I think the idea that you have twice the chance of a failure in a twin has grown from the conception that as you have two you have two engines to potentially fail which doesn’t mean twice as likely to fail.
Is this yet another fallacy? I thought we were on a roll.
If an engine fails once in every 1,000 hours, and never goes beyond a thousand hours, and the failure rate is linear, then there is twice the probability of one engine failing on twin in a given time frame because both engines eventually must fail on the twin whereas the single has only got one engine to fail.
In reality there are other factors involved that even where the same engine is concerned the probability of a failure in a twin will be different from in a single. Moreover engine usually dont fail before they become either time expired or have adequate work to enable them to continue to operate on extension where this is permitted.
In reality, if you were to plot hours against failure rates (or at least problems identified requiring remedial action) would you end up with a bell curve? If so is that also why it is common to see twins with engine hours that are not the same?