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Author Topic: 4 eng fail and flaps EICAS...  (Read 658 times)

jcomm

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4 eng fail and flaps EICAS...
« on: Wed, 9 Jan 2019 10:13 »
Whenever I simulate a 4 engine failure, on approach, before or after having started to deploy the flaps, I always get the bellow state in the EICAS, with "x"'s  in the left groups only.

.) Under these circumstances I can never tell if deployment will be asymmetric, until it is evident? Why does it only affect the trailling edge sensors of the left group ?


.) On a scenario where relight is not possible, and if it occurs during, say, the approach transition, should the APU be started ?


.) Should the Alternate Flaps deployment be used instead of the "Primary" mode under such circumstances?


John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #1 on: Wed, 9 Jan 2019 23:14 »
Quote
Why does it only affect the trailling edge sensors of the left group ?

Not sure why it was designed that way, but with Standby Power only, only certain flap position sensors are powered by the Standby Power Static Inverter.

The APU cannot be started in the air because of airplane logic. Rumour has it, that if certain circuit breakers are pulled, you can start it, but getting APU power on the busses in the air is another issue. If you are close to the airport, you have everything you need to land the aircraft (It's better not to risk what you already have). Starting the APU will also blank the instrumentation for a while (as the APU Battery powers the Captain's displays).

Alternate Flaps uses main bus power to drive the flaps. You haven't got main bus power in this situation.

Rgds
JHW


Hardy Heinlin

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Re: 4 eng fail and flaps EICAS...
« Reply #2 on: Wed, 9 Jan 2019 23:42 »
Not sure why it was designed that way ...

Perhaps to save battery power? In most cases you can probably assume the system is symmetric, so one sample on one side is enough, just like the two of twelve spoiler sensor sources on the lower EICAS.

Mechanical TE flap symmetry linkage broken and all 4 AC busses off -- that would be extremely rare, I guess?


|-|ardy

John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #3 on: Thu, 10 Jan 2019 00:20 »
Quote
In most cases you can probably assume the system is symmetric,

Good point. Because the left hand TE flaps are mechanically linked to the right hand TE flaps, only a serious mechanical failure can result in asymmetry (flap drive shafts breaking, flap gearbox failures, etc).

emerydc8

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Re: 4 eng fail and flaps EICAS...
« Reply #4 on: Thu, 10 Jan 2019 04:12 »
If I recall correctly, we had some classics that you couldn't start the APU while airborne.  On most of our old 747-200s, you could start the APU while airborne. I thought it had something to do with the design of the APU door but not sure.

jcomm

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Re: 4 eng fail and flaps EICAS...
« Reply #5 on: Thu, 10 Jan 2019 08:48 »
I did try to start the APU, the "captain displays" cleared for a while and then came back to live, but I didn't check if the APU actually started…

About the asymmetry, I believe they should at least feed some sensors on both sides. The way it is according to the screenshot, suggests there's a total blackout of the left TE sensors ? But now it makes more sense to me - I didn't yet know they were mechanically connected … Still lots of manual pages to go… :-)

John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #6 on: Thu, 10 Jan 2019 09:16 »
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I did try to start the APU, the "captain displays" cleared for a while and then came back to live, but I didn't check if the APU actually started…

Interesting. I'll have to look into this. I didn't think the starter would engage with the APU inlet door closed. The door won't open in the air (due to the airplane's air/ground system detecting that the aircraft is in the air).

The starter shares some control relays with the Battery Charger Inhibit circuit and the Static Inverter Power inhibit circuit. This is why the screens blank when the APU is started.

Hardy Heinlin

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Re: 4 eng fail and flaps EICAS...
« Reply #7 on: Thu, 10 Jan 2019 09:30 »
In that 4 E/O scenario all AC busses are unpowered and therefore certain air-ground relays which relax to ground mode will signal ground mode to some system units, e.g. the gear lever lock solenoid is relaxed, and APU systems are in ground mode etc.

APU door activity is indicated on Instructor > Analysis > Misc. -- In PSX the APU door can also open in flight (but only to 15°, not 45° like on the ground), but the APU won't start when the APU system's air-ground relay signals air mode.


|-|ardy

John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #8 on: Thu, 10 Jan 2019 10:23 »
I don't see any air/ground relays in the APU starter motor circuit. The APU controller gives the command to start based on APU switch in START, APU door open and no shutdown signal active.

I've forgotten the forum discussions on the door, but it's the door which has the air/ground relays. The primary air/ground relay "R263" is involved (which is powered by the battery/standby power system). The door will move to 15 degrees from 45 degrees if the APU door is open on the ground and the aircraft takes off. I don't see how the door will open if it is already closed in flight.

emerydc8

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Re: 4 eng fail and flaps EICAS...
« Reply #9 on: Thu, 10 Jan 2019 18:18 »
I don't know if this piece of info will help, but even on the classic airplanes that didn't have the airborne start feature, you could take off with the APU running and it would shut down automatically at 20,000' if you forgot (no electrics while airborne though).

When we were really heavy, or on a short runway, we used to have to takeoff with the APU running and only the center pack on, with the crossover duct isolated.  This would keep us from using engine bleed air for packs, so we could get the extra performance from the engines. 

John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #10 on: Fri, 11 Jan 2019 00:42 »
Here's the APU starter motor circuit  :o

APU Start Schematic



APU Controller (in dark green): outputs starter motor signal when door open, and APU switch in START.
APU Start Relay R7188 (in light blue): disables APU Static Inverter and APU Battery Charger.
APU Starter Motor (orange)

If the door is open more than 13 degrees, the APU should start.

Hardy Heinlin

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Re: 4 eng fail and flaps EICAS...
« Reply #11 on: Fri, 11 Jan 2019 00:58 »
Thanks :-)

I think that's exactly modelled in PSX?

John H Watson

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Re: 4 eng fail and flaps EICAS...
« Reply #12 on: Fri, 11 Jan 2019 04:28 »
Quote from: Hardy
therefore certain air-ground relays which relax to ground mode will signal ground mode to some system units, e.g. the gear lever lock solenoid is relaxed, and APU systems are in ground mode etc.

Ah.. now I'm with you. Although the primary circuits in the PSEU are powered by the APU Battery,  air/ground relay R263 still needs Main DC #1 bus power to energise. Since there is no main bus power in this scenario, the relay will default to ground mode.

If the system thinks the aircraft is on the ground, then the door should move to 45 degrees (fully open).

Now, can we get APU power on the busses? The books say the APU GEN AVAIL lights will not illuminate in the air, but this is just a light inhibit*. "Airmode" will stop the AGCB's closing. "Airmode" is determined by main engine IDG drive speed. As long as the aircraft isn't in a steep dive, I don't think the N3's (RB211) or N2's (GE & PW) will be windmilling fast enough to stop the APU generators coming online.

With power on the busses, R263 will energise and the door will move back to 15 degrees.

I think a previous engine guru (Mike Martin?) claimed that the 747-400 APU wasn't designed to run in a thin atmosphere, so I guess you'd have to wait until the aircraft was below 18,000'(?) before trying to start it.

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

*(EDIT: The APU GEN AVAIL lights are controlled by air/ground relay R230 which also defaults to ground mode with no power on the main busses. So, you should get illuminated AVAIL lights (checked ok in PSX).
« Last edit: Fri, 11 Jan 2019 04:53 by John H Watson »

Hardy Heinlin

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Re: 4 eng fail and flaps EICAS...
« Reply #13 on: Sun, 13 Jan 2019 20:26 »
I just split this thread re APU AVAIL light logic because it's getting complicated and I want to keep an eye on this special subject. It continues here:

http://aerowinx.com/board/index.php?topic=5072.0


|-|ardy