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When you miss the APU AVAIL lights

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Moderator
Registered: May 2009
Posts: 5140
... you most likely took off* with the APU running.

The AVAIL lights extinguish after lift-off (the ON lights extinguish when take-off power is set).

When you're back on the ground and "restart" the APU, and the AVAIL lights won't illuminate, do this:

Cycle the APU START selector to OFF, then ON.

Or cycle twice the GEN FIELD MAN RESET toggle switches APU 1 & 2 on the overhead maintenance panel.

The AVAIL lights will then illuminate.

You don't need to reboot PSX :-)


Cheers,

|-|ardy


* Or you didn't take off but moved the aircraft on the Instructor and lost ground contact and momentarily got into air mode while the APU was running.
Member
Registered: May 2012
Posts: 158
Location: Farnborough, UK
If you are referring to us yesterday. We didn't actually loose anything in PSX, we lost contact with the output boards which unfortunately are on the server pc for PSX, the only way to get the boards back is a reboot of the pc.

This mean none of the indicators on the overhead updated in the sim.

Everything will be migrated to sim stack to get away from the unreliable usb issues.

Cheers
Gary
Moderator
Registered: May 2009
Posts: 2449
Location: KTMB
Just to be sure: this APU light behaviour isn't PSX-specific, it's 744-specific, right?
Moderator
Registered: May 2009
Posts: 5140
I saw it yesterday, but it reminded me of this common problem other PSX users reported in the past and maybe still have sometimes.

This effect is the result of the documented generator field trip logic. I've implemented this logic and didn't add any hypothetical features. Whether this effect is 100% realistic, I don't know; it's hard to find someone who has, in real life, landed a 744 with the APU running.


Cheers,

|-|ardy
Member
Registered: May 2010
Posts: 843
Quote
This effect is the result of the documented generator field trip logic.


When is the field tripped in PSX in this particular situation? On landing?

I don't understand why the field should be tripped.

Looking at the wiring schematics, the APU Gen field is tripped in the following situations:

Overhead field trip switch activated (respective AGCU provides power for switch)
APU bearing failure (for 5 minutes) (Reset with field switch)
APU Fire Handle pulled (respective AGCU provides power for circuit)
APU Gen voltage too high
APU Gen voltage too low (not caused by underspeed or GCR trip)
APU Gen differential current detected

The AVAIL light should illuminate with:
APU speed above 95%
Voltage between 104 and 130volts ac
No trip signals to the GCR
Aircraft on ground (air/ground relays)

APU Gen ON switch function is disabled when engine rpms are above a certain value (with any engine GCB closed). Interestingly, the ON light circuit goes through an air/ground relay (R227 or R228 ), so whilst we know that APU power may be available under certain circumstances in flight, the ON light may not illuminate.

Rgds
JHW
Member
Registered: Jul 2009
Posts: 26
Location: Sydney
Gary Oliver wrote
Everything will be migrated to sim stack to get away from the unreliable usb issues.


:mrgreen: :mrgreen: :mrgreen:
_______________
Moderator
Registered: May 2009
Posts: 5140
John H Watson wrote
When is the field tripped in PSX in this particular situation? On landing?

I don't understand why the field should be tripped.

Looking at the wiring schematics, the APU Gen field is tripped in the following situations:

Overhead field trip switch activated (respective AGCU provides power for switch)
APU bearing failure (for 5 minutes) (Reset with field switch)
APU Fire Handle pulled (respective AGCU provides power for circuit)
APU Gen voltage too high
APU Gen voltage too low (not caused by underspeed or GCR trip)
APU Gen differential current detected

The AVAIL light should illuminate with:
APU speed above 95%
Voltage between 104 and 130volts ac
No trip signals to the GCR
Aircraft on ground (air/ground relays)

Sorry, my comment was wrong. Let me call it generator "availability" instead of "field"; the "field" is another parameter in PSX.

In PSX, the availability is event driven, i.e. by momentary signals which change a status (momentary pushbuttons, momentary toggles, bus reconfig events etc.).

E.g. in PSX the status changes from available to unavailable ...

... in the event when changing from ground to air mode.

... APU shutdown signal etc.

Vice versa, in PSX the status changes from unavailable to available ...

... during APU start, in the event when all necessary parameters rise through certain minima.

... when cycling the start switch while all parameters are normal.

... when cycling a field reset switch while all parameters are normal.

In PSX the status from unavailable to available cannot be triggered by an air-ground event. Only the reverse is possible by a ground-air event.

As we know, when in air mode the APU cannot be started. So I didn't even think of adding an air-ground event for making an already running generator available again upon touchdown. That's what I mean by "hypothetical". Do you think I should add it? E.g. when a windshear pulls the aircraft back on the ground after lift-off, the AVAIL lights should go off, then on? In the current PSX versions, they will remain off (as described above).


Cheers,

|-|ardy
Member
Registered: May 2010
Posts: 843
Quote
In PSX, the availability is event driven, i.e. by momentary signals which change a status (momentary pushbuttons, momentary toggles, bus reconfig events etc.).


I think I see what you mean.

I don't know why Boeing complicated things by making the AVAIL light logic different from the availability logic. Going back over the manuals, I see that the AVAIL light is controlled by air/ground relays (among other thigs), but availability (or selectability) is not controlled by air/ground relays, but engine (1~4) gen speed.

As far as I can see, the AVAIL light should re-illuminate with air/ground transition (if the APU remains running). The APU Gen should only be available if the engine gens are below a certain rpm.

The ON light illumination is another issue again. This looks at APB position (via APB auxiliary contacts), MD&T power and an air/ground relay (R229 for APU Gen #1). To me, the air/ground input seems pointless in a light illumination circuit. If the APB is closed, it is closed. Having the ON light not illuminated (due to a faulty air/ground relay) when the APB may be closed seems misleading or even dangerous.

Cheers
JHW
« Last edit by John H Watson on Tue, 23 Jun 2015 13:34:19 +0000. »
Moderator
Registered: May 2009
Posts: 5140
John H Watson wrote
The ON light illumination is another issue again. This looks at APB position (via APB auxiliary contacts), MD&T power and an air/ground relay (R229 for APU Gen #1). To me, the air/ground input seems pointless in a light illumination circuit. If the APB is closed, it is closed. Having the ON light not illuminated (due to a faulty air/ground relay) when the APB may be closed seems misleading or even dangerous.

I think so too, misleading and dangerous. Maybe a mistake on the diagrams only? :-)

In PSX, the ON light logic is not linked with the PSEU, i.e. in PSX the ON light tells the truth of the BCU, not of the PSEU. And, as we know, the truth is that the BCU uses its own method to determine the, in BCU language, so-called "air mode", which is set when more than 2 engines are running with N2/N3 greater than 74, 69, or 67% for GE, PW, or RR respectively. That part is fully modelled in PSX.


Quote
As far as I can see, the AVAIL light should re-illuminate with air/ground transition

I'll try to add this feature in the next beta.


Cheers,

|-|ardy
Moderator
Registered: May 2009
Posts: 5140
APU generators running in flight now become AVAIL on landing in 10.0.5-beta9 (item 0.5.0044):

http://aerowinx.com/forum/topic.php?id=2939


|-|ardy

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