NAVWEPS 00-EOT-80  OPERATING STRENGTH LIMITATIONS  life of any structural component. This fact  is certain and irreversible. Thus, the opera-  tion of the airplane, powerplant, and various  systems must be limited to design values to  prevent failure or excessive  maintenance costs  early in the anticipated service life.  The  operating limitations presented in the hand-  book must be adhered to in a very strict  fashion.  emphasize the necessity of proper maintenance  of the struts. An additional fact illustrated is  that the landing load factor varies as the square  of the touchdown rate of descent.  Therefore,  a 20 percent higher rate of descent increases  the landing load factor 44 percent.  This fact  should emphasize the need for proper landing  technique to prevent a hard landing and over-  stress of the landing gear components and  associated structure.  In many cases  of modern aircraft structures  it is very difficult to appreciate the effect of a  moderate overstress. This feature is due in  great part to the inherent strength of the  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  limit load or ~failure at 150 percent of limit  load (ultimate load is 1.5 times limit 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.  The effect of landing gross weight is two-  fold.  A higher gross weight at some landing  load factor produces a higher force in the  landing gear. The highe: gross weight re-  quires a higher approach speed and, if the same  glide path is used, a higher rate of descent  results. In addition to the principal vertical  loads on the landing gear, there are varied side  loads, wheel spin up and spring back loads,  etc., all of which tend to be more critical at  high gross weight, high touchdown ground  speed, and high rate of descent.  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. If 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  incapacitating type injury-vertical  compres-  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.)  Many of the high strength materials used in  aircraft construction have stress-strain dia-  grams typical of figure 5.6. One feature of  these materials is that the yield point is at  some stress much greater than two-thirds of  the ultimate stress. Thus, the critical design  condition is the ultimate load.  If 150 percent  of limit load corresponds  to ultimate stress of  the material, 100 percent of limit load corre-  sponds to a stress much lower than the yield  stress. Because  of the inherent properties of  the high strength material and the ultimate  factor of safety of 1.5, the limit load condition  is rarely the critical design point and usually  possesses  a large positive margin of static  strength. This fact alone implies that the  structure must be grossly overstressed  to pro-  duce damage easily vidble to the naked eye.  This lack of immediate visible damage with  “overstress”  makes it quite diflicult to recog-  nize or appreciate the long range effect.  A reference point provided on the stress  strain diagram of figure 5.6 is a stress termed  Accumulated periods of overstress  can create  a very detrimental effect on the useful service  EFFECT OF OVERSTRESS ON  SERVICE  LIFE  344 