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Aircraft maintenance: When to overhaul an alternator

Engine accessories Part 2: Alternators

The world of aircraft maintenance is a fascinating one, and I consider myself a student who is always learning.

Direct-drive alternators engage with a ring gear mounted to the crank shaft of the engine. Photo courtesy of Jeff Simon.

On one hand, we operate in a strict regulatory environment governed by the federal aviation regulations (FARs). On the other hand, our job as aircraft mechanics (A&Ps) and inspectors (IAs) is to utilize our experience and cumulative knowledge to keep our ever-aging fleet of general aviation aircraft flying safely and reliably. The FARs are the starting point, but mechanics make recommendations on many additional items based on a maintenance philosophy developed from years of experience. A good mechanic “practices the art of aircraft maintenance” just as a doctor practices medicine.

I tend to subscribe to the philosophy of “First, do no harm.” Every time a system is disassembled for maintenance, the opportunity exists to induce failures. Therefore, I am deliberately conservative about the components on an aircraft that I will proactively disassemble, inspect, overhaul, or replace without performance-based cause. For aircraft operating under Part 91, this gives us a fair amount of leeway. Manufacturer-recommended time between overhauls based on hours or calendar time, and even service bulletins, is voluntary for Part 91 operators. Research and experience guide the recommendations we make to owners regarding compliance with non-mandatory maintenance. My personal checklist for these items looks something like this:

  • Does a failure of the affected component affect safety of flight?
  • Does the affected component typically fail gradually, or catastrophically?
  • Does the component have a predictable wear trend?
  • Can the health (or wear) of the component be monitored?

Careful consideration of these questions results in my personal philosophy regarding “monitoring” versus “proactive intervention.” When it comes to engine accessories, my “proactive intervention” list is short, with magnetos and alternators topping the list. And that brings me to my most recent replacement (and upgrade): the alternator.

For anyone who flies IFR, the loss of an alternator immediately puts you on borrowed time to get on the ground before you lose critical instruments (for glass panels), communication, and navigation. Therefore, the health of the alternator is critically important. That means more than just checking and replacing worn brushes. It means ensuring that the bearings are in good shape, that the diodes are working, and that the unit as a whole is putting out its designed power capacity, something that can degrade over time.

On aircraft with gear-driven alternators (Bonanza, Baron, Cirrus, etc.), there is an additional risk in that the alternator is directly connected to the engine through a ring gear on the crank shaft. If the alternator were to seize or become unstable with failed bearings, those loads would be transferred directly to the engine if it weren’t for one critical component: the alternator drive coupling. The drive coupling is a “clutch” of sorts that mounts on the alternator shaft and connects the alternator to its drive gear through an elastomer designed to slip at a specified shear load. This “system” of alternator and drive coupling has a variety of failure modes:

  • Worn brushes or bearings can lead to reduced output or failure.
  • A worn seal can allow oil into the alternator, leading to failure.
  • A slipping drive coupling can reduce alternator output.
  • A deteriorating drive coupling (sometimes from slippage) can contaminate the engine.

The drive coupling mounts on the alternator and protects the engine should the alternator seize.  Proper installation is critical. Photo courtesy of Jeff Simon. I discussed the situation with Brett Benton, president of Quality Aircraft Accessories, and Timothy Gauntt, director of product support for Hartzell Engine Technologies. They both agree that skipping these inspections is a serious issue in the industry that they are being very proactive about. Benton noted that QAA encourages customers sending an alternator in for overhaul to leave the coupling in place on the unit. QAA will inspect the coupling along with the alternator and return the overhauled or new alternator with the inspected coupling installed. This is especially helpful to mechanics and owners because the installation of the coupling is a critical task that must be done properly. There have been enough cases of installation mistakes in the field to warrant service bulletins warning of engine failures if drive couplings are not installed properly. Adding to the case for proper inspection, Gauntt noted that many alternator replacements could be avoided if mechanics tested the coupling first. A slipping drive coupling will cause the alternator to turn more slowly and produce less power. This leads to alternator replacements that do not solve the underlying problem, along with lots of wasted time and money.

Upon close inspection of my own drive coupling, I noticed a small amount of pitting on the gears, so I sent it to QAA to inspect and repair/exchange as required. When it came to my alternator, I used the opportunity to upgrade. Over the years, I have added a significant amount of avionics and other power-consuming devices to the aircraft. This includes the new Whelen lighting I installed last year. Although I initially planned for a lower current draw, I simply couldn’t stay away from adding those powerful recognition lights and strobes. Although I was still within the current capacity of the aircraft, I could see my old 80-amp alternator working hard in the pulsing of the ammeter in sync with the strobes when everything was lit up. Benton recommended the Plane Power C14-100 alternator, and Gauntt explained that these new alternators are designed with a modern 8-pole hairpin stator (look that one up) that would deliver up to 100 amps with much smoother power output, especially at lower rpms. After installation, the difference was noticeable. With every switch turned on and the engine at idle, the ammeter is rock solid and all the lights were as bright as could be.

With the cold, dark months of winter upon us, it’s critical that your alternator perform at its peak to light up the night or help you navigate the weather. So, be proactive about this important engine accessory. If you have questions, reach out to experts such as Benton at QAA or Gauntt at Hartzell. And, if your mechanic doesn’t have the proper tools to inspect the drive coupling, have him or her reach out to me. We are working on making a drive coupling tool widely available so that mechanics can properly inspect these critical components in the field and help improve aircraft maintenance for everyone. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.

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 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
Topics: Ownership, Aircraft Maintenance
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