Starter Circuit Excessive Voltage Drop
During a morning launch, a 24 V DC starter turns slowly and the essential bus drops from 24.9 V to about 19 V, causing avionics to reset. Battery open-circuit voltage is 26.4 V. Under 150 A starter load, voltage at the battery terminal is 24.9 V and at the starter terminal 21.1 V. The starter feed and return total 8.0 m of 16 mm² copper; resistivity 0.0175 Ω·mm²/m. Terminal lugs are clean and tight at the battery, but the starter relay and airframe ground strap are warm after a 10 s crank. The maintenance manual allows 10% voltage drop maximum in the starter circuit. The supervisor wants to replace the starter. The battery has passed a capacity check and the starter has logged 900 hours since overhaul.
1. Calculate the expected cable voltage drop, the actual cable voltage drop, and compare the actual value with the 10% limit.
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Cable resistance for 8.0 m of 16 mm² copper:
R = ρL/A = 0.0175 × 8.0 ÷ 16 = 0.00875 Ω.
Expected drop at 150 A:
V = IR = 150 × 0.00875 = 1.31 V.
Actual cable drop from battery terminal to starter terminal:
24.9 − 21.1 = 3.8 V.
The 10% limit is based on the 24.9 V battery terminal voltage under load:
0.10 × 24.9 = 2.49 V.
Actual drop 3.8 V exceeds 2.49 V. The excess over the expected cable drop is 3.8 − 1.31 = 2.49 V. That excess corresponds to about 0.0166 Ω, equivalent to roughly 15 m of extra 16 mm² copper. The cable length alone cannot explain the result.
2. Interpret the warm relay and ground strap. Should the starter be replaced first?
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No. The warm relay and ground strap under a 150 A load point to a high-resistance connection, not necessarily a faulty starter. The excess 2.49 V across the circuit at 150 A dissipates about 374 W somewhere in that circuit. A normal cable and joint should not produce that much heat. A loose crimp, corroded relay contact, damaged ground strap or undersized return path can cause exactly this symptom.
Replacing the starter first would be parts swapping. The starter current is the load that reveals the fault, but the voltage is being lost before the starter. The correct sequence is to measure voltage drop across each joint and contact under load, then repair the high-resistance point. Replacing the starter may leave the same voltage drop and the same avionics resets.
3. State the isolation test and corrective action you would authorize.
Read the worked answer
With the circuit energized and the starter cranking briefly, measure millivolt drop from the battery positive post to the relay input, relay input to relay output, relay output to starter terminal, and battery negative post to the airframe ground point and starter ground. A joint showing a large drop compared with the others is the fault. Isolate the battery before physically working on the circuit.
Clean or replace the relay and ground strap, remake any suspect crimp, and recheck torque. Then repeat the loaded voltage-drop test. The total drop should be within the manual limit and the relay and strap should not heat abnormally. If the drop remains high after joints are corrected, replace the cable with the approved gauge. Do not return the aircraft to service until the avionics-reset symptom is gone during a full start.