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Statistically, when will a large twin engine jet end up in the drink?

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Statistically, when will a large twin engine jet end up in the drink?

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Old 2nd Jan 2019, 05:46
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Statistically, when will a large twin engine jet end up in the drink?

Earlier last year I started a controversial thread in relation to ETOPS. In one of the posts I quoted a professional pilot:

“The engines are very reliable these days, so the chance of even one of them to stop functioning is low.

Four engined airplanes are more expensive to maintain and even to buy than twin engine airplanes. This is not what airlines want, they want an airplane that is very economical and reliable because it is a business and business is always about making profits.”

This seemed a reasonable explanation to me and it shows clearly in this case CASA did not consider safety as the most important consideration, but airline profitability and more people being able to participate in the safest form of transport.

However I have a question to ask. With more and more EDTO throughout the world (literally millions of hours), statistically someone must have worked out when one of these aircraft is going to end up in the drink. That is, originally there is a single engine failure, and before it can get to the alternate and land , the other engine fails. I have recently been told that statistically we are over that time and the safety experts are wondering when such a disaster will happen.

For those that say the twin engine operations are just as safe, why do the ICAO regulations still require four engine operations for flights from say, Australia to South America. Surely if the twin was just as safe, it would be approved.

I wait for some to go berserk and start abusing me again as they did on the last thread. However, don’t shoot the messenger. I am bringing this forward particularly because CASA has refused to “Stop the lie and change the Act” to the words agreed to by the Shadow Minister, Anthony Albanese.

If I were a professional at CASA, I would want to be able to tell the truth rather than live a lie. I would imagine presumably these people have to go home and night and tell their children to be honest, but during the day they have to speak in a completely different way and follow the dishonest ‘groupthink’.

Then again, another section of the Canberra bureaucracy has told Canberrans that their electricity will be 100% from renewables from 2020. I suppose Canberra is the home of dishonesty. What a pity, it is so unnecessary.

Last edited by Dick Smith; 2nd Jan 2019 at 21:25.
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Old 2nd Jan 2019, 06:08
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Perhaps because, after the first engine shutdown, the distance to a safe alternate across the far south Pacific is too long, increasing the chance that the second engine would go on leave?
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Old 2nd Jan 2019, 06:15
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Dick,
Engine reliability is only part of the risk analysis. Cargo fire suppression, amongst other things, plays a part.
FYI the 4-eng aircraft in our company are required to comply with ETOPs rules beyond 180 mins flight time from an adequate airfield.
It's not the prospects of a long flight with an engine shutdown that spooks me; more the uncontained cargo fire.
VS.
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Old 2nd Jan 2019, 06:17
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I am with you Dick - as the Pantene hair shampoo advert said in the 90’s “ It won’t happen overnight but it will happen” - will be interesting to see how the first airline to be unfortunate enough to suffer this fate handles the whole thing.
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Old 2nd Jan 2019, 06:25
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Originally Posted by Veruka Salt View Post
Dick,
Engine reliability is only part of the risk analysis. Cargo fire suppression, amongst other things, plays a part.
FYI the 4-eng aircraft in our company are required to comply with ETOPs rules beyond 180 mins flight time from an adequate airfield.
It's not the prospects of a long flight with an engine shutdown that spooks me; more the uncontained cargo fire.
VS.
Whilst that is of some concern (cargo fire) I wonder how many uncontained cargo fires there have been compared to engine failures if we are going to look at what the risks are for EDOPS? Safety comes at a cost, Airlines look at cost first then safety despite all the hype!
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Old 2nd Jan 2019, 06:25
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Originally Posted by Dick Smith View Post
For those that say the twin engine operations are just as safe, why do the ICAO regulations still require four engine operations for flights from say, Australia to South America. Surely if the twin was just as safe, it would be approved.
LATAM fly a 787-9 between Auckland and Santiago and Air New Zealand fly Auckland to Buenos Aires......just saying
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Old 2nd Jan 2019, 06:37
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Mach,
Clearly cargo fires occur less frequently than engine failures. However, I would argue the odds of survival are much less. Therefore, in the risk = severity x frequency equation, engine failures are the overiding risk consideration. But I'll take an engine failure any day over cargo fire!
Cheers.
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Old 2nd Jan 2019, 06:43
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How much, if any, extra power is applied to the working engine in an engine failure scenario to maintain flight to the nearest suitable landing location?

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Old 2nd Jan 2019, 06:57
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Originally Posted by Ascend Charlie View Post
Perhaps because, after the first engine shutdown, the distance to a safe alternate across the far south Pacific is too long, increasing the chance that the second engine would go on leave?
Interesting theory.

How does the good engine 'know' the failed engine has failed?
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Old 2nd Jan 2019, 07:09
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How much, if any, extra power is applied to the working engine in an engine failure scenario to maintain flight to the nearest suitable landing location?
Can't find it now but I recall from 1976 reading in a a 737 Boeing letter to operators that it is worth considering leaving the live engine at cruise power (rather than advancing to MCT) if drift down terrain clearance is not a factor.
I took the Boeing advice after having to shut down an engine at 33,000 due loss of engine oil contents half way between Nauru and Fiji and drifted down at cruise thrust for next 1.5 hours. Psychologically it felt much more comfortable not touching the live engine than advancing to MCT when there was no need to performance-wise.
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Old 2nd Jan 2019, 07:15
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1976? I suspect there have been some technical advances since then.
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Old 2nd Jan 2019, 07:23
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Can't find it now but I recall from 1976 reading in a a 737 Boeing letter to operators that it is worth considering leaving the live engine at cruise power (rather than advancing to MCT) if drift down terrain clearance is not a factor.
How much, if any, extra power is applied to the working engine in an engine failure scenario to maintain flight to the nearest suitable landing location?
We select MCT initially to initiate a driftdown (company policy), but then have the choice of other strategies following due consideration if terrain isn't a factor . That (obviously) could be all the way back to idle for diversion to a nearby field. As an aside, drift down ceiling for the A330 is approx FL200 at MTOW, and around FL260 at typical landing weights, so any scenario not requiring flight at those levels (due terrain, lengthy diversion etc) would use less than MCT.
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Old 2nd Jan 2019, 08:35
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Originally Posted by Lead Balloon View Post
Interesting theory.

How does the good engine 'know' the failed engine has failed?
Because it’s being flogged and pushing along a bloody big aircraft on its own! Isn’t that to an extent, what happened to Wyalla?
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Old 2nd Jan 2019, 08:39
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The most likely cause of both engines failing at the same time is fuel contamination, which would also affect a four engine aircraft.
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Old 2nd Jan 2019, 08:47
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Isn't the statistical probability of the second engine failing completely independently of the first the same as having a double engine failure in a quad? Surely that would have trouble maintaining altitude as well?
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Old 2nd Jan 2019, 09:10
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Originally Posted by Squawk7700 View Post


Because it’s being flogged and pushing along a bloody big aircraft on its own! Isn’t that to an extent, what happened to Wyalla?
No, it's not being flogged. And I'm not sure how defective crankshafts and bad mixture management in a piston engine translates to a jet engine.

But apart from that, I get your point.
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Old 2nd Jan 2019, 09:11
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Old 2nd Jan 2019, 09:24
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I have recently been told that statistically we are over that time and the safety experts are wondering when such a disaster will happen.
Statistics don’t work that way. An unlikely event doesn’t become “due” just because it hasn’t happened for ages. The chances of two engines failing now are just as unlikely as they were several years ago.
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Old 2nd Jan 2019, 09:34
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I worked this out on the back of a beer coaster........
Assume that airplane engines operate independently of each other and that at least half of the engines on a plane must operate for the plane to continue flying.
A particular airplane engine fails with a probability of 1/7. which is safer, an airplane with 2 of these engines or a airplane with 4 of these engines


Consider the possible scenarios for the engines with a two engine plane.
The engine can either work (P=6/7) or fail (P=1/7).
Let's look at the probability of the independent events.
1.P(W1,W2)=P(W1)*P(W2)=6/7*6/7=36/49
2.P(W1,F2)=P(W1)*P(F2)=6/7*1/7=6/49
3.P(F1,W2)=P(F1)*P(W2)=1/7*6/7=6/49
4.P(F1,F2)=P(F1)*P(F2)=1/7*1/7=1/49
Disaster for a two engine plane is when both engines fail (1/2 or 1 engine would be OK).
That is case 4 and the probability is 1/49.
..
It works the same for a four engine plane but more outcomes (16 instead of 4).
The probabilities are the same : W(6/7), F(1/7), the denominator now is 7*7*7*7=2401.
..
1.W1*W2*W3*W4=6*6*6*6=1296/2401
2.W1*W2*W3*F4=6*6*6*1=216/2401
3.W1*W2*F3*W4=6*6*1*6=216/2401
4.W1*W2*F3*F4=6*6*1*1=36/2401
5.W1*F2*W3*W4=6*1*6*6=216/2401
6.W1*F2*W3*F4=6*1*6*1=36/2401
7.W1*F2*F3*W4=6*1*1*6=36/2401
8.W1*F2*F3*F4=6*1*1*1=6/2401
9.F1*W2*W3*W4=1*6*6*6=216/2401
10.F1*W2*W3*F4=1*6*6*1=36/2401
11.F1*W2*F3*W4=1*6*1*6=36/2401
12.F1*W2*F3*F4=1*6*1*1=6/2401
13.F1*F2*W3*W4=1*1*6*6=36/2401
14.F1*F2*W3*F4=1*1*6*1=6/2401
15.F1*F2*F3*W4=1*1*1*6=6/2401
16.F1*F2*F3*F4=1*1*1*1=1/2401
.
Disaster for a 4 engine plane is when 3 or more engines fail (2 engines failing is OK).
Look for those cases (3 F or more), they are 8,12,14,15,16.
Their probabilities add,
P(3 or more engines failing)=P(8)+P(12)+P(14)+P(15)+P(16)
P(3 or more)=(6+6+6+6+1)/2401=25/2401
.
For the two engine plane, P(disaster)=1/49=0.0204
For the four engine plane, P(disaster)=25/2401=0.0104
.
Therefore the four engine plane would be a safer bet but we all know that don't we?
By the way the calculation is even better if the plane has six, eight or ten engines...that is where a cost/benefit calculation is then made,
When does the cost of the airfare outweigh the risk that the passengers are willing to take to get to where they are going?
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Old 2nd Jan 2019, 09:42
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Aerocat. I am shocked. Are you telling me CASA has put lower cost and higher participation levels in front of improving safety?

Of course they do! That is to the powerful! Wouldn’t you then think they would tell the truth about this?

More importantly most passengers have never been informed of the lower level of safety.

Last edited by Dick Smith; 2nd Jan 2019 at 09:53.
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