Worst Case Scenario

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Worst Case Scenario Electrical system failures

AN IMPORTANT ASPECT OF all pilot training is learning how to deal with abnormal and emergency situations. Engine failures, engine fires, landing gear failures, vacuum system failures, and other failures can sometimes develop into worst case scenarios. In this article, we’ll take a look at the electrical system, and how to deal with problems that can arise during a flight. NORMAL OPERATIONS

When it comes to the aircraft’s electrical system, know the normal system indications. Unless we know what’s normal, it’s impossible to know when a situation is amiss. The two primary electrical system instruments are ammeters and voltmeters. Ammeters monitor the current or amount of electrical energy flow, measured in amperes. Voltmeters indicate the electrical potential available from the system. Think of current as “flow” of electricity and voltage as the “pressure” that causes the flow. Each type of instrumentation can provide crucial information, and monitoring the electrical system instrumentation

52 Sport Aviation June 2011

can help us identify problems before a complete failure occurs. Aircraft are typically equipped with one of two types of ammeters. A load meter tells us how much current the alternator (or generator) is producing. Often labeled as “alternator amps” or “load meter,” it usually has a “zero” on the left end of the scale, with progressively higher values to the right. When we turn on electrical components in the aircraft, the load meter registers the change, reflecting the added current draw of the particular component. Items such as radios draw little current except when transmitting, but those that produce motion (gear motors, flap motors) or heat (pitot heat, landing lights) generally have a significant current draw. If the load doesn’t change

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when a light, heater, or motor is turned on, that component probably isn’t working. A lower than normal alternator amp indication suggests the alternator is not providing enough current, and the battery is picking up the slack. Other aircraft are equipped with a “charge/discharge” type of ammeter that monitors the flow of current to or from the battery. This meter has a “zero” in the middle of the scale, with positive and negative amperage markings. A positive indication indicates the alternator is generating current and is charging the battery. A negative indication means the battery is supplying current to the electrical system. In normal cruise flight, the indication should be positive. Some aircraft are equipped with voltmeters that monitor the voltage of the battery or of the system. When the engine (or alternator) is turned off, the battery voltage will read slightly less than the rated voltage of the system. When the alternator is operating, the voltage should be slightly above the rated system voltage—e.g., 14 volts for a 12-volt system. High or low voltage readings suggest a condition that demands investigation. WARNING SIGNS

Some electrical systems incorporate warning lights to tell us when the system is not functioning properly. Illuminated low voltage, high voltage, and other indicator lights are clear signals that we have a problem and should take action to correct the situation or mitigate the effects. While a warning light might not signal an immediate emergency, we should be thinking about where to land. Even if the electrical system does not incorporate warning lights, understanding the normal readings helps us identify an abnormal condition. Low voltage indicates a battery or charging system problem. Likewise, a charge/discharge amp meter that reads in the negative indicates inadequate power generation, and the battery is making up the current shortfall. The battery can’t keep that up forever, and it will eventually be drained of energy, leaving the aircraft without electrical power. High voltage also is a problem, as it may overcharge the battery, causing it to

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ROBERT N. ROSSIER

BAT TERY BASICS

A

n aircraft’s battery is a storage device that converts chemical energy to electrical energy, and vice versa. The battery provides the electrical energy needed to spin the starter, to accommodate occasional periods of high electrical loads, and to provide emergency electrical power in the event of a generator or alternator failure. Without routine maintenance, a battery’s life will be cut short. First is maintaining the fluid level for a traditional fl ooded-cell battery. Add only distilled, demineralized water to the cells, and then charge the battery immediately after. A trickle charger should be used periodically to maintain the charge on a battery of an aircraft that is not used for extended periods. Always keep the battery clean and free of acid and corrosion, and check the battery cable connections for corrosion and tightness.

The capacity of a battery gradually degrades over time. Symptoms of a weak battery can include the inability to crank the starter long enough or fast enough, especially in the cold winter months. Another indication of reduced battery capacity is an abnormally low voltage reading during engine start; however, this can also be a symptom of loose battery cable connections or cable corrosion. Checking the specific gravity of the battery acid tells us how well it is charged, but a battery’s energy capacity is determined only by a battery load test. This test measures the total energy delivered by the battery over a 30-minute period. A battery that provides less than 80 percent of its rated capacity should be replaced. > > > To l ea r n m o re a b o u t b a t te r y maintenance see Mike Busch’s column “About Batteries” on p. 84.

overheat, spill its fluid, and suffer internal damage. High voltage may mean that too much energy is flowing into the electrical system, causing the wiring to overheat. Likewise, an abnormally high load meter or “alt amps” reading suggests an abnormal flow of current to the battery or other electrical system components. Other secondary indications also can alert us to an electrical system malfunction. For example, weak radio reception or apparent failure of a transponder can be a sign of a low voltage condition. Again, abnormal electrical system indications suggest a problem that is likely deteriorating, and we need to plan accordingly. TRIPPED BREAKERS AND BLOWN FUSES

One of the more common electrical system problems encountered in flight is that of a tripped breaker or blown fuse. Fuses and breakers can be thought of as emergency shut-off valves for the various circuits in the electrical system. If a component draws more current than it should, or more than

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the wires can safely carry, these devices shut off the flow of current to prevent damage. The difference between the two is that circuit breakers can be reset, but fuses must be replaced once they “blow.” The important thing to remember is that circuit breakers don’t trip without reason. A popped breaker or blown fuse is an indication that the electrical circuit has been overloaded. The general rule is that a circuit breaker can be reset one time if it trips off. If it trips again after being reset, it should be left alone until the system can be inspected and repaired. Similarly, a fuse can be replaced once, but if it then blows again, there is trouble. Never wrap a fuse with conductive material or replace it with a fuse of higher amperage, as this may allow too much current to pass through the wires. This can result in overheated wires, heatdamaged insulation, and an electrical fire.

worst case scenario: total electrical failure in IMC. We can keep the aircraft right side up with the vacuum instruments, but without electrical power, navigation goes out the window, as does the prospects of making an instrument approach. Consider operating with only one comm radio, use ATC vectors to keep you on course, and conserve remaining battery power in case an instrument approach is needed. Again, ATC can lighten your electrical load by providing vectors to the approach or to VFR conditions. To conserve electrical power, respond to ATC by double-keying the mic, rather than transmitting voice. Electrical failures don’t happen every day, but learning to recognize a problem and knowing what to do can help keep us in the safety zone, and avoid that worst case scenario.

aircraft’s charging system. When an alternator or generator fails or can’t produce sufficient power, the battery will carry the load for a limited period of time. That time depends on the condition of the battery and the electrical load placed upon it. The first thing to do is reduce the electrical load by turning off unnecessary electrical components (referred to as “load shedding”). The idea is to reduce the load on the battery as much as practical and conserve electrical power to meet critical needs, such as required radio calls and extending gear and flaps for landing. Markings on the circuit breakers indicate the relative current draw for each circuit and can guide us in our load-shedding process. Just remember, too, that an amp here and an amp there will quickly add up to a significant drain on the battery. The next step is to head for an alternate airport. If a charging system failure occurs in IMC, advise ATC immediately, and ask to go to the nearest airport, preferably one that is VFR. The objective is to avoid a

CHARGING SYSTEM FAILURE

Perhaps a more distressing problem for a pilot in flight is the complete failure of the

Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.

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