SK A340
Topic Author
Posts: 829
Joined: Thu Mar 09, 2000 2:44 am

Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 6:42 am

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
 
User avatar
Starlionblue
Posts: 17097
Joined: Fri Feb 27, 2004 9:54 pm

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 7:26 am

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." - John Ringo
 
calpilot
Posts: 881
Joined: Sun Oct 17, 1999 5:16 am

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 7:26 am

Incredible! Divert and land the aircraft.
 
777wt
Posts: 828
Joined: Wed Jan 19, 2005 7:45 am

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 7:40 am

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.
 
mNeo
Posts: 718
Joined: Fri Mar 26, 2004 8:12 am

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 8:22 am

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.
Powered by Maina
 
SK A340
Topic Author
Posts: 829
Joined: Thu Mar 09, 2000 2:44 am

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 10:30 pm

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
 
air2gxs
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: Why Four Engines If Three Are As Safe?

Sat Mar 05, 2005 11:14 pm

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.
 
Thrust
Posts: 2585
Joined: Wed Sep 17, 2003 12:17 pm

RE: Why Four Engines If Three Are As Safe?

Thu Mar 10, 2005 4:15 am

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 avatar
Starlionblue
Posts: 17097
Joined: Fri Feb 27, 2004 9:54 pm

RE: Why Four Engines If Three Are As Safe?

Thu Mar 10, 2005 6:40 am

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." - John Ringo
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Why Four Engines If Three Are As Safe?

Thu Mar 10, 2005 3:26 pm



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.
 
air2gxs
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: Why Four Engines If Three Are As Safe?

Fri Mar 11, 2005 12:11 am



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 avatar
Starlionblue
Posts: 17097
Joined: Fri Feb 27, 2004 9:54 pm

RE: Why Four Engines If Three Are As Safe?

Fri Mar 11, 2005 1:05 am



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." - John Ringo
 
lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

RE: Why Four Engines If Three Are As Safe?

Fri Mar 11, 2005 2:25 am

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 avatar
jumbojim747
Posts: 2426
Joined: Sat Oct 16, 2004 8:05 pm

RE: Why Four Engines If Three Are As Safe?

Fri Mar 11, 2005 12:27 pm

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
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Why Four Engines If Three Are As Safe?

Fri Mar 11, 2005 1:32 pm

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.

Who is online

Users browsing this forum: Baidu [Spider], Starlionblue and 19 guests