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CHAPTER 2

AIRCRAFT SYSTEMS

Piper Seneca 1

ELECTRICAL SYSTEM

The electrical systems in multi engine aircraft are

complicated. It is necessary to understand the electrical

system for the aircraft you intend to fly in order to be

able to cope with a partial or complete electrical failure.

The electrical schematic in Fig. 2.1 is for a

Cessna 421, but it is representative of light to medium

twins. The battery in the center left of the diagram

provides power to the battery bus, which in turn

distributes it to the starters, the avionics bus and the main

bus for the aircraft electrical systems. When the engines

are running, the alternators provide power through the

voltage regulators and overvoltage relay to the battery

bus and to the main bus.

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Electrical Schematic for a C-421

Fig. 2.1

In this aircraft, the avionics bus is normally

powered by the battery through the battery bus. There is

an emergency avionics bus switch that can power the

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avionics bus directly from the main bus. There is also an

emergency alternator field switch that will allow the

alternators to provide power through the battery bus to

the main bus in the event an alternator field circuit

breaker pops.

Fig. 2.1 also shows the external power circuit to

the battery bus. As this circuit does not include the

battery itself, the battery must be left off while the

external power is plugged in or damage to the battery

may result. Some twins require the battery to be “on”

before external power is selected “on”. You must know

which is the case for your aircraft.

Each alternator should be capable of carrying the

entire electrical load. It is wise however, to reduce the

electrical load as much as possible when an alternator

failure occurs to avoid over drawing the remaining

alternator. An ammeter (Fig. 2.2) that shows a discharge

indicates that an alternator is being over drawn or has

failed.

Ammeter Types

Fig. 2.2

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A failed alternator may be reset once. This should

also be carried out under a reduced electrical load. If it

fails to stay on line after one reset, check the alternator

circuit breakers and fuses, and if necessary select the

emergency alternator field switch, if the aircraft has one.

A popped Circuit breaker should only be reset

once. If it pops again, no attempt should be made to reset

it. Continuous resetting of a circuit breaker may start an

electrical fire.

Learn the electrical system for each aircraft you

fly, especially if you plan to fly it IFR.

FUEL SYSTEM

Fuel systems vary considerably between aircraft

types. Piper Seneca aircraft use a simple system where

all of the fuel tanks on one side are interconnected to

function as a single tank.

Older Cessna 421 (Fig. 2.3) aircraft have a much

more complicated system where all take-offs and

landings must be carried out on the main tanks (tip

tanks). Auxiliary or nacelle tanks, if installed, feed into

the tip tanks when the nacelle tank transfer pumps are

selected on, but this transfer should not take place until

the tip tanks are no more than ½ full as the nacelle tanks

feed in faster than the engines use the fuel out of the tips.

The wing tanks can be used after the nacelle tanks. The

wing tanks however, do not have a cross-feed capability.

They should be used immediately after the nacelles have

been used, so that the tip tanks are kept as a reserve in

case of a need to transfer fuel later in the flight.

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C-421 Fuel System Schematic

Fig. 2.3

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