artsyman
Posts: 4516
Joined: Wed Feb 28, 2001 12:35 pm

Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 4:46 pm

Hi Folks,
I was watching the Documentary on the Air Transat flight into the Azores a few days ago. It is one of the video's they show at Continental for recurrent training. It brought up a few tech questions for me that I was hoping a few of you could shed some light on.

1. I am aware that when you have a dual engine failure on a twin, the ram air turbine will deploy, and hopefully supply enough power to the struggling aircraft to enable basic systems to be functional. When this happens on a non fly-by-wire aircraft, the ram air Turbine will provide some power for perhaps air, and a few electrical systems, but is not totally required for the operation of rudders, flaps etc. I do understand that without much in the way of hydrolics, that the aircraft will be difficult to control, but remains controllable. What happens for a fly-by-wire aircraft in this situation ?

The fly-by-wire aircraft is reliant upon electrical power to send a charge down the wire in order to operate the control surfaces, and without this power, is this aircraft SOL ?

2. As the air transat flight was on an A33O, surely the aircraft needs a decent ETOPS rating, therefore, what are the ETOPS requirements for electrical support. I know that the 777 has an excellent APU and I am assuming that the A330 would have something similar.

I suppose the point I am trying to understand is, if the APU fails on a A330, the plane is unflyable ? versus perhaps a 757 which would be glideable ?

I have heard some pilots at Continental discussing a dislike for Airbus for these reasons along with a few other flight envelope limitations

thanks in advance

Jeremy
 
liamksa
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RE: Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 6:58 pm

I suppose the point I am trying to understand is, if the APU fails on a A330, the plane is unflyable ? versus perhaps a 757 which would be glideable ?

First the disclaimer: i don't know any specifics about the systems in the A330 or pretty much any big jet for that matter.

Maybe the RAT drives a small generator which supplies the power needed for the FBW. Maybe if the RAT supplies hydraulics it utilises a hydraulic driven generator but i doubt that given the limited supply of hydraulic power. Just like there are limited electrical systems available with dual engine failure on non FBW aircraft i'd say the first thing that would be powered by the RAT on a FBW aircraft are the control signals.

But this is all just speculation  Big thumbs up
 
gordonsmall
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RE: Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 8:26 pm

Hi Artysman,

A quick rundown of the A330 electrical system:

The A330 does indeed have an emergency generator powered by the RAT.

Like most aircraft the systems is divided into buses: AC1, AC2, AC ESS, DC1, DC2, DC BAT and DC ESS.

The AC Essential bus provides power to most vital systems, and I assume (but don't know) that it includes the 5 FBW computers (or at least some of them, even if it leaves them with degraded flight control laws). AC ESS is normally fed from the AC1 bus, but if AC1 is unpowered then AC ESS can be fed from the AC2 bus - presumably this is an automatic switchover but I don't know for sure, although I do no there is a switch on the overhead for this function.

If both AC buses are unpowered then the RAT is deployed automatically and feeds AC ESS from the emergency generator.

The DC buses are fed from their respective AC buses (i.e. AC1 powers DC1) through a Transformer Rectifier which I assume is just a fancy AC-DC inverter. The two DC buses power the DC BAT bus, the DC BAT bus among other things provides power to charge the batteries when they are below a certain voltage.

If either of the AC buses is unpowered the associated DC bus will draw power from the opposite DC bus through the DC BAT bus. If AC1 and AC2 are unpowered then DC1 and DC2 buses are lost.

The DC ESS bus is powered from the DC BAT bus (which is usually powered from DC1 and DC2), however if DC1 and DC2 buses are unpowered the DC ESS will draw power from the AC ESS bus through the ESS Transformer Rectifier, or failing that directly from the 2 batteries.

The RAT is disabled when landing gear is extended and in such situations the DC ESS bus is powered directly from batteries and the AC ESS bus is powered from the batteries through a Static Inverter. (I presume this is to ensure the continuity of supply as the aircraft slows down and the generator produces less energy - anyone tell me for sure?)

As you can see, like all modern commercial aircraft there is a lot of redundancy built into the system.

Even if the shit really hits the fan the flight crew still have direct control of the aircraft through the pitch and rudder trim, along with the throttles (which I assume would act in a mode similar to if the autothrust instinctive disconnect buttons had been held for 15 secs). I imagine the two guys up front would be very busy bees, but I have no doubt that the aircraft could be brought down with a good degree of success.

A type rated pilot or A&P will know more than me, this is just what I could muster from the limited information I have available to me - but I think it's all factually correct as it stands.

Regards,
Gordon.

[Edited 2003-10-25 13:39:00]
Statistically, people who have had the most birthdays tend to live the longest.
 
sccutler
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Joined: Thu Jan 27, 2000 12:16 pm

RE: Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 10:46 pm

In the Air Transat incident, APU would have been irrelevant, since the crisis was created by fuel starvation anyhow.
...three miles from BRONS, clear for the ILS one five approach...
 
airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 11:20 pm

FBW systems still use hydraulic actuators. The main advantage of FBW aircraft is that the control cables that connect the pilot's controls to the hydraulic power control units (PCUs) are removed, and the existing electrical transfer valve system used when the autopilot controls the aircraft, is improved by adding redundancy.

The electrical transfer valve in a PCU shuttles hyrdraulic pressure to activate the control surface. Control feedback is provided by a sensor, often an LVDT (Linear Voltage Differntial Transformer).

So....in the case of an engine failure, hydraulic and electrical power must be restored and this can be done (depending on the airplane) by having a RAT that can supply both electrical energy and hydraulic power or one that supplies electrical power exclusively that is capable of operating an electric hydraulic pump.

I an not an expert on the A330, therefore I am not familiar how this particular aircraft supplies emergency power.
 
artsyman
Posts: 4516
Joined: Wed Feb 28, 2001 12:35 pm

RE: Dual Engine Failure On Twins And Fly-by-wire

Sat Oct 25, 2003 11:43 pm

Thank you Gordon, Airplay and Liamksa. Between you, you more or less answered the question. Does an ETOPS plane not have an independent fuel supply for the APU ?, Would seem odd to link the two engines and the APU to the same source, although that said, I think I would rather have the fuel in the engine than in the APU.

Jeremy
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Dual Engine Failure On Twins And Fly-by-wire

Sun Oct 26, 2003 12:40 am

Artysman

The RAT on the B757 and B767 series is solely for Hydraulic purposes. The aircraft need hydraulics for the flight controls in order to fly. The RAT on these aircraft do nothing for the electrical system.

The RAT wil supply hydraulics to fly, but it will still be a rock...just a better rock Big thumbs up

 
VC-10
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Joined: Tue Oct 26, 1999 11:34 am

RE: Dual Engine Failure On Twins And Fly-by-wire

Sun Oct 26, 2003 1:39 am

The RAT on an Airbus drives a hyd pump that will pressurise the green system and consequently the control surfaces.

Prim Flt Ctrl comp #1 will be powered by the DC Essential & the Hot Battery Bus.

The Flap/Slat computer remaines unpowered until the landing phase is entered and Land recovery is initiated.

Pitch trim can be controled through the trim wheels.
 
gigneil
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RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 3:57 am

Transformer Rectifier which I assume is just a fancy AC-DC inverter.

Just to clarify this for anyone interested...

You rectify an AC flow to get a DC flow, and you invert a DC flow to get an AC flow.

N
 
gordonsmall
Posts: 2106
Joined: Sun Jul 01, 2001 1:52 am

RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 4:30 am

You rectify an AC flow to get a DC flow, and you invert a DC flow to get an AC flow.

Thanks Neil,

I should probably know that having studied physics for nearly three years - I must have been sleeping, unconscious or in bed recovering from a self inflicted, whisky and lager induced true Scotsman style hangover during that series of lectures.

Regards,
Gordon.
Statistically, people who have had the most birthdays tend to live the longest.
 
avioniker
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Joined: Tue Jan 01, 2002 5:38 am

RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 5:30 am

Let's not forget the 737NG which is ETOPS rated. No RAT just big battery(ies).
With no engines or APU it's just basic instruments and left side displays accompanied by adrenalin fortified pilot power for the pitch and roll controls and the same kind of F/O power to release the gear when they get where they're going.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
gigneil
Posts: 14133
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RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 5:35 am

Heh. You folks from the British Isles sure know how to do it right.  Smile/happy/getting dizzy

N
 
Shenzhen
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RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 7:37 am

Well, I sure hope those RATS provide a little hydraulics to the brakes. Would be a real bummer to get it on the ground, and not be able to stop  Smile

At what airspeed does a windmilling engine stop providing hydraulic pressure?

The 737 has tabs on the control surface, making it much easier to move. Remeber the old 707, the only hydralulic powered surface was the rudder.
 
cdfmxtech
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RE: Dual Engine Failure On Twins And Fly-by-wire

Tue Oct 28, 2003 9:23 am

Well, I sure hope those RATS provide a little hydraulics to the brakes. Would be a real bummer to get it on the ground, and not be able to stop

No, the RAT on the B757 and B767 do not provide hydraulic pressure to the system which power the brakes.

With both Engines out, and no APU...I guess the people at Boeing aren't planning for the aircraft to stop in that scenario. I guess they figure the crew should be happy the made it to the ground. Big thumbs up
Because besides the accumulated brake pressure, you've got nothing to stop the aircraft.

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