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There are many demands on aircraft engines. First they need to generate a great deal of power to provide sufficient thrust, the force that moves an aircraft forward. The engines operate at maximum power during take-off, but even during other phases of flight, such as cruise, they operate at high power settings. An engine should not add too much extra weight to an aircraft and should be of such a design and shape that minimizes drag (the reacting force caused by the passage of the airplane through the air). Engines also need to support fuel efficiency and be capable of operating effectively at extreme temperatures. Finally, there is increasing pressure on engine designers to minimize damage to the environment.
The most important requirement for an engine is that it should be reliable. Engine failure has potentially very serious consequences for a single-engine airplane. A lot of the basic training for pilots of such aircraft deals with how to make safe emergency landings in such situations, basically gliding the airplane that has lost all power. This becomes much more difficult in mountainous terrain or over water. There are thus certain areas where flying a single-engine aircraft is highly risky and in Europe it is a requirement that a passenger jet be a twin-engine.
When one engine fails on a twin-engine airplane, and despite improving levels of engine reliability this does happen on commercial jets, it is usually a fairly routine procedure to divert to the nearest airport (a pilot will always have, as part of the flight plan, a list ol suitable diversionary airports). The pilot can also sometimes reach the scheduled destination. It all depends on his / her position and fuel situation.
One of the most dangerous times for an engine to fail is during take-off, on a twin-engine aircraft the thrust is unbalanced and the pilot will have to take immediate action to correct this. In some circumstances the pilot may be able to abort take-off (this is authorized if travelling at less than the decision speed, termed V1), but if travelling at higher speed then the required (and safest) procedure is to take-off in any case. This was the case with the Concorde accident mentioned above, where the pilots knew they had a problem before lifting off. Exceptionally, a captain has the authority to go against this and abort take-off, but with the aircraft travelling too fast the consequences may be very serious and it's a decision that will need to be justified afterwards. That's not to say that the recommended decision to continue take-off might not have equally serious or worse consequences.
Failure of one engine on a four-engine airplane, at least while cruising, should not be so threatening. If a pilot is worried about the unbalanced thrust, he / she may take the decision to shut down the corresponding engine on the other wing and fly reasonably safely on two engines.
In an incident in February 2005 on a four-engine Boeing 747, a British Airways captain had to shut down one engine just after take-off from Los Angeles. Rather than deciding to return to the airport, he chose to fly on across the Atlantic on three engines and landed safely in Manchester. The US controllers expressed their surprise at this decision but the pilot had the full backing of British Airways. The incident was the subject of much debate amongst experts.
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