Vague 744 TR database question

[B767300]

I came across this TR database question, wondered if any of you could figure this one out since it's so vague. The actual answer suprised me.


With the Standby Power Selector in AUTO and the standby bus powered from the standby inverter (assume Battery Switch ON), what EICAS message should be displayed and what are the associated limitations?

a. The EICAS message ELEC AC BUS 3 is displayed and there is no time limit

b. The EICAS message ELEC AC BUS 3 is displayed and there is a 30 minute time limit

c. THe EICAS messages ELEC AC BUS 1 and . BAT DISCH MAIN are displayed and there is a 30 minute time limit

d. No EICAS message are displayed

[Hardy Heinlin]

The MAIN and APU standby systems can use power from one of three independent sources, and their priority order is this:

(1) AC bus 3
(2) AC bus 1
(3) Battery (which provides power for at least 30 minutes)

The quoted question seems to be about the MAIN standby system (not about the APU standby system).

When the MAIN standby system cannot use AC bus 3, it uses power from the MAIN battery charger; battery chargers are powered by AC bus 1 when available via the ground service bus. If the MAIN battery charger is unusable either, the MAIN standby system uses the MAIN battery alone.

The static inverter is powered by the battery bus. In your text it's not clear whether the battery bus is being powered by the charger or by the battery alone. But it's clear that AC bus 3 must be off because if AC bus 3 were on, the MAIN standby bus would be powered by AC bus 3 and not by the static inverter.

So this must be the answer:

a. The EICAS message ELEC AC BUS 3 is displayed and there is no time limit


I guess the other answers are wrong:


b. The EICAS message ELEC AC BUS 3 is displayed and there is a 30 minute time limit

Wrong because this answer excludes the message "ELEC AC BUS 1", meaning AC bus 1 is available and therefore the battery is not discharging.


c. The EICAS messages ELEC AC BUS 1 and BAT DISCH MAIN are displayed and there is a 30 minute time limit

Wrong because this answer excludes the message "ELEC AC BUS 3", meaning AC bus 3 is available and therefore the battery is not discharging.


d. No EICAS message are displayed

Wrong because this answer excludes the message "ELEC AC BUS 3". But AC bus 3 must be off in that scenario.


|-|ardy

[John H Watson]

Why would they make you work backwards like that? Pilots and engineers respond to messages. How do you know that "the standby bus is being powered by the static inverter" unless you forced it to use the inverter with the Stby Pwr switch in BAT or pulled specific circuit breakers or all the main busses were unpowered?

AFAIK, you could pull circuit breaker A25 or B24 on CB panel P6, de-energising the coil of the Stby Power Transfer relay (R7227), forcing the Main Static Inverter to feed the AC Standby Bus (it effectively simulates the loss of AC 3 power to the coil of the relay without disabling AC Bus 3). In this case, the answer would be D.

[B767300]

I came to the same conclusions as Hardy: A. But I'm curious how you (@John H Watson) got D? CB's are beyond the scope of a pilot's TR (I'd assume), so there must be some sort of malfunction thus an EICAS msg.

[John H Watson]

I came to the same conclusions as Hardy: A. But I'm curious how you (@John H Watson) got D? CB's are beyond the scope of a pilot's TR (I'd assume), so there must be some sort of malfunction thus an EICAS msg.

Pulling B24 on P6 is part of a typical Standby Power test for engineering (AMM 24-29-00). No messages are generated. I used to do this test almost every day. Engineers may forget to reset the circuit breaker.
I'm not saying A is wrong, it's just that there are other possibilities. I'm sure you won't get examined on it, but if you fly 747-400's, give it a try.

More modern aircraft might tell you if a CB has been pulled, but nothing has stopped working. AC Bus #3 is still fully powered. The Static Inverter has simply been forced to use the Battery Bus (powered by AC Bus #1). There is no battery discharge.

[Hardy Heinlin]

That "no message" condition by pulling B24 on P6 can be demonstrated in PSX. The diagrams in the Aerowinx manual on pages 209 and 210 show it. Check the yellow triangle with the "A" in it. That's the continuation point from page 209 to 210. B24 is on page 209. On page 210 it continues via A25 to the coil of the Main Standby Transfer Relay. If that relay is relaxed, the main standby bus is powered by the main static inverter.

What is the official answer to that test question?

[B767300]

That "no message" condition by pulling B24 on P6 can be demonstrated in PSX. The diagrams in the Aerowinx manual on pages 209 and 210 show it. Check the yellow triangle with the "A" in it. That's the continuation point from page 209 to 210. B24 is on page 209. On page 210 it continues via A25 to the coil of the Main Standby Transfer Relay. If that relay is relaxed, the main standby bus is powered by the main static inverter.

What is the official answer to that test question?

I'll give it a try. And the official answer is D...... Maybe they meant that the switch from AC to DC stby pwr doesn't generate a message?

[John H Watson]

And the official answer is D......

Ridiculous. Who is going to get that right (apart from an avionics engineer).

I'll give it a try.

I can't remember if any of the displays flicker during the switchover. Maybe check for that. The Static Inverter is running continuously, so there would only be Standby Bus relay switching time of milliseconds (I assume well within the ability of the displays to absorb momentary power losses)
 

[Hardy Heinlin]

What is the context of the question? Is there a previous text about CBs and maintenance tests? Is the question placed within a special chapter regarding engineering etc.?

Without context, that minimal situational information the question provides is clearly too small for a single correct answer.

[Jeroen Hoppenbrouwers]

It may be a question designed to test a specific item taught explicitly in a course, because somebody considered it of high value, possibly after an incident or expensive mistake that happened. In such a case the technical background isn't that relevant any more and it becomes dogma. Not enough to actually change documentation (which can be very expensive and very complicated) but sufficient for a training item.

[B767300]

I got it from this database: https://www.hoppie.nl/forum/var/2024-01-20_06.35.38_questions_744.htm
I believe it's from EVA airways. No real context, just a list of questions.

It may be a question designed to test a specific item taught explicitly in a course, because somebody considered it of high value, possibly after an incident or expensive mistake that happened. In such a case the technical background isn't that relevant any more and it becomes dogma. Not enough to actually change documentation (which can be very expensive and very complicated) but sufficient for a training item.

This seems the most realistic option or it was a typo. As you pointed out, maybe it's to warn pilots that there's no (clear) flightdeck indication that the system is in DC STBY. An AC BUS 3 failure msg may be a cause, not a result of the AC>DC xfer. But they stated the question ambigiuously. The >BAT DISCH msg is a clear indication....