There are three basic requirements which must be satisfied before we can start a gast turbine engine.
- The compressor/turbine assembly must be rotated to get air into the combustion chambers.
- That fuel must be provided in the combustion chambers
- That ignition must also be provided in the combustion chambers to start the air/fuel mixture burning.
There are two other requirements which satisfy servicing essentials.
- The necessity to motor over the engine with no igniters operating and this is sometimes called a 'blowout' or 'motoring over' cycle.
- The need for the igniters to be operated independent of the start cycle.
Starter Motors
Any engine starter system will have a duty cycle which is the maximum that the starter motor is allowed to be energised. The duty cycle may have to be followed by a cooling down period before the starter motor can be re-energised.
The electric starter motor was the original mean of starting gas turbine engines but it has fallen out of favor in larger and modern aircraft's due to its weight. The starter drives the compressor through the gearbox.
Any engine starter system will have a duty cycle which is the maximum that the starter motor is allowed to be energised. The duty cycle may have to be followed by a cooling down period before the starter motor can be re-energised.
The electric starter motor was the original mean of starting gas turbine engines but it has fallen out of favor in larger and modern aircraft's due to its weight. The starter drives the compressor through the gearbox.
The Air Starter Motor
It is light, simple to use and very economical. The air starter motor is attached to the accessory gearbox of the engine.
The sources of air available for engine start, in order
It is light, simple to use and very economical. The air starter motor is attached to the accessory gearbox of the engine.
The sources of air available for engine start, in order
- The aircraft auxiliary power unit
- The ground start unit
- A cross-bleed start
The Start System
Each engine is fitted with an air starter motor, when engine start is selected, each starter motor will be supplied with air from either the APU, The ground start unit or engine which is already running and capable of supplying air. APU - the aircraft's normal bleed air system ducting is utilised to pass the air to the starter motor. Non return valves prevent leaking back into the engine compressor during engine start.. |
The Relationship between RPM and EGT
(All shown in the diagram)
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The Wet Start
A wet start is indicated by the engines EGT nor rising and its RPM stabilizing at the maximum that the starter motor can achieve.
If the engine fails to light up within the required time limit, then the ignition system should be deselected and the fuel supply shut off. It can take a long time to figure if it is a wet start, during which the combustion chambers and exhaust system are becoming saturated with fuel.
Two dangers exist if this fuel is ignited
A wet start is indicated by the engines EGT nor rising and its RPM stabilizing at the maximum that the starter motor can achieve.
If the engine fails to light up within the required time limit, then the ignition system should be deselected and the fuel supply shut off. It can take a long time to figure if it is a wet start, during which the combustion chambers and exhaust system are becoming saturated with fuel.
Two dangers exist if this fuel is ignited
- it will cause a large jet of flame to issue from the exhaust system, the phenomenon called 'torching'
- The rise in EGT may well be uncontrollable and could exceed its limitation.
The hung start
The usual cause of a hung start is insufficient airflow to support efficient combustion and there could a number of reasons for this.
For instance at a high altitude airfield, starting an engine which has a contaminated compressor
Or, insufficient motive power being applied to the starter motor will prevent the compressor pushing required amount of air through the engine.
- The EGT being higher than would be expected for the RPM at which the engine has stabilized
- The RPM being lower than the engines self sustaining speed
The usual cause of a hung start is insufficient airflow to support efficient combustion and there could a number of reasons for this.
For instance at a high altitude airfield, starting an engine which has a contaminated compressor
Or, insufficient motive power being applied to the starter motor will prevent the compressor pushing required amount of air through the engine.