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Aircraft Maintenance: The importance of maintaining voltage

According to the FAA, the average age of the general aviation fleet of more than 150,000 aircraft is now more than 50 years old. That means that most of us fly airframes and engines that are little changed from the days of black and white television. What has changed immensely are the avionics in our panel and everything else that consumes electrons throughout the airplane.

Modern voltage regulators are far more reliable than the original electromechanical regulators on most aircraft. Photo courtesy of Jeff Simon.

Yet for many aircraft in the fleet, the quality of that electrical power remains reliant on ancient electromechanical devices that receive little attention until things go bad. And when things go bad with the voltage in our aircraft, it can lead to a crisis in the cockpit for pilots ill-prepared to deal with electrical system failure.

Enter the voltage regulator—the Rodney Dangerfield of the electrical system that “gets no respect” until it fails, wreaking havoc and sometimes damaging expensive avionics along with it. The voltage regulator serves a critical role in the electrical system by maintaining a steady-state voltage throughout the aircraft across varying engine speeds and electrical loads. It does this by monitoring the bus voltage of the aircraft and varying the field voltage of the alternator, effectively “throttling” the alternator output up or down as needed. Regulators often include an additional over-voltage cutout circuit to protect other systems on the aircraft as well. It’s an elegant feedback loop that works well in a healthy system.

For most aircraft owners and maintenance shops, aircraft batteries and alternators are considered routine maintenance items. However, voltage regulators are rarely given routine maintenance consideration. That’s a shame, because most original voltage regulators are electromechanical devices that rely on vibrating relays or moving carbon-pile designs that physically wear out over time. At best, they require adjustment to maintain the correct voltage, and at worst they fail entirely. They simply cannot perform at their peak forever.

All voltage regulators should be adjusted to the bus voltage specified in the aircraft maintenance manual. Photo courtesy of Jeff Simon.

Case in point: In my last article, I reviewed a case in which a jump-started dead battery caused an in-flight electrical system failure. Following the battery replacement, the aircraft bus voltage remained somewhat unstable. I reached out to Derek Grafenauer, sales and product support manager for Lamar Technologies Corp., to find out if the battery issue could have caused damage to the voltage regulator. Grafenauer explained that the resulting heat and stress caused by trying to manage charging of a bad battery, including possible voltage spikes, could have pushed the nearly 50-year-old regulator to become unreliable. Lamar manufactures modern voltage regulators and electrical components for many GA aircraft, and Grafenauer has seen similar heat-related failures before. Swapping over to a modern, solid-state unit resolved the issue completely.

While modern, solid-state voltage regulators eliminate the common wear and failure issues associated with electromechanical units, even solid-state regulators are subject to heat or stress related failures over time. 

Whether you upgrade to a solid-state voltage regulator or stick with your existing unit, you need to monitor the stability of your aircraft bus voltage and adjust it to maintain the exact voltage specified by the aircraft manufacturer. For most aircraft, that means running the engine at a high idle speed, turning on a typical electrical load, and measuring the aircraft’s bus voltage at the source using a quality multimeter (do not rely on voltage readings from an engine monitor or other avionics). Then, adjust the regulator to 14.2 volts (for most 14-volt aircraft), 28.5 volts (for most 28-volt aircraft), or the voltage specified in the maintenance manual if different.

Following adjustment, the voltage should be stable regardless of the electrical load. If you have an engine monitor or other instrument that displays voltage, you can use it to monitor the stability of your voltage. Don’t expect absolute perfection because modern electronics are more challenging for regulators to keep up with. For example, LED strobes and “wig-wag” lights put instantaneous current draws on the system as opposed to older and slower “charge and discharge” strobe systems. Even healthy electromechanical voltage regulators will fluctuate slightly in some cases. With a little attention, and possibly a modern regulator upgrade, you can ensure that your electrical system remains stable and reliable for years to come. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.

Most regulators have a built-in voltage adjustment screw for setting final bus voltage. Photo courtesy of Jeff Simon.
Jeff Simon

Jeff Simon

Jeff Simon is an A&P mechanic, IA, pilot, and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first inspection tool for geared alternator couplings available at ApproachAviation.com. Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 20,000 aviation events, hundred-dollar hamburger destinations, and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit www.SocialFlight.com.
Topics: Aircraft Maintenance, Ownership
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