Bula,
Woomera is correct, a modern engine is less likely to fail than an older one.
E.g. a PA-31 is more likely to have a double engine failure than a PC12 is to have a single engine failure.
However, if you have two or four engines from the same era, well then maths is complicated, as the mean time between failures (MTBF) is not consistent.
The MTBF tends to be higher as an engine is introduced into service, and as the engine becomes more widely used, the statistical base improves (more engines fly more hours), as does the engineering fixes to problems that reduced the MTBF when the engine was initially introduced.
Many people were suggesting that two engine aircraft were safer than a four engine due to the engineering practices employed when flying ETPOS, independent inspections, different maintenance periods etc.
However many airlines have seen the benefits of these inspection regimes, and have since employed the same techniques on the four engine fleets.
Others suggest that a two engine aircraft like the A330/B777 is safer than an four engine, as a two engine aircraft is required to have longer fire suppression than say a B744 which does not need to as its not ETOPS. This does not apply to the A340 as its has the same equipment as the ETOPS A330.
Others suggest that the number of engines is directly correlated to the number of systems or performance the aircraft will loose, this is also incorrect.
