AIR CORPS INFORMA T ON CIRCULAR PUBLISHED BY THE CHIEF OF THE AIR CORPS, WASHINGTON, D. C. November 24, 1928 ANALYSIS OF AIRCRAFT ACCIDENTS

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1 N _/ liiiliiilliillb AR CORPS NFORMA T ON CRCULAR PUBLSHED BY THE CHEF OF THE AR CORPS, WASHNGTON, D. C. Vol. V November 24, 1928 No. 633 ANALYSS OF ARCRAFT ACCDENTS UNTED STATES GOVERNMENT PRNTNG OFFCE WASHNGTON 1928

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3 ANALYSS OF AR 1. A study of airplane accidents has been completed in this office with a view to determining whether Air Corps accidents are increasing or decreasing in frequency of occurrence, and what are the major causes of accidents. 2. Accidents experienced by Reserve officers, not on extended active duty status, National Guard officers, and students have not been considered. 3. t was concluded that: (a) There has been a marked reduction in the rate of accidents per flying hours during recent years. (b) Mechanical failures of all kinds (including engine trouble) have in all years been among the less important causes of fatal accidents. (c) Since the installation of the Visual nspection System a marked reduction in accidents due to mechanical failure has been achieved. (d) Fatal accidents due to bad weather have been reduced in frequency, and this cause is now relatively unimportant. (e) Pilot errors constitute the major cause of fatal accidents. (j) There are a considerable number of pilots in the Air Corps who fly, as pilots, less than 50 hours per year. There are a large number who fly less than 100 hours per year. (g) The rate of occurrence of accidents (all kinds) is only half as great for pilots who fly 50 to 100 hours annually as for those who fly less than 50 hours. The accident rate decreases steadily with further increase in annual flying. 4. n Figure 1, the rate of occurrence of accidents of all kinds per 1,000 flying hours, during recent years, is shown, also the rate of fatal accidents per 10,000 hours. The following facts are indicated by these graphs: (a) During the period , inclusive, there was an important and remarkably uniform decrease in the rate of occurrence of accidents of all kinds (dotted line), which amounted to a 72 per cent reduction during the seven years. (b) During the same peripd there was a parallel reduction in rate of occurrence of fatal accidents, the improvement amounting to 71 per cent. 5. Based on the data used for Figure 1, it is evident that the percentage of accidents which result fatally is very nearly constant. During the seven years , inclusive, this ratio averaged about 9 per cent, the m a;i,.imum variation being less than 1 per cent. Roughly, one accident out of every ten may be expected to result fatally. 6. The steady reduction in fatalities during recent years to include June, 1926, was not due to better protection afforded personnel by stronger airplanes, for if this had been the case the percentage of accidents which resulted fatally must necessarily have decreased (1) 7. The introduction of the parachute had considerable effect in the reduction of fatalities as indicated in Figure 5. However, the accident records show so many cases where pilots have landed airplanes safely following fires in the air, collisions, structural failures, etc., that the practice of considering every parachute jump as a potential fatality can not be considered accurate from a statistical standpoint. 8. Figure 2 represents an investigation of the reasons for the steady improvement in fatal accident rates during , inclusive, and the retrogression in n this figure the fatal accidents as indicated in Figure 1 have been classified according to the nature of the accident, and the rates of occurrence per 10,000 flying hours piloted for each year. The following facts will be observed upon studying the curves: (a) Engine failures rarely result in fatal accidents unless the pilot stalls the airplane and spins into the ground. Curve "A" shows that there were two recent years in which no fatal accidents occurred in the Air Corps as result of engine failure alone. (b) Curve "B" shows that fatal accidents in which engine failure is followed by a stall or tail spin are more prevalent than any others, except those due to tail spins resulting from pilots errors only. When this cw ve is considered together with curve" A," it appears probable that most of the accidents shown by curve "B" would not have resulted fatally if the pilot had not stalled the airplane and spun in. The fact that curve "B" shows a marked improvement in 1927 is largely due to the fact that the rate of occurrence of accidents from engine troubles was greatly reduced in that year; the improvement being attributed to the improved and more systematic maintenance of aircraft. (c) Curve "C" shows that fatal tail spins due to pilots errors alone are, and have been since 1920, much more prevalent than any other kind of fatal accident. These accidents exceed in number those due, either directly or indirectly, to all mechanical troubles combined. The rate per 10,000 flying hours, which had steadily improved during four previous years, was 56 per cent worse in 1927 than in (d) Curve "D" shows that structural failure of airplanes is a relatively unimportant cause of fatal accidents. Studies of nonfatal accidents have shown that structural failure in the air is very infrequent; and that more than threefourths of accidents classified as structural failure result from breakage of landing gears. (e) t will be seen from curve "E" that collisions are third in importance among fatal accidents, and that the rate per 10,000 flying hours in 1927 was the highest registered in any year, except This curve shows all collisions while in full flight; whether with trees, wires, or other obstacles, or with aircraft.

4 (J) Curve "F" foclicates that bad landings were formerly important but at present cause almost as few fatal accidents as engine failure without tail spin. (g) Bad weather, according to curve "G, is a variable but relatively infrequent cause of fatal accidents, ranking with structural failure in importance. 9. Summing up the showing of Figure 2, it is found that: (a) The reduction in fatal accident rates for the Air Corps in , shown in Figure 1, was due to the fact that fatal accidents due to mechanical troubles, pilots errors, and miscellaneous causes all decreased in frequency. (b) n every year more than half of the fatal accidents have been due directly or indirectly to pilots errors, and the average for shows 75 per cent of accidents due to such causes. t seems certain that the greater part of the reduction of accidents. in the period was due to improved technique and the exercise of better judgment. (c) The increase in the rate of fatal accidents in 1927 was due to an increase in pilots errors, which more than offset a pronounced decrease in mechanical troubles. 10. nfluence of total flying time. (a) Figure 3 indicates that after a pilot has had approximately 1,200 hours flying e>q>erience, his accident rate per hours of flying will be stabilized at an average of one accident in each 1,000 hours, provided his annual flying is not greatly decreased in succeeding years. 11. nfluence of annual flying time. (a) Based on the data used for Figure 4, it is evident that a pilot whose annual flying time is less than 60 hours has a relatively high accident rateone accident in each 330. This rate is twice that of pilots who average between 70 and 100 hours annual flying time. 12. t is evident, therefore, that under present conditions the accident rate for an Air Corps pilot can not be brought to a satisfactory minimum until that pilot has accumulated 1,200 hours total flying time and has more than 75 hours annual flying time after that point is reached. (a) For the calendar year of 1927 the records of this office show: (1) Number of pilots with less thanper cent 1,200 hours experience 60 (2) Number of pilots who flew less than 75 hours during the year_ (b) Of these two factors, that of total annual flying time is the more important, as any increase would result in a similar increa e in total flying time. 13. Based on the data discussed in the preceding varagraphs and on an examination of the operations records at Air Corps stat.ions, it appears that at least 20 per cent of the pilots in the Air Corps do not engage in sufficient annual flying of a constructive nature to assure their proper training and preparation to perform tactical missions. 14. The records of the 59 pilots who flew less than 50 hours in 1927 show that this group had six accidents in that year. The pilots involved in these accidents included fonr majors, one captain, and one first lieutenant. One pilot was killed and one seriously injured. Causes of the six accidents included loss of control on takeoff, landing in rough part of field, losing way while on crosscountry flight, and spinning into the ground after taking off. Evidence indicated that at least four of these accidents could have been avoided had the pilot been in a better state of flying training. Had the same rate of accidents prevailed for all pilots in the Air Corps, it would have resulted in a total of 335 era hes as against a total of 240 which did occur, or an increase of 95 crashes. From past experience this would have entailed the loss of nine additional lives. TABLE 1.Resume of causes of fatal accidents, 1927 Mechanical trouble only: Per cent Engine failure without spin 0 Structural failure Mechanical trouble plus pilots errors: Engine failure with spin 6. 8 Pilots errors only: Spins and stalls due to errors only Bad landings Pilots errors plus contributory factors: Collisions, all kinds Poor weather conditions 7. 4 Miscellaneous 6. 3 Unknown and doubtful 3. 7 Total 100

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