EconoBoy
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Electrical Systems

Mon Apr 05, 2004 10:31 pm

Two questions about aircraft electrical systems:

Once, an airplane’s electrical system mostly just had to provide power for instruments, cabin lighting, flap deployment and galley equipment. Now on many planes, there are PTV’s on all seats, laptop power points and motorised seat adjustments (well, up in first class anyway). Does this mean that engine manufacturers have had to fit substantially up-rated generators to cope?

Secondly, are planes still fitted with emergency ram air turbines, or is the occurrence of total power failure considered so rare, that they are no longer deemed necessary? And how much power could an emergency turbine provide?
 
SlamClick
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RE: Electrical Systems

Mon Apr 05, 2004 11:30 pm

Airliners have been electrically overpowered for a long time. Someone will pop up with the KVA ratings for the generators (don't have my tech library handy) but they would run a small town. No upgrade required.

Reason is, they want to keep as many electrical services as possible with the loss of an engine and its generator. Most systems shed galley and entertainment power when an engine is lost, but they still have quite a bit of surplus capacity.

Ram air turnbines (RAT) are most often found on long-haul jets. Typical APUs can now provide full electrical backup service all the way up to the airplane's operating ceiling. Some planes even have an APU battery, dedicated to in-flight APU starting.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
air2gxs
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RE: Electrical Systems

Tue Apr 06, 2004 4:38 am

The B747, B757, B767, A300 all have 90 KVA generators installed. Our DC8's operate 30 KVA generators.
 
avioniker
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RE: Electrical Systems

Tue Apr 06, 2004 5:41 am

That's the newer generation of aircraft you're speaking of.
I'd have to take exception to the statement that planes have been electrically overpowered. Design considerations used keep the generator size down to limit weight.The MD80 generator is sorely taxed on pack fan startup and there's 60 or so KVA available.
The good news is that on the Next Generation aircraft most critical systems are DC controlled if not solely powered so the batteries can provide adequate power for the crew to find a nice place to set down when that bad stuff happens. I was on a DC8 once that lost all power and ten minutes later the battery went too. A dark cockpit over San Francisco Bay inbound to OAK is just not a fun place to be... ("Luckily" it was an FCF so there were no passengers screaming in the back to add to the confusion and screaming in the front.)

[Edited 2004-04-05 22:42:37]
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
DC-10Tech
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RE: Electrical Systems

Tue Apr 06, 2004 4:59 pm

Most of your semi-modern day jets pack 90KvA generators on each engine and on the APU, with any single generator capable of running all critical systems. The MD-11 comes with a 120KvA on each engine and a 90KvA on the APU. The RAT usually can only supply power to a small amount of the systems on the Emer Power bus.

Most widebody airliners I have worked on also have a load shedding capability, where non-critical systems (passenger convenience, etc) are dumped from the power circuit in emergency situations.
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EconoBoy
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RE: Electrical Systems

Tue Apr 06, 2004 7:06 pm

Interesting answers: I hadn’t realised that today’s planes had such powerful generators. They, along with bleed air, must sap power though.

My interest in the RAT was started by seeing a documentary about a Canadian twin jet Captain (767 I think) who lost both engines. This was due to running out of fuel because of a conversion mix-up by the refuelling guys. The flight deck went black and the plane became a glider. But the Captain was able to deploy a RAT which powered the instruments and the control surfaces, and presumably the landing gear hydraulics, as he made a successful landing. (This he achieved by putting the plane into a strange crab-like attitude, only straightening up just before touch down).
 
Guest

RE: Electrical Systems

Tue Apr 06, 2004 7:45 pm

Dear Econoboy -
xxx
The "crab-like" maneuver you describe is known as a side-slip. It is normally practiced by light airplanes which are not equipped with flaps, as means to increase angle of descent towards the landing field.
xxx
If I remember the AC 767 history, the captain did have (or maybe the F/O) some glider experience or background. Side slips work in ANY airplane, but if you have passengers on board... that would raise the hair on their heads...
xxx
Happy contrails  Smile
(s) Skipper

[Edited 2004-04-06 13:07:00]
 
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Starlionblue
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RE: Electrical Systems

Tue Apr 06, 2004 8:03 pm

Econoboy, the 767 landed at a disused fighter base in Gimli, Manitoba. (http://www.amazon.co.uk/exec/obidos/ASIN/066011884X/qid=1081249159/sr=1-4/ref=sr_1_0_4/026-2766194-2271655 for the report or http://www.amazon.com/exec/obidos/tg/detail/-/1875671560/qid=1081249312/sr=1-1/ref=sr_1_1/102-0914559-4126545?v=glance&s=books for the story.).

The incident became known as the "The Gimli Glider". Catchy name I guess.

There is also a (quite awful) TV movie.


On a lighter note, can you imagine being 200 minutes from the nearest field on a 777. One engine quits, and you now have to get all the way down, with no PTV to distract you!!!  Big grin
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
EconoBoy
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RE: Electrical Systems

Tue Apr 06, 2004 8:21 pm

Dear B747Skipper, yes, you're right: the Captian had done the maneuvre in a light plane, but never in a 767. As you say, the passengers had a hair-raising experience, but top marks to the Captain!

Starlionblue, yeah, I bet the incident spawned a corny film, probably featuring the likes of Troy McClure (if you are a Simpsons fan)!
 
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Starlionblue
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RE: Electrical Systems

Tue Apr 06, 2004 11:08 pm

Troy McLure did not make the cut, but it was pretty close  Big grin William Devane played the Captain in Falling from the Sky: Flight 174 (http://www.imdb.com/title/tt0113018/). Note that it was really AC 143, not "Flight 174".

Check out the "Goofs" section. It goes on and on... My fave: "Factual errors: Throughout, in straight and level flight, the Primary Flight Displays show a steep climb."


There's an article (with pics) here: http://www.casa.gov.au/avreg/fsa/03jul/22-27.pdf. Gotta love Google Big grin

[Edited 2004-04-06 16:19:10]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
SlamClick
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RE: Electrical Systems

Tue Apr 06, 2004 11:59 pm

EconoBoy in post #5 you are correct in supposing that these things do sap some power but consider this.

A B-777 or an A-330 can be rolling down the runway at maximum gross weight, lose an engine at V1 and continue the takeoff. It will maintain (at least) a positive climb gradient until the gear is retracted, a 2.4 feet per hundred gradient until the flaps are retracted and a 1.2% gradient on final climb. All that on ONE engine.

So if the engines are capable of producing that kind of power, you don't need to sweat the little bit of power loss from using air bled off the compressor stages and the chore of turning a high-output generator with all the engines running. When you lose an engine, lots of high-load appliances automatically drop off line anyway.

Fact is, jet engines take in far more air than they need to run. Even at takeoff power they are bleeding some of it off into the fan duct because they are still taking in more than they need. So up to some reasonable limit, you might say they have surplus power. In fact we can do reduced-thrust takeoffs at very heavy weights and if we lose an engine, still do not have to bring the operative engine up to normal takeoff power to get the required climb gradients in the paragraph above.

Likewise electrically, if one generator (on a two engine plane) can run nearly all the electrics on the plane (even though galley power may be shed automatically) one might say that with both generators running they are "overpowered" despite as Avioniker correctly pointed out things like pack fans produce momentary very high draws on the systems. The pack fans he mentions only run at low speeds or on the ground and only draw that kind of load when accelerating from zero to full RPM. They are a momentary peak in the demand, and not an ongoing electrical load.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
EconoBoy
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RE: Electrical Systems

Wed Apr 07, 2004 3:08 am

SlamClick, thanks for the informative reply. It helped put things in perpective, and I had often wondered about single engine performance on big twin jets, so you answered that question too!
 
bsergonomics
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RE: Electrical Systems

Wed Apr 07, 2004 8:23 am

I'm more used to single or twin engine jets, but can you still generate 'useful' current from a windmilling engine on a mtuli-engined jet? I assume so, but I've never heard a positive 'Yay or Nay".
The definition of a 'Pessimist': an Optimist with experience...
 
SlamClick
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RE: Electrical Systems

Wed Apr 07, 2004 9:17 am

Bsergonomics the answer is no, not really.

Typical twin jet airliner, the generator is 115V 400Hz AC and it is run off an accessory drive pad through a CSD or constant-speed drive.

A CSD is sort of like an automatic transmission. The driven side runs a hydraulic pump which produces pressure to operate a hydraulic motor, the torque of which spins the generator. While sampling the generator's frequency output it also spins the outside of the ring gear part of the planetary drive either "forward" or "backward" to increase or decrease the output speed - hence the frequency.

Result is that it can only keep the frequency within the desired range at a limited engine RPM range. Lets say the frequency range is 400 + or - 20Hz for a particular plane. Most CSDs will do much better than that, but the engine will have to be operating at maybe 4000-12000 N2 RPM to yield the required frequency. Outside that range and the aircraft electrical system monitoring system will drop the current offline. Most jet engines don't windmill at high enough speed.

I could imagine a DC generator in direct-drive staying on line, on the other hand.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
Guest

RE: Electrical Systems

Wed Apr 07, 2004 9:43 am

All CSDs are only capable of providing power from idle RPM (up to maximum).
By power, I mean voltage and acceptable frequency.
We have idle RPM at 56-58% RPM on our JT9Ds...
Obvious to me a generator would drop-off (underspeed) below 50%.
xxx
However to mention, a windmilling engine can provide (some) hydraulic pressure.
There is such existing procedure in 747 in case of failure of all engines.
Then we have to glide a 250 KIAS minimum to get enough windmill RPM to get hydraulic pressure.
(Remember, all flight controls are hydraulic in 747, no cables)...
This is a consideration i.e. volcanic ash with failure of all engines.
xxx
Happy contrails  Smile
(s) Skipper
 
DC-10Tech
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RE: Electrical Systems

Wed Apr 07, 2004 12:29 pm

Skipper, you are correct. The underspeed sensor will cause the generator to be taken offline. Usually below 45-50% N2. In my experience, on big turbofans, you can achieve 3000psi at approx 3% N1.
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EconoBoy
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RE: Electrical Systems

Thu Apr 08, 2004 7:16 pm

Dear B747Skipper, I think you are referring in your last sentence to the infamous incident where a BA 747 unwittingly flew into the plume of ash thrown up by a volcano in Java. The ash snuffed out all four engines – the stuff nightmares are made of. The Captain tried repeatedly to restart them by diving and wind milling them, with no luck, and was contemplating a touch down on water when the plane came through the ash, and the engines restarted. A successful landing was then made (though the Captain had to stand up to see through the only part of his window that had not been sandblasted): a tribute to the ruggedness of the plane, its engines and the professional cool of the Captain and his crew.
 
Guest

RE: Electrical Systems

Thu Apr 08, 2004 7:32 pm

Yes the incident I referred to was the volcanic ash.
It is now a part of emergency procedures in the 747 manuals...
xxx
DC-10Tech - Amazing you refer to N1 for that...
N1 3% is probably 20% on most N2 of engines (P&W or GE)...
It would take that much to get decent pressure.
xxx
Something about hydraulic pressure...
What is the "first thing" you see (or evidence) of N2 rotation...?
Is it fuel flow, EGT, N2 RPM, oil pressure, CSD oil pressure...?
Nope, first evidence, a fraction of a second before others is HYDRAULIC pressure...
xxx
Happy contrails  Smile
(s) Skipper
 
DC-10Tech
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RE: Electrical Systems

Fri Apr 09, 2004 4:43 pm

Yes Skipper, I mentioned N1 because I couldn't remember the N2, but its somewhere in the 20% range. For those not in the know, N2 is what drives the accessories, so yes, N2 speed will directly correlate to accessory speed, hence hydro pump output.
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