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Turboprop and turboshaft engines are designed to produce torque for driving a propeller. Torquemeters are calibrated in percentage units, foot-pounds, or psi. 

## N$_{1}$ Indicator 

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$_{2}$ Indicator 

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] 

## Turbine Engine Operational Considerations 

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. 

## Engine Temperature Limitations 

The highest temperature in any turbine engine occurs at the turbine inlet. TIT is therefore usually the limiting factor in turbine engine operation. 

## Thrust Variations 

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 

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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. 

## Foreign Object Damage (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. 

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. 

## Turbine Engine Hot/Hung Start 

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. 

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. 

## Compressor Stalls 

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. 

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] 

