As an aircraft owner, maintenance issues are bound to arise. When you factor in Murphy’s Law, you will eventually find yourself at an unfamiliar airport talking to a mechanic who may not necessarily be intimately familiar with your aircraft. Because of the popularity of the Bonanza, many mechanics have worked on one at one point, so chances are they can help with many issues that have you stranded away from home. But what about problems that aren’t so cut and dried? Understanding the systems on your specific aircraft will help guide the conversation with the mechanic, and possibly save you a lot of headaches.
One of my clients had just taken delivery of a G36 Bonanza and arranged to take it to an avionics shop to have the G1000 panels updated. Everything seemed normal as we taxied out to the runup area. Once we started the Before Takeoff checklist, however, we quickly found out that Alternator 1 was not functioning properly. We reluctantly taxied back to the ramp. It appeared the shop’s technician was somewhat knowledgeable about late model Bonanzas, because he was aware that there were two alternators installed in the aircraft. However, when discussing the symptoms he made a comment about swapping the alternators to help decide whether it was really the alternator at fault.
It was clear to me that he didn’t have an understanding of the G36 electrical system, which is completely different from any other Bonanza. I had a brief conversation with him to help shed light on a unique system for a single-engine general aviation aircraft. The G36 does have two alternators. But they are quite different from each other and serve very different functions. The G36 has two electrical busses, or distribution systems, normally powered by the alternators independently. It also has two batteries, one for each electrical buss.
Alternator 1 and the BUSSES TIED condition
Alternator 2
Alternator 2 is a much smaller alternator rated at 20 amps. This factory-installed unit is the same B&C Specialties unit that is sold as an aftermarket backup alternator in pre-2000 Bonanzas (the B&C became standard equipment in A36s that year). Alternator 2 will start to deliver power to Buss 2 when the output of Alternator 2 is above 2.8 amps, which occurs when the engine reaches about 2000 RPM. Once this happens, the buss tie relay opens, the current blocking diodes are taken out of the circuit, the BUSSES TIED annunciator extinguishes, and the two busses become separate and independent operating electrical systems. Alternator 1 along with Battery 1 powers Buss 1. Alternator 2 along with Battery 2 powers Buss 2. The voltage on Buss 2 shows a normal operating range since the buss is now getting its power from Alternator 2 without going through the current limiting diodes. This is the normal operating condition. It continues this way until either (1) the engine drops below 1800 RPM; or (2) Alternator 2 output drops below 2 amps.
Why this unique system?
Why engineer a system that functions this way? A look at the equipment that is powered by each buss will help answer this question. Reviewing the list of equipment on Buss 2, you will find that much of it is essential for safe flight while IFR. If there were ever a problem with Alternator 1 or an issue with any equipment attached to Buss 1, it is reassuring to know that Buss 2 can operate essential equipment completely independently. This is not to say that a failure of Alternator 1 will be a non-event or that disabling Buss 1 will go without consequence. If you don’t have power on Buss 1, a number of important things will no longer function, such as the landing gear motor. It is important to review this list of equipment so that you know what to expect. You might want to tab it in your POH binder (Section III, pages 3-12) so you can find it for a quick review in the event of Alternator 1 failure in flight. It is prudent to also understand how aftermarket additions such as tip tank fuel transfer pumps and various anti-ice systems function in the event of Buss 1 shutdown. For that you’ll need to check the POH Supplement for the modification or ask the STC holder. As advances in avionics continue, electrical systems evolve and increase in complexity. It’s up to the pilot to be aware of as many scenarios as possible so that proper troubleshooting can be performed if an issue arises. The alerts generated by the electrical system and possible equipment failures in the G36 Bonanza could leave an unknowledgeable pilot vulnerable. Having complete knowledge of the electrical system is important in knowing how to manage equipment in normal, abnormal, and emergency conditions, and to understand how these failures will affect the rest of your flight. This knowledge will also pay dividends when you find yourself at an unfamiliar airport talking with a mechanic about the unique electrical system of the G36.