King Air Ignition & Starter/Generator Diagnostics
Explore the named components, make a prediction, then connect the supplied result to the system. This workshop adds coached practice alongside the course handbook and assessed lab.
King Air: ignition branches, start power and generator buildup
Trace the B300/B300C PT6A-60A ignition and Chapter 24 start/generator interfaces. Separate a control request from the power it switches, follow the exciter into two igniter branches, and locate the field path between residual voltage and regulated generation.
Original functional map: the generator-control relay switches a control feed; the engine-start relay switches the starter power path. Ignition output and generator field buildup have separate evidence boundaries.
Recorded state: before the supplied change
Green outlines identify components named in the selected exercise record; they do not indicate a healthy component.
Named connections and what each arrow means
- Aircraft DC source and power paths → Exciter: storage and conversionElectrical power: Low-voltage ignition input
- Start selection and automatic ignition request → Exciter: storage and conversionInformation / signal: Armed torque-contact ignition request
- Exciter: storage and conversion → Two high-tension leads and ignitersElectrical power: Two high-tension discharge paths
- Start selection and automatic ignition request → Generator-control relay: three contact setsInformation / signal: ON / STARTER ONLY start-interface request
- Aircraft DC source and power paths → Generator-control relay: three contact setsElectrical power: 28 V control-feed source
- Generator-control relay: three contact sets → Engine-start relay and starter/generatorInformation / signal: Control feed to engine-start relay coil
- Aircraft DC source and power paths → Engine-start relay and starter/generatorElectrical power: Separate main-contact starter power path
- Generator-control relay: three contact sets → GCP: field buildup and output controlInformation / signal: Pin-J inhibit and shunt-field transient protection
- GCP: field buildup and output control → Engine-start relay and starter/generatorElectrical power: Pin M to terminal A: generator field excitation
- Accessory drive and cooling installation → Engine-start relay and starter/generatorPhysical relationship: Splined torque transfer in either mode
- Engine-start relay and starter/generator → Line contactor and generator busElectrical power: Generated output through separate line contactor
- GCP: field buildup and output control → Line contactor and generator busInformation / signal: Pin-H line-contactor command
Aircraft DC source and power paths
Supply the ignition and control circuits and the separate starter power path.
Supplied record: Control and main sources supplied available
How it works with the rest
The described aircraft system is nominally 28 VDC. The exciter converts its low-voltage input into ignition discharges; it does not receive starter current through an igniter lead. In the Chapter 24 diagram, the engine-start relay's main contacts connect the power distribution path to the starter/generator. That main path is separate from the control feed energizing the relay. An observed source voltage does not establish continuity through either path.
What the evidence must show
Name the observed source, the control-feed boundary and the main power-contact boundary separately. Do not infer delivered starter power from a control request.
Selected: Aircraft DC source and power paths
Follow the change through the system.
Fictional B300/B300C Chapter 24 trainer: the start-control source and main starter supply are available. The engine-start relay's main power contacts have a supplied failed-open condition that remains throughout both records. The generator-control relay's three contact functions are explicitly serviceable. The first record has the ignition/start switch OFF. Apply reveals a recorded STARTER ONLY selection, changing the control request only. No actual aircraft sequence, elapsed time or starting-current value is represented.
Before the recorded change
OFF record: generator-control relay de-energized, no engine-start-relay control feed, and no starter power delivery. The engine-start relay's main contacts have the supplied failed-open condition.
Next observation
Choose your prediction before revealing the supplied result.
Reference: MM130-590031-11: 24-30-00 p. 3 and Figure 1 Sheet 1, p. 5 (Apr 1/2018); p. 110 note 4 (Apr 15/2023).
Course applicability and study references
Formative supplement to TM-073 Rev B using the supplied B300/B300C MM130-590031-11 Chapters 74 and 24. The April 2018 description covers 300 A starter/generators; the November 2025 installation task separately identifies 300 A and 400 A branches. These fictional trainer records supply their own conditions and observations. They teach relationships, not aircraft operating or maintenance instructions. No full start sequence, starter duty limit, automatic cutout, spark energy or starting current is simulated. Existing assessed work and completion requirements remain separate.
- Ignition architecture and separate branch check
Supplied B300/B300C MM130-590031-11 Chapter 74, LEP Nov 1/2024: 74-00-00 pp. 1-2 and Figure 1; 74-10-01 p. 601 (Apr 1/2018). The manual gives discrete 9 V and 30 V isolated-branch rate comparisons; no interpolation table is introduced.
- Automatic ignition and physical interfaces
74-10-03 pp. 401 and 601 (Apr 15/2023); 74-00-00 p. 601 and 74-10-05 pp. 401-403 (Nov 1/2024). Distinct switch poles, side-specific indication, oil-interface protection and stored-energy/connector precautions remain task-specific.
- Starter/generator control and power architecture
Supplied Chapter 24, LEP Nov 1/2025: 24-30-00 pp. 1-3 and Figure 1 Sheet 1, p. 5 (Apr 1/2018). The 300 A description separates series starter winding, shunt generator field, generator-control relay, engine-start relay, GCP and line contactor. The map is original explanatory artwork, not the complete wiring diagram.
- Field-path, relay and installation boundaries
24-30-00 Table 105 p. 110 and 24-30-07 p. 501 (Apr 15/2023); 24-30-03 pp. 201-202, 601-602 (Apr 15/2023) and pp. 401-405 (Nov 1/2025). Current aircraft/engine/unit identity, applicable CMM and complete effective tasks govern physical work. Chapters 74/24 do not establish a complete Chapter 80 starting sequence for this workshop.
Original learning diagrams and supplied training records explain the stated relationships. Current aircraft-effective instructions and authorization govern actual maintenance.