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Thrust
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Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 9:49 pm

Just a little curious as to different advantages that a quadra-jet offers over a twin. THe only things I've heard about four engines vs. two are generally bad...four engined jets take longer to get a plane up to speed because of a smaller thrust to weight ratio, they burn more fuel, and they make for a poorer climb rate. My only thinking is that something about the wing has to make up for these disadvantages...something has to justify keeping the A340 in the air while the 777, a mostly superior aircraft, is still flying.
 
A342
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 10:07 pm

Car nuts will tell you that "Quadrajet" is a type of carburetor made by GM. Please note that I am not a car nut.  cheerful  The more common name is simply "quads".

Well, you see that very large aircraft, like the A380, are still being designed with four engines. Using just two would be possible, but it would difficult.

I don't quite understand the following statement...

Quoting Thrust (Thread starter):
...something has to justify keeping the A340 in the air while the 777, a mostly superior aircraft, is still flying.

 
vikkyvik
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 10:19 pm



Quoting Thrust (Thread starter):
four engined jets take longer to get a plane up to speed because of a smaller thrust to weight ratio, they burn more fuel, and they make for a poorer climb rate.

While it's true that twins are required to have more thrust if everything else is the same, that's not necessarily a good thing. Basically, you now have more thrust than you actually need, which means (probably) larger, heavier engines, and/or more fuel usage than would otherwise be required.

An airplane like the A340, with 4 engines, can mount four hairdryers on the wing, and be perfectly happy and economical on long-range cruises.

Additionally, and importantly, twins have to meet ETOPS specs, which are more rigorous than those required for tris or quads (although I hear this may be changing...?). But maintaining ETOPS certification costs money.

Quoting Thrust (Thread starter):
something has to justify keeping the A340 in the air while the 777, a mostly superior aircraft, is still flying.

I would guess that the fact that there are A340's flying, quite clearly justifies that they are good at something - perhaps better than the 777. You can't really make a blanket statement saying one airplane is superior to another.
 
roseflyer
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 10:28 pm



Quoting Thrust (Thread starter):
.four engined jets take longer to get a plane up to speed because of a smaller thrust to weight ratio, they burn more fuel, and they make for a poorer climb rate.

Poorer climb performance and longer takeoff rolls are not a very huge issue for the majority of airports and airlines. From a fuel burn perspective, you want to have the minimum amount of thrust necessary. Twins have to have more thrust because they need to be able to take off in a one engine out configuration. There is a reason why most takeoffs are derated. Lighter weight engines optimized more for cruise flight rather than single engine takeoffs are advantageous.
 
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Scooter01
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 10:28 pm

If a "quad" loses the use of one engine at any time in flight, it still has 75% of enginepower left, but if a twin-jet loses one, it only has 50% left

Scooter01
 
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Buyantukhaa
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 10:53 pm



Quoting Thrust (Thread starter):
something has to justify keeping the A340 in the air while the 777, a mostly superior aircraft, is still flying.

Besides the ETOPS issue, there is something else:

Quoting Thrust (Thread starter):
.four engined jets take longer to get a plane up

In some hot and high airports (JNB for example), four-engined planes have an advantage because of the one-engine-out scenario: if one engine goes down, performance will be so restricted that it reduces the allowed payload, and the quad does not suffer nearly as much. That may have been a reason (I am merely speculating) that SAA, a long-time B747 operator, chose the A340 over the B777.
 
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Starlionblue
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Nov 24, 2008 11:50 pm

Some points, many repeated from above.:
- Quads can have less total power.
- Quads can have much smaller engines. Very large engines are expensive to buy. Then again they can be made very efficient.
- Redundancy considerations such as ETOPS, though AFAIK the same rules will eventually apply to all aircraft, twin, triplet och quad.
- Given the extraordinary reliability of current engines, two can actually be better than one from a dispatch standpoint. Two engines give only half the number of possible faults from a maintenance standpoint.

Quoting BuyantUkhaa (Reply 5):
In some hot and high airports (JNB for example), four-engined planes have an advantage because of the one-engine-out scenario: if one engine goes down, performance will be so restricted that it reduces the allowed payload, and the quad does not suffer nearly as much. That may have been a reason (I am merely speculating) that SAA, a long-time B747 operator, chose the A340 over the B777.

Pretty likely indeed. The 340 has a big advantage in places like JNB.
 
tdscanuck
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RE: Advantages Of A Quadra-jet Over A Twin

Tue Nov 25, 2008 12:56 am



Quoting Vikkyvik (Reply 2):
Additionally, and importantly, twins have to meet ETOPS specs, which are more rigorous than those required for tris or quads (although I hear this may be changing...?).

They already do, or at least the regulations are in place. I'm not sure of the implementation date. ETOPS regulations will apply to all ETOPS flights, not just twins.

So it will still be expensive to maintain ETOPS cert, but there will be no cost advantage to the tri/quadjets.

Tom.
 
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RE: Advantages Of A Quadra-jet Over A Twin

Tue Nov 25, 2008 1:50 am



Quoting Vikkyvik (Reply 2):

An airplane like the A340, with 4 engines, can mount four hairdryers on the wing, and be perfectly happy and economical on long-range cruises.

Perfectly? Not quiteeeee....

Quoting Vikkyvik (Reply 2):
But maintaining ETOPS certification costs money.

upfront, but (when combined with other factors) saves money in the longrun... else, he majority of longrange ops wouldn't be shifting to twinjets, and you'd see more quads on the drawing board.
 
SCAT15F
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RE: Advantages Of A Quadra-jet Over A Twin

Wed Nov 26, 2008 5:14 pm



Quoting Thrust (Thread starter):
four engined jets take longer to get a plane up to speed because of a smaller thrust to weight ratio... and they make for a poorer climb rate

With the exception of the 747SP!
 
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SEPilot
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RE: Advantages Of A Quadra-jet Over A Twin

Wed Nov 26, 2008 7:53 pm

The fact that a quad requires less total thrust is counteracted by the fact that jet engines actually become more efficient as they get larger, and the drag of four small engines is greater than that of two large ones. Therefore if you can do with two engines it is almost always more efficient than four, even with the extra thrust required on the two. The fact that the 777 has outsold the A340 by a wide margin is good evidence for this, as it is difficult to find two airliners with as similar capacities, the main difference being one is a twin and one is a quad. The A380 and 748 remain quads because at present technology is not there to build an engine big enough to power them with two. But I have no doubt that eventually it will. Safety is a completely separate issue; I have a long running dispute with Zeke, who prefers flying over big oceans with four engines, while I maintain that the accident statistics actually indicate that you have a greater chance of an engine induced crash with a quad than with a twin. Side note; Zeke actually puts his butt on the line on this issue, while mine is seated at my desk. This comes from my research that shows that there have been exactly the same number of engine induced crashes on airliners with three or four engines as there have been in twins, and since far more twins are flying than tris and quads combined, that leads the the conclusion that the chance of an engine causing a crash is actually greater the more engines you have (as long as you have at least two.) The simple fact is that the single most likely event that can cause an engine to bring a plane down is an uncontained failure causing other damage, and the chance of this is proportional to the number of engines. No jetliner, to the best of my knowledge, has ever suffered two unrelated engine failures on the same flight, ever. And that goes back to the beginning of the jet age, when engines were far less reliable then they are now.
 
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RE: Advantages Of A Quadra-jet Over A Twin

Wed Nov 26, 2008 10:53 pm



Quoting SCAT15F (Reply 9):
Quoting Thrust (Thread starter):
four engined jets take longer to get a plane up to speed because of a smaller thrust to weight ratio... and they make for a poorer climb rate

With the exception of the 747SP!

The MD-11 has a higher thrust to weight ratio than the 747SP and the DC-10-30 and 747SP are roughly equal (varies slightly depending on engine option).
 
OldAeroGuy
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 12:58 am



Quoting SEPilot (Reply 10):
The fact that the 777 has outsold the A340 by a wide margin is good evidence for this, as it is difficult to find two airliners with as similar capacities, the main difference being one is a twin and one is a quad.

The actual score through the end of Oct. 2008 is (grouped by similar performance capability):

A342/3: 246
772ER: 434

A346: 102
773ER: 392


A345: 34
772LR: 49

The high, hot performance advantages of a Quad are not much of a primary selling point unless your main hub happens to be in such a location and you need to fly long range.
 
2H4
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 1:09 am

Quoting Thrust (Thread starter):
Just a little curious as to different advantages that a quadra-jet offers over a twin.

Grace, elegance, and awe.

Well, that's what I experienced earlier today in Ann Arbor, Michigan, as an old DC-8 whispered overhead during her descent into YIP.   

2H4

[Edited 2008-11-26 17:12:55]
 
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Scooter01
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 1:24 am



Quoting 2H4 (Reply 13):
Grace, elegance, and awe.

Well, that's what I experienced earlier today in Ann Arbor, Michigan, as an old DC-8 whispered overhead during her descent into YIP.

- now we are getting into "the enjoyment from watching" factor...

Scooter01  biggrin 
 
Mir
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 3:49 pm



Quoting Thrust (Thread starter):
they make for a poorer climb rate.

Actually, they make for a better climb rate where it counts - with one engine inoperative. A quad will generally have fewer takeoff restrictions than a twin when there are terrain issues involved.

-Mir
 
Thrust
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 9:48 pm



Quoting A342 (Reply 1):

The 777 and the A340 are essentially aimed at the same markets. Both planes have roughly the same range, although the 777LR models have surpassed the A345...both have roughly the same passenger capacity (this is comparing the 777-300ER to the A346 and the 772ER to the A343), but the 777 is far more fuel-efficient due to its being powered by two less engines.
 
Thrust
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 10:13 pm

The thing I do know is that a lot of four engined jets have traditionally been given more highly swept back wings for higher speeds, while twins I guess only need a moderate to conservative sweep due to I guess having more excess thrust available.
 
Northwest727
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 10:20 pm



Quoting 2H4 (Reply 13):
old DC-8 whispered overhead

Depending on what series of DC-8 you are talking about!  Smile
 
David L
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 10:32 pm



Quoting Scooter01 (Reply 4):
If a "quad" loses the use of one engine at any time in flight, it still has 75% of enginepower left, but if a twin-jet loses one, it only has 50% left

That's percentage of maximum power available, though, not percentage of power required to continue a take-off or stay in the air... hence the reason twins start off with more excess power in the first place.
 
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RE: Advantages Of A Quadra-jet Over A Twin

Thu Nov 27, 2008 10:34 pm



Quoting Thrust (Reply 17):
The thing I do know is that a lot of four engined jets have traditionally been given more highly swept back wings for higher speeds, while twins I guess only need a moderate to conservative sweep due to I guess having more excess thrust available.

Actually, I think it has more to do with advances in airfoil design that enable newer design to achieve higher optimum speeds with less sweep than older designs. Most 4 engine designs are 40 or 50 year old designs; the only recent ones are the A340 and A380. Witness the A380; it has much less sweep than the 747, yet cruises nearly as fast, and the A340 also has much less sweep but cruises faster than the 707 or DC-8 which both, I believe, had more sweep. Two or four engines do not affect it; the point is that the aerodynamics, not the thrust, determine the optimum cruising speed for each design. They can all fly faster, but at the cost of greatly increased fuel consumption.
 
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 12:37 am



Quoting SEPilot (Reply 20):
and the A340 also has much less sweep but cruises faster than the 707 or DC-8

Actually, the maximum operating speed of the 707 and DC-8 is higher than the A340 according to the FAA certification data sheets.

707 - Mach 0.887
DC-8 - Mach 0.88
A340 - Mach 0.86
 
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DocLightning
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 2:39 am



Quoting Vikkyvik (Reply 2):

An airplane like the A340, with 4 engines, can mount four hairdryers on the wing, and be perfectly happy and economical on long-range cruises.

Except the 777 is still more economical, even with its enormous engines.

But when the A340 was introduced, the 777 wasn't yet flying and both airlines and the public were still a bit skeptical about ETOPS. People weren't entirely ready to trust their lives to a twin out over the middle of the Pacific. The Atlantic was different because you almost never actually fly over open water on US-Europe routes. Now that it's been almost 15 years since the 777 was launched and the 777 has proved itself on transpac routes, quads are simply not in the future of aviation.

The ONLY thing a quad can do that a twin can't is fly over Antarctica (and a tiny patch of South Pacific off the coast of S. America). So QF, AR, LA, NZ might have a use for quads for Australiasia/S. America routes, but the rest of the world doesn't need quads anymore.

The only other thing that you can do with four engines is make a really big, honkin' plane like the A380.

So this is why the twin is the way of the future.

Why are A340's still flying? Because you don't throw a perfectly plane out just because it's a touch obsolete. You had to pay for the plane and before you replace it, it needs to pay for itself. It will still pay for itself, just not quite as quickly as a newer twin. But you already own it, so shelling out the money for a newer twin isn't the best decision.
 
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Starlionblue
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 4:34 am



Quoting DocLightning (Reply 22):
Except the 777 is still more economical, even with its enormous engines.

But when the A340 was introduced, the 777 wasn't yet flying and both airlines and the public were still a bit skeptical about ETOPS. People weren't entirely ready to trust their lives to a twin out over the middle of the Pacific. The Atlantic was different because you almost never actually fly over open water on US-Europe routes. Now that it's been almost 15 years since the 777 was launched and the 777 has proved itself on transpac routes, quads are simply not in the future of aviation.

Exactly right. If the 340 had been developed even 5 years later, it might well have been a twin. Or more accurately there would only have been a 330.

Apart from the market skepticism, engines like the GE90 weren't quite ready yet. There simply was no powerplant to power a twin in the 777-300 size over the kinds of ranges envisaged for the 340Classic.

Quoting DocLightning (Reply 22):
Why are A340's still flying? Because you don't throw a perfectly plane out just because it's a touch obsolete. You had to pay for the plane and before you replace it, it needs to pay for itself. It will still pay for itself, just not quite as quickly as a newer twin. But you already own it, so shelling out the money for a newer twin isn't the best decision.

Indeed. Also, the 340 is typically cheaper than the 777 to purchase. Finally, delivery slots play a big role. A better plane in 5 years may not be as useful as a slightly suboptimal one in 2.
 
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DocLightning
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 12:32 pm

Here's another way of thinking about it. If you buy a computer and then three months later the manufacturer announces a new model for 15% more with 15% better performance, you don't trash your perfectly good computer. You keep it around for as long as it is profitable for you to have it and then you replace it when it has lived out its useful life.
 
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 2:33 pm



Quoting Viscount724 (Reply 21):
Actually, the maximum operating speed of the 707 and DC-8 is higher than the A340 according to the FAA certification data sheets.

707 - Mach 0.887
DC-8 - Mach 0.88
A340 - Mach 0.86

Thanks for the info-I was wrong, I see. But the 340 is nearly as fast, and I believe that it has significantly less sweep than either of the older quads.
 
ovrpowrd727
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 3:21 pm

hey guys, you all have great points on your argument about twin engines v. Quads however maybe i can add this perspective, all these arguments are based on pax ops, don't cargojets have an important aspect of sales to contribute?? Without Boeing's ability to make almost all their aircraft in cargo form the quads flying today would nose-dive...Most cargojets are quads maybe for weight and distance, i'm no expert just my two cents...regards

P.S. a lot of tri-jets were preferred by pilots for their ability to keep their distance without sacrificing weight but tri-jets were/are inefficient by today's standards
 
PlunaCRJ
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 3:52 pm



Quoting DocLightning (Reply 22):
So QF, AR, LA, NZ might have a use for quads for Australiasia/S. America routes, but the rest of the world doesn't need quads anymore.

Not necessarily. LA will be replacing its A340s with 787-8/9, and use them to fly to AKL and SYD, in a slightly more northerly, ETOPS compliant route.

ANZ is also considering using some of their ordered 787s in an eventual service to S. America.


However, probably the new QF flight SYD-EZE, which goes much more south, will always need a quad, as such a detour north would make the route plainly unpractical.
 
A342
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 4:22 pm



Quoting Viscount724 (Reply 21):
Actually, the maximum operating speed of the 707 and DC-8 is higher than the A340 according to the FAA certification data sheets.

707 - Mach 0.887
DC-8 - Mach 0.88
A340 - Mach 0.86

I'd be interested to know the typical cruise speeds for the DC-8 and 707 - the A330/340 has a normal cruise speed of Mach 0.82.
 
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SEPilot
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 4:26 pm



Quoting Ovrpowrd727 (Reply 26):
Most cargojets are quads maybe for weight and distance, i'm no expert just my two cents...regards

Most freighters are converted passenger planes; weight and balance considerations are exactly the same regardless of the number of engines. Many freight operations are more concerned with acquisition cost as opposed to operating cost, and they buy what can be had cheaply. These are often quads, as DC-8's in particular last a long, long time but have become uneconomic for passenger ops. They can be acquired and converted cheaply, and so many of them are still flying freight. This is the exact same reason that many 727's, DC-10's, 747 Classics and MD-11's are flying freight. They still have life in them (the 727's are rapidly disappearing, however), can still make money flying freight, and can be cheaply acquired. Only a small percentage of freighters are built as freighters; but with high fuel prices the numbers are and will continue to increase, as the differential in operating costs increases.
 
OPNLguy
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RE: Advantages Of A Quadra-jet Over A Twin

Fri Nov 28, 2008 9:56 pm

From a dispatcher perspective, quads (as well as 3-engined aircraft) have the ability to use takeoff alternates up to two hours away from the departure airport, as opposed to the one hour limit for a twin. Fog can sometimes occur over such a wide area that every potential alternate within that one hour flying time can be below alternate minimums.

Additionally, in the event of an engine failure, a 3- or 4-engined aircraft doesn't have to land at the nearest suitable airport in point-of-time (like a twin does) and has the option of continuing to the intended destination provided certain conditions are met.

On the flip side, more engines and systems can sometimes mean that there's more that can go wrong. Like everything else, there are tradeoffs...
 
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Devilfish
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RE: Advantages Of A Quadra-jet Over A Twin

Sat Nov 29, 2008 1:23 pm



Quoting Ovrpowrd727 (Reply 26):
a lot of tri-jets were preferred by pilots for their ability to keep their distance without sacrificing weight but tri-jets were/are inefficient by today's standards


Trijets lag behind quads in terms of minimum required installed thrust but sidestep this disadvantage in some cases when the power requirement is too big for two but small for four (fast disappearing too). They will better a twin in this regard but lose their competitive edge due to the complexity and maintenance issues of a tail-mounted engine. This is where the inefficiencies come, which also erase any benefits of maintaining one less engine than a quad. The question of four, three or two engine fuel burn for a basically similar airframe I leave to those more knowledgeable.
 
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DocLightning
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RE: Advantages Of A Quadra-jet Over A Twin

Sun Nov 30, 2008 2:04 am



Quoting PlunaCRJ (Reply 27):

Not necessarily. LA will be replacing its A340s with 787-8/9, and use them to fly to AKL and SYD, in a slightly more northerly, ETOPS compliant route.

http://tinyurl.com/6bn7bd

That's going to take more than a "slight" change in course.
 
OldAeroGuy
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RE: Advantages Of A Quadra-jet Over A Twin

Sun Nov 30, 2008 5:09 pm



Quoting DocLightning (Reply 32):
Quoting PlunaCRJ (Reply 27):

Not necessarily. LA will be replacing its A340s with 787-8/9, and use them to fly to AKL and SYD, in a slightly more northerly, ETOPS compliant route.

http://tinyurl.com/6bn7bd

That's going to take more than a "slight" change in course.

The latest ETOPS rules will allow Twins to be operated on these routes using airways that are near Great Circle routings.
 
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DocLightning
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 12:12 am



Quoting OldAeroGuy (Reply 33):

The latest ETOPS rules will allow Twins to be operated on these routes using airways that are near Great Circle routings.

So when they have an in-flight engine shut-down a-la QF, they will divert to... an open patch of very cold water?
 
tdscanuck
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 2:22 am



Quoting DocLightning (Reply 34):
So when they have an in-flight engine shut-down a-la QF, they will divert to... an open patch of very cold water?

A twin with one engine operating can still get where it needs to go...the ETOPS rules are a balance on the risk of the *second* engine failing. Provide they just loose one engine, it's not a big deal. And, so far, I believe no ETOPS twin has ever suffered two independant engine failures on the same flight.

Tom.
 
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Starlionblue
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 4:48 am



Quoting Tdscanuck (Reply 35):
And, so far, I believe no ETOPS twin has ever suffered two independant engine failures on the same flight.

Indeedy. You'd in fact be hard pressed to find ANY airliner which has had unrelated engine failures. The thinking is also that in practically all cases, the cause for a dual engine failure in a twin would have stopped all engines on a quad as well. Volcanic ash and fuel starvation come to mind. Doesn't matter how many engines you have.
 
pmk
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 5:33 am



Quoting 2H4 (Reply 13):
Ann Arbor, Michigan, as an old DC-8 whispered overhead during her descent into YIP

What were you doing in YIP? My in-laws live there, I spend many countless hours watching the classics skim the roof tops.

PMK
 
ovrpowrd727
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 5:59 am

doesn't ETOPS refer to twin engine aircraft?? how do quad jets fit into this, i don't understand the relation?? if the aircraft is flying on 3 or 4 or more engines then where do you apply the ETOPS??
 
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Starlionblue
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 6:24 am



Quoting Ovrpowrd727 (Reply 38):
doesn't ETOPS refer to twin engine aircraft?? how do quad jets fit into this, i don't understand the relation?? if the aircraft is flying on 3 or 4 or more engines then where do you apply the ETOPS??

The point is that "ETOPS-style" rules will eventually apply to all aircraft regardless of the number of engines. This because twins have shown themselves to be as reliable as triplets and quads and there isn't a reason anymore for separate rule systems. Since ETOPS has been so successful, it will ironically kill itself.  Wink
 
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SEPilot
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 1:17 pm



Quoting Tdscanuck (Reply 35):
And, so far, I believe no ETOPS twin has ever suffered two independant engine failures on the same flight.

Moreover, I don't believe that ANY jet (referring to B-47's and B-52's as well as other military planes) has ever suffered two unrelated engine failures on the same flight. If any one knows of one I'd like to hear about it. The reliability of jet engines is truly one of the most amazing feats of technology.
 
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DocLightning
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 2:57 pm



Quoting Tdscanuck (Reply 35):

A twin with one engine operating can still get where it needs to go...the ETOPS rules are a balance on the risk of the *second* engine failing. Provide they just loose one engine, it's not a big deal. And, so far, I believe no ETOPS twin has ever suffered two independant engine failures on the same flight.

I am not sure any plane ever has. But then why is ETOPS there, then?
 
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 5:11 pm



Quoting DocLightning (Reply 41):
I am not sure any plane ever has. But then why is ETOPS there, then?

ETOPS was developed precisely because by the 80's it had become apparent that jet engines were reliable enough to fly long distances over water with only two engines (which was never considered for piston powered airliners, and the rules in place did not allow). To try and alleviate public concern the ETOPS regulations were developed as a way of insuring that twin engined planes so used would have the best possible maintenance, and to address any known vulnerabilities beforehand (such as insuring that if maintenance is performed on both engines, it is done by different people to try and prevent the same mistake being made on both.) The regulations have been regarded as so successful that they are now being applied to all aircraft flying long routes without suitable emergency landing sites.
 
tdscanuck
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 5:12 pm



Quoting Ovrpowrd727 (Reply 38):
doesn't ETOPS refer to twin engine aircraft??

It used to. Now it just stands for "extended range operations" or some variant thereof.

Quoting Ovrpowrd727 (Reply 38):
how do quad jets fit into this, i don't understand the relation?? if the aircraft is flying on 3 or 4 or more engines then where do you apply the ETOPS??

ETOPS has two major components...flight planning and maintenance. The maintenance is really the bigger piece, and you can apply all those maintenance principles to quads and trijets just as well as twins.

Quoting DocLightning (Reply 41):
I am not sure any plane ever has. But then why is ETOPS there, then?

The rule that necessitated ETOPS in the first place was created back in the piston age, when two independant engine failures wasn't that unusual, certainly not unlikely enough to past certification under current rules. The advent of modern jets drove reliability up enough that the rule really no longer had any statistical validity on new twins, but the FAA couldn't just cancel the old rule and leave nothing in its place, so they (sensibly) eased into it. We've gone through progressively longer ETOPS certs up to the point, now, where there's really no difference between a twin and quad and ETOPS has proven so successful that it will become the baseline way of operating far from airports, no matter how many engines you have.

Tom.
 
dw747400
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RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 5:31 pm

With regards to ETOPS, does anyone know the approx single engine TAS used for planning purposes on the 777 series, 767, and A330?
 
2H4
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Joined: Tue Oct 19, 2004 11:11 pm

RE: Advantages Of A Quadra-jet Over A Twin

Mon Dec 01, 2008 6:46 pm



Quoting Northwest727 (Reply 18):
Depending on what series of DC-8 you are talking about!

Well, it seemed to be operating at flight idle, or nearly so.

Quoting Pmk (Reply 37):
What were you doing in YIP?

That's where I grew up, and I've still got family in the area.

2H4
 
OldAeroGuy
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Joined: Sun Dec 05, 2004 6:50 am

RE: Advantages Of A Quadra-jet Over A Twin

Tue Dec 02, 2008 2:44 am



Quoting Ovrpowrd727 (Reply 38):
if the aircraft is flying on 3 or 4 or more engines then where do you apply the ETOPS??

The new ETOPS (Extended Operations) rules apply to Tri's and Quads when the nearest diversion airport is more than 180 min away.

Quoting Dw747400 (Reply 44):
With regards to ETOPS, does anyone know the approx single engine TAS used for planning purposes on the 777 series, 767, and A330?

400 KTAS works for a good rule of thumb.
 
OldAeroGuy
Posts: 3928
Joined: Sun Dec 05, 2004 6:50 am

RE: Advantages Of A Quadra-jet Over A Twin

Tue Dec 02, 2008 2:49 am



Quoting DocLightning (Reply 34):
So when they have an in-flight engine shut-down a-la QF, they will divert to... an open patch of very cold water?

330 min ETOPS solves the diverson issue. The 773ER demonstrated this type of diversion time during its certification flight testing.
 
vikkyvik
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RE: Advantages Of A Quadra-jet Over A Twin

Tue Dec 02, 2008 3:24 am



Quoting OldAeroGuy (Reply 46):
The new ETOPS (Extended Operations) rules apply to Tri's and Quads when the nearest diversion airport is more than 180 min away.

So if I understand that correctly, an airline could theoretically use a particular airplane only on routes that do not pass more than 180 mins from a diversion airport, and as a result, they wouldn't have to maintain the aircraft to ETOPS standards.

Is that correct?

Seems like that would be a relatively easy thing to do on most routes.
 
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zeke
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RE: Advantages Of A Quadra-jet Over A Twin

Tue Dec 02, 2008 3:37 am



Quoting OldAeroGuy (Reply 46):
The new ETOPS (Extended Operations) rules apply to Tri's and Quads when the nearest diversion airport is more than 180 min away.

Should clarify they are new proposed FAA rules, which will only apply to FAA registered aircraft.

Each ICAO state can (and normally do) have their own set of rules for the aircraft registered in their state, no two ETOPS requirements seem to be the same.

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