Radio Fault Isolation on a Sortie-Prepped Aircraft
A technician is asked to trace a fault in an aircraft VHF radio set before a morning sortie. The set draws from a 28 V DC bus and is protected by a 5 A fuse. With the set switched on, the fuse blows immediately each time it is replaced. A second known-good radio of the same type, when substituted, also blows the fuse. The technician measures the resistance from the radio's positive supply pin to the airframe earth with the set disconnected and reads 0.4 ohms. He also measures the supply voltage at the connector with the set isolated and reads 28.1 V. The bus itself feeds two other avionics units that are working normally. The sortie window is in 40 minutes and only one spare fuse remains.
1. Using the resistance reading and the substitution result, decide whether the fault lies inside the radio or in the aircraft wiring, and justify it quantitatively.
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The substitution test is decisive: a known-good radio also blows the fuse, so the fault is almost certainly in the aircraft wiring, not the radio. The resistance reading supports this. With the set disconnected, the technician is measuring the wiring harness from the supply pin to earth. An ideal healthy feed should read open (very high resistance) to earth. A reading of 0.4 ohms is essentially a short circuit.
Estimate the current if 28 V were applied across 0.4 ohms: I = V/R = 28.1 / 0.4 ≈ 70 A. That is far above the 5 A fuse rating, which explains the instant blowing. The radio itself is exonerated because the good unit failed identically.
2. The technician considers fitting the last spare fuse and switching on 'just to confirm'. Evaluate this action against the evidence.
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This is a bad judgement call. The evidence already predicts a ~70 A fault current into a dead short. Fitting the last fuse will simply blow it, wasting the only spare, and risks arcing, connector damage, or a fire near the harness. Confirmation has already been obtained by substitution and by the resistance measurement.
The correct action is to stop and isolate: pull the radio's circuit breaker, tag the circuit, and route the aircraft to fault-finding on the harness rather than burning the remaining fuse. The 40-minute sortie window is a pressure cue, not a reason to override electrical safety.
3. With the sortie 40 minutes away, outline the decision the technician should communicate upward.
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Report three facts plainly: (1) the radio is serviceable - a known-good unit failed the same way; (2) the aircraft wiring has a near-short to earth (~0.4 ohms, ~70 A predicted); (3) the set cannot be made serviceable by fuse replacement.
Recommend that the aircraft does not launch with this radio circuit energised, and that the discrepancy be entered in the technical log for harness repair. The technician should not be swayed by schedule pressure into a workaround such as fitting a larger fuse, which would remove the protection the wiring depends on.