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Originally Posted by Aroy 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 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 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