Avionics Bus Low Voltage After GTN 750 Installation
A Bell 407 has a 28 V DC system with a 150 A starter-generator. After installation of a GTN 750 and a GTX 345, the pilot reports that at night with the landing light and pitot heat on, the avionics bus drops to 24.1 V and the battery relay chatters. The avionics load is about 25 A. You measure 1.4 V drop from the avionics bus ground bar to the airframe ground, with a bonding resistance of 0.056 ohm. The installation manual requires bonding resistance no greater than 0.0025 ohm for avionics grounds. The new avionics were wired to a nearby airframe rib that had paint and primer under the terminal. The circuit breaker ratings are correct and the alternator output is normal.
1. Use the measured bonding resistance and load to explain the low avionics bus voltage. What is the likely cause?
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The ground path is carrying the avionics return current through a high-resistance joint. Using Ohm's law, V = I × R. With 25 A and 0.056 ohm, the drop is 25 × 0.056 = 1.4 V, which matches the measurement. That 1.4 V is lost between the avionics bus ground and the airframe, so the avionics see a lower supply voltage and the battery relay may chatter as the system voltage fluctuates.
The cause is almost certainly the new ground termination on a painted and primed rib. Paint and primer are insulators; the terminal may be held only by screw threads or a small contact area. The measured 0.056 ohm is more than 20 times the 0.0025 ohm limit. The alternator and circuit breakers are not the primary fault. The installation created a bad bonding point, and the added avionics load made the existing weakness visible.
2. What corrective actions and installation checks should be carried out?
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I would remove the ground terminal, strip paint and primer back to bare metal over the correct bonding area, treat the bare surface with the approved protective coating, and refit the terminal with the specified hardware and torque. I would verify that the ground point is structurally acceptable and not a thin or removable panel. I would inspect all new avionics grounds and the avionics bus ground bar for similar problems.
I would also check the wire gauge and terminal crimps against the installation manual, confirm circuit breaker ratings, and check for shared grounds that could carry starter or landing-light currents. After correction, I would re-measure bonding resistance and confirm it is at or below 0.0025 ohm. A load test with landing light, pitot heat and avionics on should show stable bus voltage. If the battery relay still chatters, I would check the relay itself and the battery/alternator connections.
3. What tests and records are needed before return to service?
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Before release I would record the original voltage and resistance readings, the cause, the corrective action, and the post-repair readings. I would perform a bonding resistance check at each new ground point, a voltage-drop check under specified load, and a functional check of the GTN 750, GTX 345, battery relay and avionics bus. I would confirm the alternator charges normally and that no nuisance low-voltage warnings occur at night with high loads.
The maintenance release should reference the approved installation data and the corrected bonding. If the installation manual requires a post-installation check or a compass swing due to new avionics, that should be completed. I would also make a log entry describing the paint removal and re-bonding, because it affects the aircraft's electrical bonding. Any subsequent intermittent fault should be treated as a new defect, not assumed to be fixed.