ImperialEagle
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Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 6:43 pm

In light of the BA T7 accident at LHR, just the rumor of a possible "TOTAL loss of power/avionics" leads to some interesting speculation. (Agreed---it is too soon to know what the real series of events leading up to the accident were.)

The most immediate effect I should think would be what the regulatory bodies would do in reagards to ETOPS rules.
Obviously a "TOTAL power/avionics loss" scenerio has been accounted for. No doubt the odds of such an even were considered nearly nill------untill it happens. So do they then re-think those rules, or does economics win out?

Also, the ability to control the T7 aircraft. Redundancy issues, etc.

What are your thoughts?
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tdscanuck
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 7:09 pm



Quoting ImperialEagle (Thread starter):
The most immediate effect I should think would be what the regulatory bodies would do in reagards to ETOPS rules.

If it was really a total power failure (I have my doubts) then I think they'd have to suspend ETOPS for the fleet until they determined why. However, they won't do that until they know it was really a total power failure, at which point they may have a good idea of why, so that might not be necessary.

Quoting ImperialEagle (Thread starter):
Obviously a "TOTAL power/avionics loss" scenerio has been accounted for. No doubt the odds of such an even were considered nearly nill------untill it happens.

Actually, for a 777, it's supposed to be zero (not true of all aircraft). Not that it can't happen but, if it did, it would require so much additional damage to the airframe as to render the issue moot. To completely lose power on a 777 you need both engines to die, the APU to fail, the RAT to fail, and the battery to fail. All essentially independent systems.

Quoting ImperialEagle (Thread starter):
Also, the ability to control the T7 aircraft. Redundancy issues, etc.

The FBW system is triple redundant, so if it goes down about the only way I can see that happening is an external cause. So far, there isn't really any evidence I'm aware of that suggests they lost the FBW system. Loss of engine power and/or weather could have caused this type of event even with fully operational flight controls.

Tom.
 
Tristarsteve
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 8:22 pm

He may have lost all main power supplies due to engines running down, but muat have still had battery power. You can see that the APU door has opened, from the APU battery, so their must have been some battery supply.
 
Pihero
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 8:43 pm



Quoting TristarSteve (Reply 2):
You can see that the APU door has opened, from the APU battery, so their must have been some battery supply.

... but it is also likely the sign of a running APU, which makes the total electrical power loss even more remote.
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tdscanuck
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 9:04 pm



Quoting Pihero (Reply 3):
Quoting TristarSteve (Reply 2):
You can see that the APU door has opened, from the APU battery, so their must have been some battery supply.

... but it is also likely the sign of a running APU, which makes the total electrical power loss even more remote.

Another thread brought up that the 777 has auto-APU capability. If an IDG drops offline, the APU automatically starts to pick up the load. First step is to open the door...it's possible the APU hadn't completed it's start sequence at the time of the event.

Tom.
 
Pihero
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 10:16 pm

Quoting Tdscanuck (Reply 4):
If an IDG drops offline, the APU automatically starts to pick up the load.

OK. But if I've lost TWO IDGs, the last thing I would want is an automatic APU start that would further deplete my batteries before I have a chance to assess my options. There must be something more to that effect, maybe dedicated batteries for the APU. ?..
(I've just checked : there's an APU battery, just aft of the aft cargo door..., so your post could very well be correct as to the position of the APU door.)
But I'm not buying the double engine failure scenario anyway.

[Edited 2008-01-17 14:24:03]
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zanl188
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 11:30 pm



Quoting Pihero (Reply 5):
But I'm not buying the double engine failure scenario anyway.

It's happened before....
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aviopic
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RE: Implications Of Total Avionics Loss T7---

Thu Jan 17, 2008 11:51 pm



Quoting Tdscanuck (Reply 4):
the APU automatically starts to pick up the load.

Only possible at relative low altitude.
If I am not mistaken the Boeing flight manual says not even to try an APU start at a certain altitude as it would only drain the battery without much chance of an actual APU start.
Don't know the exact details but I am sure there is a driver out there to fill me in.

Quoting Pihero (Reply 5):
the last thing I would want is an automatic APU start that would further deplete my batteries before I have a chance to assess my options

 checkmark 

Further a total PWR failure sounds very unlikely.
The emergency bus is independend of IDG's and APU.

I do know one incident with an A320(somewhere above the UK) which was unable to leave a holding pattern due to freaked out autopilot.
They had to shut down all electrical power and restart it to gain control again albeit manual this time.
Not really funny I guess as the IRS is then lost as well and won't come back online.
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XT6Wagon
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RE: Implications Of Total Avionics Loss T7---

Fri Jan 18, 2008 4:23 am



Quoting Pihero (Reply 5):
OK. But if I've lost TWO IDGs, the last thing I would want is an automatic APU start that would further deplete my batteries before I have a chance to assess my options. There must be something more to that effect, maybe dedicated batteries for the APU. ?..

I believe that the APU is required to have a pyrotechnic starter for just such a case so it wouldn't be pulling much power at all from its battery.
 
Tristarsteve
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RE: Implications Of Total Avionics Loss T7---

Fri Jan 18, 2008 8:04 am



Quoting XT6Wagon (Reply 8):
believe that the APU is required to have a pyrotechnic starter for just such a case so it wouldn't be pulling much power at all from its battery.

The B777 APU has two starter motors. Normal start is by a pneumatic starter that takes air from the aircraft pneumatic system. If this pressure is not available then there is an electric starter which takes over powered by the APU battery.
I have never seen a pyrotechnic starter on an airliner.
 
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HAWK21M
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RE: Implications Of Total Avionics Loss T7---

Fri Jan 18, 2008 8:35 am

Any Preliminary report officially declared as yet.
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Faro
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RE: Implications Of Total Avionics Loss T7---

Fri Jan 18, 2008 4:22 pm

Out of pure speculation, what if I bombard an aircraft on final approach with huge doses of electro-magnetic radiation (I believe such a weapon was once envisaged if not tested) and try to fry the electric circuits? Would it do enough damage to disable control of my engines, FBW and other electrically-controlled systems?

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tdscanuck
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RE: Implications Of Total Avionics Loss T7---

Fri Jan 18, 2008 6:39 pm



Quoting Faro (Reply 11):
what if I bombard an aircraft on final approach with huge doses of electro-magnetic radiation (I believe such a weapon was once envisaged if not tested) and try to fry the electric circuits? Would it do enough damage to disable control of my engines, FBW and other electrically-controlled systems?

An aluminum airliner makes a very nice Faraday cage. You'd probably nuke the radios and navigation equipment, but I don't know if the engines or FBW would care.

Tom.
 
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zeke
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 9:31 am



Quoting HAWK21M (Reply 10):
Any Preliminary report officially declared as yet.

AAIB released an initial report http://www.aaib.dft.gov.uk/latest_ne..._january_2008___initial_report.cfm
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HAWK21M
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 4:40 pm



Quoting Zeke (Reply 13):

Thanks.
So 600Ft & 2 miles out something caused loss of Thrust in both Engines.
regds
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BWilliams
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 6:16 pm



Quoting HAWK21M (Reply 14):
So 600Ft & 2 miles out something caused loss of Thrust in both Engines.

Interesting.

What could cause this? Obviously, the problem wouldn't be in the engine's FADEC unit, since a dual failure is a near-impossibility. Could an electrical problem (short or severed wire) in the throttle quadrant cause throttle position changes both manually and through the Auto-thottle, or are those totally independant circuits. indicating a larger problem?
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HAWK21M
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 8:32 pm



Quoting BWilliams (Reply 15):
What could cause this

Im Thinking Flock of birds.
Hows the Bird situation out there.
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Starglider
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 9:30 pm

Quoting HAWK21M (Reply 16):
Quoting BWilliams (Reply 15):
What could cause this

Im Thinking Flock of birds.
Hows the Bird situation out there.

No evidence of bird impact anywhere on the engines, wing leading edges or nose of the aircraft when viewing detailed photographs recently published .

What ever the source of the failure, it somehow may have affected engine related units interfaced to the left and right systems ARINC buses and in this case affected both buses at the same time. Units related to engine control such as:

EEC (engine electronic control),
WOW (weight on wheels cards),
ADIRU (Air Data Inertial Reference Unit),
EDIU (Engine Data Interface Unit),
ACIPS (Airfoil and Cowl Ice protection System)
are interfaced through these buses.

Left bus for the left engine and right bus for the right engine.

Next question is if the source that caused the dual failure is from an aircraft related system or external, either inside the aircraft or the outside world. Either way, since this is such a very rare event, could EMI be a factor to reccon with in this case?

I stress this is pure speculation of course, too early to draw any conclusion.


Starglider

[Edited 2008-01-19 13:36:23]
 
WingedMigrator
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RE: Implications Of Total Avionics Loss T7---

Sat Jan 19, 2008 11:24 pm



Quoting Starglider (Reply 17):
What ever the source of the failure, it somehow may have affected engine related units interfaced to the left and right systems ARINC buses and in this case affected both buses at the same time.

Are the buses 100% mechanically redundant (not just electrically redundant), i.e. could a single loose connector take both of them down? Seems very far-fetched to me... but envisioning a common avionics failure mode is quite baffling.
 
fadecfault
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 1:17 am

I'm sure I'll be flamed but any talk is just pure baseless speculation:

I'll gander it was pilot error. Pilots failed to notice A/T kicked off during final and manually pushed the throttle too late.

Like I said totally baseless but I have a hard time believing that both engines just failed to respond. Correct me if I'm wrong but the engines have their own PMA's and do not rely on a/c power off eec operation (except for alternate pwr). So even with total power failure wouldn't the engines continue to operate normally?
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Jetlagged
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 2:05 am



Quoting FADECFAULT (Reply 19):
I'm sure I'll be flamed but any talk is just pure baseless speculation:

I'll gander it was pilot error. Pilots failed to notice A/T kicked off during final and manually pushed the throttle too late.

Like I said totally baseless but I have a hard time believing that both engines just failed to respond. Correct me if I'm wrong but the engines have their own PMA's and do not rely on a/c power off eec operation (except for alternate pwr). So even with total power failure wouldn't the engines continue to operate normally?

Just where did you get this idea from? I'd suggest keeping this kind of baseless theorising to yourself.

The AAIB initial report makes no mention of the A/T being kicked off. On the contrary, it mentions that the A/T twice tried to increase thrust with no response and that the crew then manually advanced the thrust levers, also with no response.

You are implying the flight crew are covering up an error. They would be doing this in the full knowledge that the flight recorder data would find them out when analysed. Also BA would not risk parading the crew in front of the press if there was the slightest suspicion of pilot error.

The AAIB report also makes no mention of any electrical power loss, which might make this entire thread irrelevant.  Wink
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HAWK21M
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 3:49 am

The sudden loss of thrust & no response to Thrust recovery is a concern if its an Aircraft related fault.
This is one mystery.
Read somewhere that Russian Intelligence has qouted as being an Electro magnetic attack on the wrong plane rather than the PMs Aircraft.

Not sure about how that works.

regds
MEL
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FXfan
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 8:00 pm

Sorry to ask such a simple question, but could someone give a basic explanation of how the engine controls are linked to the engines themselves. Obviously fully automated but are there mechanical backups?
 
MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 8:21 pm

If the B777 works like all FADEC controlled aircraft I have so far been working on, the FADEC has two independent channels (actually two seperate computer circuit boards each with it's own processor). ERach circuit board gets powered by it's own power source (a small alternator attached to the engine gear box, actually two altenators in one housing). For gound maintenance use (when the engines are not running) there should be another alternate power source from the airframe.
The throttle levers normally operate two independent resolvers each, one for each FADEC channel. These resolvers receive their power from the FADEC. e.g. totally independent of airframe power.

Jan
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Stitch
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 8:40 pm



Quoting ImperialEagle (Thread starter):
The most immediate effect I should think would be what the regulatory bodies would do in reagards to ETOPS rules.

Three-engined and four-engined planes have had complete avionics and power losses, yet they are still allowed to fly long distances. So I don't see why a two-engined plane should suddenly be denied the right to fly long distances.

If it had been a BA DC-10 or 747, would we be demanding BA ground their TATL ops?
 
avioniker
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 10:07 pm

Anybody besides me notice that the fan is only relatively slightly fractured on the #1 engine and the #2 appears intact?
Or is there a property of the fan blades that would keep them from breaking off?

From what little I remember, doesn't it take a power signal to direct a shutdown? That as opposed to a loss of power allowing a relay to relax shutting the engine down. It seems to me that the problem would have to be in the Throttle Quatrant Module as this is the only common point to both engines even though there are separate connectors, relays, switches, and dual power supplies to the system.

It's remarkable that the FO had the presence of mind to lower the nose and get some speed. Without his having done that the plane would, I'm sure, have impacted on the roadway as the result of a stall. Very good thinking with so little time remaining.

Tonight's BBC1 & 3 Reports are saying the plane will most probably be written off after the investigation concludes. The wings are intact but badly damaged and they aren't sure the structural integrety of the spars and wing to body attach points can be assured.

 

[Edited 2008-01-20 14:11:35]
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MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 11:51 pm



Quoting Avioniker (Reply 25):
From what little I remember, doesn't it take a power signal to direct a shutdown? That as opposed to a loss of power allowing a relay to relax shutting the engine down. It seems to me that the problem would have to be in the Throttle Quatrant Module as this is the only common point to both engines even though there are separate connectors, relays, switches, and dual power supplies to the system.

On the 737NG and other aircraft, the HP valve in the HMU is closed by a solenoid, which receives power from the airframe electric system. A power failure (or defective solenoid) can not shut down the engine (We recently had the problem that a wiring fault caused no power to go to this solenoid, so that the engine could not be shut down until all the fuel in the line from the spar valve was consumed). I assume that this safety measure also exists on the B777.

Jan
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Starlionblue
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RE: Implications Of Total Avionics Loss T7---

Sun Jan 20, 2008 11:54 pm



Quoting Stitch (Reply 24):

Three-engined and four-engined planes have had complete avionics and power losses, yet they are still allowed to fly long distances. So I don't see why a two-engined plane should suddenly be denied the right to fly long distances.

Indeed. Very logical.

As I said in Part 6 the 777 fleet has been flying safely for 13 years. Now we have one crash. Doesn't it seem more likely something was wrong with this plane? If there was a serious flaw in the design you'd think we would have seen more than one manifestation of it.
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Jetlagged
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 12:20 am



Quoting Stitch (Reply 24):
Three-engined and four-engined planes have had complete avionics and power losses, yet they are still allowed to fly long distances. So I don't see why a two-engined plane should suddenly be denied the right to fly long distances.

Very true, and had an avionics problem struck the 777 in cruise it would most likely have recovered and been able to continue flying. ETOPS is primarily about system reliability and redundancy in the case of a single engine failure. The only way ETOPS could become questioned is if this turns out to be a case of two engines failing from independent causes, but it isn't looking that way at present.
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kstatepilot
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 12:34 am

What I am wondering is where anybody has said that there is a power loss? All that we know is that the engines were working, and they failed to increase when commanded to. There is no mention of a loss of power here from the article:

Quote:
At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond.

 
goboeing
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 12:53 am



Quoting Avioniker (Reply 25):
It's remarkable that the FO had the presence of mind to lower the nose and get some speed.

I don't want to sound like a jerk here but if any FO did not have the presence of mind to do just that in the situation this plane was in, they would be a waste of space in the cockpit. Airspeed must be maintained.
 
fr8mech
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 5:54 am

The list of things that can affect both engines on a twin is remarkable short.
-Fuel
-Pilots
-A/T

There may be a couple more, but none come to mind.

I haven't really gotten into the nuts and bolts of FADEC control, and please correct me if I'm wrong, but a FADEC engine has an independant power source for the EEC/FADEC. Wouldn't a loss of electrical power on the aircraft do nothing to the engine? Of course, there is no report of electrical failure, so that point is moot.

The report says significant amounts of fuel on the ground. Just out of curiousity, what's the unusable fuel on a B777?

I guess we'll have to wait for the report.
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BWilliams
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 6:20 am



Quoting Kstatepilot (Reply 29):
What I am wondering is where anybody has said that there is a power loss? All that we know is that the engines were working, and they failed to increase when commanded to. There is no mention of a loss of power here from the article

Exactly. That's why I posited the question of damage to the electronics in the throttle quadrant instead of the actual engine EEC/ECU. Would the engines have logic that, in case of a loss of input from the throttles, they would maintain their current thrust levels?
Regards, Brad Williams
 
Starglider
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 6:52 am



Quoting Fr8mech (Reply 31):
The list of things that can affect both engines on a twin is remarkable short.
-Fuel
-Pilots
-A/T

If fuel, what about fuel/pilots:

Highly unlikely but engines not responding to thrust due to some command, how about erroneously selecting both fuel control levers to off?


Starglider
 
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Starlionblue
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 8:23 am



Quoting Fr8mech (Reply 31):
The list of things that can affect both engines on a twin is remarkable short.
-Fuel
-Pilots
-A/T

There may be a couple more, but none come to mind.

You forgot alien space bats.  Wink That is, external factors. But apart from canada goose aerobatics teams, I find it hard to believe an external factor could affect both engines exactly at the same time.
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MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 8:47 am



Quoting Kstatepilot (Reply 29):
What I am wondering is where anybody has said that there is a power loss? All that we know is that the engines were working, and they failed to increase when commanded to. There is no mention of a loss of power here from the article:

Quote:
At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond.

We were trying to figure which fault would make BOTH engines not accept a increease of thrust command. Due to the explanations by some people

Quoting Fr8mech (Reply 31):
The list of things that can affect both engines on a twin is remarkable short.
-Fuel
-Pilots
-A/T

Does anybody from MX in here have the B777 on his ticket? Is the B777 following Boeing philosophy that the A/T is operating an electrical actuator in or below the centre pedestal, which mechanically moves the thrust levers, which then mechanically move the resolvers for the FADEC? Also, does RR use dual channel FADECS with an independent resolver for each FADEC channel,and each FADEC channel powered by an independent power source?
This would mean that either two independent mechanical links in the centre pedestal and/or ALL four FADEC systems went south the same time. I think if there was some mechanical issue in the throttle quadrant (e.g. a forgotten tool) which had jammed up movement of the throttles forward, it would have been stated right away. From what I understand, the pilots shoved the throttles forward and just nothing happened.
A total power failure airframe sides would have been very noticeable as well, suddenly all F/O screens go dark (the captain's too, if the standby power system failed, if the 777 is built to the same philiosophy as previous Boeing planes, with just the standby instruments running on battery power). I'll bet he would have noticed it an told about it.

Jan
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avioniker
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 3:42 pm



Quoting GoBoeing (Reply 30):
Quoting Avioniker (Reply 25):
It's remarkable that the FO had the presence of mind to lower the nose and get some speed.

I don't want to sound like a jerk here but if any FO did not have the presence of mind to do just that in the situation this plane was in, they would be a waste of space in the cockpit. Airspeed must be maintained.

I'd not say you sound like a jerk. . .

The FO stated in his interview, reprised on BBC yesterday, that he cleared the A20 by about 20ft. Lowering the nose at 600 ft above ground, which puts him less than 1/4 mi from the A20 when he did it, takes some "large ones" and since, as also stated by the FO and Captain in the same interview, the manouver isn't practiced in the Sim at anything close to this altitude I'll stand by my comments.

 Smile
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fr8mech
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 5:21 pm



Quoting Starlionblue (Reply 34):
You forgot alien space bats. That is, external factors.

You're right, I did not list external factors, because, short of bird or lightning strike, nothing external, at 600' AGL, should affect both engines in the same exact way. In my humble opinion, and opinion is all it is, something inside the aircraft failed to do its job. Whether mechanical, electrical, electronic or biological we'll leave up to the investigative folks to figure out. But, if it is the one of the first 3, operators of the B777 can except some emergency AD's in the near future.

Quoting MD11Engineer (Reply 35):
Is the B777 following Boeing philosophy that the A/T is operating an electrical actuator in or below the centre pedestal,

I made that assumption when I listed the A/T. It only makes sense to me, that you would want both throttles moving at the same rate, thus one servo driving both throttles.
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777wt
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 6:07 pm

back then BA did suffer a avionics fire in one of the 777 which landed in the US.
 
MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 6:42 pm



Quoting Fr8mech (Reply 37):
It only makes sense to me, that you would want both throttles moving at the same rate, thus one servo driving both throttles.

I should have been more specific: I meant, of course, one A/T servo for each throttle, so that they can move independently (necessary e.g. if an engine fails).

BTW, having thought about it, I think that we can exclude contaminated fuel. Since I assume that each engine was feeding from a seperate tank, it would be highly unlikely that both engines would have problems at exactly the same time.

Jan
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fadecfault
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 8:33 pm



Quoting MD11Engineer (Reply 35):
Does anybody from MX in here have the B777 on his ticket? Is the B777 following Boeing philosophy that the A/T is operating an electrical actuator in or below the centre pedestal,

Def need someone with 777 experience in here. The 757 and 767 had a TMC - thrust management computer to control A/T (don't remember if direct or indirect) not sure if the T7 has one but that could be a common link.
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MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 9:02 pm



Quoting FADECFAULT (Reply 40):
Quoting MD11Engineer (Reply 35):
Does anybody from MX in here have the B777 on his ticket? Is the B777 following Boeing philosophy that the A/T is operating an electrical actuator in or below the centre pedestal,

Def need someone with 777 experience in here. The 757 and 767 had a TMC - thrust management computer to control A/T (don't remember if direct or indirect) not sure if the T7 has one but that could be a common link.

Sure, but a failed TMC would not explain why the engines also refused to increase thrust when the throttles were firewalled manually by the pilots. One engine could be explained by a catastrophically defective EEC system (this includes the EEC, the HMU, the T/L resolver, centre pedestal switch pack, if it exists on a 777, the various EEC controlled actuators on the engine and the EEC input sensors), but not both at the same time

Jan
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fadecfault
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 9:49 pm



Quoting MD11Engineer (Reply 41):
Sure, but a failed TMC would not explain why the engines also refused to increase thrust when the throttles were firewalled manually by the pilots. One engine could be explained by a catastrophically defective EEC system (this includes the EEC, the HMU, the T/L resolver, centre pedestal switch pack, if it exists on a 777, the various EEC controlled actuators on the engine and the EEC input sensors), but not both at the same time

I agree with you, I do not understand how both engines somehow refused to give more power. All essential systems have redundent back ups (dual channel sensors, power, etc..). I don't see how it could be fuel either (unless they ran out and didn't notice the low qty OR low pressure..)

A full review of the FDR is the only thing that will tell the real story..
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tdscanuck
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 10:19 pm



Quoting WingedMigrator (Reply 18):
Are the buses 100% mechanically redundant (not just electrically redundant), i.e. could a single loose connector take both of them down?

They're supposed to be. Given the number of wires and connectors involved it's entirely possible that there is a common connector somewhere, but there shouldn't be.

Quoting FADECFAULT (Reply 19):
So even with total power failure wouldn't the engines continue to operate normally?

Yes. Although "total power failure" is a bit of a misnomer on a 777...that implies you've lost battery power too, at which point you really don't need to worry much about what the engine is doing.

Quoting Fxfan (Reply 22):
could someone give a basic explanation of how the engine controls are linked to the engines themselves. Obviously fully automated but are there mechanical backups?

Not on a 777.

Quoting Avioniker (Reply 25):
It seems to me that the problem would have to be in the Throttle Quatrant Module as this is the only common point to both engines even though there are separate connectors, relays, switches, and dual power supplies to the system.

It's not really common...it's just two identical pieces of hardware side by side. They're not sharing components.

Quoting Fr8mech (Reply 31):
The report says significant amounts of fuel on the ground. Just out of curiousity, what's the unusable fuel on a B777?

7 gallons in the center tank, about 15 gallons in each main tanks.

Quoting Starglider (Reply 33):
Highly unlikely but engines not responding to thrust due to some command, how about erroneously selecting both fuel control levers to off?

That would to it, but it would be a pretty spectacular screwup. I'm not sure I've ever heard of anyone doing that to both engines at once.

Tom.
 
MD11Engineer
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RE: Implications Of Total Avionics Loss T7---

Mon Jan 21, 2008 11:00 pm



Quote:
(From Yanqui67 in the BA 777 crash thread #7 in the civ av forum)
Answers:
EEC- Electronic Engine Control, it is mounted on the engine and its the brain of the engine. Each engine has one. It is a computer that is used to do all the functions for the engine. it receives data from all the engine components and uses the data to make the engine operate at its optimum settings. It has two channels A and B. Every time the start switch is actuated the EEC channel changes. Only one channel is in control at a time. The EEC is always ready to take over in case of failure with the other channel. The other channel is monitoring the active channel to make sure they are both receiving the same data. The EEC operates on the ARINC 429 avionics data bus. The EEC is powered by the aircraft once the start switch is actuated or the EEC maintenance switch is on. Once the engine reaches a certain N1 or EPR setting then the EEC alternator powers the EEC. Meaning the engine itself is self powered by the alternator that is spinning.

FADEC-Full Authority Digital Engine Control, This is what is called when an engine is Fly By Wire. That is, no mechanical controls. The EEC controls all engine operations. The goal of FADEC is to operate the engine at its highest efficiency in all flight aspects.

AIMS- Aircraft Information Management System- On the 777 this is the brain of the aircraft. All data is brought in and through the AIMS system. There are two AIMS cabinets left and right. They are essentially the EEC of the 777 aircraft. The AIMS system makes the 777 what it is. It eliminates many avionics black boxes because the AIMS does those functions. As an example the Autothrottle computer which is found on most airliners is eliminated on the 777. The function of A/T is a function of AIMS. Other functions include, Primary Flight Displays, Flight Data Recorder System, Aicraft Conditioning System and other functions. The AIMS system receives and transmits data on four 629 buses. I could go on and on with AIMS. It is very complex and I just gave a quick description of it.

EIDU- Engine Data Interface Unit- On the 777 there are two. One each for each engine. This box is what lets the AIMS system interface with the EEC. The EECs are on the 429 bus while the AIMS is on the 629 bus. In order for the system to communicate with each other the EDIU takes care of it. It converts the data coming from the EEC to a 629 signal for AIMS to use and vice versa convert 629 to a 429 signal for the EEC to use. This is the sole way for the flight deck to make the engine do what you want it to do. You bump the throttle up and the signal is sent to the AIMS which in turn sends it to the EDIU and then to the EEC which tells the engine to add some gas. The feedback is then sent back the same way it came. The EDIUs are three channel A, B, and C.

I hope that answers some questions. The 777 is very complex and I could be writing forever. Like I said before, I am waiting for the results of the investigation, I will not speculate.

It seems from this information that the B777 is a completely different bird than all those I have worked on before.
The way I understand is that throttle information is being transfered from the throttles through the EIDU´s in a digital format, not as analog voltage changes as per conventional transmission per resolver directly to the EEC.
Frankly so much on modern aircraft is software controlled, where a traditional AME / mechanic has virtualy no chance to understand what really goes on in the boxes.
All he can do is a BITE test and hope that the box isn't lying to him (as hs happened before, from my own experience).
On a classic aircraft, like the 727, I had a chance to figure almost everything out, using wiring diagrams, schematics and a multimeter (and maybe a TIC box for certain avionics receivers, but an analog VOR receiver or an ILS receiver are quite simple).
With modern aircraft, even if we would be able to understand it, we wouldn't get the code of the software used in the various boxes anyway, so there is no chance of us knowing what really goes on.

Jan
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Starglider
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RE: Implications Of Total Avionics Loss T7---

Tue Jan 22, 2008 1:24 am



Quoting Tdscanuck (Reply 43):
That would to it, but it would be a pretty spectacular screwup. I'm not sure I've ever heard of anyone doing that to both engines at once.

Like said, highly unlikely. I can't imagine knocking those switches to the cutoff position by accident, even though the fuel control module is right behind the throttle quadrant on the pedestal there is quite some space between the throttles and fuel control switches.

I do, however, remember an incident whereby a pilot on a different type of aircraft, during the landing roll, selected flaps up . .errr . . he thought he did . . . . but it was the gear handle and the aircraft came to rest on its belly. The 2 handles were not near each other either.

Back to fuel control. I remember first hand years ago, again on a different type of aircraft, that a batch of fuel shutoff valves (fire wall shutoff valve, normally open, in the engine feed line. Not the same as the engine fuel valve for starting or shutting down the engine) were suspect of closing un-commanded. As a temporary measure we had to select the valve switch to the open position, pull the c/b and safety wire the manual handle on the valve in the open position. The electric motor on the valve was sensitive to EMI and could potentially close without being commanded to do so by the pilot. And incidents had happened before the EMI problem surfaced as the cause. It became a temporary inspection item to ensure the breaker was pulled and the valve was safety-wired open before flight. Soon after, all the valves were replaced by a higher dash number and the inspection was lifted.

Due to that experience the question did cross my mind, what if on the T7, the fuel valves had a similar type of problem? Far fetched . . .i know . . . .


Starglider
 
Tristarsteve
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RE: Implications Of Total Avionics Loss T7---

Tue Jan 22, 2008 9:49 am



Quoting MD11Engineer (Reply 44):
It seems from this information that the B777 is a completely different bird than all those I have worked on before.

Yes.
I work B777, I have an EASA B1 licence on it. I try to help with the discussions here, but because the systems are so complicated it is difficult to be absolutely certain of what happens. You won't get me discussing how the autothrottle signals get to the EECs because I really don't know.
Working on B777, 90pc of non cabin work is trusting the computors. You get Warning and Status messages, you go to the Maintenance Access Terminal and get a maint message number. You open the FIM and look up the number and see what it says. Usually you start by ignoring this and cycling the breakers like any other aircraft.
It takes a long time to get confident, because on the line you get few defects and most of them go away with resets.
 
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HAWK21M
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RE: Implications Of Total Avionics Loss T7---

Tue Jan 22, 2008 4:47 pm



Quoting TristarSteve (Reply 46):
I work B777, I have an EASA B1 licence on it.

You seem to be the right person to have a more realistic view on what could have gone wrong.
regds
MEL
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tdscanuck
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RE: Implications Of Total Avionics Loss T7---

Tue Jan 22, 2008 4:55 pm



Quoting Starglider (Reply 45):
I can't imagine knocking those switches to the cutoff position by accident, even though the fuel control module is right behind the throttle quadrant on the pedestal there is quite some space between the throttles and fuel control switches.

There's also a detent on the cutoff levers so you need to move them back then sideways then down to close the valves. Dumber things have been done on the flight deck, but you'd really have to work at it.

Quoting Starglider (Reply 45):
Back to fuel control. I remember first hand years ago, again on a different type of aircraft, that a batch of fuel shutoff valves (fire wall shutoff valve, normally open, in the engine feed line. Not the same as the engine fuel valve for starting or shutting down the engine) were suspect of closing un-commanded.

This was known to happen on some older 737's because the fuel valve and landing light wires when through the same connector. If the connector got wet in just the right way, the engine would shut down when you turned on the landing lights. A service bulletin years ago move the wires to separate connectors.

Quoting Starglider (Reply 45):
Due to that experience the question did cross my mind, what if on the T7, the fuel valves had a similar type of problem?

The 777 fuel valves are the same as those used on the 737 and 767. Not to say they might not have a problem, but you'd expect it to have shown up elsewhere first just due to the number of valves in the 737 fleet.

Tom.
 
WestWing
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RE: Implications Of Total Avionics Loss T7---

Wed Jan 23, 2008 5:55 am

Quoting Starglider (Reply 45):
I do, however, remember an incident whereby a pilot on a different type of aircraft, during the landing roll, selected flaps up . .errr . . he thought he did . . . . but it was the gear handle and the aircraft came to rest on its belly. The 2 handles were not near each other either.

Which is of course why, from a human factors engineering standpoint, the landing gear lever feels like a wheel and different to the touch than the flaps lever which feels like...a flap. The hope is that some neurons will wake up as your palm/fingers touch the control lever and scream at you through your mental fog about what you are about to do wrong.


Back to the BA38 incident. Has it been confirmed that there was a loss of electrics during approach? I know that tug driver may have reported that this is what the Captain told him, but was there any other source? Is it possible for the APU intake door to have been jolted open because of impact?

[Edited 2008-01-22 22:14:28]
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