APU

What are the primary functions 

  • Heating, Bleed Air, Electrical Power

What is in the box back there?

  • Dual gas turbine APU’s

  • Provides electrical power and bleed air for engine start and heating the aircraft on the ground. 

  • Electrical power is supplied by a 25 KVA (Kilovolt-Ampere) generator which can be installed in either APU

    • How can you tell what APU has the generator?

      • The forms, the light, the wires in the enclosure

  • Right Hydraulic system provides pressure. Can cross-over if necessary

Checklist 

  • Steps 1-7 says “may be accomplished by the pilot team.” 

  • What do the pilots do if the APU battery checks good? 

    • Rings bell - Approximately 24 Volts

      • If battery voltage is below 18 volts, APU fire extinguisher will discharge when APU  

      • Show that it doesn't go below 18 on a picture in the Reference in Electrical F 1-236

    • Say the APU battery doesn't check good, can I use another battery?

      • Yes the pilots would be able to disengage the battery and connect to the aircraft battery

      • Does the bell actually Indicate the battery is good?

        • No, it is a part of their checklist. Our pilots are good about not ringing unless it is good but you may not always be flying with our great pilots.

Electric Current Buzzing Generator

Enclosure Cabinet Breakers Generator

  • APU enclosure 

    • Hydraulic and fuel 

    • What is the caution?? 

      • Do you have to check on an EWO start or emergency taxi?

        • NO

  • APU equipment cabinet 

    • Hydraulic fluid

  • Breakers 

  • Generator Switch

  • Start accumulators 

    • Started with hydraulic motor operated by pressure from two accumulators 

      • After each start attempt, the accumulators must be recharged to:

    • How can we charge them?

      • 1 min - Engine driven 

      • 6 min - Accumulators 

      • 500 Full strokes roughly 30 mins

    • Each Accumulator pressurized for 1 start

      • Pressure: 3000 +/- 350

  • Surge damper 

    • What is a surge damper

      • Dampens the shocks from hydraulics. We don’t want all the pressure to hurt our system.

    • We are told to check it Against chart

      • Where are the charts?

    • What is it filled with?

      • Nitrogen 

    • Maintenance will have to refill

    • Relief Valve is only to be used my maintenance

  • Fire/Dslbd light doesnt come on.

    • Get into checklist (APU Fire detector test failure) 

    •  FIRE/DSBLD switch is pressed.

      • If the battery is disengaged due to low voltage, the 3 APU BATTERY CHARGER circuit breakers must be opened to avoid an APU battery fire.

Operating Modes

  • Gen Only - 150 pph

    • Gives electrical power only

      • Can start in Aft APU

  • Normal - 250 pph

    • Gives electrical power and bleed air

    • Where can we start in normal?

      • Crew Entry, Pilot, Aft APU

      • Starts both apu if from the crew entry chute

        • This switch is safety wired 

  • EWO Mode - 275 pph

    • Used for minimum engine start time

    • Upgrades both APU’s

      • Can start at Pilot APU Control Panel

  • How would I upgrade?

  • How do I downgrade?

    • What about EWO?

  •    -- Surge bleed pulsing occurs after ready light comes on shutdown APU

Fuel System

  • Where does the APU fuel from?

    • DC powered Boost Pump in Engine #2

  • What happens if I can’t get fuel from number 2

    • Bypass Port for fuel suction 

APU Control System

  • APUs are controlled by a (ESCU) Electronic Sequencing Control Unit

    • Aft Control Panel Cabinet, 1 for each APU

    • Controls the following functions:

      • Starting sequence, acceleration, on speed operation, operation within the temperature limits, inlet guide van control, surge valve control, engine shutdown

      • The brains of the APU governs start-up, temp limits, on-speed, shutdown etc.

      • Incorp a BIT capability which monitors and stores shutdown data and provides failure indications

      • Dash-1 contains BIT codes that allow another start attempt -- all else required MX action. BIT codes are black/white - read white i.e. 1-3-6

  • APU Doors

    • Hydraulically operated inlet & exhaust doors

      • Mechanically linked together

    • Manual Operation

      • How do we do it? Refer to (Manual Operation of APU Doors Checklist)

        • Each APU is equipped with a hydraulic operated inlet and exhaust doors - can be operated manually.

          • If they are opened manually they must be closed manually

  • What are the Operating Temps?

    • Cold weather?

      • OAT is Below 40 degrees F or 4 C  and visible moisture

    • Hot weather?

      •    -- There are time limitations associated with operations in NORMAL - fuel vapors

      • Normal mode operating timing

      • Why do we care?

        • In the deid really hot fixing engines may have to shut down the APU

    • Only applies in normal 

APU Starting System

  • BIT Codes

    • Indicated a problem within the APU

  • How would you know what action is required?

    • Read paragraph in section 3

    • Shutdown for fire = no restart (Mx Approval)

    • When in doubt, ask Mx

  • When does the ready light come on?

    • When the APU reaches 100% in normal or EWO mode

  • When does it go out?

    • When the APU is downgraded to the Gen Only mode or when a shutdown is initiated

Fire Protection System

  • Abnormal operation of the APUs are limited due to auto shutdown for malfunctions

  • Each APU has its own fireproof compartment

    • 2 fire doors for access

    • Emergency stop switch - closes fuel valve

    •  What situation would I use the emergency stop?

      • Yes the emergency stop buttons

  • What if it Senses a fire/ overheat condition 

    • Automatically discharges

  • Dual loop Fire detection sensors

    • When activated, the loop signals the fire detector system

      • Illuminates the fire lights

      • Signals the electronic sequence control unit to shut down the APUs. The ESCU initiates the shutdown sequence and illuminates the DSLBD lights

      • Closes the fire control shutoff valves to remove the oxygen source from the fire

      • Discharges the respective fire extinguishers

    • Talk about the electricity needing enough voltage to reach the end of the loop, if not it may create a shortage.

  • What is the Note?

    • If the battery voltage reads at or below 21 volts the APU fire extinguisher may activate when the detection system is checked

  • How long does it remain energized

    • 5 minutes after APU shutdown