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Beginning at power lever positions just aft of flight
idle, propeller blade pitch angles become progressively
flatter with aft movement of the power lever until they
go beyond maximum flat pitch and into negative pitch,
resulting in reverse thrust. While in a fixed shaft/
constant-speed engine, the engine speed remains
largely unchanged as the propeller blade angles
achieve their negative values. On the split shaft PT-6
engine, as the negative 5° position is reached, further
aft movement of the power lever will also result in a
progressive increase in engine (N1) r.p.m. until a
maximum value of about negative 11° of blade angle
and 85 percent N1 are achieved.
Operating in the beta range and/or with reverse thrust
requires specific techniques and procedures depending
on the particular airplane make and model. There are
also specific engine parameters and limitations for
operations within this area that must be adhered to. It
is essential that a pilot transitioning to turboprop
airplanes become knowledgeable and proficient in
these areas, which are unique to turbine-enginepowered
airplanes.
TURBOPROP AIRPLANE ELECTRICAL
SYSTEMS
The typical turboprop airplane electrical system is a
28-volt direct current (DC) system, which receives
power from one or more batteries and a starter/
generator for each engine. The batteries may either be
of the lead-acid type commonly used on pistonpowered
airplanes, or they may be of the
nickel-cadmium (NiCad) type. The NiCad battery
differs from the lead-acid type in that its output
remains at relatively high power levels for longer
periods of time. When the NiCad battery is depleted,
however, its voltage drops off very suddenly. When
this occurs, its ability to turn the compressor for engine
start is greatly diminished and the possibility of engine
damage due to a hot start increases. Therefore, it is
essential to check the battery’s condition before every
engine start. Compared to lead-acid batteries, highperformance
NiCad batteries can be recharged very
quickly. But the faster the battery is recharged, the
more heat it produces. Therefore, NiCad battery
equipped airplanes are fitted with battery overheat
annunciator lights signifying maximum safe and
critical temperature thresholds.
The DC generators used in turboprop airplanes double
as starter motors and are called “starter/generators.”
The starter/generator uses electrical power to produce
mechanical torque to start the engine and then uses the
engine’s mechanical torque to produce electrical power
after the engine is running. Some of the DC power
produced is changed to 28 volt 400 cycle alternating
current (AC) power for certain avionic, lighting,
and indicator synchronization functions. This is
accomplished by an electrical component called an
inverter.
The distribution of DC and AC power throughout the
system is accomplished through the use of power distribution
buses. These “buses” as they are called are
actually common terminals from which individual
electrical circuits get their power. [Figure 14-9]
Buses are usually named for what they power (avionics
bus, for example), or for where they get their power
(right generator bus, battery bus). The distribution of
DC and AC power is often divided into functional
groups (buses) that give priority to certain equipment
5 GEAR WARN
5 TRIM INDICATOR
3 TRIM ELEVATOR
5 TRIM AILERON
5 STALL WARNING
5 ACFT ANN-1
5 L TURN & BANK
5 TEMP OVRD
5 HP EMER L & R
5 FUEL QUANTITY
5 L ENGINE GAUGE
5 R ENGINE GAUGE
5 MISC ELEC
5 LDG LT MOTOR
5 BLEED L
3 WSHLD L
3 LIGHTS AUX
5 FUEL FLOW
POWER DISTRIBUTION BUS
Figure 14-9.Typical individual power distribution bus.
Ch 14.qxd 5/7/04 10:09 AM Page 14-8
14-9
during normal and emergency operations. Main buses
serve most of the airplane’s electrical equipment.
Essential buses feed power to equipment having top
priority. [Figure 14-10]
Multiengine turboprop airplanes normally have
several power sources—a battery and at least one
generator per engine. The electrical systems are
usually designed so that any bus can be energized by
any of the power sources. For example, a typical
system might have a right and left generator buses
powered normally by the right and left engine-driven
generators. These buses will be connected by a
normally open switch, which isolates them from each
other. If one generator fails, power will be lost to its
　
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