The
APU is a constant speed (± 49.000 RPM) gas turbine engine that can supply AC
power and pressurized air. The starter/generator is powered from either
directly the main battery (28VDC) or transfer bus 1 (115VAC) where either
source is converted into 270VDC for starter operation. At 95% starter operation
reverses to a 90 KVA generator indicated by the blue APU OFF BUS light. (90 KVA
until 32.000 ft and 66 KVA until 41.000 ft) Starter sequence is automatically
determined by the Electronic Control Unit (ECU) that needs the battery
switch to be in the ON position to energize.
The APU can be used for air and AC power until 10.000 ft, just air to 17.000 ft and just AC power until 41.000 ft. That is also the maximum starting altitude although recommended at 25.000 ft.
Air takes the biggest performance from the APU as it takes air from the load compressor which is mounted on a common shaft with the combustion compressor. The more air taken, the lower the performance of the APU. That is why there is a restriction in altitude use, specially with air and when the demand is large (high EGT), air use is squeezed by IGV’s toward the load compressor.
When on suction feed the APU draws fuel from tank #1 and when operating for an extended time select a fuel pump to pressure feed which extends the lifetime of the APU.
The ECU protects the APU and shuts down with a low oil pressure, over speed or when a FAULT light illuminates. The latter represents more than just the foregoing, including ECU failure, loss of DC power, APU fire, overtemp (during start), High oil temp and many more. The start limit is 2 minutes and a FAULT light illuminates when the start is aborted through a protection or when the generator malfunctions. A blue MAINT light illuminates when oil quantity is low or a generator malfunction occurred, the APU is still allowed to operate.
APU compartment and oil cooling is accomplished by exhaust air used as an eductor to draw outside air into the compartment from an inlet just above the exhaust.
When the APU is stopped by placing the switch to OFF, the ECU determines a cooling cycle of 1 minute before the APU actually stops. The cooling cycle closes the APU BAV and trips the generator OFF line. By doing so it reliefs the APU from load and decreases the EGT preventing so called cooking of the nozzles.
(residual fuel forms carbon on the hot nozzles which can
affect the flame pattern)
Delay switching the Battery to OFF to 2 minutes after selecting the APU to OFF, this allows the inlet door to close.
Delay switching the Battery to OFF to 2 minutes after selecting the APU to OFF, this allows the inlet door to close.
The door closes when the APU decelerates to ± 30%
preventing the inlet duct to collapse. The 1 minute is by-passed when the APU
shuts down through a malfunction, the Fire Switch is activated or when the
Battery Switch is selected to OFF.
