AMT Reciprocating Engine Ignition Systems Questions and Answers 1 — Questions and Answers
Question 1: What is the primary reason for using a dual ignition system on a certified aircraft reciprocating engine?
- To reduce engine operating temperatures and decrease fuel consumption.
- To provide redundancy for safety and improve combustion efficiency for better performance. (Correct answer)
- To eliminate the need for an impulse coupling during engine start.
- To decrease the electrical load on the aircraft's main battery and alternator.
Correct answer: To provide redundancy for safety and improve combustion efficiency for better performance.
A dual ignition system uses two separate magnetos, harnesses, and spark plugs for each cylinder. This provides redundancy, meaning if one entire system fails, the engine can still operate on the other. Additionally, having two spark plugs ignites the fuel-air mixture from two points, leading to a faster, more complete combustion process, which improves engine performance and power output.
Question 2: During a magneto check at the specified RPM, a technician observes a significant RPM drop on the right magneto that exceeds the manufacturer's limits, while the left magneto check is normal. The engine runs rough when switched to the right magneto only. What is the most likely cause?
- One or more fouled spark plugs on the right magneto's ignition harness. (Correct answer)
- A broken P-lead wire connected to the left magneto.
- Incorrect internal E-gap timing in the left magneto.
- A sheared impulse coupling on the right magneto.
Correct answer: One or more fouled spark plugs on the right magneto's ignition harness.
An excessive RPM drop isolated to one magneto, accompanied by rough running, is a classic symptom of a faulty or fouled spark plug (or plugs) in that magneto's system. The engine is losing the contribution of one or more cylinders when operating only on that magneto. A broken P-lead on the left magneto would cause it to be 'hot' at all times, resulting in no RPM drop. An impulse coupling only functions during starting at very low RPMs.
Question 3: In a magneto ignition system, what is the specific purpose of an impulse coupling?
- To advance the ignition timing as engine RPM increases.
- To pressurize the magneto for high-altitude operations.
- To filter out radio frequency interference from the high-tension leads.
- To produce a hot, retarded spark for engine starting. (Correct answer)
Correct answer: To produce a hot, retarded spark for engine starting.
An impulse coupling is a spring-loaded mechanical device that gives the magneto's magnet a sharp, fast rotation during low-speed engine cranking. This produces a strong spark when the magneto would otherwise be turning too slowly. It also incorporates a lag angle to retard the spark timing, preventing engine kickback during start-up.
Question 4: The internal timing of a magneto is set by adjusting the breaker points to open at the 'E-gap' position. What does the E-gap represent?
- The physical air gap between the rotating magnet and the pole shoes.
- The angular position of the rotating magnet a few degrees past neutral where the magnetic field flux is strongest. (Correct answer)
- The clearance between the distributor rotor finger and the terminal.
- The total electrical resistance of the primary winding.
Correct answer: The angular position of the rotating magnet a few degrees past neutral where the magnetic field flux is strongest.
The 'E' in E-gap stands for efficiency. It is a specific angular position of the rotating magnet, just past its neutral position, where the magnetic field is at maximum strength. Opening the breaker points at this precise moment causes the most rapid collapse of the magnetic field, inducing the highest possible voltage in the secondary coil and creating the strongest spark.
Question 5: An aircraft engine operating at consistently high power settings is experiencing pre-ignition. A technician suspects the spark plugs may be a contributing factor. Which action is most appropriate?
- Installing spark plugs with a colder heat range. (Correct answer)
- Installing spark plugs with a hotter heat range.
- Increasing the spark plug electrode gap.
- Decreasing the spark plug electrode gap.
Correct answer: Installing spark plugs with a colder heat range.
Pre-ignition is caused by a hot spot in the combustion chamber igniting the fuel-air mixture before the timed spark. A spark plug that runs too hot can become such a source. A 'cold' heat range spark plug is designed with a shorter insulator nose, which allows it to transfer heat more quickly to the cylinder head, thus running cooler and preventing its tip from becoming an incandescent point that could cause pre-ignition.
Question 6: What is the primary function of the P-lead wire connected to a magneto?
- To supply primary voltage from the aircraft bus to the magneto.
- To carry the high-tension voltage to the distributor.
- To ground the primary circuit, thereby disabling the magneto. (Correct answer)
- To provide a signal to the cockpit tachometer.
Correct answer: To ground the primary circuit, thereby disabling the magneto.
The P-lead (Primary lead) connects the primary winding of the magneto to the ignition switch. When the ignition switch is turned to the 'OFF' position, it completes a circuit that grounds the P-lead. This shorts the primary circuit, preventing the magnetic field from collapsing when the breaker points open, which in turn stops the induction of high voltage in the secondary coil and kills the spark.
AMT Reciprocating Engine Ignition Systems Questions and Answers 1
What is the primary reason for using a dual ignition system on a certified aircraft reciprocating engine?