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DC Bus failure - no HYD PRESS DEM x caution?

Started by sbergert, Mon, 2 Oct 2017 07:38

sbergert


Hi everybody,

I was curious to see PSX EICAS messages compared to a panel trainer and noticed that there is no HYD Press DEM ... EICAS caution. There are quite a lot of cautions and I learned that this special EICAS Message is a good cue for a DC Bus failure (in the bus Equipment list it's listed as well).

Am I overlooking something? Maybe the panel trainer is wrong?


John H Watson


sbergert


I nearly think it should be with any DC bus - the failure I had was DC Bus 4 with the EICAS HYD PRESS DEM 4 -message. I was not able to reproduce this with PSX.

Since DC-Bus faults are not indicated as such (only a lot of EICAS cautions pop up), I was told that a good indication for a DC Bus fault is, between all those seemingly unrelated EICAS messages, the HYD PRESS DEM x -caution

Hardy Heinlin

Hi,

as far as I recall, HYD PRESS DEM 3, for example, is not shown on the real EICAS during preflight when both DC bus 3 and AC bus 3 are unpowered. But it may be like this solely because the message is inhibited by the HYD PRESS SYS 3 and ENG 3 SHUTDOWN messages. I can imagine it should be shown when the respective engine is running.


|-|ardy


John H Watson

#4
You would have to figure out what generates the messages. The HYDIM cards are powered by the DC busses. E.g. DC Bus 2 powers some of the circuits for the #2 demand pump ELCU but also the #2 HYDIM card.

See following messages...

Multiple DC bus failures may not give the expected results.

Cheers.
JHW

sbergert


Maybe I will have the chance to verify the messages in the SIM the next days...
Coming from Airbus I'm surprised that the DC Busses are not monitored like the AC Busses where you get the correct EICAS message.

I always thought the PSX- EICAS cautions are complete (except for those stated in the manual)... Could also be that the Panel Trainer is wrong off course, but I guess the instructors hint with the HYD PRESS DEM in case of a DC Bus failure is mainly given for later in the SIM or real aircraft.

Bastien

#6
Hello guys,

I've just checked the manual, and actually HYDIM are cross powered by DC Buses :

HYDIM1 powered by DC Bus 4
HYDIM2 powered by DC Bus 3
HYDIM3 powered by DC Bus 2
HYDIM4 powered by DC Bus 1

So one DC Bus failure should gives expected result, but as said by John, multiple DC Buses failure may be more difficult to predict.

This have to be tested ....

Hardy Heinlin

Hello,

I don't think the HYDIMs are the cause of the message; it wouldn't make sense anyway as each HYDIM doesn't use the same DC bus that the related demand pump control uses. I think it's just the demand pump control alone that triggers the message.


|-|ardy

Bastien

#8
Hello,

Yes indeed, if HYDIM are not cross monitoring demand pump status, it doesn't make sense. I agree.
I've done some more tests to understand and see what is going on.

We assume the following situation :
On the ground with 4 engines running
No system failure
No external power
All systems configured for takeoff
(Demand pump command : AUTO)

Pull out the demand pump X breaker > EICAS message is not displayed
Set demand pump X to OFF or ON > EICAS message is displayed
Set demand pump X back to AUTO > EICAS message disappear
Now generate EDP X failure > demand pump X is still unpowered and can't provide pressure > EICAS message is not displayed (inhibited by HYD PRESS SYS X)
Set demand pump X to ON > EICAS message is not displayed

My conclusion is when Demand Pump X is unpowered EICAS message is only displayed when demand pump X selector is set to OFF or ON but not to AUTO

Question to the specialists : Is this working as expected ?
Thanks


Hardy Heinlin

Hello,

as far as I recall without checking the maintenance diagrams, the DEM pump selector's OFF position state goes directly into the HYDIM causing the message. This is just one of several conditions causing the message. The message tells that the DEM pump is inoperative.

When the selector is not in OFF but the DEM pump control is inoperative, the DEM pump is also inoperative. Hence this triggers the message as well (needs to be modified in PSX).

The message will not occur when it's inhibited by the HYD SYS or ENG SHUTDOWN message, or when the related HYDIM is inoperative.


Regards,

|-|ardy

Bastien

Hi Hardy,

That sounds clear to me !

Thanks,
Bastien

John H Watson

#11
QuoteI've just checked the manual, and actually HYDIM are cross powered by DC Buses :

HYDIM1 powered by DC Bus 4
HYDIM2 powered by DC Bus 3
HYDIM3 powered by DC Bus 2
HYDIM4 powered by DC Bus 1

On further investigation, the HYDIMs appear to have multiple DC power sources (i.e. two circuit breakers for each card). If you look at the wiring schematics for engine pumps, the DC inputs are as Bastien states. If you look at the schematics for the demand pumps, there appears to be a 15v power supply in each Hydim card powered by the corresponding DC Bus. I'm trying to figure out what the functions of the multiple power sources are. It was probably done during beta testing (but I can't remember). The Maintenance Manual specifies pulling two circuit breakers in the cockpit when changing the HYDIM cards (plus numerous CB's in the MEC).

My training manuals clearly show the HYDIMs triggering the fault messages and overhead lights. The HYDIMs contain some of the message filtering.

John H Watson

QuoteWhen the selector is not in OFF but the DEM pump control is inoperative, the DEM pump is also inoperative. Hence this triggers the message as well (needs to be modified in PSX).

In the ON position, the pump is commanded on directly (via a different solenoid on the pump). The hydim card needs to be powered to put on the overhead lights and send fault messages to the EIUs. I think the multiple DC power supply functions need to be sorted before any changes are made to PSX.

Cheers
JHW

John H Watson

QuoteOn further investigation, the HYDIMs appear to have multiple DC power sources (i.e. two circuit breakers for each card).

Three CB's if you include the AUX pump circuits.

Pressure indications also need 28Vac circuit breakers. Analogue pressure signals are processed in the Hydim cards.

John H Watson

After further investigation...

The Hydim CB's on overhead circuit breaker panel P7 provide power to main processing circuits in the HYDIM cards (from busses as stated in Bastien's post). This would include the EICAS message and annunciator light logic. These CBs provide +15dc/-15vdc/5vdc power to the main processing circuits.

The demand pump CBs on P7 (which use the respective dc busses) only power the Hydim circuits which control the demand pump on/off logic.

Ref: Boeing Wiring Schematics 29-12-05

This would seem to suggest that, for example, a complete DC Bus 2 failure would stop demand pump 2 working and also pump 3 working (at least in AUTO mode)

Hardy Heinlin

This all is already modeled in PSX.

Also the HYD PRESS DEM () advisory appears in PSX when the respective demand pump control is inoperative, BUT only when the demand pump selector is ON. (It's a disagree alert.)

So the only thing I need to remove is the "demand pump selector ON" condition in the above part of the logic. (Not a disagree alert, but a power loss alert.)


Cheers,

|-|ardy

Hardy Heinlin

#16
Quote from: sbergert on Mon,  2 Oct 2017 07:38
... I learned that this special EICAS Message is a good cue for a DC Bus failure ...

There are many messages when a DC bus fails. It may take a while to review them all. A good indication may also be the respective demand pump PRESS light. However, No. 3 will not illuminate when DC bus 3 is unpowered as DC 3 powers the indicator lights of No. 1, 2, and 3.


Regards,

|-|ardy


The effect is now implemented in PSX 10.2: http://aerowinx.com/board/index.php?topic=4191.0

(Actually, that effect was mentioned in the first Aerowinx manual already.)

sbergert


I had a chance to see a DC Bus 1 failure yesterday in the Full Motion sim... the mentioned EICAS caution did show (for System 1 in this case).
Thanks for implementing it in the next update (and also for the stab trim with hydraulic 2&3 off - fix earlier).

Hardy Heinlin

It's already implemented in the current update 10.2. See link above.