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Six sigma is a concept usually only used in manufacturing operations to manage quality. I don't see how it can be applied to aviation except for aircraft factories.For airplane operations the notion of safety is handled somewhat differently. There is not acceptable level of flaws or failures. Instead there is a level of redundancy in airplane systems and constant training for the weakest link in the safety chain - the pilots.In the small planes that I fly there is generally no reason or excuse for fatal or very serious accidents. Such accidents are nearly always traced to considerable failure on the part of the pilot in command. Even when there are significant mechanical or other system failures the pilot should be able to handle it and still have everyone in the plane walk away after landing.Small planes become gliders when the engine quits. They fly slow enough to be able to make a credible landing just about anywhere except on water. For that reason many pilots refuse to fly over water unless they are within gliding distance of land. Safety for engine losses comes from altitude. The higher the plane is when an engine quits the more choices the pilot has for a place to make an emergency landing. In most cases there is a real possibility of damage to the plane but the occupants should get through the experience with nothing more than perhaps a few bumps and scrapes.Less critical equipment problems are much easier and less dramatic for the pilot to handle. For example a navigation radio might fail. In that case, the pilot needs to use a different form of navigation to reach his destination or an alternate destination that is easier to find. He can also get on the radio and seek help from air traffic control. Large multi-engine planes are even safer than small single engine planes. They can react to an engine loss and continue flying at somewhat reduced performance. For some small twins this might mean a gradual descent while navigating to a nearby airport. For airliners it usually means continuing to the planned destination possibly at a reduced speed. Even with airliners the pilot in command might choose to land at a nearby alternate rather than continuing to the expected destination.In all cases the only kind of failures that cannot be handled by the pilot tend to be either total structure failures for the airplane or total failure of the pilot such as a sudden heart attack. Airliners always have a second pilot to deal with such medical problems. Regular passengers in small planes (such as the pilot's spouse) often seek sufficient pilot training so they can land the plane in a case where the pilot becomes incapacitated.Back to the safety numbers question where this all started - each year there is a significant report written that gives important summaries of all the aviation accidents that occur. Accident rates and fatal accident rates are expressed in numbers per 100,000 flight hours. For a typical light plane type this might be something like 7 accidents per 100,000 hours with 2 fatal accidents. Since these are broken down by aircraft type it is possible to discover which planes are prone to accidents more than other designs. Even the poorest designs typically have accident rates of perhaps 20 per 100,000 hours and/or 5 fatal accidents. This kind of aviation is more dangerous than airline flight but is still very safe.Airline accidents are extremely rare. With some 10,000 airliners in the air any time of the day in the USA and many years between accidents there isn't any practical way, in my opinion, to apply numeric analysis to this issue.
