Turboprop, Turbofan Engines and Starting Procedures - Ground Movement of Aircraft | Aircraft Systems

Turboprop, Turbofan Engines and Starting Procedures - Ground Movement of Aircraft

Turboprop Engines

The starting of any turbine engine consists of three steps that must be carried out in the correct sequence. The starter turns the main compressor to provide airflow though the engine. At the correct speed that provides enough airflow, the igniters are turned on and provide a hot spark to light the fuel that is engaged next. As the engine accelerates, it reaches a self-sustaining speed and the starter is disengaged.

The various covers protecting the aircraft must be removed. Carefully inspect the engine exhaust areas for the presence of fuel or oil. Make a close visual inspection of all accessible parts of the engines and engine controls, followed by an inspection of all nacelle areas to determine that all inspection and access plates are secured. Check sumps for water. Inspect air inlet areas for general condition and foreign material. Check the compressor for free rotation, when the installation permits, by reaching in and turning the blades by hand.

The following procedures are typical of those used to start turboprop engines. There are, however, wide variations in the procedures applicable to the many turboprop engines. Therefore, do not attempt to use these procedures in the actual starting of a turboprop engine. These procedures are presented only as a general guide for familiarization with typical procedures and methods. For starting of all turboprop engines, refer to the detailed procedures contained in the applicable manufacturer’s instructions or their approved equivalent.

Turboprop engines are usually fixed turbine or free turbine. The propeller is connected to the engine directly in a fixed turbine, resulting in the propeller being turned as the engine starts. This provides extra drag that must be overcome during starting. If the propeller is not at the “start” position, difficulty may be encountered in making a start due to high loads. The propeller is in flat pitch at shut down and subsequently in flat pitch during start because of this.

The free turbine engine has no mechanical connection between the gas generator and the power turbine that is connected to the propeller. In this type of engine, the propeller remains in the feather position during starting and only turns as the gas generator accelerates.

Instrumentation for turbine engines varies according to the type of turbine engine. Turboprop engines use the normal instruments—oil pressure, oil temperature, inter-turbine temperature (ITT), and fuel flow. They also use instruments to measure gas generator speed, propeller speed, and torque produced by the propeller. [Figure 1] A typical turboprop uses a set of engine controls, such as power levelers (throttle), propeller levers, and condition levers. [Figure 2]

Figure 1. Typical examples of turboprop instruments

Figure 2. Engine controls of a turboprop aircraft

The first step in starting a turbine engine is to provide an adequate source of power for the starter. On smaller turbine engines, the starter is an electric motor that turns the engine through electrical power. Larger engines need a much more powerful starter. Electric motors would be limited by current flow and weight. Air turbine starters were developed that were lighter and produced sufficient power to turn the engine at the correct speed for starting. When an air turbine starter is used, the starting air supply may be obtained from an APU onboard the aircraft, an external source (ground air cart), or an engine cross-bleed operation. In some limited cases, a low-pressure, large-volume tank can provide the air for starting an engine. Many smaller turboprop engines are started using the starter/generator, that is both the engine starter and the generator.

While starting an engine, always observe the following:
  • Always observe the starter duty cycle. Otherwise, the starter can overheat and be damaged.
  • Assure that there is enough air pressure or electrical capacity before attempting a start.
  • Do not perform a ground start if turbine inlet temperature (residual temperature) is above that specified by the manufacturer.
  • Provide fuel under low pressure to the engine’s fuel pump.

Turboprop Starting Procedures

To start an engine on the ground, perform the following operations:
  1. Turn the aircraft boost pumps on.
  2. Make sure that the power lever is in the “start” position.
  3. Place the start switch in the “start” position. This starts the engine turning.
  4. Place the ignition switch on. (On some engines, the ignition is activated by moving the fuel lever.)
  5. The fuel is now turned on. This is accomplished by moving the condition lever to the “on” position.
  6. Monitor the engine lights of the exhaust temperature. If it exceeds the limits, shut the engine down.
  7. Check the oil pressure and temperature.
  8. After the engine reaches a self-sustaining speed, the starter is disengaged.
  9. The engine continues to accelerate up to idle.
  10. Maintain the power lever at the “start” position until the specified minimum oil temperature is reached.
  11. Disconnect the ground power supply, if used.

If any of the following conditions occur during the starting sequence, turn off the fuel and ignition switch, discontinue the start immediately, make an investigation, and record the findings.
  • Turbine inlet temperature exceeds the specified maximum. Record the observed peak temperature.
  • Acceleration time from start of propeller rotation to stabilized rpm exceeds the specified time.
  • There is no oil pressure indication at 5,000 rpm for either the reduction gear or the power unit.
  • Torching (visible burning in the exhaust nozzle).
  • The engine fails to ignite by 4,500 rpm or maximum motoring rpm.
  • Abnormal vibration is noted or compressor surge occurs (indicated by backfiring).
  • Fire warning bell rings. (This may be due to either an engine fire or overheat.)

Turbofan Engines

Unlike reciprocating engine aircraft, the turbine-powered aircraft does not require a preflight run-up unless it is necessary to investigate a suspected malfunction.

Before starting, all protective covers and air inlet duct covers are removed. If possible, head the aircraft into the wind to obtain better cooling, faster starting, and smoother engine performance. It is especially important that the aircraft be headed into the wind if the engine is to be trimmed.

The run-up area around the aircraft is cleared of both personnel and loose equipment. The turbofan engine intake and exhaust hazard areas are illustrated in Figure 3. Exercise care to ensure that the run-up area is clear of all items, such as nuts, bolts, rocks, shop towels, or other loose debris. Many very serious accidents have occurred involving personnel in the vicinity of turbine engine air inlets. Use extreme caution when starting turbine aircraft.

Figure 3. Engine intake and exhaust hazard areas

Check the aircraft fuel sumps for water or ice. Inspect the engine air inlet for general condition and the presence of foreign objects. Visually inspect the fan blades, forward compressor blades, and the compressor inlet guide vanes for nicks and other damage. If possible, check the fan blades for free rotation by turning the fan blades by hand. All engine controls must be operational. Check engine instruments and warning lights for proper operation.

Starting a Turbofan Engine

The following procedures are typical of those used to start many turbine engines. There are, however, wide variations in the starting procedures used for turbine engines, and no attempts are to be made to use these procedures in the actual starting of an engine. These procedures are presented only as a general guide for familiarization with typical procedures and methods. In the starting of all turbine engines, refer to the detailed procedures contained in the applicable manufacturer’s instructions or their approved equivalent.

Most turbofan engines can be started by either air turbine or electrical starters. Air-turbine starters use compressed air from an external source as discussed earlier. Fuel is turned on either by moving the start lever to “idle/start” position or by opening a fuel shutoff valve. If an air turbine starter is used, the engine “lights off” within a predetermined time after the fuel is turned on. This time interval, if exceeded, indicates a malfunction has occurred and the start must be discontinued.

Most turbofan engine controls consist of a power lever, reversing levers, and start levers. Newer aircraft have replaced the start levers with a fuel switch. [Figure 4] Turbofan engines also use all the normal instruments speeds, (percent of total rpm) exhaust gas temperature, fuel flow, oil pressure, and temperature. An instrument that measures the amount of thrust being delivered is the engine pressure ratio. This measures the ratio between the inlet pressures to the outlet pressure of the turbine.

Figure 4. Turbofan engine control levers

The following procedures are useful only as a general guide and are included to show the sequence of events in starting a turbofan engine.
  1. If the engine is so equipped, place the power lever in the “idle” position.
  2. Turn the fuel boost pump(s) switch on.
  3. A fuel inlet pressure indicator reading ensures fuel is being delivered to engine fuel pump inlet.
  4. Turn engine starter switch on. Note that the engine rotates to a preset limit. Check for oil pressure.
  5. Turn ignition switch on. (This is usually accomplished by moving the start lever toward the “on” position. A micro switch connected to the leveler turns on the ignition.)
  6. Move the start lever to “idle” or “start” position, this starts fuel flow into the engine.
  7. Engine start (light off) is indicated by a rise in exhaust gas temperature.
  8. If a two-spool engine, check rotation of fan or N1.
  9. Check for proper oil pressure.
  10. Turn engine starter switch off at proper speeds.
  11. After engine stabilizes at idle, ensure that none of the engine limits are exceeded.
  12. Newer aircraft drop off the starter automatically.