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Flying Handbook Menu > Transition to Turbopropeller Powered Airplanes > Operational Considerations
As previously stated, a turboprop airplane
flies just like any other piston engine airplane of comparable
size and weight. It is in 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 the most common cause of aircraft damage or mishap.
The time of maximum vulnerability to pilot error in any gas
turbine engine is during the engine start sequence.
Turbine engines are extremely heat sensitive.
They cannot tolerate an overtemperature condition for more than
a very few seconds without serious damage being done. Engine
temperatures get hotter during starting than at any other time.
Thus, turbine engines have minimum rotational speeds for introducing
fuel into the combustion chambers during startup. Hypervigilant
temperature and acceleration monitoring 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 must 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 will occur
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 r.p.m. 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. Depending on conditions, takeoff power may be limited
by either torque or by engine temperature. Normally, the power
lever position on takeoff will be 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. [figure14-11]
The takeoff and departure profile should be in accordance with
the airplane manufacturer’s recommended procedures as
outlined in the 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.
figure14-11. Example—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. [figure14-12]
figure14-12. Example—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 reference from the AFM/POH—usually
1.25 to 1.30 times the stall speed in approach configuration.
The descent rate is stabilized from 500 feet per minute to 700
feet per minute 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 will dramatically reduce the
need for braking in comparison to piston airplanes of similar
weights.
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