensor behind it, so a low oil or fuel pressure indication is still troubleshot by isolating the pressure switch, the wiring and the indicator, aided by the maintenance pages and stored faults of the alerting and digital engine control systems.

    Source: FAA-H-8083-32, Aviation Maintenance Technician Handbook — Powerplant, annunciator and crew alerting indicationsReport a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Powerplant (FAA-H-8083-32). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the POWERPLANT written test only; the General and Airframe written tests and the oral and practical tests are separate. Torque values, running clearances, compression limits, magneto timing angles, spark plug gaps, temperature and pressure limits, servicing quantities, and overhaul intervals always come from the manufacturer's current maintenance data, the type certificate data sheet, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →