Aircraft Maintenance: Reducing resistance
If your engine power was reduced by a hidden restriction in the fuel line, chances are that you’d notice and fix it immediately. But what if a similar loss of power is happening in your aircraft’s electrical system? It’s a surprisingly common problem that’s relatively simple to diagnose and fix.
To understand the challenge facing aircraft electrical system efficiency, we need to start with some basics of electricity. Ohm's law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit, where voltage equals current multiplied by resistance (V=I x R). If you equate an electrical system to the plumbing in your home, you can think of voltage as the water pressure, current as the water flow rate, and resistance as the size of the pipe or a restriction in the plumbing.
As time marches on, corrosion and wear inevitably take their toll on the aircraft’s electrical system, adding blockages in the form of resistance throughout the system. Something has to give, and the result is increased power demands on the battery and alternator with less power delivered to the starter and electronics. Keep adding more avionics, lights, portable device ports, and other loads, and the stress on the system increases until pulsing ammeter gauges, electrical gremlins, and issues starting the engine begin to appear.
When power is restricted by resistance, voltage drops and the energy is dissipated as heat. Using Ohm’s Law, the more current flowing through the system, the greater the impact of the resistance and the more voltage is lost along the way. The starter circuit for the aircraft is the most extreme example of resistance sensitivity in a high-current situation. A typical starter motor might have 1 ohm of resistance in the motor itself (an ohm is a measure of resistance). Add a similar 1 ohm of resistance in an old cable from the battery to the starter and you’ve doubled the amount of power required to get the starter turning at high amperage. Not only can this reduce the performance of the starter, it can generate a lot of heat at the point of resistance. For example, poor battery connections have been known to cause melting of the battery posts and even fire in extreme cases.
Richard Bogert, founder of Bogert Aviation, recognized this issue back in 1986 when he saw Piper owners struggling to get their engines started. Piper Aircraft used aluminum power cables in many of the company’s aircraft to save weight, but the combination of long cable runs, fatigue, and corrosion in the crimped fittings introduced significant resistance in the system. This meant that the starter motors were trying to operate on voltage well below what they were designed for. Bogert saw an opportunity and began producing all-copper, heavy-gauge, low-loss cable kits that virtually eliminated the problem. Today, Bogert Aviation is the go-to source for high-quality replacement cables for many aircraft makes and models (in addition to battery boxes, jacks, and innovative strut servicing tools).
Resistance develops most often due to corrosion at wiring and cable terminals, but it can also be caused by mechanical fatigue and carbon buildup within switches, solenoids, and circuit breakers. The best way to identify issues in the system is to measure the voltage loss at each critical point. As noted earlier, the voltage loss is relative to the current flow. Therefore, you need to measure the loss with the maximum load on each circuit to identify problems.
For the starter circuit, this can be accomplished by measuring the voltage at the battery versus at the starter while running the starter (always use safe maintenance practices and consult your mechanic when performing these tests). The voltage should be nearly identical in a healthy system. Most 14V starter systems will show about 10V under load and a healthy system might show 0.2V of loss at the starter, while an unhealthy system could show several volts lower at the starter compared to the battery. The problem could be the cable, the starter solenoid, or at any of the connections along the way.
Perform similar tests for the main power bus and avionics bus. With the aircraft on battery power, turn on all available devices to draw as much current as possible. Then measure voltage at the battery and at every major point along the way to the bus bars in the cockpit. It’s not uncommon to find that an old master solenoid has a high resistance under heavy load. Similarly, the circuit breakers themselves can introduce internal resistance as they age. Simply measuring voltage going into and out of the solenoids, breakers, and switches under load can help you spot issues. In my case, I found a 0.3V difference between the main bus and the avionics bus. The only item connecting the two: a 50-year-old avionics relay.
Identifying the source of power losses in your aircraft’s electrical system usually makes easy work of fixing the problems. Cleaning up connections and swapping out cables, switches, circuit breakers, solenoids, and switches are relatively simple fixes with measurable and satisfying results. With a little effort, you can rest easy that your aircraft’s electrical system is as healthy as possible…with a steady ammeter to prove it. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.
