The well-known maxim “ know your enemy ” is as sound in aircraft design as it is in war. Application of this, doctrine might perhaps modify the conclusions in Mr. Tye's interesting paper, “ Factors of Safety –or of Habit? ” published in the November issue of the Journal. He discussed design factors and strength requirements from the aspects of V-g records and of structural variation. The V-g records with very few exceptions have been obtained from aircraft which have not broken up.