Type Course > ATA 49 APU > Flashcards

ATA 49 APU Flashcards

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1
Q

What does the APU provide?

A
  • Electrical power for the aircraft systems
  • Bleed air for engine starting (MES), air conditioning (ECS) and wing anti-ice testing on ground
  • Bleed air for air conditioning and pressurisation in flight until the specified manufacturer limit
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1
Q

When does the intake flap open and close?

A

When the APU master switch is ON and closes when the master switch is selected to OFF

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2
Q

Where is the ECB located?

A

Aft cargo compartment, RH side

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3
Q

What is installed between the APU mount brackets and the tie rods and what is their purpose?

A

Vibration isolators

Reduce vibrations transmitted between APU and aircraft

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4
Q

To what door is the air intake duct attached?

A

Right access door

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5
Q

When is the APU drain mast emptied?

A

When the aircraft is above 200 knots

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6
Q

Describe the APU fuel pump?

A

Centrifugal pump driven by a single phase AC motor

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7
Q

When does the APU fuel pump run?

A
  • APU master switch P/BSW ON
  • Pump inlet pressure < 21.8 psi, stops when pressure > 23.2 psi
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8
Q

How is the APU fuel LP valve driven?

A

Two DC motors

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9
Q

When is the APU fuel LP valve automatically closed?

A
  • APU master P/BSW has been released out and before ECB de-energised
  • ECB commands a protective shutdown in case of abnormal APU parameters
  • ECB commands an emergency shutdown in case of APU fire or overheat detection
  • APU fire P/BSW is released out in the cockpit
  • APU shut off P/BSW has been pressed on the external power control panel
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10
Q

When does the pressure switch at the inlet connection of the fuel control unit display a FUEL LOw PRessure message?

A
  • APU speed > 7% RPM and the APU is supposed to run
  • Fuel pressure < 15.8 psi
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11
Q

What are the components mounted on the APU accessory gearbox? (Honeywell 131-9)

A
  • Starter motor assembly
  • Cooling air fan assembly
  • Lubrication unit with the oil pump assembly which drives the FCU
  • AC generator
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12
Q

When is the electrical supply to the starter stopped? (Honeywell 131-9)

A

APU is around 50%

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13
Q

What happens if the APU speed drops below 95% due to flame-out? (Honeywell 131-9)

A

ECB starts the ignition until the APU reaches 100% again

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14
Q

What is fuel muscle used for? (Honeywell 131-9)

A
  • IGV actuator
  • Surge control valve actuator
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15
Q

What parameters does the ECB use to control the fuel system during starting? (Honeywell 131-9)

A
  • Speed
  • EGT
  • Air inlet pressure and temperature sensors
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16
Q

What components does the ECB operate to control the fuel system? (Honeywell 131-9)

A
  • Fuel metering valve
  • Flow divider solenoid valve
  • 3-way shut-off solenoid valve
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17
Q

At what level of pressure does muscle pressure operate the surge control valve actuator for air release to exhaust and the load-compressor IGV actuator? (Honeywell 131-9)

A

250 psi

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18
Q

What is the purpose of the flow divider? (Honeywell 131-9)

A

Supplies fuel in sequence to the primary and secondary nozzles

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19
Q

What does the ECB do during cold start or high altitude operation in the fuel system? (Honeywell 131-9)

A

Energises the solenoid valve to close during the first 30% speed of the APU and all fuel flow is pushed through the primary nozzles for better engine acceleration

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20
Q

At what fuel pressure are the secondary nozzles supplied with fuel? (Honeywell 131-9)

A

125 psi

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21
Q

What happens to the fuel in the system after the 3-way shutoff solenoid valve is brought to the closed position? (Honeywell 131-9)

A

2 fuel manifolds are still filled up with remaining fuel

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22
Q

Describe the EGT probes? (Honeywell 131-9 & APIC 3200)

A

Chromel-alumel device which has two probes of different lengths

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23
Q

When will the ECB use the highest value of EGT? (Honeywell 131-9)

A

If there is a difference of 65°C between the two signals and an APU speed of above 95% N1

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24
Where is the speed sensor located? (Honeywell 131-9)
Bottom-right side of the gearbox
25
What does the speed sensor measure? (Honeywell 131-9)
Compressor-turbine shaft
26
Why does the ECB electrically open the de-oiling solenoid valve? (Honeywell 131-9)
Make sure of cold start conditions and to decrease the load on the pressure pump
27
At what RPM is the de-oiling solenoid valve maintained open during engine start-up? (Honeywell 131-9)
Up to 60%
28
At what RPM is the de-oiling solenoid valve maintained open during engine shutdown? (Honeywell 131-9)
50% to 7%
29
What oil parameters will cause the ECB to automatic shutdown the APU? (Honeywell 131-9)
* Gearbox high oil temp threshold - 162.78°C * Oil low pressure - 33 psi
30
For how long can the APU continue to operate when the oil level is at the 'ADD' mark? (Honeywell 131-9)
10 hours maximum
31
Lubricating oil from where is returned to the reservoir by one scavenge pump? (Honeywell 131-9)
Generator sump
32
How is the load control valve controlled and actuated? (Honeywell 131-9)
Electrically controlled, pneumatically actuated
33
Through what does the APU BLEED P/BSW ON signal reach the ECB? (Honeywell 131-9)
BMC
34
Why is the surge control valve hydraulically operated (using high pressure fuel)? (Honeywell 131-9)
For accurate and fast operation
35
What parameters does the ECB use to control the air system? (Honeywell 131-9)
* APU inlet pressure (P2) and temperature (T2) sensors * Load compressor discharge pressure sensor (PT) and differential pressure sensor (DP) * Command and feedback signals from the actuators and the valves (IGV, load control valve and surge control valve) * Aircraft demands - MES and ECS
36
What colour indication is there on the P/BSW when the APU bleed is switched to on? (Honeywell 131-9)
Blue
37
What sensors are used to send valve position feedback to the ECB with regards to bleed air? (Honeywell 131-9)
Microswitch for LCV and LVDT for SCV
38
What position will the IGVs and the SCV be in when the bleed air is initially required following bleed and pack P/BSW pressed? (Honeywell 131-9)
IGV - More open position SCV - Fully closed position
39
What position will the IGVs and the SCV be in when the MES master switch is moved to the ON position? (Honeywell 131-9)
IGV - Fully open SCV - Modulate and discharge a certain mass of air into the APU exhaust
40
At what altitude will the ECB de-energise the start contactor at 58%? (Honeywell 131-9)
Above 20,000 ft
41
What happens at 60% rpm during the APU start sequence? (Honeywell 131-9)
* ECB de-energises the ignition unit * ECB de-energises the de-oil solenoid, if it is open
42
What happens at 95% rpm during the APU start sequence? (Honeywell 131-9)
* IGVs move to 22° open position to prevent overheating * ECB controls and monitors the APU automatically * Blue ON light in the START P/BSW goes off * Green AVAIL legend in the START P/BSW comes on * APU generator can operate and APU bleed be switched to ON
43
When does the ECB reduce the speed of the APU to 99%? (Honeywell 131-9)
If the APU supplies the air conditioning system on a normal day and while the aircraft is on the ground
44
What happens to the air system's valve positions when the APU is shutdown? (Honeywell 131-9)
* IGVs go to minimum opening of 15° * LCV closes * SCV opens
45
What does the APU do on every shutdown? (Honeywell 131-9)
Tests its overspeed protection circuit by simulating an overspeed condition, which causes the 3-way shut-off solenoid in the FCU to close, and thereby start the APU roll-down
46
Describe the APU cool-down cycle? (Honeywell 131-9)
APU operates at a low stabilised temperature for 60 seconds, although will shutdown immediately if the LCV was not in the open position
47
At what speed of APU shutdown will the AVAIL light go off? (Honeywell 131-9) & (APIC 3200)
95%
48
At what speed will the de-oil solenoid valve open during normal shutdown? (Honeywell 131-9)
50%
49
What happens during normal APU shutdown when the RPM gets to 7%? (Honeywell 131-9)
* De-oil solenoid closes * APU fuel LP valve closes * Data memory module is updated * Air intake closes
50
How many igniter plugs and fuel nozzles are in each APU system? (APIC 3200) & (Honeywell 131-9)
APIC 3200 Igniter plugs - 2 Nozzles - 6 main injectors, 3 pilot injectors Honeywell 131-9 Igniter plugs - 1 Nozzles - 10 dual orifice, each having a primary and secondary fuel orifice
51
At what speed does the ECB stop the supply to the start contactors and the ignition system? (APIC 3200)
55%
52
What inhibits APU starting during RAT extension? (APIC 3200 & Honeywell 131-9)
BCLs
53
What is between the LP pump and the HP pump? (APIC 3200)
Filter, impending bypass DPI and bypass valve
54
How does the ECB control the fuel servo valve and the 3-way solenoid valve? (APIC 3200)
Fuel servo valve - TM 3-way solenoid valve - Solenoid
55
What is fuel muscle pressure used for? (APIC 3200)
Operate the bleed control valve actuator and the load compressor IGV actuator
56
How is excessive fuel pressure from the HP pump removed from the system? (APIC 3200)
Constant ΔP valve
57
Describe the flow divider during the start sequence? (APIC 3200)
Pilot injector valve opens when the fuel pressure reaches 20 psi to initially supply the 3 pilot fuel injectors Main injector valve opens as fuel pressure increases to supply fuel to the 6 main fuel injectors as well
58
How does the ECB ensure positive fuel shut-off when the APU shutdown is initiated? (APIC 3200)
De-energises the 3-way solenoid valve
59
How is the remaining fuel in the pilot injectors after APU shutdown purged? (APIC 3200)
To the APU exhaust by combustion chamber pressure
60
What is the PMG installed on the cooling fan for? (APIC 3200)
* Back-up overspeed protection * Emergency power supply to the ECB
61
Where are the speed sensors fitted? (APIC 3200)
Gearbox casing
62
What happens if there is a failure of the two speed sensors? (APIC 3200)
ECB commands an automatic shutdown
63
What parameters in the oil system will cause an automatic shutdown of the APU? (APIC 3200)
* Gearbox high oil temperature threshold: 135°C * AC generator high oil temp: 180°C * Low oil pressure: 35 psi
64
For how long can the APU continue to run when the oil is at the 'ADD' mark? (APIC 3200)
At least 60 hours
65
Where are the ΔP switches in the oil system? (APIC 3200)
* Scavenge filter * Pressure filter
66
What components have the oil returned to the sump by scavenge pumps? (APIC 3200)
* Generator * Rear bearing
67
When would the de-oiling valve close during starting? (APIC 3200)
Above 55%
68
When would the de-oiling valve open during shutdown? (APIC 3200)
Between 90% and 7%
69
For how long will the PMG supply the emergency power to the ECB for? (APIC 3200)
230 ms
70
How is feedback obtained from the BCV? (APIC 3200)
LVDT
71
When are the IGVs commanded to the fully open position? (APIC 3200)
During MES
72
How is protection against load compressor surge achieved? (APIC 3200)
By controlling and reading the position of the bleed control valve, according to the inlet temperature, and differential and discharge pressure signals of the load compressor
73
How is over temperature avoided? (APIC 3200)
Controlling the IGVs to closed
74
For how long is the APU run at a low stabilised temperature during shutdown? (APIC 3200)
120s
75
What components are mounted on the gearbox? (APIC 3200)
* Cooling fan and PMG * AC generator * Starter * Lubrication unit and fuel control unit
76
How would you purge the APU fuel line for maintenance?
A vent APU fuel line P/B, located on the front firewall allows the APU fuel feed line to be purged during ground maintenance. As long as the P/B is held pressed in, the APU fuel LP isolation valve is open and the APU fuel pump runs
77
Name the three modules of the APU?
* The power section * The load compressor * The accessory drive gearbox
78
Within the Fuel Control Unit (FCU), when should the maintenance engineer change the fuel filter element?
Fuel filter replacement is recommended every 8500 Flight Hours
79
During start, what happens when RPM > 55% plus 5 seconds?
The backup start contactor is de-energized.
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