Hi. Was just wondering whether there are any differences in terms of maintenace costs, reliability issues, costs, fuel efficiency, engine lifespan etc. between aircraft with two or four engines?
When i was growing up, I remember my opa telling me that (in cars), the bigger the engine the longer the lifespan. Does this principal apply to aircraft as well (the two engines on a twin e.g B777 are so much bigger than the quads on equivalents e.g. A343/747)?
In a quad, when one engine flames out, 25% of power is lost while on a twin 50% of power is lost. How reliable does a twin engined aircraft become under these circumstances?
Thanks in advance for any answers.
Difference Between Quads and Twins
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Was just wondering whether there are any differences in terms of maintenace costs, reliability issues, costs, fuel efficiency, engine lifespan etc. between aircraft with two or four engines?
Planes with 4 engines have higher maintenance cost. This is due to the fact that you have top replace 4 time the same component, while on a twin jet you only need to replace 2 components.
The reliability of an engine (by itself) has nothing to do with the number of them installed on the plane. The engines for twin engined planes are different than those on 4 engined ones. They were designed for the different usuage. That's why in my opinion, the reliability has onlt to do with the engine type and manufacturer itself.
That's partially true. But as far as I know (correct me if I'm wrong) Twin's need to be able to climb with only one engine. (In case of an engine failure after V1). When you have 4 engines and one of them fails then you still have 3 engines to climb.In a quad, when one engine flames out, 25% of power is lost while on a twin 50% of power is lost. How reliable does a twin engined aircraft become under these circumstances?
That's why a Twin engine aircraft would have more power than the identical plane with 4 engines. That's also the reason why twin's often do de-rated T/O (when the situation permits it), because they don't need the extra power and can spare the engines.
I hope this answers some of your questions
Greetz
Chris
8)
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Sabena_690
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Chris: although your explanation sounds very logic, I have my doubts honnestly...
Not too long ago, I was talking with an engineer of SR Technics after he was involved in a discussion at airliners.net about maintenance.
I don't know which reasons he gave, but saying that 4 engines causes much more costs etc was not very true, a lot of factors play a role into this.
Are there any engineers on this board who can shed some light onto this?
Frederic
Not too long ago, I was talking with an engineer of SR Technics after he was involved in a discussion at airliners.net about maintenance.
I don't know which reasons he gave, but saying that 4 engines causes much more costs etc was not very true, a lot of factors play a role into this.
Are there any engineers on this board who can shed some light onto this?
Frederic
Brussels Airlines - Flying Your Way
In three years, yesSabena_690 wrote:Are there any engineers on this board who can shed some light onto this?
but saying that 4 engines causes much more costs etc was not very true, a lot of factors play a role into this
I agree that a lot of factors may play a role, but I was only talking about the parts. An engine is an engine. You have the same parts in every engine, and if you have 4 of them you'll have to replace twice more parts as far as I know.
Maybe you have to replace parts more frequently on a twin
, but it would be strange, since I assume it all depends on the cycles flown. It would indeed be great if some engineers or technicians could explain us the real reason. I'm thinking of Lame, or Fiero .....
Greetz
Chris
8)
My opinion as an engineer who graduated 38 years agoAvro wrote:In three years, yesSabena_690 wrote:Are there any engineers on this board who can shed some light onto this?
but saying that 4 engines causes much more costs etc was not very true, a lot of factors play a role into this
:If you have 4 engines, the load on each of them will be lower, so that could last much longer that twins on which a maximum of power is requested at each take-off.
A 5Y-KQV correctly stated, it is the same as on a car. When you look at those American 8-cylinder 5-litre engines running at max 3000 rpm, they will run forever. Same for a plane with four (over-rated) engines.
André
ex Sabena #26567
ex Sabena #26567
Guys thanks a million for the responses so far. They have been very enlightening.
Looks like maintenance of a quad is more expensive as per Avro's posts. I am not sure but I think there are airlines that prefer to have only twin engined planes and some only four engined planes for their longhaul operations. (e.g. KQ go with twins for longhauls while SA are going with quads). If the maintenance costs are twice as much, what is the incentive for airlines to invest in four engined aircraft over twins?
Looks like maintenance of a quad is more expensive as per Avro's posts. I am not sure but I think there are airlines that prefer to have only twin engined planes and some only four engined planes for their longhaul operations. (e.g. KQ go with twins for longhauls while SA are going with quads). If the maintenance costs are twice as much, what is the incentive for airlines to invest in four engined aircraft over twins?
If you would ask this question to Airbus (comparing the a340 and the 777), they'll say a 340 is cheaper because of a hundred of reasons. But the strangest one is the fact that in case of an engine replacement, for a 777 you need to rent an Antonov, because it is hard to get the engine in an other aircraft. So Airbus also includes the price for the rent of this aircraft, the time it takes fore the engine to be replaced (so the 777 will be grounded for a long time), and the profit lost due to the grounded aircraft. So Airbus includes all these in the price comparison! I don't know how credible this explenation is, but it's what airbus says! 8O
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Sabena_690
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One point in favor of the twins is that with only two engines thats half as many things that can break. Remember that much of the "maintenance cost" is actually the loss of revenue when the aircraft is on the ground. With less engines (considering the reliability for each is more or less the same) thats half as many unexpected maintenance events.
Half the engines, half the problems (too bad you can't dispatch a quad-engine plane with one engine inop ).
In terms of fuel efficiency, for the most part the larger the engine the better (lower) the SFC (lb/h of fuel per lb of thrust).
Peter
EDIT: Also, this:
Half the engines, half the problems (too bad you can't dispatch a quad-engine plane with one engine inop
).In terms of fuel efficiency, for the most part the larger the engine the better (lower) the SFC (lb/h of fuel per lb of thrust).
Peter
EDIT: Also, this:
Is way off. Twins are designed so they can climb on one engine (as previously mentioned). Quads are designed to climb on three engines. This means engines are operating much closer to maximum rated power on a quad then on a twin during a normal takeoff.If you have 4 engines, the load on each of them will be lower, so that could last much longer that twins on which a maximum of power is requested at each take-off.
Hi all, finally I managed to get here upon recommendation by Sabena 690! 
First of all, I work in a CFM56 overhaul facility supporting CFM56-5C overhauls technically from an engineering point of view.
Now to the topic....
Also it seems to be important to me to mention that a twin engine, GE90 for example, is always bigger than it's quad rival (CFM56-5C or Trent 500). This has as a consequence, that for examples the test cell required to test the engine needs to be much larger than for a smaller Trent 500 or CFM56-5C engine. This means that the test cell is more expensive tu run and consequently the maintenance costs are increasing. This is also valid when it comes to the piece parts. The larger parts require bigger handlin and repair facilities that also increase maintenance costs.....
As you can see, there is no such simple equation that says half the amount of engines equals half the amount of maintenance cost. Even Rolls Royce agrees to that....
http://www.rolls-royce.com/civil_aerosp ... efault.jsp
Of course also operational parameters play a part in this "game" but there I can't provide any insight as I'm not familiar with operations.....
Not to forget about the operators maintenance policies that also can drives cost high. Let's imagine the following scenario: An operator doesn't comply with Service Bulletins to improve reliability of the engine, in order to save money. Then he might save the money at first, but if heis fleet suffers an IFSD due to such a "saving", the saving might become a boomerang. In that case (especially if an ETOPS aircraft is involved) it is possible that the authority will request the operator to modify his fleet as soon as possible. Consequence is, additionally to the costs of the IFSD and diversion, the operator has to pay for the modification and also might have to have additional costs for lease engines to support a roll over programme to get his fleet modified......
I hope I could give you a brief overview to show you, that maintenance costs are quite a complex kind of thing.......
Cheers, Thomas
First of all, I work in a CFM56 overhaul facility supporting CFM56-5C overhauls technically from an engineering point of view.
Now to the topic....
It isn't that simple when you look into details, and those details cost the money! For example, a CFM56-5C powering the A340 family consists of 1 stage high pressure turbine and 5 stages low pressure turbine (of course there are also comprsssors and other hardware involved, but the turbine and combustor are the "hot" parts and those parts are resposible for the main share when it comes to maintenance cost.) while the GE 90-94B powering the 777 has 2 stages high pressure turbine and 6 stages low pressure turbine. Usually, low pressure turbines are not that frequestly overhauled and therefore the difference in costs overhauling them is not that significant for the whole picture in my opinion. If we limit the focus on the high pressure turbine, there you see that even not comparing the "price tag" involved, the number of stages is double on the twin side, so this makes up the "advantage" that a twin has to a quad on the cost base. Of course there are more important issues to consider, but I would just stick to this simple example to illustrate that the quoted statement is not true per se.Planes with 4 engines have higher maintenance cost. This is due to the fact that you have top replace 4 time the same component, while on a twin jet you only need to replace 2 components.
Also it seems to be important to me to mention that a twin engine, GE90 for example, is always bigger than it's quad rival (CFM56-5C or Trent 500). This has as a consequence, that for examples the test cell required to test the engine needs to be much larger than for a smaller Trent 500 or CFM56-5C engine. This means that the test cell is more expensive tu run and consequently the maintenance costs are increasing. This is also valid when it comes to the piece parts. The larger parts require bigger handlin and repair facilities that also increase maintenance costs.....
As you can see, there is no such simple equation that says half the amount of engines equals half the amount of maintenance cost. Even Rolls Royce agrees to that....
http://www.rolls-royce.com/civil_aerosp ... efault.jsp
Of course also operational parameters play a part in this "game" but there I can't provide any insight as I'm not familiar with operations.....
Not to forget about the operators maintenance policies that also can drives cost high. Let's imagine the following scenario: An operator doesn't comply with Service Bulletins to improve reliability of the engine, in order to save money. Then he might save the money at first, but if heis fleet suffers an IFSD due to such a "saving", the saving might become a boomerang. In that case (especially if an ETOPS aircraft is involved) it is possible that the authority will request the operator to modify his fleet as soon as possible. Consequence is, additionally to the costs of the IFSD and diversion, the operator has to pay for the modification and also might have to have additional costs for lease engines to support a roll over programme to get his fleet modified......
I hope I could give you a brief overview to show you, that maintenance costs are quite a complex kind of thing.......
Cheers, Thomas
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Sabena_690
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