Chapter 6: Engine Electrical Systems

Engine Electrical Systems

Overview

This chapter covers the fundamental principles, components, and maintenance practices associated with engine electrical systems on both reciprocating and turbine-powered aircraft. It encompasses the generation, storage, and distribution of electrical power for engine operation, including starting systems, charging systems, and associated control and protection devices. The material emphasizes systematic troubleshooting, adherence to approved data (AC 43.13-1B, manufacturer's manuals), and compliance with 14 CFR regulations. A thorough understanding of these systems is critical for the safe return-to-service of aircraft.

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Key Concepts and System Components

1. Power Generation: Generators and Alternators

Aircraft engines use either DC generators or AC alternators to produce electrical power.

• DC Generators: These are typically found on older or smaller aircraft. They produce direct current using a commutator and brushes. The output voltage is regulated by a voltage regulator that controls the generator's field current.
• AC Alternators: Modern aircraft predominantly use alternators. They generate alternating current, which is then rectified to DC. Alternators are generally lighter, more reliable, and can produce more power at lower engine RPMs than generators.

Troubleshooting Low or No Output:

When a generator or alternator fails to produce its specified voltage, a systematic approach is essential. The first step is always to verify the mechanical drive system. A slipping, worn, or broken belt will prevent the unit from turning and is a common, simple cause of failure. Only after confirming the mechanical drive and all electrical connections are sound should you suspect the voltage regulator or the unit itself.

2. Electrical Storage: Batteries

The battery serves two primary functions: providing power to crank the engine during start and acting as a stabilizer for the electrical system, absorbing voltage spikes and supplying power during periods of high demand.

• Battery Types: Common types include lead-acid and nickel-cadmium (Ni-Cad). Each has specific maintenance and servicing requirements.
• Battery Inspection and Maintenance: Corrosion on terminals, posts, and hold-down brackets is a common finding. This corrosion creates high resistance, leading to poor electrical contact and potential system failures.
• The correct procedure for treating corrosion is to neutralize acid deposits with a solution of baking soda and water, thoroughly clean the affected areas, and then apply an approved protective finish.
• Battery Capacity Check: A visual inspection or electrolyte level check does not verify a battery's ability to deliver its rated current. A capacity check, using a high-rate discharge test, is required per the manufacturer's instructions to ensure the battery can perform its intended function.
• Troubleshooting Slow Cranking: If an engine cranks slowly, the cause can be a weak battery, high resistance in the starter circuit, or a faulty starter motor. A fully charged battery with slow cranking points to high resistance, often from corroded or loose cable connections. A battery that drops voltage significantly under load is likely defective and cannot supply the necessary current.

3. Starting Systems

Engine starting systems consist of a starter motor (or starter-generator), a solenoid or contactor, a start switch, and the associated wiring.

• Starter-Generators: Common on turbine engines, these units combine the functions of a starter motor for starting and a generator for producing power after the engine is running.
• Solenoids and Contactors: These are heavy-duty relays that use a small control current to switch the large current required by the starter motor. A faulty relay is a common cause of intermittent or no-crank conditions. If the start switch is activated and the solenoid does not energize, the fault is likely in the control circuit (start switch, wiring, or the solenoid coil itself).
• Troubleshooting a No-Crank Condition:
• Check the Battery: Verify the battery has adequate voltage and is fully charged.
• Check the Control Circuit: If the solenoid is not energizing, test the start switch and the wiring to the solenoid coil.
• Check the Power Circuit: If the solenoid energizes but the starter does not operate, test for voltage at the starter motor. If voltage is present but the motor does not run, the starter motor is faulty (e.g., open field winding).
• Contactor Maintenance: Pitted or severely arcing contactor points increase resistance, which can cause excessive current draw and heat. The correct action for a severely pitted or arcing contactor is replacement with an approved part. Cleaning contact points with sandpaper is not an approved practice.

4. Electrical Wiring, Bonding, and Protection

• Wiring Inspection: Wiring must be inspected for chafing, cracking, and damage, especially where it passes through grommets or rubs against brackets. Exposed conductors create fire and short-circuit hazards. Damaged wiring must be repaired using approved methods (e.g., soldering with proper connectors) and protected from future chafing by replacing worn grommets or adding protective sleeving. Temporary fixes like tape are not acceptable for return to service.
• Electrical Bonding: Bonding ensures a low-resistance electrical path between metallic components and the aircraft structure. This is critical for:
• Lightning Protection: Providing a path for lightning current to travel safely to the aircraft's discharge points.
• Static Discharge: Preventing the buildup of static electricity.
• System Operation: Ensuring proper operation of electrical and electronic systems.
• Corroded or high-resistance bonding straps must be cleaned, replaced if damaged, and verified with an ohmmeter to ensure low resistance.
• Circuit Breakers and Fuses: These are protective devices designed to prevent damage from overcurrent conditions. A circuit breaker that trips repeatedly indicates a real fault, likely a short circuit. Simply resetting it or installing a larger breaker is unsafe and violates the approved design. The fault must be properly identified and corrected.

5. Electrical Instruments and Indicators

• Ammeters: An ammeter indicates the flow of current into (charge) or out of (discharge) the battery. A discharge indication during normal cruise, when the alternator is producing rated output and the battery is charged, suggests an instrument error or incorrect wiring.
• Gauges (Oil Pressure, Temperature): Wildly fluctuating gauges with smooth engine operation indicate a faulty sensor or gauge, not a mechanical engine problem. Faulty senders are common and should be tested before suspecting mechanical issues.

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Important Procedures and Regulations

1. Systematic Troubleshooting

Effective troubleshooting follows a logical, systematic process to identify the root cause of a fault before replacing components. This includes:

1. Verifying the simple causes first: Check the mechanical drive (belts), electrical connections, and circuit breakers.
2. Using wiring diagrams: Consult the aircraft's wiring diagram to understand the circuit and identify test points.
3. Testing components in a logical order: For example, if a starter does not crank, verify power is reaching the solenoid, then the starter motor, before condemning the starter itself.
4. Referencing AC 43.13-1B: This advisory circular provides accepted methods, techniques, and practices for aircraft inspection and repair.

2. Approved Parts and Data

All maintenance must be performed using approved parts and data. This includes:

• 14 CFR 43.2(a): Prohibits using automotive parts or making modifications that alter the type design without proper approval.
• Manufacturer's Maintenance Manuals: These provide specific procedures, torque values, and inspection criteria.
• AC 43.13-1B: This document serves as an acceptable means of compliance for many maintenance and repair procedures.

3. Return to Service and Documentation

• 14 CFR 43.9: Requires a logbook entry for any maintenance performed. The entry must include a description of the work, the date, and the signature and certificate number of the person approving the aircraft for return to service.
• 14 CFR 43.12: Prohibits returning an aircraft to service with a known defect that would make it unsafe. Deferring a known critical defect is a violation of this regulation.

4. Post-Lightning Strike Inspection

After a lightning strike, the aircraft structure and electrical system must be inspected for damage. This includes checking all bonding and static discharge paths to ensure they are intact and functional. Any damaged areas must be repaired to ensure continued airworthiness.

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Common Relationships and Troubleshooting Scenarios

| Slow cranking with significant battery voltage drop | Weak or defective battery | Perform a battery capacity check or load test. |

| Starter engages intermittently | Faulty relay/solenoid, loose wiring | Test the relay/solenoid and check all control circuit connections. |

| Circuit breaker trips repeatedly | Short circuit in the component or wiring | Do not reset repeatedly. Inspect the circuit for a short to ground. |

| Ammeter shows discharge during cruise | Faulty ammeter, incorrect wiring, charging system failure | Verify alternator output and battery condition, then test the ammeter. |

| Gauges fluctuate wildly with smooth engine operation | Faulty sensor/sender, faulty gauge | Test the sender and gauge per the manufacturer's instructions. |

| Corroded battery terminals | High resistance, poor electrical contact | Clean with baking soda solution, apply protective coating, and retighten to spec. |

| Corroded bonding strap | High resistance, potential for arcing and interference | Clean surfaces, replace strap if damaged, and verify low resistance with an ohmmeter. |

| Pitted starter contactor points | High resistance, excessive current draw and heat | Replace the contactor with an approved part and perform a functional test. |