2 Engines and 4 Engines
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2 Engines and 4 Engines
Hello,
I read on the other forums on PPRuNe that Virgin Atlantic will order 26 Airbus A340s. If you haven't read it, this is the link to the thread - http://www.pprune.org/forums/showthr...hreadid=140132
There's something that I'm a bit confused about and this might be a stupid question to some of you. I'd like to know what's the difference between two engines and four engines as Virgin Atlantic chose the A340 becaues it had 4 engines which the same size 777 didn't. Does the number of engines have something to do with enhancing a/c performance such as better climb performance, cruise, approach, landing etc.?
I remembered from memory I read on Aircraft & Aerospace May Edition I think something about 2 vs. 4 engines and the number of accidents between them. If anyone has the time to explain to me, It'd be greatly appreciated. Wouldn't mind knowing about 3 Engine a/cs, 1 engine a/cs, etc. too. Thank you
Kind Regards,
Capt. J (Still a newbie
)
I read on the other forums on PPRuNe that Virgin Atlantic will order 26 Airbus A340s. If you haven't read it, this is the link to the thread - http://www.pprune.org/forums/showthr...hreadid=140132
There's something that I'm a bit confused about and this might be a stupid question to some of you. I'd like to know what's the difference between two engines and four engines as Virgin Atlantic chose the A340 becaues it had 4 engines which the same size 777 didn't. Does the number of engines have something to do with enhancing a/c performance such as better climb performance, cruise, approach, landing etc.?
I remembered from memory I read on Aircraft & Aerospace May Edition I think something about 2 vs. 4 engines and the number of accidents between them. If anyone has the time to explain to me, It'd be greatly appreciated. Wouldn't mind knowing about 3 Engine a/cs, 1 engine a/cs, etc. too. Thank you
Kind Regards,
Capt. J (Still a newbie
)
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Gee, how couldn't I have thought of that? Sorry for my stupid post! Of course! If one engine fails, you still have 3, if 2 fail, you still have 2! On a 2 engine plane, if 1 fails, you're left with 1, if that one fails you're left with NONE!
Thanks for even considering my post. It's greatly appreciated!
Cheers!
Thanks for even considering my post. It's greatly appreciated!Cheers!
There was once a four engined airliner, the Stratocruiser I think, not sure.
Anyway, it had four radial engines. Each engine had four banks of cylinders. The rear bank was highly susceptable to over heating due to not getting enough cooling air. Also, it struggled to maintain height with one engine out! It was essentially a single with four times the chance of an engine failure. An aircraft like that wouldn't get certified these days, but it just goes to show that more isn't necessarily better.
Anecdote provided by an Ernest Gann (sp?) book via many years residing in my foggy memory, any untruths or mistakes have been introduced during the years-of-foggy-memory rather than coming from the book's author.
Anyway, it had four radial engines. Each engine had four banks of cylinders. The rear bank was highly susceptable to over heating due to not getting enough cooling air. Also, it struggled to maintain height with one engine out! It was essentially a single with four times the chance of an engine failure. An aircraft like that wouldn't get certified these days, but it just goes to show that more isn't necessarily better.
Anecdote provided by an Ernest Gann (sp?) book via many years residing in my foggy memory, any untruths or mistakes have been introduced during the years-of-foggy-memory rather than coming from the book's author.
Actually Capt. J, it's not really that stupid a question. Boeing and Airbus can't agree which is the better way to go ie. A340-500/600 vs. 777-200/300ER.
ETOPS (Extended Twin Ops) is a very complex field and there's no easy answer to your question. Some of the things to consider:
*ETOPS aircraft have to demonstrate greater reliabilty than 4-eng aircraft with respect to engine and critical systems failures. Theoretically, this makes them as safe as four engine aeroplanes. Whether it does or not is open to debate. Things to consider are not only the number of engines but the availability of hydraulic and pneumatic systems, electrical power etc.
* Twin engine aircraft have to have more powerful engines than four engine aeroplanes. On takeoff an engine failure on a 4-eng aeroplane - lose 1/4 of your thrust, on a two engine aeroplane - lose 1/2. That two (now one) engine aeroplane still has to meet the minimum required performance gradients. This requires more powerful and thus heavier engines. This tends to impact negatively on range.
Conversely, because the engines are more powerful when both engines are working the twin engine aircraft, generally speaking, has bucketloads of excess thrust and thus performance, often better than 4-engine aeroplanes.
4 engine aeroplanes, however, require greater maintenance and this increases costs so it's a tradeoff.
* Control problems tend to be minimised on two engine aeroplanes as the thrust lines of the engines tend to be closer to the longitudinal axis of the aeroplane. This generally requires smaller rudder and vertical stabiliser assemblies than on four engine aircraft - tending to reduce weight.
*Certainly, if two engines fail, you're in a better position in a four engine aeroplane than a two. The aircraft is quite controllable. Two engine ceiling is up to about FL220 depending on weight. Performance leaves a lot to be desired but the aircraft will fly and can be handled quite safely. Lots of considerations but not too bad.
ETOPS (Extended Twin Ops) is a very complex field and there's no easy answer to your question. Some of the things to consider:
*ETOPS aircraft have to demonstrate greater reliabilty than 4-eng aircraft with respect to engine and critical systems failures. Theoretically, this makes them as safe as four engine aeroplanes. Whether it does or not is open to debate. Things to consider are not only the number of engines but the availability of hydraulic and pneumatic systems, electrical power etc.
* Twin engine aircraft have to have more powerful engines than four engine aeroplanes. On takeoff an engine failure on a 4-eng aeroplane - lose 1/4 of your thrust, on a two engine aeroplane - lose 1/2. That two (now one) engine aeroplane still has to meet the minimum required performance gradients. This requires more powerful and thus heavier engines. This tends to impact negatively on range.
Conversely, because the engines are more powerful when both engines are working the twin engine aircraft, generally speaking, has bucketloads of excess thrust and thus performance, often better than 4-engine aeroplanes.
4 engine aeroplanes, however, require greater maintenance and this increases costs so it's a tradeoff.
* Control problems tend to be minimised on two engine aeroplanes as the thrust lines of the engines tend to be closer to the longitudinal axis of the aeroplane. This generally requires smaller rudder and vertical stabiliser assemblies than on four engine aircraft - tending to reduce weight.
*Certainly, if two engines fail, you're in a better position in a four engine aeroplane than a two. The aircraft is quite controllable. Two engine ceiling is up to about FL220 depending on weight. Performance leaves a lot to be desired but the aircraft will fly and can be handled quite safely. Lots of considerations but not too bad.
Last edited by DirectAnywhere; 6th Aug 2004 at 22:07.
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Thanks for the reply DirectAnywhere. Very good explanation (although I still have lots to learn from you guys) It's greatly appreciated!
Once again, I say thanks to all replies
Kind Regards,
Capt. J - Wannabe
Once again, I say thanks to all replies
Kind Regards,
Capt. J - Wannabe
As Branson put it .... 'QUADS do it better'
Just remember with any aircraft the greater the number of engines you have the more likely you are to have an engine failure... 4 engines.. four times as likely to have an engine failure... more true with piston engines however because of the reliability of a modern Jet engine compared to a pistion but the law of probability still apply.
The only thing you really have to consider is the amount of engines which require overhauls and the greater amounts of fuel you will burn based on ETOPS considerations......... What will be more expensive in the long run?
Just remember with any aircraft the greater the number of engines you have the more likely you are to have an engine failure... 4 engines.. four times as likely to have an engine failure... more true with piston engines however because of the reliability of a modern Jet engine compared to a pistion but the law of probability still apply.
The only thing you really have to consider is the amount of engines which require overhauls and the greater amounts of fuel you will burn based on ETOPS considerations......... What will be more expensive in the long run?
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Bula, interesting comment you made "the greater the number of engines you have the more likely you are to have an engine failure" but you are not correct. The mathematics of probabilities - rather like the old question "Is the glass half full or half empty".
The risk of engine failure does not increase in proportion to the number of engines, as you imply. The risk of an engine failure on a four engine aircraft is no greater than the risk of engine failure on a single engine aircraft.
It's the number of engines you have left after an engine failure that makes life interesting.
And Desert Duck is right on the money. A PC12 would have infinitely less chance of engine failure than a Heron (particularly with Gipsy IV engines). But then the mathematics of probabilities begs the question: Is a PC12 safer than a Heron?
Back to your original question - two factors to consider, the power available from present technology engines (less than 100,000 pounds thrust maximum) and required to move, for example 500 passengers; and the possible additional flight time and cost required for twin engine ETOP operations.
Horses for courses..........
Woomera
The risk of engine failure does not increase in proportion to the number of engines, as you imply. The risk of an engine failure on a four engine aircraft is no greater than the risk of engine failure on a single engine aircraft.
It's the number of engines you have left after an engine failure that makes life interesting.
And Desert Duck is right on the money. A PC12 would have infinitely less chance of engine failure than a Heron (particularly with Gipsy IV engines). But then the mathematics of probabilities begs the question: Is a PC12 safer than a Heron?
Back to your original question - two factors to consider, the power available from present technology engines (less than 100,000 pounds thrust maximum) and required to move, for example 500 passengers; and the possible additional flight time and cost required for twin engine ETOP operations.
Horses for courses..........
Woomera
4 times the components... four times the chance of one of them giving up...... well thats my thought...
Woomera.... um to use an old paying.. "please explain"
I guess you could say that this is one of those things that everyone will never agree on.... no matter what mathmatics you put behind it
Woomera.... um to use an old paying.. "please explain"
I guess you could say that this is one of those things that everyone will never agree on.... no matter what mathmatics you put behind it
Bula,
Woomera is correct, a modern engine is less likely to fail than an older one.
E.g. a PA-31 is more likely to have a double engine failure than a PC12 is to have a single engine failure.
However, if you have two or four engines from the same era, well then maths is complicated, as the mean time between failures (MTBF) is not consistent.
The MTBF tends to be higher as an engine is introduced into service, and as the engine becomes more widely used, the statistical base improves (more engines fly more hours), as does the engineering fixes to problems that reduced the MTBF when the engine was initially introduced.
Many people were suggesting that two engine aircraft were safer than a four engine due to the engineering practices employed when flying ETPOS, independent inspections, different maintenance periods etc.
However many airlines have seen the benefits of these inspection regimes, and have since employed the same techniques on the four engine fleets.
Others suggest that a two engine aircraft like the A330/B777 is safer than an four engine, as a two engine aircraft is required to have longer fire suppression than say a B744 which does not need to as its not ETOPS. This does not apply to the A340 as its has the same equipment as the ETOPS A330.
Others suggest that the number of engines is directly correlated to the number of systems or performance the aircraft will loose, this is also incorrect.
Woomera is correct, a modern engine is less likely to fail than an older one.
E.g. a PA-31 is more likely to have a double engine failure than a PC12 is to have a single engine failure.
However, if you have two or four engines from the same era, well then maths is complicated, as the mean time between failures (MTBF) is not consistent.
The MTBF tends to be higher as an engine is introduced into service, and as the engine becomes more widely used, the statistical base improves (more engines fly more hours), as does the engineering fixes to problems that reduced the MTBF when the engine was initially introduced.
Many people were suggesting that two engine aircraft were safer than a four engine due to the engineering practices employed when flying ETPOS, independent inspections, different maintenance periods etc.
However many airlines have seen the benefits of these inspection regimes, and have since employed the same techniques on the four engine fleets.
Others suggest that a two engine aircraft like the A330/B777 is safer than an four engine, as a two engine aircraft is required to have longer fire suppression than say a B744 which does not need to as its not ETOPS. This does not apply to the A340 as its has the same equipment as the ETOPS A330.
Others suggest that the number of engines is directly correlated to the number of systems or performance the aircraft will loose, this is also incorrect.
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SWH is on the money - but, um, how do I explain the mathematics of probability in relation to multi engine aircraft engine failures.......? 
Here's a try......
The PT6A-114 installed in Cessna 208 Caravans and the PT6A-67 installed in the PC12 have exhibited engine failure rates less than 1 in 100,000. In effect, assuming all maintenance requirements are complied with, an engine failure should not occur in less than 100,000 hours of fleet time. It would be true to say that the greater the world wide fleet size, the greater the chance of engine faulure, solely because there are more examples of the type.
Mathematically, the C208 and PC12 should be more reliable than (for example) the Heron, with four Gypsy engines (or four Lycoming engines if modified), which do not meet the same failure testing criteria.
Similarly, with advances in engine technology and reliability, it would probably be reasonable to assume the chances of engine failure in an older technology Boeing 707, would be greater than the chances of engine failure in a late model Boeing 747.
Back to the mathematics of probability: If you were comparing a Boeing 767 (2 engines) with a Boeing 747 (4 engines), the chances of an engine failure on the 767 are probably identical to the chances of an engine failure on the 747.
Because the 747 has double the number of engines, does not double the chance of engine failure.
As I said above, it's not how many engines you have - it's how many are left after one engine fails......
And that's probably why the 146 has five engines..........
I know I haven't explained this very well, but, hey, it's early Sunday morning..........
Woomera
Here's a try......
The PT6A-114 installed in Cessna 208 Caravans and the PT6A-67 installed in the PC12 have exhibited engine failure rates less than 1 in 100,000. In effect, assuming all maintenance requirements are complied with, an engine failure should not occur in less than 100,000 hours of fleet time. It would be true to say that the greater the world wide fleet size, the greater the chance of engine faulure, solely because there are more examples of the type.
Mathematically, the C208 and PC12 should be more reliable than (for example) the Heron, with four Gypsy engines (or four Lycoming engines if modified), which do not meet the same failure testing criteria.
Similarly, with advances in engine technology and reliability, it would probably be reasonable to assume the chances of engine failure in an older technology Boeing 707, would be greater than the chances of engine failure in a late model Boeing 747.
Back to the mathematics of probability: If you were comparing a Boeing 767 (2 engines) with a Boeing 747 (4 engines), the chances of an engine failure on the 767 are probably identical to the chances of an engine failure on the 747.
Because the 747 has double the number of engines, does not double the chance of engine failure.
As I said above, it's not how many engines you have - it's how many are left after one engine fails......
And that's probably why the 146 has five engines..........
I know I haven't explained this very well, but, hey, it's early Sunday morning..........
Woomera
Last edited by Woomera; 8th Aug 2004 at 02:16.
A four-engined airliner was flying along when the captain announced "sorry folks, we have an engine failure; don't worry, there are 3 left but our trip will be a little longer than scheduled".
The passengers were a little concerned initially but soon settled down.
A while later, the captain comes on again and says "sorry folks, another engine has failed...again, no problems because these things are designed to handle such occurrences, but our trip will now take an extra hour."
Then lo and behold, after another few minutes, the captain announces "Sorry ladies and gentlemen, number 3 engine has just failed. Now don't worry because we can fly quite well on only one engine, but now it's going to take us just that bit longer to reach the destination."
The passengers are now a bit concerned, and Paddy turns to Fergus and says "Begorrah, I hope the fourth engine doesn't fail, otherwise we'll be up here all day!"
Boom boom
The passengers were a little concerned initially but soon settled down.
A while later, the captain comes on again and says "sorry folks, another engine has failed...again, no problems because these things are designed to handle such occurrences, but our trip will now take an extra hour."
Then lo and behold, after another few minutes, the captain announces "Sorry ladies and gentlemen, number 3 engine has just failed. Now don't worry because we can fly quite well on only one engine, but now it's going to take us just that bit longer to reach the destination."
The passengers are now a bit concerned, and Paddy turns to Fergus and says "Begorrah, I hope the fourth engine doesn't fail, otherwise we'll be up here all day!"
Boom boom
Fair enough...... going have to mull over that for for a little while
In general on the whole yes true. compare the rates of failure among the engine type and previous experience and the chances of having a failure are still slim to none... I've got that one.
I think i need to hit my head a bit longer and i might get some sense out of it..... or look at my year 12 maths again ... (I dont think that will be happening any time soon hehe) to get around a single aircraft example.. just that aircraft by itself having twice the chance of a failure........ .....
BANG BANG BANG BANG... still aint helping... time to talk to the man upstairs for some wisdom
In general on the whole yes true. compare the rates of failure among the engine type and previous experience and the chances of having a failure are still slim to none... I've got that one.
I think i need to hit my head a bit longer and i might get some sense out of it
..... or look at my year 12 maths again ... (I dont think that will be happening any time soon hehe) to get around a single aircraft example.. just that aircraft by itself having twice the chance of a failure........ ..... BANG BANG BANG BANG... still aint helping... time to talk to the man upstairs for some wisdom
Perhaps this viewpoint will help:
The probability of failure that gets quoted is a fleet average (or possibly a median) for ALL engines of that type in use. This means that some engines will fail after a shorter time in service, some will fail later etc. In other words the reliability is probability, not a guarantee, and it doesn't predict any particluar engine's failure time.
Whether any particular engine is mounted on your wing, or on someone elses , doesn't change the likelyhood of it failing**. It's no more likely to fail just because engine #3 or #4 was added to the clutch of engines on your airframe, than if it was left running somewhere else. Since the overall chance of failure hasn't changed by moving engines to your wing, you can't be at a higher risk of failure by having more of them.
**provided extraneous factors aren't a consideration eg poor maintenance etc.
The probability of failure that gets quoted is a fleet average (or possibly a median) for ALL engines of that type in use. This means that some engines will fail after a shorter time in service, some will fail later etc. In other words the reliability is probability, not a guarantee, and it doesn't predict any particluar engine's failure time.
Whether any particular engine is mounted on your wing, or on someone elses , doesn't change the likelyhood of it failing**. It's no more likely to fail just because engine #3 or #4 was added to the clutch of engines on your airframe, than if it was left running somewhere else. Since the overall chance of failure hasn't changed by moving engines to your wing, you can't be at a higher risk of failure by having more of them.
**provided extraneous factors aren't a consideration eg poor maintenance etc.
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Well I was trying to keep out of this, but I cannot take any more.
There MUST be more chance of AN engine failure on a 4 engined aircraft, than a 2 engined aircraft, PARTICULARLY if they have the same engine type.
Okay, on a 4 you have 3 left, where on a 2 you are down to only 1.
NOT saying which is safer, or which I prefer, but there MUST be more chance of AN engine failure on a 4 engined aircraft than a 2.
There would be much LESS chance of losing ALL engines on a 4, as compared to the 2 on a twin, but MORE chance of losing AN engine.
To me it is a matter of the odds.
IF you had say 400 pax on one aircraft, and 200 pax on another aircraft, (assuming there were all in similar condition, like the engines are in similar condition to each other) the odds of ONE passenger having say a heart attack would be greater on the aircraft with 400 pax, however the odds of EVERY pax having a heart attack would be less on the one with 400 pax.
There MUST be more chance of AN engine failure on a 4 engined aircraft, than a 2 engined aircraft, PARTICULARLY if they have the same engine type.
Okay, on a 4 you have 3 left, where on a 2 you are down to only 1.
NOT saying which is safer, or which I prefer, but there MUST be more chance of AN engine failure on a 4 engined aircraft than a 2.
There would be much LESS chance of losing ALL engines on a 4, as compared to the 2 on a twin, but MORE chance of losing AN engine.
To me it is a matter of the odds.
IF you had say 400 pax on one aircraft, and 200 pax on another aircraft, (assuming there were all in similar condition, like the engines are in similar condition to each other) the odds of ONE passenger having say a heart attack would be greater on the aircraft with 400 pax, however the odds of EVERY pax having a heart attack would be less on the one with 400 pax.