Question 1: In a "mid-aged" NG, how often does a random failure happen?Question 2: In a NG, what is the probability of an engine failure during take off or on flight?Thanks,Teo Halfen

1. No idea for the 737NG but I recall a friend on the A320 saying he sees the 'occasional transient indication' with an outright failure being a less-than-annual event.2. I'm going from memory here but it's in the order of one every million hours I think. The CFM's are a pretty reliable engine.Bear in mind that most of us would be doing well to fly 200 hours a year in FS. Airline pilots fly a LOT more than that.So, if you're looking for a failure to occur at a reasonable interval in FS so as to be interesting, you'll need to set it according to how often you usually fly.

1) Hard to say... the electrical boxes are strange things... they can break when new, or will break after 10 years... You can have months without failures, and a day with more than one. In most cases a reported failure is not really a failure but a momentary glitch on the system.Mechanical failures are prevented with correct maintenance (also electric one).Same for engines, they are revised, changed at intervals, failures are random, but mostly present when engine is near to be changed and revised. The critical part for the engine is the take off when the aircraft have full power, expecially when derate and flex are not used.

Not sure about the general system failures, but I seem to recall one of thepilots of the Sioux City DC-10 crash saying that engine outs are fairly rare.He said many pilots can fly a whole career and not lose an engine.Of course, some do lose one from time to time..I was listening to Hobby tower one day, and heard a guy blow an enginesoon after takeoff. He decided to just continue on to Dallas with one engine,rather than returning to Hobby.I'm not sure what he was flying, but judging from the company he wasworking for, it may have been a MD-82, etc.. I don't think it was a Boeing.

If it was an MD80 it was equipped with JT8 engines... pretty old, louder, and less reliable than actual CFM engines.