Landing Light Voltage Drop and Nuisance Breaker Trip
A flight school reports that the landing light on a 14-volt trainer is dim and that the 15-ampere landing-light circuit breaker trips after about 15 minutes of continuous operation. With the lamp illuminated, the mechanic measures 12.4 volts at the bus and 10.9 volts at the lamp. The run from the bus through the relay to the lamp is 12 feet each way, and the installed wire is 14 AWG copper. The lamp is rated 10 amperes at 14 volts. The mechanic also finds a slightly loose ground stud at the firewall and a relay terminal that is discolored. The aircraft has no other electrical discrepancies. The mechanic must decide whether to replace the wire, repair connections, or replace the breaker before returning the aircraft to service.
1. Calculate the voltage drop and compare it with the acceptable value for a 14-volt continuous load. What does the result tell you about the wire and the connections?
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Acceptable continuous voltage drop for a 14-V system is generally no more than 0.5 V bus-to-load. Measured drop: 12.4 - 10.9 = 1.5 V.
Wire estimate: 14 AWG is about 2.5 ohms per 1,000 ft. The conductor run is 24 ft total, so R_wire ≈ (24/1000)(2.5) = 0.060 ohm. At 10 A, wire drop ≈ 10 × 0.060 = 0.60 V. Even a clean 14 AWG run is marginal and already exceeds the 0.5 V target. The extra ≈0.90 V must come from resistance at the discolored relay terminal, the loose ground stud, or both. Clean and tighten all terminals and the ground, then re-measure before condemning the wire.
2. If the wire is replaced, what gauge should be selected and why? Show the expected drop for 12 AWG.
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12 AWG is about 1.6 ohms per 1,000 ft. For the same 24-ft run: R ≈ (24/1000)(1.6) = 0.038 ohm; drop at 10 A ≈ 0.38 V, which is below 0.5 V. Use 12 AWG or larger, but verify the terminals, breaker rating, wire support, and approved data. Upsizing wire does not fix a bad connection, so correct the terminals and ground first and confirm the measured drop. If the load is continuous and the original wire was marginal, a 12 AWG replacement gives a defensible margin.
3. The breaker still trips after the voltage-drop repair. How should the mechanic proceed, and what return-to-service judgment applies?
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A 10 A lamp on a 15 A breaker should not trip in normal continuous operation. After fixing voltage drop, measure actual current with a clamp meter and inspect the breaker terminals, wire routing, and relay for heat or chafing. Compare the breaker part number and rating with approved data. Do not install a larger breaker to stop tripping; that removes circuit protection. Replace the breaker only if it is approved and shown defective. Return to service only when current, voltage drop, and circuit protection all match the design.