Ask Captain Lim

Hello Captain Lim,

A lot of people claim that twins are more economical than planes with four engines.

Can you explain the reason for that?

I know that twins have to be rated at double the amount of thrust normally needed, in case of engine failure at take off, and that quads only have to be rated at 33.3 percent more than normally needed, in case of the above mentioned.

So the engines on a wide body twin have to be huge compared to those of a quad. One should think that would also make them consume more fuel than the smaller quads?

Or do they win in the long run by the fact that they might need a lower thrust setting than quads while in cruise?

Thanks for a great web site.

Tonny,
Denmark

Hi Tonny,

Since you are asking a question about the twin versus four engine planes, it is only fair that we look at two planes of a similar category ? the Boeing 777 and the Airbus A340 - not against the 550-seats Airbus A380.

Yes, the debate is quite fierce between the two manufacturers - starting with the Airbus* billboard advertisement at the edge of a runway at the 2002 Farnborough Air Show that read, "A340 ? 4 engines 4 long haul", which suggested that twinjets like Boeing 777 was not as safe or reliable as the four-engine airplane like the Airbus A340.

Proponents of four-engine jets claim that, if a twinjet loses one engine, the airplane and passengers could get stranded at an alternate airport that is remote and inhospitable whereas a four-engine jet could continue to its destination.

In reality, when an engine fails, depending on the circumstances, the safest option is to land at the nearest airport rather than to continue. Also, it is pertinent to note that, more than 90 percent of airplane diversions have nothing to do with engine failures but rather one involving bad weather, a sick passenger or other non-technical reasons.

And anyway, according to the Boeing records, the four-engine A340 has had twice as many diversions and turn backs than the twin-engine Boeing 777. Further, the A340 has a lower average dispatch reliability rate than the 777.

Why so? Twin-engine planes operating on ETOPS routes are required by regulations to operate at a higher standard and reliability than four-engine ones. As a result, ETOPS rules have been increased to more than three hours today.

Records have also shown that twin-engine planes are more efficient, economical and more reliable than four-engine ones. Remember, airlines around the world have flown almost 4 million ETOPS flights to date. Since operating with twin-engines is proven safe and reliable, it only makes sense to take advantage of the increased efficiency they offer. They consume less fuel, weigh less, have fewer emissions and operate more efficiently (5% to 9% lower in operating costs)

Why the twins are more economical? One reason is attributed to the engines designed and installed on them. Secondly, it could be due to the structural design of the plane (the Boeing 777 is the first plane to be fully designed by computers).

Yes, the engines on a wide-body twin are huge as compared to those of a quad. So, it natural for them to consume more fuel than the smaller quads but overall, they are more or less comparable.

Do the twins win in the long run by the fact that they might need a lower thrust setting than quads while in cruise?

Engines are designed by the manufacturer to operate at optimum settings whether installed on a twin or quad. So a twin might not be operating at a lower thrust setting but rather at the recommended one for a particular cruising level.

Remember, the normal cruising speed of a Boeing 777 is 0 .84 Mach whereas an Airbus A340 is at 0.82 Mach and yet, the former plane still operates at a lower cost!