Generator Will Not Parallel After Engine Start
After the left engine start on a twin-turboprop with a 115 V AC 400 Hz parallel bus system, the left generator line contactor (GLC) does not close. The right 30 kVA generator picks up both main buses through the closed bus tie and indicates 85% load. The left generator control unit (GCU) built-in test shows no fault, and the generator control switch is ON. Ground power is disconnected. You measure 0 V at the left generator output terminals, but the generator field resistance is within the manual limit. The APU 20 kVA generator is available and the APU is running. The aircraft has a no-break power transfer feature. Ambient is 38 °C, and the aircraft is scheduled for a night freight run. The maintenance manual includes a GCU connector pin-out and a generator on-line test procedure.
1. Explain the sequence you would use to determine whether the left GLC is deliberately inhibited or has failed to close.
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First confirm the bus configuration: right generator online, bus tie closed, ground power off, and no differential or overcurrent protection latched. Read the GCU fault and status data, not just the built-in test pass. Measure generator output voltage and frequency at the GCU sense terminals; if the GCU does not see voltage and frequency within its window, it will inhibit the GLC. Check the GLC coil supply and the interlock inputs from the bus tie and no-break transfer logic. A closed bus tie may inhibit closing the GLC depending on the system design, so follow the schematic rather than assuming. Inspect the GCU connector pins for spread or corrosion, and perform a wiggle test while monitoring. Do not manually force the contactor closed; that risks out-of-phase paralleling. Only after the inhibit inputs are proven healthy would I treat the GLC or GCU as failed.
2. The right generator is at 85% load and the APU generator is available. Assess the options and the risks.
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One 30 kVA generator at 85% is about 25.5 kVA, leaving little margin for landing gear, flaps, anti-ice, or a go-around. The 20 kVA APU generator can offload the right generator, but only if the no-break power transfer conditions are met: APU generator voltage and frequency must be stable and within limits, and the bus tie and contactor logic must be designed for that transfer. Paralleling an unsynchronised generator can cause large circulating currents and damage. The APU generator may also have altitude or operating-time limitations. A safe option is to keep the APU generator online to supply its permitted bus and reduce right-generator load, then troubleshoot the left system on the ground. If the load cannot be kept within limits, the aircraft should not dispatch on one generator. Record the load calculation and the APU limits used.
3. The fault is traced to a loose pin in the GCU connector. What maintenance and certification actions follow?
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Replace the affected contact with the approved part, using the correct extraction and insertion tools. Crimp with a calibrated tool, pull-test to the manual value, and confirm pin retention. Inspect the mating connector for damage and check for debris. Reconnect, then perform the generator on-line test: bring the generator online, check voltage and frequency, close the GLC, parallel with the other generator, and verify load sharing and trip protection. Record the connector location, pin number, part number, crimp tool calibration, pull-test result, and on-line test values. Reference the approved maintenance data. If the loose pin could have caused arcing, inspect for heat damage and report it. Sign the release only after the on-line test and any required duplicate inspection are complete. The handover should note the intermittent nature and the exact repair.