NAVWEPS 00-EOT-80 
OPERATING STRENGTH LIMITATIONS 
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. 
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.) 
EFFECT OF OVERSTRESS ON SERVICE 
LIFE 
Accumulated periods of overstress can create 
a very detrimental effect on the useful service 
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. 
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. 
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-