DC Electrics - Aircraft Electrical Power Systems — Page 132, Lesson 131

I want to walk you through the DC electrical power system on an aircraft, starting with the instruments that tell us what the electrical system is doing. The number and type of indicating devices fitted depends on the type of aircraft and the nature of its electrical installation, but there are some standard arrangements. For each possible source of power, we normally provide one ammeter — or what we sometimes call a load meter. And for each DC system, we provide a single voltmeter that can be switched to measure voltage at multiple points. Now, there are basically two types of ammeter. The first is the charge/discharge ammeter, also called a centre-zero ammeter. The second is the generator ammeter or load meter, also called a left-zero ammeter. Let me explain what each one tells you. The charge/discharge ammeter — the centre-zero type — shows you the current flowing into or out of the battery. If the needle is to the right of zero, that means the alternator is working. It is supplying power to the electrical system and it is charging the battery. If the needle is to the left of zero, then the battery is discharging. That tells you the alternator is not supplying power to the electrical system. The load meter — the left-zero type — shows the actual current draw, or system demand, from the alternator. If the load meter reads zero, the alternator is not supplying power to the system. In a single-engine aircraft, that leaves the battery as the sole source of power. Now, here is a critical scenario. If an alternator fails in flight, all the operating electrical equipment begins to deplete the battery. The pilot must immediately assess the situation to determine what equipment is absolutely essential to the safety of flight at that moment, and turn off everything else to conserve battery power. This procedure is called load shedding. Let me also explain how these instruments are connected. An ammeter is placed in the current flow so that it measures the current flowing through it — that means it is connected in series. An ammeter has a low internal resistance. To increase its indicating range, we may fit a shunt in parallel with the meter. A voltmeter, on the other hand, is connected to the two points between which we want to measure the potential difference — that means it is connected in parallel. A voltmeter has a high internal resistance. To increase its indicating range, we may fit a multiplier in series with the meter. Now let's talk about the battery itself. The battery on an aircraft is normally a 12-volt or 24-volt lead-acid or alkaline battery. It has two main jobs: starting the engines, and supplying electrical power in the event of a generator or alternator failure.