text: In many cases of modern aircraft structures 
  it is very difficult to appreciate the effect of a 
  moderate overstress. This feature is due in 
  the inherent strength of the 
  great part 
  materials used in modern aircraft construction. 
  As a general airframe static strength require- 
  ment, the primary structure must not expe- 
  rience objectionable permanent deformation at 
  load or ~failure at 150 percent of limit 
  limit 
  load (ultimate 
  load). 
  To satisfy each part of the requirement, limit 
  load must not exceed the yield stress and ulti- 
  mate load must not exceed the ultimate stress 
  capability of the parts. 
  
  to 
  
  The function of the landing gear as a shock 
  absorbing device has an important application 
  when a forced landing must be accomplished 
  on an unprepared surface. 
  the terrain is 
  rough and the landing gear is not extended, 
  initial contact will be made with 
  relatively 
  solid structure and whatever energy is ab- 
  sorbed will be accompanied by high vertical 
  accelerations. These high vertical accelera- 
  tions encountered with a gear-up landing on 
  an unprepared surface are the source of a very 
  compres- 
  incapacitating type injury-vertical 
  sion fracture of the vertebrae. Unless some 
  peculiarity of 
  the configuration makes it 
  inadvisable, it is generally recommended that 
  the landing gear be down for forced landing on 
  an unprepared surface. (NOTE: for those prone 
  to forget, it is also recommended that the gear 
  be down for landing on prepared surfaces.) 