APU Flashcards
Fuel Burn
120 lbs/hr
Can it power the De-Ice?
No, bleed air goes to De-Ice system, but to provide 18 PSI for the rear outflow valve. Helps the plane to not pressurise on the ground.
DC Load Remains >1 after start?
APU starter has possibly welded shut. Aircraft needs to be shutdown.
Gen Info?
Constant Speed, Gas Turbine.
Installed next to A/C packs.
Elec Power for Start?
RHS 28V DC bus, from main or external power.
Fuel Supply?
LHS Tank, from the collector bay
ESU?
Electronic sequence unit, controls the APU start/running etc.
Start Circuit?
Armed when fuel pressure is sufficient for start (sensed by ESU)
APU Bleed Air
De-ice system, Air Conditioning (allow APU to run for 1 min prior to on)
Start Bypass Valve
Controls pressure during start, it prevents compressor overpressure by bleeding off excess air.
Starter Limitations
2 mins between retries (5 mins for Skips)
Try 1: 1 min on, 2 mins off
Try 2: 1 min on, 2 mins off
Try 3, 1 min on, 30 mins off.
Starter/Generator - Electrical output
- Operates ALL DC, Fixed Freq AC and charges both batteries.
Start/Gen - Engine Starts
Can assist up to 300 amp limit.
When does starter cutout during start.
70% RPM (GCU signals starter to cutout, starter light goes out, ignition de-energised).
What happens at 90% RPM during APU start.
- ESU signals FCU for auto RPM governing
- Arms bleed air and gen systems
- Starter light out, run light on.
GCU Functions
- Provides voltage reg, contractor control and auto fault protection to starter/gen.
Bus Logic?
The DC logic relays close the main bus ties, and close the secondary to main bus ties. All DC buses are then tied to the APU.
Limitations (Big List)
- Ground use only
- Fire detection test must be conducted before 1st flight of day
- APU off before takeoff
- Max operational temp is 51 degree
- Same fuel as A/C
- During Px refuelling, flight crew/eng must be in cockpit
- Not on during grav refuelling.
Rear Bay Overtemp
- 2 temp sensors, one above and forward, one above and aft, of the APU.
- Operates at 121 degrees.
- Auto shutdown initiated and RBY OHT light on APU panel.
ESU monitoring
Detects high EGT, high oil temp, low oil press, under/over speed. ESU will autoshutdown the APU.
Gen Fault?
If detected GCU will remove DC gen from R main feeder bus, and open the main to secondary bus tie.
Gen light goes on, warn light on, with APU caution light and master caution light on.
APU Overspeed
If detected the ESU illuminates:
- FLR on APU PWR Switch
- APU Caution Light
- Master Caution Switch Light
- Normal APU S/Down initiates.
Duct/Pack O/Temp - System & What Happens
- Fire loop around exhaust and APU shroud
- Fully automatic extinguish system
- Discharges under APU for compartment fire, and inside APU shroud for internal fire.
- ESU initiates automatic shutdown.
ESU Fire Shutdown Indications
- Red fire warning light on O/Head panel
- APU caution light on
- Master warning and caution lights on.
Extinguisher discharge delay
15 seconds after fire detected (to allow APU to shutdown and fuel to be used/drain)
If the discharge fails, there is a manual release available, powered from the battery bus, orange BTL light indicated problem.
APU fire test switch lights up
Fire, Fault, BTL, Fuel Valve Closed, Master Warn/Caution light
APU Compartment Vent
Fully automatic
- Opens at 94 degrees for oil temp, 66 degree for rear bay temp
- Cools APU and allows RBay cooling
- Blower pulls in air through rear bay, through the oil cooler and out the exhaust ports.
