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2, 3, Or 4 Engines

Thu Nov 16, 2000 7:56 am

How many engines would you prefer an airliner to have and what makes it better?

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Photo © Paul Arnold

2 engines like the 777

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Photo © Vincent Gury

3 engines like this yak-42

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Photo © Frank Schaefer

or 4 engines like this 747
If u ever feel depressed about urself, just remember that at one time you were the most vicious sperm in ur group.
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RE: 2, 3, Or 4 Engines

Fri Nov 17, 2000 11:41 am

Well, 4 engines were needed when the first jets came about only because the engines barely made enough thrust to get the plane off the ground so four was needed to do the job. The same applies to the DC-10 & tristar; also the the early large fan engines were un-reliable.

I see no real reason to have more than 2 engines nowadays except on extremely heavy airliners. Although, flying very long distances over water, I prefer 3 or more engines just for a piece of mind if one engine fails.

I am just curious, what costs more, 2 98,000lb thrust GE-90 engines or 4 50,000lb class CF6-80 engines?

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Yak-42 Three Engined?

Mon Nov 20, 2000 2:10 am

I think that Yak-42 have two engines (I have flew with them four times). Yak-40 have three. Lithuanian Airlines haven't Yak-40, but their domestic airline "Air Lithuania" have. So there is a possibility, that here is not a Lithuanian Airlines Yak-42, this is Air Lithuania's Yak-40. I can't see airline name or aircraft lenght from this angle.


RE: 2, 3, Or 4 Engines

Mon Nov 20, 2000 3:40 am

FOUR! 4!
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RE: 2, 3, Or 4 Engines

Mon Nov 20, 2000 4:40 am

4 of course. And not only because of looks.

Even in times of increasing reliability and ETOPS four engines make sense, especially on very big airliners. Engines bigger than on a 777 will mean the aircraft will have to have a very high undercarriage, will look awful and so on. And I doubt the long-term-reliability of Big Twins. They´re just to new to be compared with DC-10s, B747s and L-1011 and the like.

I´ll choose a grand old 747 a hundred times over a 777.
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RE: 2, 3, Or 4 Engines

Mon Nov 20, 2000 6:23 am

Four engines are at least one too many.

Three is perhaps the best compromise, but is hard to implement without serious weight penalties. Perhaps when blended wing body designs appear, a return to three will ocur.

Two is the best configuration moving forward. There are a multitude of advantages to twins that one should consider. I offer the following as food for thought. I received this secondhand and the author is unknown to me.

>>After reading the recent thread, I thought I'd offer my opinion as to why 2
engines are safer than 4 engines on any aircraft.

The main issue is performance. Each aircraft is required to meet certain
climb performance requirements on takeoff in the event of an engine
failure. The climb performance requirements are virtually the same,
regardless of whether the aircraft has 2, 3, or 4 engines. A 2 engine
aircraft will therefore have about the same performance on 1 engine as a 4
engine aircraft has on 3 engines (each has one engine failure).

Most accidents do not involve engine failures. In situations where all the
engines are operating, 2 engine aircraft will have much better climb
performance than 4 engine aircraft. This enables 2 engine aircraft to have
a much better chance at surviving encounters with windshear, or perform
GPWS escape maneuvers (to avoid hitting the ground). They will also have
better performance during a go-around.

There are also problems with 4 engine aircraft. Many people point to
instances of dual engine failures on 4 engine aircraft as an argument
against 2 engine aircraft. In reality, I would argue the opposite point.
The dual engine failures on 4 engine aircraft that I can think of are: El
Al in AMS, the United 747 in HNL, and Evergreen in Alaska. In each of
these cases the 2 engine failures were related. In both the El Al and
Evergreen cases, the explosive failure of the #3 engine (inboard right
side) caused the failure of the #4 engine. In the United case, debris from
the cargo door, etc. caused both the #3 and #4 engines to fail.

If these same events occurred on a 2 engine aircraft, only 1 of the engines
would fail. While it might appear that both aircraft have lost half of
their engines, the 4 engine aircraft is in a much more serious situation.
The 2 engine aircraft still has 100% of it's required engine out
performance; the 4 engine aircraft only has 66%. The 4 engine aircraft
doesn't even meet takeoff climb performance requirements. If the
initiating event occurs very soon after takeoff, it is quite possible the
aircraft will not be able to clear surrounding terrain.

The argument is also made that flying with 2 engines over the water is not
safe. In the modern era, there has never been an aircraft accident due to
a loss of thrust from an engine during cruise (I'm specifically excluding
UA 232, which resulted from a hydraulic problem, not a loss of thrust).
There has also never been an instance of dual, unrelated engine failures.
Under ETOPS rules, there are strict limits on the operation of the aircraft
which seriously lessen the risk of related engine failures. In fact, a 3
engine Eastern L1011 almost ditched in the water off of Miami due to
related engine problems with all 3 engines (which would not have happened
if the aircraft was operated under ETOPS rules).

In my opinion, overwater flight under ETOPS is safer. I'm not worried
about flying on 1 engine after a failure; the airplane does that just fine.
I am worried about not having an alternate to which to divert. Under
ETOPS, there are strict rules on diversion airports, including the weather
at the alternate, and the time required to get there. There are no such
limitations for non-ETOPS flights. If an airplane is on fire out over the
water, it won't matter how many engines there are. The availability of an
alternate airport might make a difference, however.

2 engine aircraft are much better equipped to survive windshear and CFIT
encounters, two of the leading accident causal factors in recent years.
Unrelated independent engine failures is not a leading accident causal
factor. In addition, 2 engine aircraft under ETOPS are better protected
against related engine failures than 4 engine aircraft. That is why I feel
safer boarding a 2 engine aircraft.<<


Re: Sonic / Yak-42

Tue Nov 21, 2000 12:26 am

the yak-42 has got three engines, believe me. the only russian jetliner with two engines at the tail is the tu-134, as far as i know  

in any case, i would say that three engines look best, especially tail-mounted ones, so that the wing is clean. jetliners like the 727 or the tu-154 look just great. 2 engine aircraft with wing-mounted engines look just boring, every company is building them...come on boys, be wittier please!
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RE: 2, 3, Or 4 Engines

Tue Nov 21, 2000 9:12 am

Tarantine wrote:
Well, 4 engines were needed when the first jets came about only because the engines barely made enough thrust to get the plane off the ground so four was needed to do the job.

Nope Tarantine, it isn't so. Half of the four very earliest jet airliners - the Tu-104 and the Caravelle - were twins. Only (what at that time were considered) long range jets - the Comet and the 707 - had four holes. The Caravelle flew nicely on two RR Avons - same as the four on the Comet 4, but not nearly as far.
But of course you are right in some way. The three first US jets - 707, DC-8 and CV-880 - were all four engined. In the 50'es a US jet without coast to coast capability made no sense, while a Caravelle did make sense in small western Europe. And the Tu-104 was nothing but a hastily converted bomber plane (Tu-16 Badger) which itself was little more than a russified German Junkers Ju EF 132 project, which started "life" as a Ju-287 project in 1942 and was halted only temporarily in early 1945 when Soviet troops overran the Junkers plant in Dessau, East Germany.
The major change from Ju EF 132 to Tu-16 was in fact that the six Junkers Jumo 012 engines of 5500 lbs thrust were substituted by two Russian engines of almost 20,000 lbs thrust. The six Jumo 012s would have been buried in the wing root in Comet style.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs

RE: 2, 3, Or 4 Engines

Tue Nov 21, 2000 4:26 pm


Excellent defence for 2 engine aircraft, although i dont agree that 2 engine aircraft are safer in CFIT situations.

Any comments?
Posts: 1893
Joined: Wed Sep 27, 2000 10:11 am

RE: 2, 3, Or 4 Engines

Wed Nov 22, 2000 2:47 pm

2 engines. One reasons, there is one half the chance of engine failure. Why do I want to double my chances of a possible in flight engine failure in a less over powered aircraft with four engines???

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RE: 2, 3, Or 4 Engines

Thu Nov 23, 2000 1:06 am

One engine is one less engine to go wrong in flight than in a twin. If the logic of some of you is right, why don´t you opt for one-engined airliners in the future?
If I can avoid a twin and have a four-engined plane instead, I´ll go for that. One of the worst things I can think about when flying long-distance is that sitting in a Twin one engine will fail and you´ll have to pray...

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