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Old 29th June 2014, 12:36   #61
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Default Re: Factors Affecting Reliability

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Originally Posted by Jeroen View Post
ETOPS plays a big role and is ultimately all about reliability. Plain all weight is a factor too. You need a certain amount of thrust to get a 747 into the air. More engines, more thrust.
Not necessarily. You just use a bigger engine with double the thrust and you replace every two with one.

The reason long distance planes had four engines, is that there was a legislature (or was it a requirement) that planes flying across long stretches of water have minimum of 4 engines. Not only that they were supposed to take off with at least three engines and cruise with two. So it was a case of redundancy to have four engines. In contrast today many long distance planes have only two engines.
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Old 29th June 2014, 13:48   #62
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Post Re: Factors Affecting Reliability

Quote:
Originally Posted by Jeroen View Post
ETOPS plays a big role and is ultimately all about reliability.
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Originally Posted by Aroy View Post
The reason long distance planes had four engines, is that there was a legislature (or was it a requirement) that planes flying across long stretches of water have minimum of 4 engines.
Hi Aroy, this is ETOPS, for 'Extended Twin Engine Operations' or 'Extended Range Twin Operations', which Sutripta & Joroen have mentioned.
If I remember correct, it's for aircraft with two engines & limits flight time from an airport on a single engine.
Since over water your airport options are limited ( unless you consider aircraft carriers ) it came to be more applicable over water than not.
You did have wide-body aircraft with three engines that were allowed ETOPS, like the L-1011.

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Originally Posted by Aroy View Post
In contrast today many long distance planes have only two engines.
The aircraft makers convinced the administrators ( FAA & the like ) that reliability had improved to the point that they could be operated like 3 or 4-engined aircraft.
So, the ETOPS duration was increased, allowing twin-engined aircraft ( 767, etc. ) access to routes they were denied earlier.
Basically reliability has offset, or negated, redundancy.

Last edited by im_srini : 29th June 2014 at 13:51.
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Old 30th June 2014, 09:01   #63
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Default Re: Factors Affecting Reliability

Quote:
Originally Posted by Aroy View Post
Not necessarily. You just use a bigger engine with double the thrust and you replace every two with one.

The reason long distance planes had four engines, is that there was a legislature (or was it a requirement) that planes flying across long stretches of water have minimum of 4 engines. Not only that they were supposed to take off with at least three engines and cruise with two. So it was a case of redundancy to have four engines. In contrast today many long distance planes have only two engines.
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Originally Posted by im_srini View Post
Hi Aroy, this is ETOPS, for 'Extended Twin Engine Operations' or 'Extended Range Twin Operations', which Sutripta & Joroen have mentioned.
If I remember correct, it's for aircraft with two engines & limits flight time from an airport on a single engine.
Since over water your airport options are limited ( unless you consider aircraft carriers ) it came to be more applicable over water than not.

So, the ETOPS duration was increased, allowing twin-engined aircraft ( 767, etc. ) access to routes they were denied earlier.
Basically reliability has offset, or negated, redundancy.
Some good answers. Let's elaborate a little more!

First back to the question Sutripta asked:

Quote:
Originally Posted by Sutripta View Post
OT OT OT.
Could someone rank in order of 'reliability' (and explain why!): Single engined, twin engined, and four engined planes.
Let's start by looking at it from a theoretical model point of view, similar to the NA/TC discussion. Have a look here: http://www.eventhelix.com/realtimema...m#.U613LRZ-FFc

This is all about availibillity in parallel. Look at what it does to the overall availibillity. With respect to our plane and engine reliability issue it clearly shows that adding more eninges and the overall availibillity/reliability goes up! Its what the essence of redundancy is/does.

So, at least in theory, there is a case to be made that the more engines a plane has, the more reliable it is likely to be. Back to the real world and that is more or less how it played out.

In practice as pointed out there were requirements/legislation that put limits on how far, in flying time, you can be away from the nearest suitable airport on one engine. ETOPS, Extended range Twin Operations. Before ETOPS came into play (mid 80s I believe) there were similar requirements, but named differently. Started with a 60 minute rule. Since ETOPS rules have steadily increased the the f'ying or rather diversion time to 120, 180, 240 and now even up to 330 minutes.

Note, ETOPS is not about flying over water, it is the flying time to reach the nearest suitable airport. Suitable airport is not a static parameter in the equation. Expected weather is a real factor. There are some routes with limited diversion airports that meet the diversion time requirement. As part of the flight planning you need to take into consideration the expected weather, runway conditions etc. to determine wheter the airport is suitable. If not and if there aren't any alternates you can't take off.

Jet engine reliability has come a very long way. Jet engines are incredibly reliable. Over several decades a huge amount of emperical data has been gathered on engine reliability. Nett result is ever increasing ETOPS diversion times.

Still engine problem do happen, if reading about aviation incidents make you nervous, don't click!: http://avherald.com/h?search_term=en...62&search.y=15

Let there be no mistake. Whether you're flying a 2,3 or 4 engine plane. An engine failure is always a very serious incident. Crews will, in most cases, try and land asap. Pilots are taught to think in scenario's. So if an engines fails, you bring the situation under control, back to stable flight. But then you start thinking through different scenario's. What if something else fails? What options do I have. If you're in a 2-engine plane with one failed engine the options in case of another engine failure are limited.

Some years ago a BA 747 suffered an engine failure a few hours after departing the USA. They chose to continue onto their route to the UK on three engines. They made it, but not to London as they were running low on fuel. Fuel efficiency on three engines is less, because you can't get to the most economic cruising altitudes anymore and you have some extra drag due to assymetrical thrust. To date the debate amongst pilots whether that BA crew made the right call is still ranging. Many feel they should have returend back to the USA, as they could have landed within two hours, rather then to fly at cross the whole ocean.

Weight is a also factor as I pointed out for having four rather then two engines. No so much about reliability, but thrust. If you look at the current max thrust of jet engines such as the GE90 series they can produce some 115.000lb. You'll find them on the 777 Boeing series and they are considered the most powerfull commerical jet engines. If you look at the thrust of a 747-8, which uses a derivate of the GE90 it's thrust is 67.000lb. On the Airbus engine thrust is even higher, close to 80.000lb. four times 67.000lb or four times 80.000 is still a lot more then two times 115.000.

To put it differently, the thrust required to get a Boeing 747 or an Airbus 380 into the air is such that has always required four engines. No two engines, in the past or in the present, have the necessary combined thrust. There are other engine and plane design restriction as well (how big an engine can you still get under a wing?) and commercial considerations as well, what is the market for such a a huge engine? The GE for instance is designed initially for two engine operations and subsequently a slightly smaller and modified version is made for four engine operation. I dont know all the details, but I can well imagine that it is a more economical route than the other way around.

Jeroen

Last edited by Jeroen : 30th June 2014 at 09:11.
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Old 30th June 2014, 21:43   #64
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Default Re: Factors Affecting Reliability

So what's the bottomline to my query?

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Sutripta
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Old 1st July 2014, 06:03   #65
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Default Re: Factors Affecting Reliability

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Originally Posted by Sutripta View Post
So what's the bottomline to my query?
In practical terms; it doesn't matter. 2,3,4 engine planes are incredibly reliable. Although you would have to do a full detailed system calculation, taking the complete system design into consideration, I would think that theoretically the four engine plane comes of better for the reason pointed out before.

As with the NA/TC example, you do need to take into consideration the reliability of the individual plane and engines. I.e. are engines used on 2-engine planes more reliable then engines used on 4-engine planes? But even then I don't think there is much difference between them.

I'm trying to think of (fatal) incident in commercial aviation involving engine out scenario's, but I can't think of any other then the ElAl 747 crashing in the Netherlands many years ago. http://en.wikipedia.org/wiki/El_Al_Flight_1862

I read somewhere that during the ETOPS era, not a single passenger was killed due to engine failures.

So for practical purposes it doesn't seem to matter anymore.

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Old 1st July 2014, 22:36   #66
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Default Re: Factors Affecting Reliability

^^^
No practical difference, but still theoretically where would we stand?
How would the figures play out if the engines were less reliable?

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Old 2nd July 2014, 11:59   #67
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Default Re: Factors Affecting Reliability

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Originally Posted by Sutripta View Post
^^^
No practical difference, but still theoretically where would we stand?
How would the figures play out if the engines were less reliable?

Regards
Sutripta
In theory you will need to do the math;
You need to compare a system with two parallel components to a system with four parallel components. Should be easy to calculate for a given reliabity of X for two engine plane what the reliabilty Y for a four engine plane need to be to be identical, or vice versa, but I'm too lazy for that.

A few other thoughts that you need to factor in as well.

We really need to define what "reliable" means. I've put it in the context that it means, as a result an incident resulting in a crash with fatalities. If we just define it as an engine stopping it becomes very different, obviously. But there is a fundamental difference between a two and a four engine plane having an engine failulure.

Two engine planes will do fine on one engine, even during take off, past the decision point V1, they will still take off. Similar for four engine planes. Not a big problem with one engine less, even during take off.

Even though take off is the toughest flight regime on the engine (highest thrust settings) it only lasts for a very short while, just a few minutes and then thrust is usually reduced for the climb. So planes and therefor engines spend most of their time in the cruise regime. Look at the link I provided on engine out incidents. All of them during cruise, possible during the climb out. So the likelyhood of engine out appears to much higher in cruise. Every flight is multiple hours in cruise versus a few minutes take off and maybe 15-25 mintues climb, 15-30 minutes descending with engines close to idle if well planned and ATC accomodates.

A two engine plane will cruise fine on one engine, that's why the ETOPs rules get longer times. A four engine plane will cruise fine on three engines as well. Both will have a reduced cruising altitude and they will use more fuel due to lower altitude and additional drag due to assymetric thrust that needs compensating. But if the second engine fails, the two engine plane will go down whereas the four engine plane can still fly. Lots of restrictions on how it will handle and what it can do, but still flyable. So in terms of "survivability" of multiple engine failures, obviously the more engines you have the better your chances are.

So really you need to factor that into the reliability calculations as well. So you can see the math getting more complicated. I'm definitely not doing the math. Gives me a headache, just thinking about it.

Finally, I'm not sure if engines on a four engine plane are less reliable. Somebody would have to look that up. But here's my thinking: These days a lot of the engines for four engine planes are derived from engines that were originally designed for two engine planes. So they produce a little less thrust, probably some more modifications, but essentially a down tuned version of the original. Very often that leads to favourable characteristics in terms of reliabillity (e.g. slightly over designed). But this is just guessing on my part. I really don't know.

So if you add that all up and use the definition of reliable as preventing a crash you can see that, at least in theory, engines on a four engine plane can be quite a bit less reliable then engines on a two engine planes to get to the same over reliability. All theory of course, as per the NA/TC discussion any aviation engine gets desgined towards a certain specification that is part of a larger spc (i.e, the whole plane).

In both general aviation as well as commercial aviation engine problems are rare. Fatalities due to engine out are even more rare. Exception to the rule are of course the one engine planes (i.e. the ones I can pilot! And although rare it might happen and that's why I prefer to fly a plane such as the Cirrus that comes with it's own handy parachute, just in case of an engine out scenario.
See http://cirrusaircraft.com/caps/

In fact, OT, but on my next trip to Europe I'll be flying one of these again. Just made the booking yesterday!

Last edited by Jeroen : 2nd July 2014 at 12:06.
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