Smart Troubleshooter: Taming Alternator Whine

It’s a common pitfall. You spend an arm and a leg on the most modern avionics upgrade, including the latest and greatest audio panel, only to be disappointed when you fetch the plane from the shop because you still have that annoying high-pitched whine in the headsets. You thought the upgrade would fix it, especially when the avionics shop told you the aircraft’s audio panel was left over from the Reagan era. You’re overlooking step one of any major electronics upgrade: cleaning up the charging system.

All That Noise

Truth is, alternator whine is pretty common and sometimes we live with it because it isn’t always that crippling. But many times it’s a real nuisance, especially when you’re wearing high-end headsets and listening through a modern audio system. We define alternator-specific whine as a high-pitched audio noise through the headsets (and cabin speaker, if equipped) that varies in intensity and frequency with the engine rpm. In other words, the higher the rpm the higher the pitch. It sneaks into the intercom audio, VHF comm radio audio, and aux audio including entertainment input channels.

You can pinpoint alternator noise by turning the alternator master switch off (or pulling the field circuit breaker if there isn’t a dedicated switch), which will stop the offending noise, but of course you will be drawing on the aircraft’s battery power alone—that won’t fly for long. Also, in some cases some types of solid-state voltage regulators can create a slight whine, but there is an audible difference between the two. With a voltage regulator whine problem the pitch and intensity will change with the current load at a steady rpm. In other words, a heavy current load increase (like switching on an incandescent landing light) at a fixed rpm will cause a change in a voltage regulator-induced whine, but not with an alternator-based problem.

Why It Whines

The cause of the alternator whine is from the rectification process of AC to DC voltage in the diodes. This process generates a slight degree of AC ripple being conducted into the electrical bus and on to the radio circuits. A stator winding going bad with a partial internal short can cause this noise as well. Normally there is not enough ripple to cause a problem, but an oscilloscope can detect this ripple easily. You can also rig up some voltmeters to detect this AC ripple voltage. Set it to the low AC (or auto) setting and detected AC voltage should read under one volt.

The two conditions that make the alternator-induced whine audible in the aircraft’s audio system are failed (or failing) diodes because this increases the magnitude of AC ripple in the circuit. This is a common cause of whine and almost always requires the replacement of the diodes. Failed or failing diodes can also cause a drop in the system voltage or intermittent tripping of circuit breakers, especially under a high current load, so this might be another clue beside the audio whine. In the bad old days, some would get the alternator fixed at a local auto alternator shop, but that wasn’t and still is not considered an approved repair. The proper FAA airworthiness return-to-service paperwork wouldn’t follow the repair and despite what many people think, automotive repair parts are not always identical. Talk with your A&P when dealing with these repairs for certified applications. In the experimental world, anything goes, of course, but do make sure an alternator swap or repair is compatible.

Sometimes charging system whine deserves troubleshooting beyond the diodes. A high circuit impedance (circuit impedance is something not readily measured with a voltmeter in the resistance function, even though the unit of measure is in ohms). A circuit can have very low DC resistance but still have unacceptably high impedance. The DC ground resistance should be under 0.2 ohm, something you can measure. Old aircraft short on maintenance are often the culprit. This circuit impedance can be worsened by poor electrical connections, corroded connectors (especially at the battery or alternator ground), or a bad alternator filter capacitor, which is always installed on or as close to the alternator as possible for maximum effectiveness for this type of electrical noise problem.

Last, you might have to look no farther than the aircraft’s battery. The battery itself also acts as a capacitor, so if it’s not up to rated capacity it may contribute to whine or other audible electrical noises such as from strobes flashing. Logical troubleshooters ensure that all airframe-to-engine grounding straps are in place, not corroded at the connections, and are of the flat braid design as this is far more effective than regular wire for noise suppression.

Wrap It Up

The advice is simple. Before spending big on an avionics and audio system upgrade, be sure to have someone qualified evaluate the aircraft’s charging system—especially if you’re hearing noise in the existing audio circuits before the upgrade. What might have been OK to live with when listening through the old systems may be unbearable with a modern audio panel and high-end headsets where you’ll hear all of the imperfections.