Fly By Wire Electrical Question

[Deleted member 27505]

FBW pilots, I'm looking for some info and would appreciate your help.
In the statistically almost zero probability event that a FBW aircraft loses all power generation capability and goes to ship's batteries, what happens to flight controls?

 

[Der_Meister]

The computers go onto the batteries as well. Then hope you can make it on the ground with in the remaining time. Some aircraft the the ability via a RAT to generate either power or Hydraulic pressure. The 757 (some hadthem some didn't) hfor example had also a Hydraulic driven generator that was on the same system the RAT was should you loose it all.

 

[Deleted member 27505]

Funny you mentioned the 757. It was a 757 deal that lead me to ask the question. I read about a 757 that lost everything including the HMG. That's never supposed to happen, but somehow it did. According to the article the 757 has ~30 minutes on batteries in EMER batt with whatever equipment is available in that mode. In the deal mentioned, it sounded like if they hadn't somehow gotten the APU started enroute to their divert, it would have been a very ugly evening. Yet, as I understand it, the 757 is not full FBW, so I'm not sure what they would have lost in basic airplane control.

Do the "real" FBWs have more/bigger batteries to keep the computers up? A separate battery bus/ emer bus for the computers, or what? I would think the computers would be a rather large draw. And after the batteries drain, you're a paper airplane with flapping trailing edges, is that correct?

 

[Roger Roger]

Pretty sure somewhere in very dusty memories I recall something about a permanent magnet generator in at least one aircrafts FBW system (maybe 777?) devoted just to powering enough equipment to keep the aircraft flyable.

 

[Adler]

We have dedicated a fly by wire backup battery that give us 15 minutes of life, though we aren't aileron fly by wire. The permanent alternators power our FADEC.

 

[Der_Meister]

Funny you mentioned the 757. It was a 757 deal that lead me to ask the question. I read about a 757 that lost everything including the HMG. That's never supposed to happen, but somehow it did. According to the article the 757 has ~30 minutes on batteries in EMER batt with whatever equipment is available in that mode. In the deal mentioned, it sounded like if they hadn't somehow gotten the APU started enroute to their divert, it would have been a very ugly evening. Yet, as I understand it, the 757 is not full FBW, so I'm not sure what they would have lost in basic airplane control.

Do the "real" FBWs have more/bigger batteries to keep the computers up? A separate battery bus/ emer bus for the computers, or what? I would think the computers would be a rather large draw. And after the batteries drain, you're a paper airplane with flapping trailing edges, is that correct?

If i remember correctly for the 757, it was quoted at 30min line you said and then if you turned off one of the probe heats you could get an extra 5 min.

On the 320 you can turn of one of the IRs for another 5 min. I think the 320 is also 45 min on battery power. Though I haven't looked at that part of the book in a few months.

 

[Deleted member 27505]

We have dedicated a fly by wire backup battery that give us 15 minutes of life, though we aren't aileron fly by wire. The permanent alternators power our FADEC.

What I'm really trying to determine is what happens to basic flight controls (pitch, roll, yaw) in the event of a total power failure in the FBWs. No control whatsoever? And how much time to dead backup batteries? 15 min seems perilously short. The 757 referenced earlier would not have made it. I don't want to assume anything, but it would seem sensible that in transoceanic A/C, there'd be a larger backup envelop, especially if the plane becomes a rock after complete electrical loss. (Granted, it would be very unlikely ever to get to total power loss with all the intermediary backup systems, but still)

 

[Roger Roger]

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

 

[inigo88]

Here's an example for the A320. Guys in the discussion below probably aren't A320 typed like a variety of people who post here, so hopefully they can correct if necessary. Your 757 example is not FBW. Think A319 thru A380, B777 and B787.

https://aviation.stackexchange.com/...rolled-in-case-of-complete-electrical-failure

If FBW computers fail and you lose direct law, and somehow still had hydraulic pressure, supposedly you can still fly with stab trim and rudder.

 

[Zapphod Beblebrox]

inigo88 is correct. With a total loss of electrical power, and none of the 7 flight control computers online, you still have a powered rudder and trim to control pitch. You are in mechanical back up. (Man Pitch Trim) in red appears on the PFD. This situation is designed to be temporary so that you can get any electrical source back to power any one of the 7 computers. If you can get back an ELAC, Elevator, Aileron Computer or a SEC, Spoiler Elevator Computer, you have normal flight controls back but you are in what Airbus calls (Alternate Law).

The Airbus AC system is backed up by a RAT or Ram Air Turbine, that can be deployed to give you back up hydraulics, blue system and electrics. I suppose you can as "what if" questions to infinity and put the aircraft in a un-flyable condition but there are 7 flight control computers 2(ELAC's) 3 (SEC's) and 2(FAC's) (flight augmentation computers) that keep things going. They can all operate on straight DC if required off the battery.

There are three hydraulic systems (blue) (green) (yellow) and the blue system is also backed up by the RAT. They told us in ground school that you can fly the Bus for a while without computers, but you can't fly it without hydraulics.

 

[Deleted member 27505]

inigo88 is correct. With a total loss of electrical power, and none of the 7 flight control computers online, you still have a powered rudder and trim to control pitch. You are in mechanical back up. (Man Pitch Trim) in red appears on the PFD. This situation is designed to be temporary so that you can get any electrical source back to power any one of the 7 computers. If you can get back an ELAC, Elevator, Aileron Computer or a SEC, Spoiler Elevator Computer, you have normal flight controls back but you are in what Airbus calls (Alternate Law).

The Airbus AC system is backed up by a RAT or Ram Air Turbine, that can be deployed to give you back up hydraulics, blue system and electrics. I suppose you can as "what if" questions to infinity and put the aircraft in a un-flyable condition but there are 7 flight control computers 2(ELAC's) 3 (SEC's) and 2(FAC's) (flight augmentation computers) that keep things going. They can all operate on straight DC if required off the battery.

There are three hydraulic systems (blue) (green) (yellow) and the blue system is also backed up by the RAT. They told us in ground school that you can fly the Bus for a while without computers, but you can't fly it without hydraulics.

Thanks.

"With a total loss of electrical power... you still have powered rudder and pitch trim. ...(Man Pitch Trim) in red appears on the PFD." That's a different kind of "total loss" than the one in which I'm interested. I mean, if I have no electricity, how would I get an electrically powered annunciator on an electrically powered PFD?

Let me try to be more clear about the FBW condition I'm trying to understand:
1. Electrical Power Loss
2. Total
In other words, the ship's electrical system and batteries are gone. All the backups failed. All the batteries are dead.

In that condition*, What's left as far as flight controls (aileron, elevator, rudder)?

Maybe this is easier.... In the above condition, what are you?
A rock?
A rock with thrust?
A glider?
An airplane?




*Yeah, as I said before, I understand it's an outlier condition that's never supposed to happen. But my question disregards probability.

 

[SlumTodd_Millionaire]

I’ll stick with Douglas and old Boeing as long as they’re still around, thanks.

 

[Landis]

So, in this hypothetical situation we've had an EMP that's knocked out all electrical systems and backups? On the E175 you're a rock with zero controls. But I suspect you're a rock with zero controls on almost every aircraft these days. Lose the FADEC, lose the engines, lose all hydraulics. RAT doesn't work since it's still powering electrical pumps that are knocked out by the EMP. No hydraulics, no power, no nothing.

Give me some backups that will continue to function in a variety of different situations and we'll discuss. But what's led to this electrical loss and what do you mean by total? So many different paths we can take on so many different aircraft.

 

[Deleted member 27505]

So, in this hypothetical situation we've had an EMP that's knocked out all electrical systems and backups? On the E175 you're a rock with zero controls. But I suspect you're a rock with zero controls on almost every aircraft these days. Lose the FADEC, lose the engines, lose all hydraulics. RAT doesn't work since it's still powering electrical pumps that are knocked out by the EMP. No hydraulics, no power, no nothing.

Give me some backups that will continue to function in a variety of different situations and we'll discuss. But what's led to this electrical loss and what do you mean by total? So many different paths we can take on so many different aircraft.

I'll be the first to admit this is an outlier case we're considering. Oh, wait, I already stated that clearly.
I'll also be the first to admit that for various reasons, I'm the kind of guy whose hackles go up the second I hear somebody say, "That can never happen."
In the case of the 757 I mentioned in the IP, they were down to batteries. That's not hypothetical; That happened. They would have been down to nothing in a few minutes if they had not gotten their APU started. Just for emphasis, yes, it was over water at night. (Continental B-757 Flt 120 Anchorage to Seattle on Aug. 12, 2000). AFAIK no EMP was involved. At least nothing was mentioned regarding EMP. I suspect if EMP had been involved, it's highly likely it would have been mentioned. In fact, I think its likelihood of mention would have been inversely proportional to the likelihood of them finding themselves with no backup electrical generating capability.

 

[Landis]

I'll be the first to admit this is an outlier case we're considering. Oh, wait, I already stated that clearly.

I'm not saying anything like that. Just saying that there are so many "what if's" to the situation in question and the definition of 'total' that the question is nearly impossible to answer.

On the E175 one of the more entertaining scenarios in the type training was the Electrical Emergency. Basically it's a 10-minute airplane. You lose power and you've got 10 minutes of batteries to re-establish electricity in some way before things get *really* entertaining. That power that you can bring back includes the RAT (that should automatically deploy) and the APU (as long as you're in the envelope for starting and have sufficient battery power to start 'er up). RAT deploys automatically when you enter your Electrical Emergency and brings power back within 8 seconds to the FBW computers, and the backup pump on hydraulic system 3 which can power all 3 axes of the primary flight controls.

Either of those fail and you'll eventually lose the FBW aspect of things - dedicated batteries give the FBW computers up to 15 minutes. Engines keep turning as long as the 'Permanent Magnet Alternator' on each side didn't get lost in this hypothetical as the fuel is driven mainly by motive flow. Each engine has a mechanical hydraulic pump that should continue to function giving you systems 1 and 2 and control over the 3 axes of the primary flight controls. You re-establish electricity and you've got your spoilers, flaps, and trims and you're no longer in this Electrical Emergency. You don't and you've got 15-minutes before you lose control of the tail. At that point you're basically flying the Sioux City Scenario except you've got ailerons. But if you lose your engines, and the RAT doesn't work, and you can't get the APU online, you've lost hydraulics too. And in that case you're a rock.

So in essence on my plane you've got 10-minutes to get electricity back. That should be plenty. If you can't, you're SOL. I know you've mentioned the 757 and are probably thinking more along the lines of the newer Boeing and the Airbus families, but in the (mostly) all electric Jungle Jet this is a very serious question and why there's so many different ways that this *shouldn't* happen.

 

[Zapphod Beblebrox]

Thanks.

"With a total loss of electrical power... you still have powered rudder and pitch trim. ...(Man Pitch Trim) in red appears on the PFD." That's a different kind of "total loss" than the one in which I'm interested. I mean, if I have no electricity, how would I get an electrically powered annunciator on an electrically powered PFD?

Let me try to be more clear about the FBW condition I'm trying to understand:
1. Electrical Power Loss
2. Total
In other words, the ship's electrical system and batteries are gone. All the backups failed. All the batteries are dead.

In that condition*, What's left as far as flight controls (aileron, elevator, rudder)?

Maybe this is easier.... In the above condition, what are you?
A rock?
A rock with thrust?
A glider?
An airplane?




*Yeah, as I said before, I understand it's an outlier condition that's never supposed to happen. But my question disregards probability.

You are an airplane with very limited control. You have pitch control via manual stabilizer trim and you have roll control via rudder input. You say total electrical so that means you still have hydraulics and you have engines. You are an airplane but I don't think you can land in that configuration. It is designed only to allow time to re-establish electrical power somehow.

 

[JordanD]

I
So in essence on my plane you've got 10-minutes to get electricity back. That should be plenty. If you can't, you're SOL. I know you've mentioned the 757 and are probably thinking more along the lines of the newer Boeing and the Airbus families, but in the (mostly) all electric Jungle Jet this is a very serious question and why there's so many different ways that this *shouldn't* happen.

They must have calculated that best lasts less than 10 minutes anyway so screw it.

All that is one of those "welp, maybe today just isn't my day" scenarios where you just have to accept it.

 

[Adler]

All that is one of those "welp, maybe today just isn't my day" scenarios where you just have to accept it.

"yea it's me, not looking good, will you send up that girl from 4a please"

 

[Autothrust Blue]

I'm not saying anything like that. Just saying that there are so many "what if's" to the situation in question and the definition of 'total' that the question is nearly impossible to answer.

On the E175 one of the more entertaining scenarios in the type training was the Electrical Emergency. Basically it's a 10-minute airplane. You lose power and you've got 10 minutes of batteries to re-establish electricity in some way before things get *really* entertaining. That power that you can bring back includes the RAT (that should automatically deploy) and the APU (as long as you're in the envelope for starting and have sufficient battery power to start 'er up). RAT deploys automatically when you enter your Electrical Emergency and brings power back within 8 seconds to the FBW computers, and the backup pump on hydraulic system 3 which can power all 3 axes of the primary flight controls.

Either of those fail and you'll eventually lose the FBW aspect of things - dedicated batteries give the FBW computers up to 15 minutes. Engines keep turning as long as the 'Permanent Magnet Alternator' on each side didn't get lost in this hypothetical as the fuel is driven mainly by motive flow. Each engine has a mechanical hydraulic pump that should continue to function giving you systems 1 and 2 and control over the 3 axes of the primary flight controls. You re-establish electricity and you've got your spoilers, flaps, and trims and you're no longer in this Electrical Emergency. You don't and you've got 15-minutes before you lose control of the tail. At that point you're basically flying the Sioux City Scenario except you've got ailerons. But if you lose your engines, and the RAT doesn't work, and you can't get the APU online, you've lost hydraulics too. And in that case you're a rock.

So in essence on my plane you've got 10-minutes to get electricity back. That should be plenty. If you can't, you're SOL. I know you've mentioned the 757 and are probably thinking more along the lines of the newer Boeing and the Airbus families, but in the (mostly) all electric Jungle Jet this is a very serious question and why there's so many different ways that this *shouldn't* happen.

With a complete loss of hydraulic power (not supposed to be possible), I was told that you basically get one control input and that might be the last thing you do.

 

[seagull]

WIth the EMP scenario you're down to not a lot. On the 777 you're left with a couple of spoiler panels and the stabilizer trim levers on the console to manage an aircraft that has pushed the boundaries in terms of longitudinal stability. Not a pretty scenario at all!