ok lets look at it this way.... if an aircraft has an MTBF of 1:100 000, if you have 2 engine sthere is more chance of that engine being on your aircraft. For example lets consider say a lycoming O-360 example as a whole... quite a popular make of engine on both singles and twin engine aircraft alike...... who has the greatest chance of that engine being on their aircraft? true of however only slight.
Secondly, if an aircraft operates on four engines with a set MTBF.. does that mean because you clock up 4 engine hours per flight hour and if you compare it to an aircraft with two engines (both engine types having simliar MTBF's) that per flight hour the chance of an engine failing on the 4 engines aircraft if increased because the engine logs more hours over a shorter period of time....... hence greater the engine, greater the chance... even using that MTBF formula i saw before.....
