Alternator trip and avionics brownout during gear retraction
A 24 V DC light twin has one 70 A engine-driven alternator, a 24 Ah lead-acid battery and a 20 A avionics bus. During takeoff, when the 25 A landing-gear motor starts, the avionics flicker and the alternator offline light illuminates. Battery voltage drops to 22.5 V and alternator field voltage falls to 0 V. On the ground the alternator passes a bench test. The main bus measures 23.8 V at a 50 A load, with 0.9 V drop from alternator B+ to the bus and 0.3 V drop across the engine ground strap. The manufacturer's wiring limit is 0.5 V total between alternator B+ and main bus. The battery capacity check shows 60 percent of rated capacity. The AME must decide whether the alternator or the wiring is at fault and whether the aircraft can be released.
1. Use the voltage-drop figures to identify the likely fault. Show your reasoning.
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The total voltage drop is 0.9 V plus 0.3 V, or 1.2 V, against a manufacturer limit of 0.5 V. At 50 A, 1.2 V corresponds to about 0.024 ohm in the main feed and ground path, which is excessive. The alternator passed a bench test, so the evidence points to the aircraft wiring or connections rather than the alternator itself. A corroded or loose B+ terminal, a damaged cable, a bad engine ground strap or an undersized repair splice could all produce this. The transient during gear retraction is important: the gear motor draws a large current, the already high-resistance feed drops more voltage, the bus collapses, and the alternator may drop offline or the warning logic may trip. I would measure voltage drop across each connection and cable segment under load rather than condemn the alternator.
2. What tests and corrective actions are required before release?
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I would isolate the circuit, inspect and clean the B+ terminals, ground strap connections and any splice, then torque to the manual values. If the cable or ground strap fails a resistance or voltage-drop test, replace it with the approved part. After repair, repeat the load test at 50 A and confirm the total drop is within 0.5 V. I would also run the gear retraction on the ground and monitor bus voltage and avionics operation, because the fault appeared during a transient load. The battery at 60 percent capacity needs comparison with the manufacturer's minimum; if 60 percent is below the release limit, it must be charged or replaced. The alternator control and warning circuit should be checked for correct operation. Release only after the voltage drop, transient load and battery capacity all meet the approved data.
3. If the alternator output is normal but the offline light still flickers, what next?
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I would not keep replacing the alternator. A flickering offline light with normal output points to the sensing or warning circuit: the field circuit, overvoltage or reverse-current relay, sense wire, connector pins, or the warning logic itself. I would check the alternator control unit connections and the sense lead voltage under load, then wiggle-test the harness while monitoring the light. A loose pin or chafed wire can produce an intermittent field interruption that a bench test will not reveal. I would also confirm that the light is not responding to the gear motor transient through a shared ground. The fault must be reproduced and corrected before release, because an intermittent field circuit can leave the aircraft on battery power with no warning.