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APU Protective/Emergency Shutdowns and Faults
Figure 05.10.10
AUXILIARY POWER UNIT Description Vol. 2 05.10.20
Oct 05/06
Description
APU compressor bleed air is used to pressurize the left side of the 10th-stage bleed air manifold, providing pneumatics for engine starting and air conditioning. Reverse airflow protection from the main engines into the APU is provided by a one-way check valve and the 10th-stage bleed air switch/lights interlock control logic (Refer to Chapter 19, Pneumatic System, for additional information on the 10th-stage interlock circuit).
The APU pneumatic control system consists of two main components: the load control valve (LCV) and the surge control valve (SCV).
Components and Operation
Bleed Air Supply
The APU delivers a portion of its compressor discharge air to the 10th-stage manifold when the LCV is opened. This diversion takes pneumatic energy from the turbine, and some of the cooling air from the APU’s combustion system. As with shaft loads, the rpm drops, and the APU recovers full governed speed by commanding an increase in fuel flow. The loss of pneumatic energy is replaced with thermal energy (fuel), and EGT increases.
Two variables influence EGT under bleed load conditions:
.
Aircraft demand for bleed air (air conditioning packs or engine starting)
.
Ambient air temperature and air density
If the EGT reaches its operating limit, the ECU reduces the bleed air load by commanding the LCV to cycle toward the closed position.
Load Control Valve
The APU LCV is located above and to the right of the APU turbine section. The valve is controlled by the APU ECU when the APU LCV switch/light on the BLEED AIR panel is selected to the open position. The LCV controls APU bleed air supply to the left side of the 10th-stage manifold. The ECU ensures that system priority is given to electrical loads over the bleed air demand, by cycling the LCV closed (reducing bleed load) when the exhaust gas temperature approaches operating limits.
Load control valve operation is indicated by illumination of the OPEN legend of the APU LCV switch/light. When the ECU cycles the LCV for EGT limit protection, the OPEN legend extinguishes. If the APU LCV has failed open with the 10th-stage switch/light interlock protection activated, the APU LCV FAIL caution message is displayed on EICAS.
Surge Control Valve
To prevent compressor surge during high altitude shaft operation in flight, unused bleed air is vented through the SCV into the APU tailpipe. SCV operation is automatically controlled by internal logic within the ECU. The valve’s open/closed schedule is based on pressure altitude and APU inlet temperature. The surge control valve is only required during in-flight APU operation, at altitudes above 15,000 feet.
AUXILIARY POWER UNIT Description Vol. 2 05.10.21
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