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4 Engine Aircrafts VS 2 Engine Ones  
User currently offlinemitris From United States of America, joined Oct 2012, 24 posts, RR: 0
Posted (1 year 8 months 5 days 22 hours ago) and read 5383 times:

I'm sure this has been discussed before... However, I can't find any recent topic about it.

Why do airlines prefer 2 engine planes rather than the 4 engine ones?

Do they really consume so much less fuel?

What are the advantages and disadvantages?

I always wondered, what happens if one engine goes off power on a two engine plane? Can the plane continue its flight smoothly? If an engine goes down on a A340, or a B747, or even an Avro RJ you know you have 3 more engines to keep the flight going or make an emergency landing. I would not feel safe on a two engine airplane if that scenario ever happened.

9 replies: All unread, jump to last
 
User currently offlinestarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 1, posted (1 year 8 months 5 days 20 hours ago) and read 5342 times:

Caveat: My comments apply to modern large widebodies like 777 and 330/340. This is the "cross-over zone", where there are 2 and 4 engine designs. On smaller designs, two engines are definitely the way to go.

Quoting mitris (Thread starter):
Why do airlines prefer 2 engine planes rather than the 4 engine ones?

They are typically cheaper to maintain, and generally speaking they use less fuel. However a lot depends on the mission. And there is a point where four engines become necessary. There are no current engines that can power a twin in 747/380 size.

Quoting mitris (Thread starter):
Do they really consume so much less fuel?

The mission has a lot to do with it, and fuel burn is not everything in the economic equation. Hard to make an exact comparison but in general yes.

Quoting mitris (Thread starter):
What are the advantages and disadvantages?

Twin advantages.
- Two engines instead of four to maintain.
- In general lower fuel burn. However this is very mission dependent.
- Better performance due to more "excess" thrust*. However a quad often has better performance in hot and high scenarios.
- Catastrophic engine failure less likely to affect another engine due to debris etc.

Quad advantages.
- More wing bending relief from the outboard engines and weight distribution along the wing, meaning the wing can be made lighter.
- Less yaw moment in engine out scenarios, meaning the vertical tail can be made smaller and lighter.
- Much higher size/weight limit on designs. You can't build a 380 with four engines. This is only an advantage for a small market segment.
- Can hold higher altitude in case of engine failure.
- Cheaper engines in the case of large designs.
- Regs may allow more direct routing further from alternate airports. However currently few routes need this. A 777 can fly 3-4 hours away from an alternate at one-engine cruising speeds. Even flying across the North Pole you're not that far away.

* Twins needs to fly on one engine in case of an engine out, while quads need to fly on three. So all things being equal, tins have 50% more thrust.

Quoting mitris (Thread starter):
I always wondered, what happens if one engine goes off power on a two engine plane? Can the plane continue its flight smoothly?

This case is extensively trained for.

A twin can climb out on one engine at max weight from the worst case failure moment (on the runway taking off). If an engine fails in cruise, the aircraft will have to descend since it cannot hold the same altitude on one engine.

So while it may not be a "smooth" continuation, certainly the aircraft will manage to get to the closest alternate, even if several hours away.

Quoting mitris (Thread starter):
If an engine goes down on a A340, or a B747, or even an Avro RJ you know you have 3 more engines to keep the flight going or make an emergency landing. I would not feel safe on a two engine airplane if that scenario ever happened.

Jet engines are ridiculously reliable. Most current pilots will never experience an engine failure in their entire career spanning decades. Since the advent of jet airliners, there has not been a single case where two engines failed for unrelated reasons. In cases where multiple engines failed, the cause has been common (e.g. fuel starvation), so the number of engines would not have mattered.

In a twin, one engine is plenty to continue and land. Modern long haulers are tested and certified to fly for hours on one engine.

[Edited 2012-11-25 03:51:08]

[Edited 2012-11-25 04:35:48]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinethegeek From Australia, joined Nov 2007, 2638 posts, RR: 0
Reply 2, posted (1 year 8 months 4 days 4 hours ago) and read 4921 times:

I'd add to the twin advantages faster climb due to higher thrust/weight. This is a fuel saving, but on the longer sectors it is a lesser proportion of the fuel burnt.

Quads in theory burn less fuel at cruise, as the engines should be working slightly harder.

Regarding the diversion rules Starlionblue referred to, I'd refer to this article: http://en.wikipedia.org/wiki/ETOPS


User currently offlinemitris From United States of America, joined Oct 2012, 24 posts, RR: 0
Reply 3, posted (1 year 7 months 4 weeks 9 hours ago) and read 4442 times:

Quoting thegeek (Reply 2):
Since the advent of jet airliners, there has not been a single case where two engines failed for unrelated reasons.

Wow, you answered all my questions! Thank you!
But what about this scenario: http://en.wikipedia.org/wiki/US_Airways_Flight_1549

[Edited 2012-12-02 15:21:13]

User currently offlinerfields5421 From United States of America, joined Jul 2007, 7607 posts, RR: 32
Reply 4, posted (1 year 7 months 4 weeks 8 hours ago) and read 4426 times:

Quoting mitris (Reply 3):
But what about this scenario

That wasn't 'unrelated causes'

The first reported bird strike was with Orville Wright at the controls in 1905. The first confirmed fatal bird strike was aviation pioneer Cal Rodgers near Long Beach CA in 1912.

Four engined aircraft have been downed by birds in the past, including a Lockheed Electra at BOS - Eastern Airlines Flt 375 - the current largest number of fatalities from a bird strike. Both Ryanair Flt 4102 and US Airways Flt 1549 could easily have exceeded that toll if the pilots hadn't done exactly the right things.

Military aircraft are not exempt. Some of the larger four engined planes include a Belgian Air Force C-130 in 1966, two USAF E-3 (B707 airframe, a USAF B-1B and a RAF Nimrod have crashed due to bird strikes.

Four engined aircraft have had all four engines fail from other contamination causes such as the BA 747 that flew through a volcanic dust cloud.


User currently offlinestarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 5, posted (1 year 7 months 3 weeks 6 days 20 hours ago) and read 4313 times:

Quoting mitris (Reply 3):
Quoting Starlionblue (Reply 2):
Since the advent of jet airliners, there has not been a single case where two engines failed for unrelated reasons.

Wow, you answered all my questions! Thank you!
But what about this scenario: http://en.wikipedia.org/wiki/US_Airw..._1549

As rfields5421 says, the engines did not go down for unrelated causes. Furthermore, with that many birds it is by no means a given that a quad would have fared better.

You also have to think of risk vs reward. Using only quads to be better prepared against bird strikes (assuming that helps) and other events MIGHT decrease risk by a very small amount. However it would increase cost by a very large amount. Flying is ridiculously safe anyway.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAA757MIA From United States of America, joined May 2008, 253 posts, RR: 0
Reply 6, posted (1 year 7 months 3 weeks 2 days 7 hours ago) and read 3943 times:

Might be a stupid question, but, I tried searching for it and couldn't find any answer... Can a quad take off on two engines? I mean, if something goes wrong when taking off.

User currently offlinestarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 7, posted (1 year 7 months 3 weeks 2 days 4 hours ago) and read 3918 times:

Quoting AA757MIA (Reply 6):
Can a quad take off on two engines? I mean, if something goes wrong when taking off.

Maybe, if it is very light. However I don't think it is a certification requirement.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineandrewtang From United Kingdom, joined Jun 2001, 461 posts, RR: 10
Reply 8, posted (1 year 7 months 1 week 6 days 20 hours ago) and read 3286 times:

Quoting AA757MIA (Reply 6):
Can a quad take off on two engines? I mean, if something goes wrong when taking off.

An Airbus A340-300 taking off at MTOW will still be able to climb to 5000ft, albeit very slowly if two engines failed right after take-off.

[Edited 2012-12-17 03:53:42]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 9, posted (1 year 7 months 1 week 6 days 20 hours ago) and read 3269 times:

Quoting andrewtang (Reply 8):
Quoting AA757MIA (Reply 6):
Can a quad take off on two engines? I mean, if something goes wrong when taking off.

An Airbus A340-300 taking off at MTOW will still be able to climb to 5000ft, albeit very slowly if two engines failed right after take-off.

Even with the gear out? That is, would it be able to climb with gear and climb flaps on two engines if the two other engines failed at V1?



"There are no stupid questions, but there are a lot of inquisitive idiots."
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