Turboprop and turboshaft engines are designed to produce  torque for driving a propeller. Torquemeters are calibrated  in percentage units, foot-pounds, or psi.  N$_{1}$ represents the rotational speed of the low pressure  compressor and is presented on the indicator as a percentage  of design rpm. After start, the speed of the low pressure  compressor is governed by the N$_{1}$ turbine wheel. The N$_{1 }$ turbine wheel is connected to the low pressure compressor  through a concentric shaft.  N$_{2}$ represents the rotational speed of the high pressure  compressor and is presented on the indicator as a percentage of  design rpm. The high pressure compressor is governed by the  N$_{2}$ turbine wheel. The N$_{2}$ turbine wheel is connected to the high  pressure compressor through a concentric shaft.  [Figure 7-27]  The great variety of turbine engines makes it impractical to  cover specific operational procedures, but there are certain  operational considerations common to all turbine engines.  They are engine temperature limits, foreign object damage,  hot start, compressor stall, and flameout.  The highest temperature in any turbine engine occurs at the  turbine inlet. TIT is therefore usually the limiting factor in  turbine engine operation.  Turbine engine thrust varies directly with air density. As air  density decreases, so does thrust. Additionally, because air  density decreases with an increase in temperature, increased  A compressor stall is an imbalance between the two vector  quantities, inlet velocity, and compressor rotational speed.  Compressor stalls occur when the compressor blades’ AOA  exceeds the critical AOA. At this point, smooth airflow  is interrupted and turbulence is created with pressure  fluctuations. Compressor stalls cause air flowing in the  compressor to slow down and stagnate, sometimes reversing  direction.  [Figure 7-28]  Compressor blades are small airfoils and are subject to the  same aerodynamic principles that apply to any airfoil. A  compressor blade has an AOA that is a result of inlet air  velocity and the compressor’s rotational velocity. These two  forces combine to form a vector, which defines the airfoil’s  actual AOA to the approaching inlet air.  If the engine fails to accelerate to the proper speed after  ignition or does not accelerate to idle rpm, a hung or false start  has occurred. A hung start may be caused by an insufficient  starting power source or fuel control malfunction.  When the EGT exceeds the safe limit of an aircraft, it  experiences a “hot start.” This is caused by too much fuel  entering the combustion chamber or insufficient turbine rpm.  Any time an engine has a hot start, refer to the AFM/POH or an  appropriate maintenance manual for inspection requirements.  Prevention of FOD is a high priority. Some engine inlets  have a tendency to form a vortex between the ground and  the inlet during ground operations. A vortex dissipater may  be installed on these engines. Other devices, such as screens  and/or deflectors, may also be utilized. Preflight procedures  include a visual inspection for any sign of FOD.  Due to the design and function of a turbine engine’s air inlet,  the possibility of ingestion of debris always exists. This  causes significant damage, particularly to the compressor  and turbine sections. When ingestion of debris occurs, it is  called foreign object damage (FOD). Typical FOD consists  of small nicks and dents caused by ingestion of small objects  from the ramp, taxiway, or runway, but FOD damage caused  by bird strikes or ice ingestion also occur. Sometimes FOD  results in total destruction of an engine.  temperatures also results in decreased thrust. While both  turbine and reciprocating powered engines are affected to  some degree by high relative humidity, turbine engines will  experience a negligible loss of thrust, while reciprocating  engines a significant loss of brake horsepower.  N$_{1}$ Indicator  Foreign Object Damage (FOD)  N$_{2}$ Indicator  Turbine Engine Operational Considerations  Turbine Engine Hot/Hung Start  Engine Temperature Limitations  Thrust Variations  Compressor Stalls  Figure 7-27.  Dual-spool axial-flow compressor.  7-23  Low pressure  compressor (N$_{1}$)  High pressure  compressor (N$_{2}$)  Low pressure compressor drive shaft  High pressure compressor drive shaft 