Why do commercial airlines have different number of engines?

Question

why does an A380 have 4 engines but carrys nearly twice as much as a 747 which also has 4 engines? why does an A340 have 4 engines but a777 has 2 engines but has a longer range and the difference in payload is not very much? the 707,727 and the 737 have more or less the same range and payload but they have 4,3 and 2 engines respectively. why does the 787 have 2 engines and is faster and uses less fuel? who knows about the A350 or the 747-8?

Answer 1

The number of engines a plane has is directly connected to how big/heavy the plane is, and the length of the routes the plane is expected to fly.

When the 707 was designed, the biggest/ strongest engines they had for the size of plane made it necessary to use 4 engines to get enough power to safely fly. Then a few years later the 727 came along, the world and the engines in it had evolved and they were able to use just 3 for the same task. The 737 with 2 engines was the same reason.

Fewer but bigger/ more efficient/ powerful engines means less fuel burnt and less "dead" weight" on the plane. Two small engines will always use more fuel than one, due to twice the amount of movable parts, and a weight reduction and less maintenance for the same reason. So the bean counters like as few engines as possible.

For longer flights we have to take in to account that places like the Atlantic, the Pacific and Eastern Russia are very remote, so if one engine stops we want to be sure we can reach a safe landing place. For that reason a plane with 2 engines have restrictions on how far away from a "suitable aerodrome" (legal term) they can fly, while a plane with 3 or more engines stands more free to fly direct routings, again saving time and money for the bean counters, and in the end the passengers.

All this should mean that we use 737/A320 with 2 engines for shorter flights and flights close to populated areas, while we use 747/A380/A340 for flights over the big oceans. But it's not that easy...!

Because the bean-counters wanted to have cheaper planes to operate they got the various regulators in the Aviation industry (FAA, JAA, ICAO) to make rules that mean 2 engined planes can fly far away from land. This has been the case for the last 15 years or so and have proven very successful.

The 777, 787 and A330 can all fly up to 3 hours from the nearest airport, meaning that they can fly in a straight line almost anywhere in the world. This is called ETOPS; Extended Twin Engine OperationS. It is an approval each operator (airline) need to have to be able to use it, and it can be costly and time consuming to maintain this approval.

So the A 340 can fly unlimited, with no approval anywhere with the same amount of passengers/cargo as a 777, but will use more fuel.

As anything in the aviation world it is a trade-off between different things.

The 747 again has 4 engines for long haul flights, and could today fly with 2 bigger engines, but this would require a complete new wing to fit it.

The A380 has 4 engines each producing about 70 000 lbs of thrust, 280 000 lbs in total, while the strongest engine in production today produces about 115 000 lbs, so a power deffict of about 50 000 lbs would exist with just 2 engines, meaning it wouldn't be able to bring 800 passengers 8 hours away.

Engines are made bigger and more efficient all the time, and so are planes, so we'll always have planes with different amounts of engines.

Btw the A350 will have 2 engines, because of the efficiency, and the 747-8 will have 4 due to it's size.

So there it is, it all depends of what and where the bean-counters want the plane to fly.

:)