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in the background. The lighting system was then modified to reduce this confusion, and it is significant that there were no more similar accidents.

DESIGN

Furthering this principle of outside influence as a cause of "pilot error", it would appear that the airplane design should have quite an effect. McFarland(2) has said:

"The fact that accident rates vary from model to model suggests that the airplane itself may play as important a role as the human element.... It is commonly believed that certain air transports are generally safer to operate than others. The evaluation of such a statement is almost impossible because of the many variables involved or the lack of a common base line of comparison."

Since this statement was made, statistics have only recently become available through the CAB which make it possible to attempt such an analysis. These are presented in Figure 2, "A Comparison of the Accident Rates of Widely Used U.S. Transport Aircraft." The figures presented here apply only to scheduled flying by U.S. domestic airlines, the international figures not being available. Although there are many factors involved, it is felt that the variables of crew competence, maintenance, and operational

techniques are largely eliminated through the close CAA supervision and regulation of the scheduled airlines.

In analyzing the data, it is significant in the case of the twin engine planes to note the marked correlation between the accident rate and the accumulated mileage, and the lack of correlation between the accident rate and the period of years over which the planes flew. This leads to the conclusion that it is the original design and its development as accumulated mileage shows up weak points that determine the safety level rather than the skill of the pilots. If the latter were a major determining factor there should be some correlation of years of service with the accident rate. It is a fact, however, that piloting technique becomes soon stabilized on each type of plane and varies but little over the years and the pilots quickly become proficient in it.

In looking at the fatal accident rates, it is noteworthy that there are four types with perfect safety records. Most significant of these is the CV-240, although the accumulated mileage of the other three is sufficient to take their combined record out of the realm of pure chance. By all averages the CV-240 should have had three fatal accidents.# In fact, it has had six very serious accidents but in all the survival rate had been 100%. This suggests strongly that