Twin v Four engines

[NWSRG]

Why did Airbus build the 340 with four smaller engines, and the 330 with two larger engines? The 340 has longer range, and this obviously requires more power to lift the additional fuel off the ground, and up to cruising altitude. But could this not have been achieved with two larger engines (as per the 777)? Is there any reason why four smaller engines would cruise more economically at altitude than two large ones? The ownership costs must be higher for four engines with the additional maintenance burden etc.

[747FOCAL]

4 engine aircraft have less restrictions on them for over water flight. :D

[OldAg84]

747Focal

Having recently just sat for 12 hours from Los Angeles to Auckland in a 777- I'm curious. There don't seem to be too many more remote overwater routes.

I spent the entire time hoping GE's quality department was up to snuff. I would have much prefferred a 747-400. Interestingly, the gentleman next to me, who makes the trip 3-5 annually said," the interior and layout of the 777 is far superior from a comfort standpoint (we were in coach), but I would still prefer a 747."

The trip was great, the service on UA was good, but two engines over that much water...if I ever get a better fare on a 747, i.e. Qantas/Air NZ, customer loyalty goes out the window.

[NorthernSky]

Don't wish to pour water on anyone's bonfire, but the debate should not revolve around multipe engine failures over water. Twins are safer, because if you get close to killing yourself in one, you've got loads of thrust to escape with (eg CFIT, microburst), when all engines are operating. In these circumstances, the four-engined aircraft loses because its excess thrust is minimal. Aircraft are lost, not because of multiple engine failures, but because of other, avoidable, situations, which are best escaped in a twin.

[Localiser Green]

I think there is generally a bit of a public perception problem on the "4 engines are safer than 2" concept, particularly when flying over the Atlantic.

It is pretty fair to say that the chance of both engines on a twin failing at the same time for mechanical reasons is effecitvely zero. If the chance of even a single engine failure is 1 in a million, then if I remember correctly from doing probability at school, the chance that both engines will fail at the same time (for purely mechanical reasons) is 1 in a million multiplied by 1 in a million, which is 1 in 100,000 billion.

Generally, multiple engine failures occur due to fuel starvation / contamination or flight into volcanic ash. In both of those cases, a 4-engined jet will not help you, all 4 engines will fail.

I don't think my argument is flawed, do you?

NorthernSky highlights some other interesting safety enhancements with twins which I had never previously considered.

One thing is for sure though, I would much rather fly across the Atlantic in a nice new A330 or 777 than the likes of a DC-10 or 747 classic.

[OldAg84]

I "hear" what you are all saying, it's all true and I believe it...but somehow 4 "seems" better than 2.

[wysiwyg]

I positioned on a 146 a few years ago. Engines 1 and 2 were started but 3 had a starter problem. We went tech and suffered the inevitable delays but I remember thinking that we would have already been there if we'd only had 2 engines to start!

[jtr]

There is no difference b/t the two until you lose one 179 min from the closest ERA. Then assess the pucker factor!

[lomapaseo]

> Having recently just sat for 12 hours from Los Angeles to Auckland in a 777- I'm curious. There don't seem to be too many more remote overwater routes.

I spent the entire time hoping GE's quality department was up to snuff. <

But wern't you aboard a P&W powered B777:confused:

[mutt]

jtr,

There is no difference b/t the two until you lose one 179 min from the closest ERA. Then assess the pucker factor!

That’s true, but remember that the twin has enhanced fire suppression systems, the quad isn’t required to. The twin also has a declared ERA within that prescribed time where you know the weather is under constant surveillance, does the quad?

Pros and Cons both ways!

Mutt.

[jtr]

Mutt, I cannot speak for the 777, but when comparing the -400 and 340, to the 330, there is no difference.

Fire suppresion is a mute point. The fire either goes out or it doesn't.
If it goes out ....fine. You are now going to the closest bit of bitumen (and in a hurry) 280 min of bottle on an Airbus, or 195 on a -400. Irrelevant, you will be on the ground before it empties.

If it doesn't go out, the process is quite simple. You have 12-15 minutes to blow up your floaties, and ditch.

Weather. Hmmmm, I haven't done too many ETOPS flights, but I can assure you I have not seen too many people check the ERA WX after departure.

If the excrement hits the cooling apparatus, WX, RFF, ATC, all become somewhat irrelevant.

I have had an engine burp on a twin, 25 min away from a suitable strip, and I have had similar on a quad, 700 miles from Shemya in winter. I will take the latter every time.

Assuming the WX is reasonable, and you have the gas and terrain clearance, you can lose an eng 1 hour into a 15 hour flight, and still continue to dest (Legally. Though obviously not too desirable.)

Lose on on a twin, and you're making a distress call!

[Ignition Override]

From what I've read about interesting evaluations of certain divert airports in Siberia, I would not want to be flying almost three hours on one engine through winter skies with a tribe of women/young children onboard while heading to an airport which might have the electricity shut down, planning to land a very heavy plane from Japan onto a runway (with functioning navaids and lights?), which we assume has been plowed free of snow, and a cozy, warm (?) little terminal with food and water available, possibly an airport fire department and medical clinic...

Heck, many of these things might not always available or reliable at certain dispatch-required driftdown airports in the western US! How about in poor, bankrupt countries? The airlines might have been better off using planes with at least three engines, but then some airlines became unhappy with the limitations of old DC-10s (possibly paying three cockpit crewmembers, versus two), and the payload/range figures on the MD-11s, despite having one less pilot to pay.

[crackerjack]

Localiser Green,

You say that:

It is pretty fair to say that the chance of both engines on a twin failing at the same time for mechanical reasons is effecitvely zero. If the chance of even a single engine failure is 1 in a million, then if I remember correctly from doing probability at school, the chance that both engines will fail at the same time (for purely mechanical reasons) is 1 in a million multiplied by 1 in a million, which is 1 in 100,000 billion.

I have to say that I'm less than convinced by your 'O' level probability theories, what if the engine fails due to a faulty component? I would suggest that the probability that the other engine has the same part fitted is somewhat better than one in one hundred thousand billion.

So yes, I do think your argument is flawed.

[Fresca]

Crackerjack.

True...the same part would also be on the other engine. But that argument works also for a quad, there you have 3 same parts that could fail therefore the liklyhood of an another engine failure is much more.

An ETOPS check is done before each ETOPS flight to make sure everything is up to par on a twin. Is there such a requierment for quads?

[delarocha]

Four engines are best for very long ranges

The reason why a four-engined aircraft is better at long ranges is essentialy twofold. First, all airliners are designed to continue to fly safely after suffering an engine failure during take-off. This means that each engine of a twin must have enough power available to allow the aircraft to continue its take-off. Carrying such an excess of power (in the form of heavier engine with more drag) as a marginal effect over moderate ranges, but becomes an unacceptable penalty over longer ranges. By contrast, a four engines aircraft loses only a quarter of its power when it suffers an engine failure, so the additional power required from each of the remaining three is not as great as from the remaining one on a twin.
Secondly, the structure of the wing of a four engine can be lighter than that of a comparable twin, since the location of the four engines better compensates the normal upward bending of the wing-root during flight. Conversely, with a common wing the MTOW of the four engined version can be 20 per cent higher that of the twin, as is the case with A340 and A330.

Regards

[Groundloop]

Congratulations, delarocha, for bringring this thread back on track and away from the inevitable "4 engines is safer than 2" debate that it had veered off on. Your response is the correct answer to NWSRG's question. You only just beat me to it!

[jtr]

If the odds of both engines failing on a twin are so slim, then what was the reasoning behind CX grounding their RR powered 330's a few years ago (after an increase in engine failures)

[Slasher]

Having flown the 747, give me 4 donks for any long over-water crossing anyday. Twins are for long haul over-land or within 60 mins @ SE GS in my personal safety book!

[PAXboy]

The point raised about the same component existing on the second engine and so forth ... on an ETOPS thread a few months ago, someone kindly spelt out the details of ETOPS.

Things like - each engine must be worked on by a different team, so that the same human factor mistake cannot take place.

I have to say, after reading the long list, I was greatly reassured but ETOPS is driven only by money. If it is cheaper - they will do it.

[OldAg84]

lomapaseo

you are right- I stand corrected... it was UAL with P&W's (I double checked on the web) my mistake...on the plus side I might look ignorant but I don't look like an anorak, eh...

All the 12 hours worrying about nuthin'......jees...

Or I couldv'e just told you it was a charter.....

[lomapaseo]

Old Age84 < you are right- I stand corrected... it was UAL with P&W's (I double checked on the web) my mistake...on the plus side I might look ignorant but I don't look like an anorak, eh...

All the 12 hours worrying about nuthin'......jees...<

No worries, the GE Quality Dept could use some prayers
:D

>If the odds of both engines failing on a twin are so slim, then what was the reasoning behind CX grounding their RR powered 330's a few years ago (after an increase in engine failures)
<

The concern was that with such a small fleet with just another two or three IFSD that the probability could have worsened by a factor of a thousand. What you do in a case like that is *restrict* the operations while you sort out the suspect population. Not a big deal with small fleets BTDT

[jtr]

Lomapeso, sorry, I was playing devils advocate with a rhetorical question.

The reason they grounded the fleet was due to a component failure (step aside geasbox drive shaft) which related to a metallurgical problem created during manufacture by a RR contractor.

You are correct in your analysis of the statistical side of things though. There was a figure floating around on the odds of a multiple fail, based on the failure rate at the time. Can't remember the #, but I recall it wasn't pretty!

[Max Angle]

Well I fly a twin jet and have never flown a 4 engine aircraft, the company flies ETOPS on A330's (well two of them anyway) and I would love to get on the fleet and have a go.

BUT

Having a lost an engine mid atlantic would I be happier flying a large jet single or a large tri-jet?, easy answer I reckon.