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The twin-engine versus four-engine plane debate

 
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Birendra
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PostPosted: Mon Jul 16, 2007 7:35 pm    Post subject: The twin-engine versus four-engine plane debate Reply with quote

I just read an article in Flight Global magazine Arrow "A340 — 4 engines 4 long haul", which suggested that twinjets like Boeing 777 was not as safe or reliable as the four-engine airplane like the Airbus A340.

A lot of people claim that twins are more economical than planes with four engines.

I know that twins have to be rated at double the amount of thrust normally needed, in case of engine failure at take off, and that quads only have to be rated at 33.3 percent more than normally needed, in case of the above mentioned.

So the engines on a wide body twin have to be huge compared to those of a quad. One should think that would also make them consume more fuel than the smaller quads?

Or do they win in the long run by the fact that they might need a lower thrust setting than quads while in cruise?

Since this is a question about the twin versus four engine planes, it is only fair that we look at two planes of a similar category – the Boeing 777 and the Airbus A340.
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stealthpilot
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PostPosted: Tue Jul 17, 2007 8:19 am    Post subject: Re: The twin-engine versus four-engine plane debate Reply with quote

Birendra wrote:

So the engines on a wide body twin have to be huge compared to those of a quad. One should think that would also make them consume more fuel than the smaller quads?

(Just thoughts and ideas, not meant to be proven fact)

Per engine I would assume any 777 engine would burn more fuel than an a340 engine. But overall twins are more efficient. Remember, even if the 777 engines consume 90% more fuel than a 340 engine its more efficient overall. In fact due to the increased drag of the 4 engines on the aircraft, I would guess that even if the twin engines consumed double the quad engine did it would still work out more efficient. I am sure some engineers in this forum can add/correct these thoughts.

Back in the day when engines weren’t as reliable then yes, quads might have been a safer bet. (Not to mention the fact that engines were under powered and you didn’t have a choice but to slap 4 of them on to carry a certain amount of ppl)

As technology improved engine reliability improved and today’s engines are very very safe. I don’t ever recall an incident or accident which was attributed to both engines failing mechanically (feel free to correct)

Was the article in Flight Global Magazine recent? I find it strange that one can call a 340 safer than a 777 in today’s world. If you look at it in pure percentage terms then ok- I guess the 340 has a lower chance of having all engines fail. But the point is that the probability of both engines failing in a twin is so miniscule that it’s considered safe.
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Birendra
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PostPosted: Tue Jul 17, 2007 8:45 am    Post subject: Reply with quote

Quote:
Was the article in Flight Global Magazine recent? I find it strange that one can call a 340 safer than a 777 in today’s world.


No this is not a recent article. I got this mag from the local old paper mart for 10/- and read this article therein.
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Birendra
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PostPosted: Tue Jul 17, 2007 9:35 am    Post subject: Reply with quote

Some more inputs:

The debate is quite fierce between the two manufacturers - starting with the Airbus’ billboard advertisement at the edge of a runway at the 2002 Farnborough Air Show that read, "A340 — 4 engines 4 long haul", which suggested that twinjets like Boeing 777 was not as safe or reliable as the four-engine airplane like the Airbus A340.

Proponents of four-engine jets claim that, if a twinjet loses one engine, the airplane and passengers could get stranded at an alternate airport that is remote and inhospitable whereas a four-engine jet could continue to its destination.

In reality, when an engine fails, depending on the circumstances, the safest option is to land at the nearest airport rather than to continue. Also, it is pertinent to note that, more than 90 percent of airplane diversions have nothing to do with engine failures but rather one involving bad weather, a sick passenger or other non-technical reasons.

And anyway, according to the Boeing records, the four-engine A340 has had twice as many diversions and turn backs than the twin-engine Boeing 777. Further, the A340 has a lower average dispatch reliability rate than the 777.

Why so? Twin-engine planes operating on ETOPS routes are required by regulations to operate at a higher standard and reliability than four-engine ones. As a result, ETOPS rules have been increased to more than three hours today.

Records have also shown that twin-engine planes are more efficient, economical and more reliable than four-engine ones.

Remember, airlines around the world have flown almost 4 million ETOPS flights to date. Since operating with twin-engines is proven safe and reliable, it only makes sense to take advantage of the increased efficiency they offer. They consume less fuel, weigh less, have fewer emissions and operate more efficiently (5% to 9% lower in operating costs)

Why the twins are more economical?
One reason is attributed to the engines designed and installed on them. Secondly, it could be due to the structural design of the plane (the Boeing 777 is the first plane to be fully designed by computers).

The engines on a wide-body twin are huge as compared to those of a quad. So, it natural for them to consume more fuel than the smaller quads but overall, they are more or less comparable.

Do the twins win in the long run by the fact that they might need a lower thrust setting than quads while in cruise?
Engines are designed by the manufacturer to operate at optimum settings whether installed on a twin or quad. So a twin might not be operating at a lower thrust setting but rather at the recommended one for a particular cruising level.

Remember, the normal cruising speed of a Boeing 777 is 0 .84 Mach whereas an Airbus A340 is at 0.82 Mach and yet, the former plane still operates at a lower cost!
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HAWK21M
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PostPosted: Tue Jul 17, 2007 1:16 pm    Post subject: Reply with quote

With Reliability build in.I'd say Twins are better.
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stealthpilot
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PostPosted: Wed Jul 18, 2007 1:07 pm    Post subject: Reply with quote

I thought the “4 engines 4 long haul” was just an advertising gimmick?

Nowadays I highly doubt Airbus or any manufacturer will claim that a twin is less safe. The 340 and 380 might very well be the last of the quads.
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gauravprakash007
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PostPosted: Fri Aug 03, 2007 12:43 am    Post subject: Reply with quote

Quote:
The 340 and 380 might very well be the last of the quads.


Quite possible....
With the 772 and 773s doing so well....
i think the same way....
quad engines might be a thing of the past!
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ssbmat
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PostPosted: Tue Aug 07, 2007 7:28 am    Post subject: Reply with quote

There are a few points worth highlighting which have been briefly covered in relation to this debate.
1) High-Bypass turbofan like JT9/GE CF6/RB 211 that first went into service on the B747 and later widebodies like DC10/Tristar/A300 TRULY revolutionized air travel. Without that development, the 747 would be like the B52 bomber with probably 8 or 10 underpowered and somewhat unreliable 'fanjets' as they called 'em back in the 60s.

The long range DC-10 and Tristar-500 had already made their case. And the performance and reliability of the turbofans on the 767-200ER and 767-300ER only further provided impetus to Boeing, Pratt GE and RR to expand this to a twin-engine long range widebody.

Airbus applied a slightly different concept by trying to expand on the PW2000 capabilities. I believe they were not successful with that engine (the SuperFan issue) and the CFM-56 (uprated ) was a technical compromise that achieved just the right economic capabilities to a good airframe. That the airframe itself is good is evinced by the super success of the A330.
Is the A340 more reliable than B777 or vice-versa? I think it is hard to tell. To the casual observer, 4 vs 2 is an important difference. For 99.99% of the people, it hardly matters as to whether there is a CFM56 tubofan or a big GE-90 underneath the wing, so the debate about 50% vs 33% overpowering is moot.
To the technical buff, the 777 flight dynamics (M.84 etc) are compelling. To the Airline accountant, nothing matters more than the fuel bill.
At the end of the day, both SQ and LH have been successfully operating the 777 and A340 respectively, with no significant issues.
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