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Old 31st Dec 2007, 10:36   #41 (permalink)
 
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Leave the hydraulic systems on, don't touch stick and rudder, use eng thrust to try it in the sim.

Edited:

Ooops, I'm afraid this doesn't work in the 320 SIM due to the Flight Control Logic, sorry guys and gals.

(Apart from that it works out in the A300 SIM, but that's another story).

hetfield

Last edited by hetfield; 1st Jan 2008 at 07:43.
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Old 31st Dec 2007, 13:05   #42 (permalink)
 
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Re CHORNEDSNORKACK`s post. The DHL A300 did not loose its port wingtip, it lost the outer 2 flaps. The aileron stayed on the wing Cockpit controls effect would have been the same if on an A320 ie zilch.
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Old 31st Dec 2007, 14:08   #43 (permalink)
 
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Quote:
The DHL A300 did not loose its port wingtip, it lost the outer 2 flaps. The aileron stayed on the wing Cockpit controls effect would have been the same if on an A320 ie zilch.
Ah, so an extended part of wingtip was directly damaged. But the cockpit controls were affected in the sense that, e. g. the structurally intact elevator, rudder and opposite wing were not controllable because of no hydraulic fluids.
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Old 31st Dec 2007, 14:09   #44 (permalink)
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Well, let's try some *very* crude estimates to give people an idea of what they can conclude without even leaving the armchair to look up the systems. And let me estimate without leaving *my* armchair to go look anything up.

Malinge told the CPI that the A320 family had accumulated about 60 million flight hours.

There have been dual hydraulic failures. Let's suppose there have been 60 of them to make the arithmetic easy. That makes one dual-failure in 1 million flight hours.

Suppose in each of these cases the two systems failed independently. We can take the posterior reliability of each, the MTBF to be 1,000 hours (1,000 x 1,000 = 1 million).

So what is the expected MTBF for three systems? 1,000 x 1,000 x 1,000 = 10^9 hours. So we can expect another
940 million flight hours to go before seeing one.

60 million under the belt, 940 million to go. How many hours a year does an A320 fly, and how many of them are
there? Well, they have been in service 19 years. Let us assume a constant rate of production, x aircraft per year, and that all of them carry on flying for ever at identical levels, say y hours per year. Then there will be xy hours
flown in the first year, 2xy in the second year, 3xy in the
third year, and so on. And we know
xy + 2xy + 3xy + ..... + 19xy = 60 million.

xy + 2xy + ..... +19xy = xy(1 + 2 + 3 + ... + 19) = xy.190 = 60 million

So xy = about 316,000.

Now we want to know how many years it is going to be before the fleet accumulates 1 billion hours. We want to
solve for n in
316,000(1 + 2 + 3 + .... + n) = 1,000,000,000

That is, (1 + 2 + .... + n) = about 3,165.

Now, (1 + 2 + .... + n) = n.(n+1)/2 so we want to solve for
n where n.(n+1) = 6,330. Now, n.(n+1) is a little over n^2, so let's just take the square root of 6,330, say of 6,400, which is about 80 (since 8^2 = 64). So we are round about 80 years of service life before we expect to see a triple failure, and we have had 20 years already, so we can expect another 60 years without one. Don't explain that plane to be flying in line service in 60 years.

All that without leaving the armchair to get a calculator!

Now, about those dual failures. There are, as has been mentioned, dual failures with common cause, namely concerning the PTU. The third system is independent of this common cause, so assuming all failures (including dual failures) are independent is a conservative assumption.

The only non-conservative assumption I see in this is that A320 family aircraft are not produced at a constant rate per year since service intro, but at an increasing rate. However, look at one of the other assumptions. Is is *really* true that one loses one hydraulic system every one thousand flight hours in an A320? I don't think so, I think it is *much, much* more rarely than this. That more than makes up for any increased production rate.

As NoD and T-t-o have said, don't expect the sim, even the Iron Bird, to accurately portray what's going to happen for real in this scenario, for the reasons they have mentioned.

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Old 31st Dec 2007, 14:30   #45 (permalink)
 
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How many hours had the 747 be flown until a QUAD HYD LOSS with JAL?

How many hours had the DC 10 flown until the tripple Loss at Sioux?

How was that calculation?
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Old 31st Dec 2007, 14:35   #46 (permalink)
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Gee, hetfield, I'd forgotten about that!

you're right. All that math can't help. It's going to happen tomorrow!

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Old 31st Dec 2007, 14:36   #47 (permalink)
 
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I don't hope so.

But if it happens, YOU are not the person at the controls.
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Old 31st Dec 2007, 15:00   #48 (permalink)
 
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While that calculation is good, you're assuming that hydraulic system failures are independent events, but they're not. Most (all?) of A320 G&Y failures have been caused by inadventent PTU selection. All total hydraulic failures on commercial aircraft have been caused by structural damage. A strengthened floor would have saved THY981 (along with McDD listening to Convair/General Dynamics' concerns about the floor, not getting the FAA to change their mind about the floor strength, fitting the supporting plate to prevent the torque tube from deforming, not falsifying records &c.) The addition of a fourth hydraulic system probably wouldn't have helped. Four hydraulic systems didn't save JAL123.

Of course, the proliferation of EHAs in new designs means that ship-wide hydraulic failure won't condemn future aircraft (and the A380).
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Old 31st Dec 2007, 15:20   #49 (permalink)
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Just out of interest: Can anybody explain to me how exactly the PTU can cause either the G or Y system to fail in case of a hyd low rsvr lvl ?

I don't quite see the logic?

Thanks in advance
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Old 31st Dec 2007, 15:41   #50 (permalink)
 
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@PGA

If you have a GREEN SYS loss due to RSVR LOW LVL the Y PTU shouldn't run. Otherwise you will have YELLOW RSVR OVHT and so a DUAL LOSS.

This happened many times due to wrong switching as well as PB failures.
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Old 31st Dec 2007, 15:50   #51 (permalink)
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@ Hetfield,

Thanks I got that bit but I fail to understand why the say Y system would then overheat?
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Old 31st Dec 2007, 15:54   #52 (permalink)
 
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There is no "load" on the PTU cause on the green side no fluid, it speeds up like hell.
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Old 31st Dec 2007, 15:58   #53 (permalink)
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Quote:
Originally Posted by violator
While that calculation is good, you're assuming that hydraulic system failures are independent events
Correct. Explicitly. So now we are getting into what the figures can mean. They are a decision guide. In order to let such figures guide one in making decisions, one does indeed need a feel for what such calculations can say, and what they can't. And the independence assumption is the trickiest of the lot.

There are obviously people here who don't know what such calculations can say and what they can't, and I don't see how to give a feel for this which can be packed into a dozen words.

What the figures are good at showing is that, even if you know about or have experienced a simultaneous failure of two systems, that does not necessarily mean you or anyone else needs seriously to worry about a failure of three.

Quote:
Originally Posted by violator
but they're not.
I think you mean that not all hydraulic failures need be caused by independent subsystem failures. Correct. Being hit by a missile might be a common cause failure of all three hydraulic systems. The calculation obviously does not account for failures caused by external events, such as missiles or mid-air collisions or such.

It is also the case that there have been features of certain architectures that slipped through the regulators, such as the common-cause failure near Sioux City. But that was a glaring design error which should have been caught at review time by the hazard analysis. Throwing blades was not exactly an unknown event. And when doing the hazard analysis obviously either nobody had asked what the worst outcome could have been when number 2 throws a blade, or had done so and not answered the question correctly.

Calculations of likelihood don't help when significant design-analysis errors are made

On the other hand, when it comes down to it I don't actually know what hazard analysis techniques were current when that AC was designed.

Quote:
Originally Posted by violator
Most (all?) of A320 G&Y failures have been caused by inadventent PTU selection.
Note that I took account of that specific common-cause failure, and it played a conservative role in the argument.

[/QUOTE]All total hydraulic failures on commercial aircraft have been caused by structural damage.[/QUOTE]

Right, as far as I know.

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Old 31st Dec 2007, 16:08   #54 (permalink)
 
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Quote:
All total hydraulic failures on commercial aircraft have been caused by structural damage. A strengthened floor would have saved THY981 (along with McDD listening to Convair/General Dynamics' concerns about the floor, not getting the FAA to change their mind about the floor strength, fitting the supporting plate to prevent the torque tube from deforming, not falsifying records &c.) The addition of a fourth hydraulic system probably wouldn't have helped. Four hydraulic systems didn't save JAL123.
But a 747 DID have the same kind of accident like THY981. A cargo door opened, the floor duly failed and 9 passengers were blown into Pacific Ocean.

Unlike the DC-10 which fell out of the sky, the 747 kept flying and landed happily, minus the nine passengers sucked out. Something must have saved the 747.
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Old 31st Dec 2007, 16:31   #55 (permalink)
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Since the PTU comes on when the differential in a 3000psi nominal system is 500 psi, and the direction is automatic, then more has to be wrong for there to be such low pressure on one side such that both the PTU is working and it has low enough load for it to overheat.

What is that "lot more that has to be wrong"?

Obviously if G is leaking, you don't want to be pumping fluid over. But if everything else is OK, it doesn't happen. What else has to fail?

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Old 31st Dec 2007, 16:33   #56 (permalink)
 
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Quote:
Obviously if G is leaking, you don't want to be pumping fluid over.
Do you know the 320 systems?
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Old 31st Dec 2007, 16:34   #57 (permalink)
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Quote:
Originally Posted by chornedsnorkack
But a 747 DID have the same kind of accident like THY981. A cargo door opened, the floor duly failed
The flight control lines are routed differently on a B747.

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Old 31st Dec 2007, 16:38   #58 (permalink)
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hetfield,

I'd be glad of a precise answer in one message, if there is one and you are willing to give it. I don't want to go back and forth with one-sentence interactions.

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Old 31st Dec 2007, 16:40   #59 (permalink)
 
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To my limited knowledge there is no "fluid to pump over" on 320, 340, 330, 300.

Oh yes, FUEL.

But that's not what we are talking about.
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Old 31st Dec 2007, 17:19   #60 (permalink)
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Hetfield,

understand you're unwilling to answer the question. Thanks anyway

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