8
THE ELECTRICAL SYSTEM, THE FADEC & RELATED INSTRUMENTS, ENGINE MASTER
SWITCH
General
The operation of the electrical system is very much the same as prior to the engine retrofit, the most
important difference being the addition of the FADEC system. As the operation of the engine depends
largely on the proper operation of the FADEC, the important thing to be aware of is that the TAE 125-
1 engine, besides fuel, also needs electrical power to keep it running.
According to the “Supplement”, the alternator field is supplied by a 12V excitation battery, whose
secondary purpose it is to supply the FADEC (at least for some time) in case of failure of the main
battery. The complete battery arrangement, main battery and excitation battery, is located in the
fuselage, behind the baggage compartment.
Note that, unlike the main battery, the excitation battery is not recharged by the alternator.
Consequently, besides regular inspections by Maintenance, the excitation battery must be replaced
once every 12 months.
The master switch is a single battery switch controlling the main battery only, the alternator being
controlled by an adjacent circuit-breaker supposed to remain pushed in all the time. Selecting the
battery switch “ON” supplies all busses and their associated users, except the navigation aids which
require the selection of the avionics power switch. Neither does it power ALL of the FADEC
components, some of these being activated by the engine master switch (see below).
The FADEC
FADEC stands for Full Authority Digital Engine Control, i.e. the heart for the functioning of the TAE
125-01 engine. Just as is the case on gasoline power plants where the magneto system is dual, the
FADEC consists of two identical redundant halves labelled A and B or, to put it simply, two computers
operating independently, one being the “active” system the other being the “standby” system, ready
to take over automatically should the “active” fail. FADEC A is normally the active one. Both these
computers are located behind the right hand part of the instrument panel. As said earlier, the FADEC
system is designed to operate on a tension of 14V.
Each FADEC registers a total of 16 signals from sensors in the various parts of the engine. The
sensors are in communication with a total of 9 so-called actuators, i.e. the users of the various
signals, through 5 main control loops. These loops are activated partly by the main battery switch,
partly by the engine master switch.
As an example, let us consider the sensors related to the barometric pressure, the manifold air
pressure, the outside air temperature and the thrust lever’s potentiometer. All these signals allow the
FADEC to calculate the exact amount of fuel to be injected in the cylinders in order to obtain the
required power. Similarly, a propeller signal (sensor) allows the FADEC to select the most suitable
propeller blade angle (actuator). In other words, as the pilot must not be worried by fuel/air mixture
problems, there is no mixture control lever, and, as said before, there is no propeller lever either. In
fact, the pilot has no direct control on the engine: he orders a certain power setting through the
thrust lever and its associated potentiometer, the FADEC performs the required calculations and
executes.
Controller Area Network, CED, AED
The FADEC is fitted with a so-called Controller Area Network system, abbreviated “CAN”. The purpose
of the CAN system is to ensure fast and efficient communication between the various FADEC sensors
and actuators, thus improving the global efficiency of the electronic system. The CAN also allows a
laptop computer with suitable program to be connected to the FADEC. This feature is for maintenance
only, and allows the rapid detection and location of possible FADEC malfunctions. Incidentally, the
laptop computer also shows the total engine running time.
Versie oktober 07
