The start of a jet engine involves a set of steps that are required to be performed in an orderly manner to ensure that the engine does not suffer from a major failure. Jet engines are very expensive and thus, they need to be well taken care of. A simple mistake could very well write off an engine.
GE90 /Boeing 777 engine startup(listen to the sounds,)
As discussed above, to start a jet engine, the N2 compressor needs to be rotated. To do so, air needs to be sent to the air starter motor. If the aircraft is equipped with an APU this air called bleed air can be provided by the APU. During ground operations, the APU supplies air for the air conditioning units. However, during the startup, the air to these units is shut off so that the air is available for engine start.
When the pilot engages the starter, the start valve opens, which allows the bleed air to be passed into the turbine of the starter motor. The starter motor then turns the N2. As the N2 builds up, the pilot monitors this from the cockpit instruments. At about 20% N2 rotation, fuel is introduced into the combustion chamber by the pilot using the fuel switches. The ignitors then fire up the fuel and air mixture, which causes the engine temperature to rise. In most jet engines, this temperature is sensed at the turbine stages or exhaust, and it is called Exhaust Gas Temperature (EGT).
Start and run of a freshly installed GE90-115B on a 777-300ER. The engine goes immediately to flight idle power due to a test. The little smoke trail at the start is the preservation oil on the new turbine blades (LPT 6 was replaced) that burns, common on new/overhauled engines. Recorded around 15-10 meters of the engine.
As fuel is first introduced, there is a sharp rise in EGT due to the presence of excess fuel in the combustion chamber compared to air. Less air means less cooling. As the engine accelerates, more fuel is introduced, which progressively increases the EGT. At some point, the engine reaches a self-sustaining speed where at which the engine can continue to accelerate without the assistance of the starter motor. When this point is reached, the starter motor automatically de-clutches from the N2 compressor, and the igniters are switched off. The EGT then peaks at a value and then drops as the fuel and air become balanced in the combustion chamber. This ends the startup procedure.
The peak EGT is an important value. A high peak EGT shows possible engine degradation. This can also be due to a faulty engine starter motor. Either way, abnormally high peak EGTs during the start should be discussed with the maintenance before it becomes a part of a bigger problem. The EGT itself shows the temperature of the turbines, and as they are sensitive to heat, there is a start EGT limit that must never be exceeded. If exceeded, the engine should be immediately shut down, and the aircraft handed over to maintenance.
When the APU is unserviceable or not available, a ground start unit can be hooked up to the aircraft. A long pipe from the start unit is connected to the adapter, which supplies the air from the unit to the engines. When this method is used, pilots start one of their engines at the gate with the start unit. Once the engine is running it is disconnected from the aircraft. Then the air from the fired-up engine can be diverted through a cross-bleed valve to start up the remaining engine(s). This type of starting is called a cross-bleed start.
For those engines started with an electrical generator or a motor, the same process applies, except there is no requirement to provide air for the startup. The Boeing 787 is the only large aircraft that uses electricity to start the engine. This requires a lot of power, and without an APU, two external ground power units are required to be connected to the aircraft for engine start.
WE&P by: EZorrillaM.