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The Actuator
3rd Aug 2018, 22:17
Iím trying to understand an issue with an APU. Is there some guidance from Boeing under what conditions the APU Bleed Valve will close automatically during a load demand during start? As soon as the fuel cut off switch is positioned to run during an auto start the duct pressure starts dropping and then the APU bleed valve closes. It cannot be reopened!
Anyone have some pointers.

ACMS
4th Aug 2018, 01:24
It would have to be a fault, it won’t normally close during the start for obvious reasons!!

Willit Run
4th Aug 2018, 03:16
Maintenance may have installed a starter that does not meet the effectivity codes.
We have installed the wrong starter on some of our planes, and during the starts, the duct pressure would
drop very low. The big wigs fix, was to turn off all packs during engine start, instead of installing the proper starter.
This was an improper inter mix between JAL Birds and early Malaysian birds.

Thats my only suggestion.

eckhard
4th Aug 2018, 04:54
Are you sure that the Wing anti-ice switch is OFF?

spannersatcx
4th Aug 2018, 16:15
APUC settings?

N2000
4th Aug 2018, 18:08
Close the cross-bleeds as well as checking Packs and wing-antice off. Also, check that the hydraulic demand pumps are off.

The Actuator
4th Aug 2018, 22:12
So far the starter theory might make sense. We have tried all suggestions here. My question is under what conditions would the APU bleed valve modulate to closed uncommanded upon creating a bleed demand?

So even if switches like hyd pumps, anti ice and opposite isol valve were in correct or incorrect positions ( no idea what cross bleeds are on 74!) what would cause the APU Bleed valve to modulate closed? Is it to protect the APU from excessive load?
If so then turning one APU Gen, hyd pumps, packs, one isol valve and anti ice off would ensure minimum load...not so? Tried that still goes closed. Weird thing is you can only open it again if you completely de power the aircraft or use an air cart for start then shut APU down - subsequent APU start allows APU bleed valve to open until you try and start again?
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CCA
5th Aug 2018, 03:13
I'm guessing it's closing for auto shutdown protection any CMC message? I remember on the classic it was easy to cause an auto shutdown due to a rapid EGT rise by placing tto much demand on the bleed system all at once, so I guess you need to search for what's causing a potential Auto shutdown, is it a clapped out APU?

The classic reasons:
​​​
- Low oil pressure with RPM above 95%
- High oil temperature
- Cooling air valve fails to open
- High EGT
- Overspeed, 110% RPM
- Excessive pressure drop across APU inlet screen
- Duct failure overheat
- Loss of airplane battery power
- Loss of APU sensor signals: EGT, Air inlet temperature, Oil temperature, Oil pressure

The Actuator
5th Aug 2018, 04:16
CMC had a message about IGV torque motor failure - since relaced. Changed the Bleed Valve yesterday no effect. All good reasons for an APU to shutdown above - Except the APU does not shutdown - just the APU bleed valve closes.

CCA
5th Aug 2018, 05:53
A bit rusty on the 744 I believe the valve closes to help prevent an auto shutdown so I guess as soon as the signal from the eng start lever is received and the APU begins its ramp up for the 40psi that it's meant to deliver something happens and the valve is commanded closed to protect the APU from an auto shutdown. Any new CMC message after the torque motor replacement? Maybe the IGV's aren't moving as required?

NSEU
5th Aug 2018, 13:58
Since the APU Bleed Valve is motorised, it can only close because of a loss of DC control power to the Isolation Valve relay R7745 ("AIR SUPPLY APU VALVE CONTROL).

DC control power goes through the APU start latch relay R7730, the APU fire switch, the APU bleed valve overhead switch and the APUC (rpm>95%) circuit.

Have you tried a new APUC?

Are the rpms actually dropping to 95%? Does the APU know the engine is being started? i.e. Does the maintenance page show MEC (Main Engine Start) for OP MODE during start? Is it the same for more than one engine? (There are four separate inputs from the start circuits to the APUC).

Do you have any CMC snapshots?

NSEU
5th Aug 2018, 15:46
Are you sure that the Wing anti-ice switch is OFF?

The wing anti-ice should only operate in the air.

what would cause the APU Bleed valve to modulate closed?

Probably best not to use the term "modulate".... It only has two basic positions. Open or closed. Well, not including "in transit". No spring-loading, no servo air. If it loses AC power, it freezes in the last position. If it loses DC control power (only), it closes. Ref SSM 36-11-03

I can't see it being commanded closed to stop the APU shutting down. With a tired APU, all I've seen are duct pressures going down and down until you start getting yellow numbers on the ECS synoptic.

Changed the Bleed Valve yesterday no effect.

I can't see how it would cause a problem like this. It's just a DC motor attached to a butterfly. It needs an external command to move.