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Why Four Engines If Three Are As Safe?  
User currently offlineSK A340 From Sweden, joined Mar 2000, 845 posts, RR: 2
Posted (9 years 6 months 3 weeks 16 hours ago) and read 3122 times:

There's been a lot of discussions going on about flying a 744 with only three (or perhaps two) engines. This discussion is only an example: British Airways 747 Flies Again On Three Engines (by Jacobin777 Mar 4 2005 in Civil Aviation)

I assume that it is not as safe to fly a 744 (or any four-engine aircraft) with three engines as it is with four but if it is as safe as stated in the posts, why do the manufacturers (read Boeing and Airbus) put the extra engine on the plane? If it is as safe as stated (not only by a.netter, but also by "experts") the extra cost of buying and dragging around an extra engine would be much higher than the costs of the extra safety gained by the fourth engine. And please, don't answer with "four engines are safer than three, therefore four is better". With that argument every four engine jet would have five engines and every five engine jet would have six engines etc. etc.

So, why have four engines if three are as safe?

/Micke

14 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17040 posts, RR: 66
Reply 1, posted (9 years 6 months 3 weeks 15 hours ago) and read 3048 times:

Three engines are, as you point out, a neater solution in some ways that four. Fewer engines has some advantages.

However triplets have one huge problem. Where the heck do we put the third engine. Obviously it has to be along the longitudinal center, so it ends up in the tail. Next problem: S-Duct (Trident, 727, Tu-154, TriStar) or passthrough (DC-10, MD-11).

S-Duct:
- Shorter tail = less drag and less dutch roll tendency.
- Thrust close to center of mass line.
- Easier servicing.
- Less complex fin.

Passthrough:
- Easier tail construction.
- Easier to upgrade to a larger engine, although eventually you run into problems here too, as MD discovered when they tried to stretch the MD-11 into the (triplet) "MD-12X".

In any case it's just much much easier to make it a quad. Which is why we don't see widebodies with three engines anymore.

[Edited 2005-03-04 23:36:54]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineCalpilot From United States of America, joined Oct 1999, 998 posts, RR: 13
Reply 2, posted (9 years 6 months 3 weeks 15 hours ago) and read 3047 times:

Incredible! Divert and land the aircraft.

User currently offline777WT From United States of America, joined Jan 2005, 877 posts, RR: 1
Reply 3, posted (9 years 6 months 3 weeks 15 hours ago) and read 3023 times:

Other than those details as listed by Starlionblue, another reason is that the aircraft is required to takeoff with 3 engines, simulating one engine failure after V1.

So a triple engined aircraft would have to continue takeoff with 2 engines with one simulated engine failure.

Then it comes down to twin's where one enigne would have to continue takeoff with an simulated failure of the other one.


User currently offlineMNeo From Bulgaria, joined Mar 2004, 776 posts, RR: 1
Reply 4, posted (9 years 6 months 3 weeks 14 hours ago) and read 2981 times:

well look at it this way. If an T7 could be in the air with 1 ENGINE for 330min why cant a B747 be up in the air with 3 ENGINES for 11 hours. the 747 was certified to fly with 3 engines. weather or not a carrier wants to fly it that way its up to them. with 3 engines more drag is produced thus making the plane less efficent. But the cost to turn around the plane in an away airport will be tremendeous.


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User currently offlineSK A340 From Sweden, joined Mar 2000, 845 posts, RR: 2
Reply 5, posted (9 years 6 months 3 weeks ago) and read 2798 times:

So, to sum it up: it's more difficult to construct an airplane with three engines than with four engines. The aircraft makers don't want this cost so they let the airlines pay for it.  Wink

/Micke


User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 6, posted (9 years 6 months 2 weeks 6 days 23 hours ago) and read 2780 times:

Quoting SK A340 (reply 5):


There's more to it than that. A 4 engined aircraft must be able to safely take-off and fly with 3 engines. Now, if you decide to eat the design costs and build the same airplane with 3 engines, you will not get it certified. Why? Because the airplane will not be able to TO and continue flight with 2 engines.

It's a redundancy feature.


User currently offlineThrust From United States of America, joined Sep 2003, 2690 posts, RR: 10
Reply 7, posted (9 years 6 months 2 weeks 2 days 18 hours ago) and read 2544 times:

Does anybody know how much the speed of a 744 would be reduced if one its engines is shut down? I assume this would cut it by a quarter?


Fly one thing; Fly it well
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17040 posts, RR: 66
Reply 8, posted (9 years 6 months 2 weeks 2 days 16 hours ago) and read 2511 times:

There's more to it than that. A 4 engined aircraft must be able to safely take-off and fly with 3 engines. Now, if you decide to eat the design costs and build the same airplane with 3 engines, you will not get it certified. Why? Because the airplane will not be able to TO and continue flight with 2 engines.

It's a redundancy feature.


Well all you have to do is make the three engines more powerful than the previous four. But the cost, weight and complexity penalties of a triplet just make it much cheaper to build a quad.


Does anybody know how much the speed of a 744 would be reduced if one its engines is shut down? I assume this would cut it by a quarter?


Not at all. The thrust of all four engines is only used at takeoff. We would need Philsquares or someone to corroborate, but the speed (and max altitude) with three engines is probably not much below that with four. The three remaining engines would run at a higher thrust level to compensate of course.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 9, posted (9 years 6 months 2 weeks 2 days 7 hours ago) and read 2458 times:



Quoting SK A340 (Reply 5):
So, to sum it up: it's more difficult to construct an airplane with three engines than with four engines. The aircraft makers don't want this cost so they let the airlines pay for it.

Nope. The airlines don't want to pay the cost of the difficult construction of an aircraft with three engines so they buy four-engined aircraft from manufacturers offering those instead.

Another factor is if you need x N of thrust but can't find suitable engines which provide x/3 N or more... four donks it is, then.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 10, posted (9 years 6 months 2 weeks 1 day 22 hours ago) and read 2405 times:



Quoting Starlionblue (Reply 8):
Well all you have to do is make the three engines more powerful than the previous four. But the cost, weight and complexity penalties of a triplet just make it much cheaper to build a quad.

But then you have a triplet capable of taking off on 2 engines. Where does it end?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17040 posts, RR: 66
Reply 11, posted (9 years 6 months 2 weeks 1 day 22 hours ago) and read 2395 times:



Quoting Air2gxs (Reply 10):
Quoting Starlionblue (Reply 8):
Well all you have to do is make the three engines more powerful than the previous four. But the cost, weight and complexity penalties of a triplet just make it much cheaper to build a quad.

But then you have a triplet capable of taking off on 2 engines. Where does it end?

All triplets need to be able to climb out with an engine out at V1 or later.

My point was that it's much simpler (cheaper) to build a quad than a triplet. You can take it down to a twin, but 777 class engines haven't been availale that long. And that's why we have the 340 instead of a 777-like 330.

2 or 4 (or even 6) engines= relatively straightforward construction.
3 engines= headache



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 12, posted (9 years 6 months 2 weeks 1 day 20 hours ago) and read 2381 times:

Quoting SK A340 (Thread starter):
Why Four Engines If Three Are As Safe?

No airplane in the world exists just to be safe, they are designed to be efficient while delivering a specified payload to a specified destination. Big grin

747 is safe on three or even two, I donno; but it is only efficient on four that was why it was designed with four.

[Edited 2005-03-10 18:27:00]


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineJumboJim747 From Australia, joined Oct 2004, 2464 posts, RR: 44
Reply 13, posted (9 years 6 months 2 weeks 1 day 10 hours ago) and read 2328 times:

If a 747 was to run on say 3 engines say for example number 3 is on idle but and the other 3 are on cruise .
Then number 4 gives way and is running down the thrust from the number 1 and 2 would be so great that the aircraft would turn to the right at an alarming rate .
It wouldn't be safe i would rather have 4 running at cruise then 3 anyday.
Correct me if im wrong if in this situation how long has a pilot got to react and power up the idling engine before its too late



On a wing and a prayer
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 14, posted (9 years 6 months 2 weeks 1 day 9 hours ago) and read 2328 times:

Transport jet aircraft are designed and certified for a specific mission profile.
For example, the Lockheed TriStar was originally designed as a twin, but engines were not available at that time to enable the aircraft to meet the weight/range/payload requirements required by customer airlines, so the aircraft was re-designed as as a three-engine aircraft, to meet customer requirements.
3 and 4 engine jet transport aircraft have to meet the same engine-out second segment certification requirements.
Twins however, meet another set of guidelines
In each case, three or four engine types CAN, if the operator so chooses, continue with one engine inoperative, to the scheduled destination, provided certain enroute conditions are met, subject to the driftdown performance specified.
Twin engine aircraft however, should an engine fail in cruise, must divert to an enroute alternate, subject to certain conditions, not the least of which is the weather conditions at the selected enroute alternate.

In addition, the Lockheed TriStar will, if an engine fails enroute, driftdown to a lower altitude (dependant on weight and ambient temperature aloft), will cruise at a slower airspeed (ballpark figure, 430 knots TAS) and burn, on average, an additional 500kgs of fuel per hour.
The Boeing 707 is quite similar.
One incident that I recall many years ago was a B747, enroute BAH-ATH, where one engine failed enroute over Saudi Arabia.
The flight crew decided to continue to ATH, as would normally be the case.
However, on descent, another engine failed, and the aircraft was successfully landed with the two remaining engines apparently operating normally.
At the stand, just prior to normal engine shutdown, one of the two operating engines flamed out.
Upon investigation, severe fuel contamination was found...the fuel last uplift having occured in BAH.
It is noteworthy to keep in mind that engines fail enroute for a variety of reasons, and the cause at the time might not be readily apparent.


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