The “feathered” position is the highest pitch angle obtainable. [Figure 15-8C] The feathered position produces no forward thrust.
The propeller is generally placed in feather only in case of in-flight engine failure to minimize drag and prevent the air from using the
propeller as a turbine.
Figure 15-8C. Propeller feather (maximum forward pitch angle) characteristics.
In the “reverse” pitch position, the engine/propeller turns in the same direction as in the normal (forward) pitch position, but the
propeller blade angle is positioned to the other side of flat pitch. [Figure 15-8D] In reverse pitch, air is pushed away from the
airplane rather than being drawn over it. Reverse pitch results in braking action, rather than forward thrust of the airplane. It is used
for backing away from obstacles when taxiing, controlling taxi speed, or to aid in bringing the airplane to a stop during the landing
roll. Reverse pitch does not mean reverse rotation of the engine. The engine delivers power just the same, no matter which side of flat
pitch the propeller blades are positioned.
Figure 15-8D. Propeller reverse pitch characteristics.
With a turboprop engine, in order to obtain enough power for flight, the power lever is placed somewhere between flight idle (in
some engines referred to as “high idle”) and maximum. The power lever directs signals to a fuel control unit to manually select fuel.
The propeller governor selects the propeller pitch needed to keep the propeller/engine on speed. This is referred to as the propeller
governing or “alpha” mode of operation. When positioned aft of flight idle, however, the power lever directly controls propeller blade
angle. This is known as the “beta” range of operation.
The beta range of operation consists of power lever positions from flight idle to maximum reverse. 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 also results in a progressive increase in engine (N1) rpm
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 particula r
airplane make and model. Specific engine parameters and limitations for operations within this area should be adhered to. It is
essential that a pilot transitioning to turboprop airplanes becomes knowledgeable and proficient in these areas, which are unique to
turbine-engine powered 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 are either lead-acid, nickel-cadmium (NiCad), or Lithium-ion. When
battery voltage is low, 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. The diff erent battery
types have different operating characteristics depending on the specific aircraft installation and operational environment.
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 15-9]
Figure 15-9. Typical individual power distribution bus.
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 during normal and emergency operations. Main buses serve most of the airplane’s electrical equipment. Essential bus es
feed power to equipment having top priority. [Figure 15-10]
Figure 15-10. Simplified schematic of turboprop airplane electrical system.
Multiengine turboprop airplanes normally have several power sources —at least one generator per engine and at least one battery for
the airplane. The electrical systems are usually designed so that any bus can be energized by any of the power sources. For example, a
typical system has a left and right engine generator-powered bus. While these buses are normally isolated, they may be fed from other
power sources. However, in the event of a short-circuit, the bus remains isolated. Pilots should refer to the appropriate checklist when
an electrical fault occurs.
Power distribution buses are protected from short circuits and other malfunctions by a type of fuse called a current limiter. In the case
of excessive current supplied by any power source, the current limiter opens the circuit and thereby isolates that power source and
separates the affected bus from the system. If this occurs, pilots should refer to the appropriate checklist.
Operational Considerations
As previously stated, a turboprop airplane flies just like any other piston engine airplane of comparable size and weight. It is the
operation of the engines and airplane systems that makes the turboprop airplane different from its piston engine counterpart. Pilot
errors in engine and/or systems operation are common causes of aircraft damage or loss of aircraft control. There are two engine-
related issues that should be considered when a pilot transitions to turboprop operations.
The first issue concerns the split-shaft/free turbine engine, where power output lags for several seconds when the pilot moves the
power lever from flight idle to a high power setting. This delay may surprise a pilot who has only flown airplanes with a piston
engine (or a fixed-shaft turboprop). Certain operations such as firefighting and agricultural application require maneuvering close to
the ground while operating at or near flight idle. Although smooth power applications are still the rule, the pilot should be aware that
a greater physical movement of the power levers is required as compared to throttle movement in a piston engine. The pilot should
understand the lag and anticipate and lead the power changes more than in the past and should keep in mind that the last 30 percent of
engine rpm represents the majority of the engine thrust. Below that setting, the application of power has very little effect.
A second consideration for transitioning pilots concerns turbine engine heat sensitivity. A turbine engine cannot tolerate an over
temperature condition for more than a very seconds without experiencing serious damage. Engine temperatures get hotter during
starting than at any other time. Thus, turbine engines have minimum rotational speeds for introducing fuel into the combustio n
chambers during startup. Vigilant monitoring of temperature and acceleration on the part of the pilot remain crucial until the engine is
running at a stable speed. Successful engine starting depends on assuring the correct minimum battery voltage before initiating start or
employing a ground power unit (GPU) of adequate output.
After fuel is introduced to the combustion chamber during the start sequence, “light-off” and its associated heat rise occur very
quickly. Engine temperatures may approach the maximum in a matter of 2 or 3 seconds before the engine stabilizes and temperatures
fall into the normal operating range. During this time, the pilot should watch for any tendency of the temperatures to exceed
limitations and be prepared to cut off fuel to the engine.
An engine tendency to exceed maximum starting temperature limits is termed a hot start. The temperature rise may be preceded by
unusually high initial fuel flow, which may be the first indication the pilot has that the engine start is not proceeding normally. Serious
engine damage occurs if the hot start is allowed to continue.
A condition where the engine is accelerating more slowly than normal is termed a hung start or false start. During a hung start/false
start, the engine may stabilize at an engine rpm that is not high enough for the engine to continue to run without help from the starter.
This is usually the result of low battery power or the starter not turning the engine fast enough for it to start properly.
Takeoffs in turboprop airplanes are not made by automatically pushing the power lever full forward to the stops. As stated earlier,
depending on conditions, takeoff power may be limited by either torque or by engine temperature. Normally, the power lever position
on takeoff is somewhat aft of full forward.
Takeoff and departure in a turboprop airplane (especially a twin-engine cabin-class airplane) should be accomplished in accordance
with a standard takeoff and departure “profile” developed for the particular make and model. [Figure 15-11] The takeoff and
departure profile should be in accordance with the airplane manufacturer’s recommended procedures as outlined in the Federal
Aviation Administration (FAA)- approved Airplane Flight Manual and/or the Pilot’s Operating Handbook (AFM/POH). The
increased complexity of turboprop airplanes makes the standardization of procedures a necessity for safe and efficient operation. The
transitioning pilot should review the profile procedures before each takeoff to form a mental picture of the takeoff and departure
process.
Figure 15-11. Example of a typical turboprop airplane takeoff and departure profile.
For any given high-horsepower operation, the pilot can expect that the engine temperature will climb as altitude increases at a
constant power. On a warm or hot day, maximum temperature limits may be reached at a rather low altitude, making it impossible to
maintain high horsepower to higher altitudes. Also, the engine’s compressor section has to work harder with decreased air density.
Power capability is reduced by high-density altitude and power use may have to be modulated to keep engine temperature within
limits.
In a turboprop airplane, the pilot can close the throttles(s) at any time without concern for cooling the engine too rapidly.
Consequently, rapid descents with the propellers in low pitch can be dramatically steep. Like takeoffs and departures, approach and
landing should be accomplished in accordance with a standard approach and landing profile. [Figure 15-12] However, when flying
an airplane equipped with a split shaft/free turbine engine, the pilot should anticipate the demand for power and account for any lag in
“spool-up” time.
Figure 15-12. Example of a typical turboprop airplane arrival and landing profile.
A stabilized approach is an essential part of the approach and landing process. In a stabilized approach, the airplane, depending on
design and type, is placed in a stabilized descent on a glidepath ranging from 2.5 to 3.5°. The speed is stabilized at some r eference
from the AFM/POH —usually 1.25 to 1.30 times the stall speed in approach configuration. The descent rate is stabilized from 500
fpm to 700 fpm until the landing flare.
Landing some turboprop airplanes (as well as some piston twins) can result in a hard, premature touchdown if the engines are idled
too soon. This is because large propellers spinning rapidly in low pitch create considerable drag. In such airplanes, it may be
preferable to maintain power throughout the landing flare and touchdown. Once firmly on the ground, propeller beta range operation
dramatically reduces the need for braking in comparison to piston airplanes of similar weight.
Training Considerations
The medium and high altitudes at which turboprop airplanes are flown provide an entirely different environment in terms of
regulatory requirements, airspace structure, physiological requirements, and even meteorology. The pilot transitioning to turboprop
airplanes, particularly those who are not familiar with operations in the high/medium altitude environment, should approach
turboprop transition training with this in mind. Thorough ground training should cover all aspects of high/medium altitude flight,
including the flight environment, weather, flight planning and navigation, physiological aspects of high-altitude flight, oxygen and
pressurization system operation, and high-altitude emergencies.
Flight training should prepare the pilot to demonstrate a comprehensive knowledge of airplane performance, systems, emergency
procedures, and operating limitations, along with a high degree of proficiency in performing all flight maneuvers and in-flight
emergency procedures. The training outline below covers information used by pilots to operate safely at high altitudes.
Ground Training
1. High-Altitude Flight Environment
a. Airspace and Reduced Vertical Separation Minimum (RVSM) Operations
b. Title 14 Code of Federal Regulations (14 CFR) part 91, section 91.211, Requirements for Use of
Su
pplemental Oxygen
2. Weather
a. Atmosphere
b. Winds and clear air turbulence
c. Icing
3. Flight Planning and Navigation
a. Flight planning
b. Weather charts
c. Navigation
d. Navigation aids (NAVAIDs)
e. High Altitude Redesign (HAR)
f. RNAV/Required Navigation Performance (RNP) and Receiver Autonomous Integrity Monitoring (RAIM)
rediction
4. Physiological Training
a. Respiration
b. Hypoxia
c. Effects of prolonged oxygen use
d. Decompression sickness
e. Vision
f. Altitude chamber (optional)
5. High-Altitude Systems and Components
a. Oxygen and oxygen equipment
b. Pressurization systems
c. High-altitude components
6. Aerodynamics and Performance Factors
a. Acceleration and deceleration
b. Gravity (G)-forces
c. Mach Tuck and Mach Critical (turbojet airplanes)
d. Swept-wing concept
7. Emergencies
a. Decompression
b. Donning of oxygen masks
c. Failure of oxygen mask or complete loss of oxygen supply/system
d. In-flight fire
e. Flight into severe turbulence or thunderstorms
f. Compressor stalls
Flight Training
1. Preflight Briefing
2. Preflight Planning
a. Weather briefing and considerations
b. Course plotting
c. Airplane Flight Manual (AFM)
d. Flight plan
3. Preflight Inspection
a. Functional test of oxygen system, including the verification of supply and pressure, regulator operation,
xygen flow, mask fit, and pilot and air traffic control (ATC) communication using mask microphones
4. Engine Start Procedures, Run-up, Takeoff, and Initial Climb
5. Climb to High Altitude and Normal Cruise Operations While Operating Above 25,000 Feet Mean Sea Level (MSL)
6. Emergencies
a. Simulated rapid decompression, including the immediate donning of oxygen masks
b. Emergency descent
7. Planned Descents
8. Shutdown Procedures
9. Postflight Discussion
Chapter Summary
Transitioning from a non-turbopropeller airplane to a turbopropeller-powered airplane is discussed in this chapter. The major
differences are introduced specifically handling, powerplant, and the associated systems. Turbopropeller electrical systems and
operational considerations are explained to include starting procedures and high temperature considerations. Training considerations
are also discussed and a sample training syllabus is given to show the topics that a pilot should become proficient in when
transitioning to a turbopropeller-powered airplane.