All Electric Plane Electric Failure....

[TheDude]

Hi, i'm new here but i have been thinking about something that has been bugging me because i can not find a solution to it, so i figured i would ask you guys...
I'm currently instrument rated and checked out in a sr22 which is an all electric plane with two battery systems and two alternators. What this means is there is no vacuum pump powering the back up instruments, and if there was a full electrical failure then i would loose everything except the backup altimeter and airspeed (the backup attitude is electric). Now if i were in IMC conditions in a situation like this i could manage my pitch with the airspeed and altimeter but how would i manage the roll of the plane? The only solution that i have come up with so far is to monitor the floating compass on the dash, and try to maintain it a level orientation, but i'm not sure how reliable this would be...
What do you guys think?

 

[BoeingBoy]

I know nothing about an SR22 except what it looks like. According to the Sirrus catalog it looks like the backup attitude indicator incorporates an inclinometer (the ball portion of the old needle and ball turn instrument). That works without electricity (other than for lighting) or vacuum and can be used to keep the wings relatively level. The key is not trying to make coordinated turns when the normal instruments are lost - in a coordinated turn the ball will stay centered even in a banked turn. By not using the rudders, a wing down attitude would cause an uncoordinated turn and the ball would move off center.

Not the best solution, but better than nothing.

Jim

 

[TheDude]

I know nothing about an SR22 except what it looks like. According to the Sirrus catalog it looks like the backup attitude indicator incorporates an inclinometer (the ball portion of the old needle and ball turn instrument). That works without electricity (other than for lighting) or vacuum and can be used to keep the wings relatively level. The key is not trying to make coordinated turns when the normal instruments are lost - in a coordinated turn the ball will stay centered even in a banked turn. By not using the rudders, a wing down attitude would cause an uncoordinated turn and the ball would move off center.

Not the best solution, but better than nothing.

Jim

I had forgotten about the ball... Of course it would indicate to the right in a low speed high rpm situation and would indicate to the left in a high speed low rpm situation, but for in cruise flight it could work pretty well as an indicator.

Thanks for the reply though, I had just figured this thread was long gone and no one was going to reply to it...

James

 

[BoeingBoy]

I would certainly think that Sirrus would make some provision for standby attitude indicator operation on one battery and nothing else. I'd have to check, but suspect that the FAA has some regulation specifying such a requirement. If so, "total electrical failure" in the sense of losing both generators would still provide some time to find a suitable landing site.

All airliners are like the SR22 - no vacuum system and electric only. Boeing uses what they call the battery bus and hot battery bus to provide battery power to essential flight instruments if the generators fail and it would be surprising if Sirrus doesn't do something similar.

Of course, if you fail to recognize loss of generators and deplete the battery you're down to what you'd have in the Sirrus - standby altimeter/airspeed and the ball.
Jim

 

[TheDude]

I would certainly think that Sirrus would make some provision for standby attitude indicator operation on one battery and nothing else. I'd have to check, but suspect that the FAA has some regulation specifying such a requirement. If so, "total electrical failure" in the sense of losing both generators would still provide some time to find a suitable landing site.

All airliners are like the SR22 - no vacuum system and electric only. Boeing uses what they call the battery bus and hot battery bus to provide battery power to essential flight instruments if the generators fail and it would be surprising if Sirrus doesn't do something similar.

Of course, if you fail to recognize loss of generators and deplete the battery you're down to what you'd have in the Sirrus - standby altimeter/airspeed and the ball.
Jim

My original question was just about a worst case scenario of dual alternator and dual battery failure... Cirrus has the electrical system split into two different buses, the main bus (alt1, bat1) and the e-bus (alt2, bat2), or essential bus. So if alt1, alt2 and bat1 fail we still have essential instruments; if alt1,alt2 and bat2 fails then i believe everything will still be functional, but it would be a good idea to start pulling breakers to slow down the discharge.

So if there were a total electrical failure in the sense of both alternators failing, then the pilot would still have access to the attitude on the pfd and backup.

As far as a battery failure goes, i have a feeling it is a pretty rare occurence, and should be easily noticable before flight, but i have heard from other pilots about their own personal battery failures because the mechanic did not refil the water level in the battery at the last inspection.

Another question though, i was watching seconds from disaster, and they were talking about an airliner that ran out fuel while crossing the pacific. When the engines shut down they needed to drop a little fan generator out of the bottom of the plane to provide power.
Do these large airliners carry the large heavy batteries that would be necessary to power there large avionics systems, or do they just rely on a backup generator like i mentioned for alternator failure?

 

[BoeingBoy]

While I certainly don't have knowledge of all ETOPS airliners, and think those are the ones you're talking about, the RAT or Ram Air Turbine is normal for twin-engine ETOPS planes. Because of the allowed distances from suitable landing sites with only two engine driven generators plus APU generator, an alternate source of electrical power is provided since the battery won't last long enough to reach a landing site in the worst case.

Most times the battery is rated for 30 minutes of power after loss of all generators, the same as other smaller jets like the 737. I forget who, but one carrier recently had electrical problems on one of their flights and for some reason the crew elected to press on - don't know if they didn't have indications of total generator failure or what. The battery lasted about 1-1/2 hours before they lost everything as I recall.

Jim

 

[TheDude]

While I certainly don't have knowledge of all ETOPS airliners, and think those are the ones you're talking about, the RAT or Ram Air Turbine is normal for twin-engine ETOPS planes. Because of the allowed distances from suitable landing sites with only two engine driven generators plus APU generator, an alternate source of electrical power is provided since the battery won't last long enough to reach a landing site in the worst case.

Most times the battery is rated for 30 minutes of power after loss of all generators, the same as other smaller jets like the 737. I forget who, but one carrier recently had electrical problems on one of their flights and for some reason the crew elected to press on - don't know if they didn't have indications of total generator failure or what. The battery lasted about 1-1/2 hours before they lost everything as I recall.

Jim

Interresting, i'm surprised that the battery could last for hour longer then it is rated for. Either they were shutting all kinds of stuff off in the cockpit, or the battery theres just a huge margin of safety when it comes to those battery ratings.

 

[BoeingBoy]

or the battery theres just a huge margin of safety when it comes to those battery ratings.

I'd vote for that answer, given that the published limits must take into account the worst case. A new fully charged battery could well last well in excess of the 30 minute requirement (as that one did) while a battery nearing the end of it's life might barely make 30 minutes.

Of course, in twin-engine airplanes there's an awful lot of automatic load shedding as you lose generators. If you're down to just the battery, you've got only rudimentary backup flight instruments, enough cockpit lighting to see those instruments at night, one radio, and fire protection (but not detection). Cabin lighting comes from the emergency exit lights. Far different from the older planes like the 727 where the flight engineer handled electrical loads.

Jim

 

[TheDude]

I'd vote for that answer, given that the published limits must take into account the worst case. A new fully charged battery could well last well in excess of the 30 minute requirement (as that one did) while a battery nearing the end of it's life might barely make 30 minutes.

Thats the explanation that i have to agree with more as well...

Of course, in twin-engine airplanes there's an awful lot of automatic load shedding as you lose generators. If you're down to just the battery, you've got only rudimentary backup flight instruments, enough cockpit lighting to see those instruments at night, one radio, and fire protection (but not detection). Cabin lighting comes from the emergency exit lights. Far different from the older planes like the 727 where the flight engineer handled electrical loads.

Jim

So if a modern airliner is down to just a battery, then that should be fairly obvious to the pilot, and passengers, as things have automaticaly shut down leaving the bare minimum. But if that is the case then why would the airliner in your example decide to continue for over an hour after failure? Is it just not that big of a safety risk? I always pictured these new big airliners with all their fancy electronic systems being difficult to fly with everything shut down as it seems that the plane would have been designed to fly pretty much only with all these various computer systems up and running. Now of course they would have to make the plane flyable with no electronic systems just for the off chance there was a failure, but it just seems like it would be more difficult for the pilot and create a safety risk that could be avoided by just aborting the flight.

Or, from the airliners point of view, is the risk to safety, in this situation, less important compared to the cost and bad PR that would come from aborting the flight and landing at the nearest airport capable of handling this large of an aircraft?

 

[BoeingBoy]

As far as I know, the NTSB is still investigating the flight that lost all electrical power - it was on a transcon and landed at ORD, BTW. It was a 757 - glass cockpit - which I've never flown so don't know what indications they'd have. I don't really know enough about that airplane to guess, so can only assume that the crew wasn't aware of the seriousness of the problem until the battery died.

If you want to read the preliminary report here's the link:

NTSB Preliminary Report

Jim

 

[TheDude]

As far as I know, the NTSB is still investigating the flight that lost all electrical power - it was on a transcon and landed at ORD, BTW. It was a 757 - glass cockpit - which I've never flown so don't know what indications they'd have. I don't really know enough about that airplane to guess, so can only assume that the crew wasn't aware of the seriousness of the problem until the battery died.

If you want to read the preliminary report here's the link:

NTSB Preliminary Report

Jim

wow, i just read that article, and its a lot more intense then what i expected when you first mentioned it, mainly because i hadn't heard about it all.

But what it does say is that they received the AIR/GND SYS message elected to switch to running solely on BAT after consulting both the QRH and their maintenance center. So it sounds like they knew there was a problem, but i'm wondering if they were not fully aware of what configuration they had put the plane into, setup to run only on the battery, to get rid of the error messages...

Then again, if you say you have no room to guess, then i definitely have no room to guess...

 

[BoeingBoy]

Switching standby power to the BAT bus doesn't necessarily mean you're down to battery power, although it did in this case. If the battery charger is powered, it'll keep the battery charged even with BAT bus selected. At least that's the way the Boeings I've flown worked - with everything normal you can select standby to the BAT bus and all you've done is change the power source for the standby bus from the normal bus to the battery bus. Nothing else changes and the battery is still being charged.

They apparently thought that they still had power to the battery charger, although that's guesswork on my part.

One big difference between airliners and general aviation airplanes is the complexity. A twin-engine airliner will have 2 normal AC buses (AC 1 & AC 2), each powered by an engine driven generator (or the APU generator) and powering normal DC buses (DC 1 & DC 2) thru static inverters. The normal buses normally power the standby buses (AC and DC), although the BAT bus can be selected to power the standby buses. The BAT buses (AC & DC) are normally powered by the standby buses, but the BAT buses can be switched to the battery if the generators aren't operating (or engines & APU not running) as long as the battery switch is on. The Hot BAT buses are powered by the BAT buses in normal operation, but switch to the battery at all other times (even with the battery switch off). The electrical system is really two independent systems each powered by their own generator until you get to the standby buses. From there down it's one system with alternate power sources available. The separate systems are what gives load shedding - as buses are lost the components powered by those buses lose power except for some critical components (like #1 radio) that switch to another power source.

Jim

 

[PHL]

I've flown the Cirrus SR20, which has the same electrical system layout as the SR22. While the loss of both alternators and then both batteries is highly unlikely, there is an approved aftermarket option for older Cirrus aircraft to update the attitude indicator with a newer model that includes it's own battery backup that is designed to give 1 hour of time. I believe new ones have this variant installed standard now.

So in a total cockpit blackout, you'd still have the Airspeed, Altimeter, Compass and Attitude indicator with the backup power.

 

[PHL]

I've flown the Cirrus SR20, which has the same electrical system layout as the SR22. While the loss of both alternators and then both batteries is highly unlikely, there is an approved aftermarket option for older Cirrus aircraft to update the attitude indicator with a newer model that includes it's own battery backup that is designed to give 1 hour of time. I believe new ones have this variant installed standard now.

So in a total cockpit blackout, you'd still have the Airspeed, Altimeter, Compass and Attitude indicator with the backup power.