APU AVAIL light logic

[John H Watson]

For some reason, I can't get APU Gen #2 online in this scenario. Once APU Gen #1 is online, APU Gen #2 AVAIL light extinguishes, but this is expected. The switch should still function, shouldn't it?

Any ideas?



Edit HH: Thread split from http://aerowinx.com/board/index.php?topic=5069.msg54163#msg54163

[Hardy Heinlin]

I'm trying to remember why we got the idea that we had to pull CBs to get the APU to operate in flight.

You're probably thinking of the PSEU CBs. The APU's to-ground-mode-relaxing relays are those that are powered by F22 and H8.

For some reason, I can't get APU Gen #2 online in this scenario. Once APU Gen #1 is online, APU Gen #2 AVAIL light extinguishes, but this is expected. The switch should still function, shouldn't it?

F23 power enables the air-ground relay for APU generator operation to indicate air mode. Before GEN 1 starts powering the AC busses, F23 is unpowered (air mode indication disabled), thus allowing GEN 1 operation.
When GEN 1 powers the AC busses and F23, GEN 2 operation is inhibited because air mode indication is enabled and the PSEU mechanisms are in air configuration.

The old chess game :-)

[John H Watson]

I don't remember this being so complicated 

The AVAIL light indication circuit is relatively simple. It's just based on undervoltage, overvoltage, underspeed  sensing, etc, and secondary air/ground relay R230.

F23 power enables the air-ground relay for APU generator operation to indicate air mode.

As far as I can see, it's AIRMODE (engine gen rpm-based*) logic, not the PSEU and air/ground relays, which affect APB closure. However, the Gen 1 & 2 "ON" light illumination will be inhibited by Secondary air/ground relay "R229". So, you will have power on the bus, but the light will not be on. Ref Engineering Elec Book 2, pages 120/122

*N3>67.4%

In PSX, if all engines are off in the air and I start the APU, the AVAIL lights appear (as expected). Then I select APU Gen 1 ON. Assuming the SSB is closed or closes (I just discovered that AIRMODE also affects SSB operation... See Engineering Elec Book 2 page 65), APU Gen 1 should power the whole ship. However, the APU Gen 1 "ON" light should not be illuminated: "ON" illumination is produced by a sent of contacts on the Aux Power Breaker, secondary air/ground relay R228 (which is now powered to the AIR mode) and Master Dim & Test power.
Similarly, if I select APU Gen 2 on, Aux GCB should close (unless SSB operation inhibits it?). 

AIRMODE also affects the Ground Handling Relays.

Normally, there is a power interrupt if APU Gen 1 power is on both sides of the SSB and you select APU Gen 2. If I decided to power the APU in the air, I would think twice about selecting APU Gen 2.

[Hardy Heinlin]

I have the BCU's RPM related air mode condition in the connection logic of APB 1, APB 2, and SSB.

For the GEN run command, which occurs when the APU has spooled up, I have the ground signal from that F23 thing, not from the BCU's air mode. Should this be on the BCU as well? Note that this is not APB stuff, but generator activation.

However, the APU Gen 1 "ON" light should not be illuminated: "ON" illumination is produced by a sent of contacts on the Aux Power Breaker, secondary air/ground relay R228 (which is now powered to the AIR mode) and Master Dim & Test power.

If you remember our "F22/H8" etc. air/ground categories, in which category is "R228"?

[John H Watson]

Are you saying the APU lights shouldn't even illuminate in flight during a normal MD&T test?

It will be ok with the MD&T test. As you say, lamp voltage comes from the MD&T. All that is needed is an earth to illuminate the light. The MD&T provides the earth for the light test. However, for normal indication purposes, the earth signal (provided by Aux Power Breaker auxilliary contacts) goes via an air-ground relay.

I don't know if you have the schematic for this, but it's 24-22-03 page 101 sheet 1.

For the GEN run command, which occurs when the APU has spooled up, I have the ground signal from that F23 thing, not from the BCU's air mode. Should this be on the BCU as well? Note that this is not APB stuff, but generator activation.

I'll have to figure out your interpretation of "GEN run command". The generators are always mechanically connected to the APU gearbox. The generator field is controlled by the generator control relay (GCR) inside the AGCU. The GCR is normally closed (otherwise the FIELD OFF light will be illuminated on the overhead maintenance panel).
The generator field can be tripped by the AGCU for:
undervoltage NOT caused by underspeed (APU < 95%)
differential current fault (on the phases)
fire switch pulled
overvoltage

The BCU can also trip/close the field (by sending a signal to the AGCU) for:
APU fire switch input (into the BCU)
Overhead APU Gen field toggle switch input into the BCU
APU Generator bearing fail input into the BCU.

As far as I can see, it's just the APB which normally controls whether the power goes to the aircraft or not (other than the power which goes to the Ground Handling Bus, which has it's own set of logics).

Hopefully it's just a case of removing the line of PSX code for apu speed from your "Gen Run Command". APU speed (>95%) only goes into the AVAIL light logic and APB logic (and, in part, to the AGCR logic "undervoltage NOT caused by underspeed" mentioned above) 

The AVAIL light also looks at:
gen field not tripped
voltage greater than 104v (+/-1.5)
voltage less than 130v (+/-3)
APB not closed.

Hope this makes sense.

I'm not sure where we got the F23 air/ground logic from for generator operation.

[Hardy Heinlin]

By "GEN running" I mean "GEN AVAIL", i.e. the system status when APU RPM is up and the field is connected.

It's been almost 10 years when I coded this. I just rediscovered that PSX includes two air mode checks in the above system status:

F23 air mode and BCU air mode.

This is because there are two kinds of events that control the GEN AVAIL status:

(A) PSEU ground/not-ground events
(B) APU RPM up/not-up events

In (A) events, GEN AVAIL control just depends on F23 air mode; it doesn't check for BCU air mode.
In (B) events, GEN AVAIL control depends on both F23 air mode and BCU air mode.

Do you think I should entirely remove the F23 check from the GEN AVAIL stuff and let (A) and (B) events just check the air mode of the BCU?

It could be that the F23 stuff is a relict of a very old version, before I introduced the BCU air mode. Perhaps I should've replaced all the F23 stuff by the BCU stuff, and not just some of it.

The MD&T provides the earth for the light test. However, for normal indication purposes, the earth signal (provided by Aux Power Breaker auxilliary contacts) goes via an air-ground relay.

And this air-ground relay is not related with F23?

My search function cannot find 24-22-03, unfortunately.

[John H Watson]

And this air-ground relay is not related with F23?

Correct. (EDIT) Incorrect... see following messages.

By "GEN running" I mean "GEN AVAIL", i.e. the system status when APU RPM is up and the field is connected.

OK. As you know, AVAILability does not necessarily mean selectability. During the takoff roll, the engines increase in rpm (AIRMODE becomes active) and the APBs trip. However, the AVAIL lights (falsely) remain illuminated until liftoff. The APU generators are still producing power (they are spinning and the field has not tripped), but the power is not going anywhere.

I still don't understand why Boeing put in the air/ground F23 circuit to disable the ON light.  If the APB is somehow closed in flight (the APU would have to be running, the gens would have to be manually selected with engines in idle or off), wouldn't you want an indication of it?

[Hardy Heinlin]

And this air-ground relay is not related with F23?

Correct.

By "GEN running" I mean "GEN AVAIL", i.e. the system status when APU RPM is up and the field is connected.

OK. As you know, AVAILability does not necessarily mean selectability. During the takoff roll, the engines increase in rpm (AIRMODE becomes active) and the APBs trip. However, the AVAIL lights (falsely) remain illuminated until liftoff. The APU generators are still producing power (they are spinning and the field has not tripped), but the power is not going anywhere.

I still don't understand why Boeing put in the air/ground F23 circuit to disable the ON light.  If the APB is somehow closed in flight (the APU would have to be running, the gens would have to be manually selected with engines in idle or off), wouldn't you want an indication of it?

So is that air-ground relay related with F23 indeed?

I asked: "And this air-ground relay is not related with F23?"
You replied: "Correct."

:-)

Cheers,

|-|ardy

[John H Watson]

Ugh....  I think I missed the NOT (or got confused by the different scenarios).

F23 is related to the ON illumination (i.e. it's in the circuit).

earth > relay R229 (F23 logic) > APB contacts > "ON" > MD&T volts.

The APB opens when engine N3 increases. So no power available with higher than idle N3's. However, the AVAIL light is only affected by APU >95% speed (and faults and GCR trips) and F23.

I'm trying to work through different scenarios. e.g.

Prior to takeoff, the APU gens are selected back ON (by mistake). During takeoff (with the engine N3 high):
AVAIL off (because ON was selected), ON is off (because the APB trips). The AVAIL light should appear when the APB trips (and should extinguish on liftoff).

It's the 67% N3 value for AIRMODE which is confusing. It's above idle, but I'm not sure if it's always above high idle. Can the APU gens be selected ON in flight with the engines running (at idle or high idle), even though the AVAIL light is inhibited?   

AIRMODE is engine gens at 6100rpms.

The IDG input speed 6100 rpm for the three engine
types are respectively:
• PW 73.3% N2
• GE 74.6% N2
• RR 67.4% N3
NOTE: The normal ground idle speed is 5700 – 5800 RPM.

[John H Watson]

Thought experiment:

A freighter takes off with APU running. Aircraft levels off at 8,000' and the cabin is manually depressurised. All IDG's are then disconnected. Air/Ground CB "F22" pulled. Will the APU provide power to the #2  cargo busses? If so, can you open the nose cargo door or will the door motor stall?

I see the #2 Cargo Bus relays are affected by BCU AIRMODE and primary Air/Ground Relay R910 (which will relax to GND mode with F22 pulled).

I've forgotten.... Does PSX prevent any door opening in flight (including the cockpit overhead hatch and MEC door)?

[ScudRunner]

Air loads alone would surely prevent the door doing anything more than unlatching even if the `smarts' did not.

[Hardy Heinlin]

Does PSX prevent any door opening in flight (including the cockpit overhead hatch and MEC door)?

Normal door opening is impossible in PSX when the differential pressure is 0.4 psi.

But you can open a door on the Malfunctions > Bleed page by activating "Cabin depress - gradual" in Severe mode. That will be a rapid decompression due to an open door.


|-|ardy

[John H Watson]

Normal door opening is impossible in PSX when the differential pressure is 0.4 psi.

That's why I depressurised the aircraft manually. However, I just read that the door motor may stall in 40kt winds. The side cargo door should be easier to open in flight ...as long as there are no strong crosswinds and the logic is similar... and I haven't overlooked anything.

But you can open a door on the Malfunctions > Bleed page by activating "Cabin depress - gradual" in Severe mode. That will be a rapid decompression due to an open door.

Thanks,

Cheers
JHW

[John H Watson]

ah.. I didn't wait long enough during my tests... .. the doors started to open in flight (including the nose cargo door).

The nose door adds quite a bit of drag. It's a really fast way of doing a loop )))

[Hardy Heinlin]

You just discovered an Easter Egg I forgot it existed :-)



What did you do to get nose door power?