number of engines on planes

[pilotho]

hey this has been bugging me for a while now and i have to write an essay on it now!

basically, why does a B777 have 2 engines while an A340 have 4. they are both very similar aircrafts in terms of weight and cruise speed, so what are the advantages of both designs?

[Gargleblaster]

To be certified, an aircraft needs to have enough power to continue flight safely if one engine fails just after takeoff.

Say a given aircraft needs 100 units of thrust to fulfil this.

If you equip it with 2 engines, they need to yield 200 units of thrust total.

If you equip it with 4 engines, they only need to yeld 133 units of thrust. So you have some savings here. People wiser than me will elaborate on weight and fuel burn implications :-)

Also, 2 engined aircraft have some limits imposed upon them for flight over water, they need to stay withing X minutes of flight to a suitable airport. Think the regulations are called ETOPS.

[pilotho]

sounds like it would be wiser to have future planes to have 4 engines right? since the engines can be of smaller ones, also it doesnt have to comply with ETOPS. yet not that many planes have 4 engines......

i think boeing built the 777 to compete with the a340 right? so they must have a reason to design it with 2 engines and also no winglets! which i dont understand the reason to.

[fyrefli]

sounds like it would be wiser to have future planes to have 4 engines right? since the engines can be of smaller ones

Hmm... Methinks you might what to come at this one from the other direction :)

Just whisper it in Toulouse ;)

Cheers,

Rich.

[Piltdown Man]

Remember that rules and legislation come from people called politicians. These people are lawyers, not scientists - just 21st century Snake Oil salesman. They'll listen to the press, their friends etc. but rarely to scientific argument. As consequence, they believe that more engines equals more safety and therefore less stringent operating conditions can be allowed. Therefore, aircraft are still built with four engines when two would have done. Unfortunately, the most likely reason for diversion is icky pax and other non-technical reasons which are nothing to do with the plane. And when you look at four engined ops, what is the most worrying? Pressurisation failure at the wrong place!

[Mad (Flt) Scientist]

Swings and roundabouts, different compromises, is the answer.

As noted, 2 engined a/c by regulation end up with significantly more thrust when all engines are operating than a 4-engine aircraft, assuming both meet the mninima for one engine inop.

That makes two engined aircraft less close to the design goal for AEO ops, since they are 'overengined'. That's usually bad. (in terms of design optimization).

It also gives a typical two engine aircraft more performance margin for AEO - e.g. for windshear. that's good.

Four engined aircraft can afford to lose two engines. Two engined can't, obviously. Four engines good.

But four engines instead of two means roughly twice the inflight shutdown rate. So four engined aircraft have more engine failures. That's bad.

Four engined aircraft may well end up lighter (due to less thrust excess by design). That's good.

But 4 sets of engine 'stuff' - gearboxes and nacelles and the like - may weigh more than two sets of bigger stuff. That's bad.

And twice the number of engines may cost more to operate due to maintenance and the like.

In the end, someone waves their hands in the air and does a few sums and tries to guess which will be better. Sometimes it just comes down to what's available - if you're designing a new a/c and don't want a new, expensive, engine, you may have to choose it off the shelf. that may limit your choices such that only one of the two options is viable.

[faultygoods]

so they couldn't make up their minds in the case of the tri star, L1011 and DC10 eh?????????????

Winglets.....a more recent innovation, designed and perfected a little too late for the B777:=

[Old Smokey]

so they couldn't make up their minds in the case of the tri star, L1011 and DC10 eh?????????????
So they had 90,000 Lb thrust engines available back in the 70's when these aircraft were designed, did they?

Given that the aircraft mentioned had 3 engines of APPROXIMATELY 45,000 Lb thrust each, 90,000 Lb (2 engines) would be required to meet the OEI requirement. To make the L1011 and DC10/MD11 two engined aircrafrt would have required engines of a little less than 90,000 Lb thrust each.

Nice post, Mad (Flt) Scientist, a good summary in simple terms. I think that you neglected to mention that the IFSD (In-Flight Shutdown) Rate for the 4 engined aircraft is somewhat more than twice that for the 2 engined aircraft. In Normal operations (i.e. the 99.999% of the time that an engine DOESN'T fail), the 2 engined aircraft is operating with 100% thrust excess above the minimum requirement, whilst the 4 engined aircraft has a mere 33% excess. Thus, the 4 engined aircraft is operating at high thrust levels (Takeoff and Climb) for a MUCH longer period, and, as most turbine engine failures are due to cumulative stress, and the cumulative stress for the 4 engined aircraft is much higher, with a commensurately higher IFSD Rate per engine than for the twin.

"Four engined aircraft can afford to lose two engines" - True, but not on Takeoff, or early in the flight at high gross weights, unless you can dump fuel fast enough before impact.

"Four engined aircraft may well end up lighter (due to less thrust excess by design). " - Absolutely true, plus lighter wing structure needed to provide wing bending relief due to the outboard engines. The 3 engined aircraft is WORST in this respect.

"And twice the number of engines may cost more to operate due to maintenance and the like." - Absolutely true again. Increased size does not necessarily mean increased labour for overhaul. Roughly, twice as many engines cost twice as much to overhaul, irrespective of size.

Mad (Flt) Scientist, not in any way a criticism of your post, it was an excellent summary provoking further thought and comment.

Given that 99.999% of the flights that I've operated in 2, 3, and 4 engined aircraft did NOT suffer an engine failure, my clear preference for safety is for the aircraft with the greatest performance margin to get out of a sticky situation - the twin.

Pray that the IFSD Rate never reaches ZERO, because when it does, the accountants will be calling for single engined aircraft:*

Regards,

Old Smokey

[vapilot2004]

so they couldn't make up their minds in the case of the tri star, L1011 and DC10 eh?????????????

......and the Trident and 727....... All of the tri-jets were designed and built in a time where more is better - fuel costs were less important - the industry was fat with cashflow and image was everything.

A large 2-engined airliner was unthinkable back then - not only due to the prevailing beliefs of the designers and the engines that were available to them - it was also the collective public memory of the not-so distant piston powered days would make a transoceanic twin a most unattractive conveyance for an airline to sell to passengers at that time.

[Gargleblaster]

A large 2-engined airliner was unthinkable back then

Isn't this still true ? I'm thinking about the B747 and the A380.

Is there an engine powerful enough today so that a pair can drive such beasts ?

[B Fraser]

Also, 2 engined aircraft have some limits imposed upon them for flight over water, they need to stay withing X minutes of flight to a suitable airport. Think the regulations are called *ETOPS.

*ETOPS = Engines Turning Or Passengers Swimming

Politics does indeed play a part, the A340 uses French engines supplied by Moulinex. Earlier this year, I was on an Air Jamaica A340 from Heathrow to Kingston and I am sure we ran a red light in Hounslow High St ;) Thankfully, the larger -600 has good old RR engines.

Interesting point about the 727 and Trident, the Trident 3B had four engines with the extra donkey being housed above the tailpipe of number two. I vaguely recall the spare engine being used to give crews a four engine qualification.

Getting back to the 2 v 4 discussion, all twins tend to look the same these days which makes the world a dull place. Thankfully the A380 looks distinctive....fugly but distinctive

[pilotho]

in my essay i have actually finally decided to build the new plane, which should be heavier than a a380 with a 3 engine design.

that may sound mad but i have taken into account "asymmetric thrust" into main consideration. i have thought that 2 engine would not generate enough thrust whereas 4 engines is the main contributor to "asymmetric thrust" if an engine was to fail.

so i was thinking with 3 engines, the rudder could be smaller then and also with 3 engines the weight would be further reduced. i know the wing would have to be further reinforced but i was wondering if the other 2 factors would outweigh that.

finally, did the decision of the a340 having 4 engines was influenced by politicians?

[Old Smokey]

did the decision of the a340 having 4 engines was influenced by politicians?
More likely influenced by the fact that there are no engines big enough to make it a twin.....YET!

Regards,

Old Smokey

[PAXboy]

pilotho you have no idea the debate that you have reignited as powerfully as Concorde's reheat (and that's 4 engined re-heat ;) ) Just for the record, here is the definitive answer.

Two engines only look good on small machines. The 767, 777 and 787 are swollen beyond what nature intended and cannot ever look good.

Tri-motor machines have always been a horrendous abberation and the sooner the last ones are retired the better.

Four engined machines are where it's at, however, this is not to say that all x4s are good - the 380 is ugly as sin and even if I overcome my conern about evacuation times from it, I would not want to be travelling in something so hideous.

The 747 has a natural grace and authority to it that is timeless. The DC8 + 707 tried but, ultimately, only looked average. Which, since Concorde is in a class of beauty all her own, leaves us with the VC-10. Yes, that is the one to hold in your mind's eye as the reason that man created four bl@@dy strong engines and strapped them on to the place where they would look the best.

I can now leave PPRuNe, content in the knowledge that i have insulted almost everyone!

--------------------
"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.

[john_tullamarine]

One point which I don't recall seeing noted in the earlier posts.

Pre-ETOPS there was an operational and design restriction on distance unless the aircraft could operate with two engines shutdown. Hence twins were limited other than in areas with a reasonable aerodrome density. Three motor jobs were a cheaper way to get around this problem than were four motor jobs.

[Wizofoz]

It should also be noted that the ONLY reason any four engined aircraft is currently in production is that, at the time it was designed, there weren't engines with sufficient thrust to make it a twin. Before anyone jumps up and down, name one quad which COULD have been a twin?

Nobody uses a four engined aircraft where a twin will do the job.

[wonderboysteve]

A340-200/300. Less maximum thrust than an A330-200/300.

[Wizofoz]

Less thrust, but 45 tonnes extra max AUW due to the different rules for 4 vs 2 engines as explained above. There were no engines capable of lifting a 275 tonne aircraft in twin configuration, so they HAD to go for a quad.

Now better technologies are available, what are they developing? the TWIN engine A350, with no plans for a 4 engine derivative.

[yachtno1]

VA will be running out of options soon ! :D

[ICT_SLB]

Don't forget that both the 727 and Trident were not designed for optimum efficiency but to make very specific - and taxing - performance requirements. The '27 was built to a United requirement for a transcontinental aircraft that could get out of La Guardia (short runways, high temps in Summer) and the Trident to meet similar problems with Heathrow/Mediterranean. The ultimate Trident IIE added the RB166 lift engine (the 4th) just to do the hot weather performance with increased loads.

[Ignition Override]

Was the much smaller Bae-146/RJ-70 Avro (60-75 passengers)designed with four engines because of an RAF requirement to transport Her Majesty the Queen over water, and having two engines created too much concern?

If this was the case, then lesser mortals are deemed an acceptable risk for the majority of newer aircraft which all, except for the MD-11 and A-340, Boeing 747, just plain economics?

The US FAA would call it cost/benefit ratio during the regulation, certification process.:ouch:

[archae86]

Most designs must pick from a rather small range of suitable engines likely to be available in the design timeframe. Old engines are out--noise, specific fuel consumption, so the menu is shorter than you'd think.

Just imagine that an A380 design person had the bright idea of making it a twin. Other issues aside, the prospect of working over the engine manufacturers to design something so far outside their current range for a modest total units shipped would have stopped this thought promptly.

Though I don't know the history, I'd guess the BAe-146 choice of four engines was likely driven by engine availability, as its short range and other characteristics are firmly in a slot dominated by twins.

You'll notice that long-range aircraft are those more likely to have more engines. With the rule that takeoff must succeed with n-1 engines at the awkward moment implying higher engine count gives less engine weight, the weight benefit of more engines is more helpful at long range (review the Breguet range equation if this point seems obscure).

[CAT1]

The B727 was originaly designed with 3 engines due to approach minimas at that time being more restrictive for 2 engne aircraft than 4. Boeing persaded the FAA to grant the same minima as a 4 engine aircraft for the 72.

[Paradism]

I think the basic driver on the number of engines on an aircraft is simply financial.

There are all kinds of arguments being put forward about safety, ETOPS, take-off parameters etc., but surely the aircraft manufacturer will provide an aircraft that can comply with the requirements at least cost and lowest running costs.

It is naturally possible to expand the argument in all sorts of directions but to cut to basics, two engines will cost less than four, they will be more efficient and therefore use less fuel.

Do you suppose that the Airbus A380 would have two engines if there were large enough power units available? I think it quite possible, if the Certification Specifications could be met.

[ElNino]

The 146 has 4 due to limited availability at the time (late 70's/early 80's). It had to be quiet so the option of putting 2 JT8D's on it was not feasible and at the time there wasn't any other choice other than a tank engine.

[Doctor Teeth]

ICT_SLB
As far as my memory goes, the Trident IIE was like all the other Tridents in that it had 3 Speys. It did however have some help in the in hot weather by using water injection. It also had a fin fuel tank to give it more range. It didn't have the Boost Engine though.
The IIIB was the the one with the Boost Engine (the RB162) which was needed because the poor old Speys were flat rated to 15 degrees C and needed all the help they could get

[barit1]

747 or A380 as a BIIIIG twin?

I think it could be done - theoretically - with today's GE90-115 or the like.

But it wouldn't operate at anything like the TOGW of the current designs. It would be a short-range plane for hopping around Japan or the US E. Coast or Western Europe.

[Old Smokey]

I heard it in the pub, and the pub was probably where the story began, but.......

Is there any truth in the story that, as 30,000 Lb thrust (or so) engines became available, Lufthansa investigated the possibility of re-engining their B727s with 2 X 30000 Lb engines with a totally new (Boeing built) Section 48 to match the entire assembly?

Or was it just another pub story?:p

Regards,

Old Smokey

[Wizofoz]

One of the last designs MD looked at before being taken over by Boeing was a twin engine MD-11 derivative.

[barit1]

Lufthansa investigated the possibility of re-engining their B727s with 2 X 30000 Lb engines with a totally new (Boeing built) Section 48

Yes - it was one of many re-engining schemes on the drawing board in late 70's/early 80's. The DC-8 series 70 was the only one that made it as a retrofit, and of course the 737-300 totally revitalized that bird.

[brain fade]

Aren't the A330 & A340 essentially 2 & 4 engined versions of, basically, the same aircraft?

Thus proving that each has merit, or am I talking sh1t again?

[ICT_SLB]

The 146 started life as the HS681 - a VSTOL airlifter that was intended to support the P1154 (Supersonic Harrier II) force. It was powered by twin Pegasus with the efflux (one each side of each Peggy) being entrained by the flaps. To get the same effect required 4 small conventional jets. Again engine power, as such, had little to do with the choice of number of engines.

[CV880]

Regarding the DC10 and L1011 I recall they grew out of an American Airlines requirement for a 250 seater that could operate non-stop across the USA off La Guardia's runways and AA envisaged a big twin. Both Douglas and Lockheed looked at future growth options and hedged their bets by going for 3 engines. Douglas did look into a twin version of the DC10 later but it was not economically viable and was dropped.
AA supposedly said some years later that the A300 was what they were after, the DC10 being too big, but it came along a few years too late although AA did eventually buy some A300-600's.
I can remember Boeing stating at a presentation back in the late 80's/early 90's that no-one would build a 3 engined aircraft again as it was not economical. In their opinion all new conventional designs would be either twin or quad. The MD11 was an exception as it was a derivative and could be cost justified. Airbus said more or less the same thing.

Brain Fade,
Yes, the A330 and A340 were designed as a common airframe. The system architecture is common and the wing aerodynamics are the same. They are built on a common assembly line and share a single serial number sequence ie. the first A330 was MSN 12 the previous 11 being A340\'s.
The A340 was supposed to be the long range model and A330 the intermediate range one however there became a demand for a longer range A330 so the A330-200 appeared.

[barit1]

Strangely enough the A300 (which Eastern leased and later bought) was not permitted to operate from LGA. Something obscure like tire loads in lowspeed taxi turning. Had EAL been able to operate the Bus into LGA their demise might not have been so quick.