Originally Posted by
bjornhall
The usual error of trying to draw conclusions from statistics using only mean values. One needs higher order moments as well in order to draw any kind of meaningful conclusions from the data itself (at the very least some variance measurement). If one were to do that, I guess one would find that the variation among pilots is so high that the mean values are quite pointless.
The other big problem is that the sample sizes are too large. People try to create large sample sizes by grouping together all sorts of pilots undertaking all sorts of operations in some belief that large sample sizes make the statistics better. What one ends up with is a fairly accurate value of something nobody is interested in measuring. If I want to know, say, my probabilities of having an engine failure, I want a sample consisting of pilots similar to me doing similar things in similar planes. The proper sample size is one individual, and the result will not be available until my career is over!
The third rather obvious point is that engine failures are not random events. An engine fails for a reason. If one has a perfectly good, healthy engine that is handled properly, the chances of it failing is close to zero. If there is a fatigue crack in a rod with a remaining life of some 3000 cycles, the chances of the engine failing in the next five minutes is about 100%. Since one does not know if one belongs to the former or the latter cathegory, one fundamentally has no idea of the probability of having an engine failure during that flight. Statistics is useful for all sorts of things, but not for making predictions of individual events.
I agree that one has to be carefull not to attribute too much accuracy to a statistical value but I think the accident trends provide usefull information. The 80% number came from light aircraft that had an engine failure and had to execute a off airport forced landing or were damanged. I think it is safe to say that the majority of light aircraft hours flown are either as training or as a private flight. The fact is that the majority of crumpled airplanes sitting in a field got there because they ran out of gas or let carb ice stop the engine. Very few had an engine failure because of a "fatigue crack in the connecting rod", or any other catastrophic internal engine failure. To reduce risk one must be able to assign an appropriate level of risk to possible events. I think the engine failure statistic is of value in reminding all pilots that they are in direct control of the major risk factors that are the most likely to cause a pilot to experience an engine failure.
I suppose if you are convinced that you will always takeoff with sufficent uncontaminaded gas with the fuel selector always correctly set and never let carb ice develop then the accident statistics that show many pilots have in the past failed to take those steps and suffered the resultant engine failure...... you would feel that the information is of no value to you. Obviously each pilot must decide for themselves how much weight to put on the statistics and whether or not the trends that they may reveal would prompt a reexamination of how they operate their aircraft.
BTW of the private light aircraft engine failure accidents that I have personal knowledge of and which resulted in off airport landings with the aircraft substantially damaged/destroyed; 2 were "ran out of gas"(C150,C172), one was the result of leaving the fuel selector on one tank untill it ran dry, even though there was fuel in the other tank (C172) 1 one was carb ice (C150)and the last one ironically was a fatigue failure of a connecting rod (C172).