419 

Federal Aviation Administration, DOT 

§ 25.533 

§ 25.531

Hull and main float takeoff 

condition. 

For the wing and its attachment to 

the hull or main float— 

(a) The aerodynamic wing lift is as-

sumed to be zero; and 

(b) A downward inertia load, cor-

responding to a load factor computed 
from the following formula, must be 
applied: 

n

C

V

W

TO

S

=

⎛

⎝

⎞

⎠

1

2

2
3

1
3

tan

β

where— 

n=inertia load factor; 
C

TO

=empirical seaplane operations factor 

equal to 0.004; 

V

S1

=seaplane stalling speed (knots) at the de-

sign takeoff weight with the flaps ex-
tended in the appropriate takeoff posi-
tion; 

b=angle of dead rise at the main step (de-

grees); and 

W=design water takeoff weight in pounds. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5673, Apr. 8, 
1970] 

§ 25.533

Hull and main float bottom 

pressures. 

(a) 

General.  The hull and main float 

structure, including frames and bulk-
heads, stringers, and bottom plating, 
must be designed under this section. 

(b) 

Local pressures. For the design of 

the bottom plating and stringers and 
their attachments to the supporting 
structure, the following pressure dis-
tributions must be applied: 

(1) For an unflared bottom, the pres-

sure at the chine is 0.75 times the pres-
sure at the keel, and the pressures be-
tween the keel and chine vary linearly, 
in accordance with figure 3 of appendix 
B. The pressure at the keel (psi) is 
computed as follows: 

P

C

K V

k

S

k

=

×

2

2

1

2

tan

β

where— 

P

k

=pressure (p.s.i.) at the keel; 

C

2

=0.00213; 

K

2

=hull station weighing factor, in accord-

ance with figure 2 of appendix B; 

V

S1

=seaplane stalling speed (Knots) at the de-

sign water takeoff weight with flaps ex-

tended in the appropriate takeoff posi-
tion; and 

b

K

=angle of dead rise at keel, in accordance 

with figure 1 of appendix B. 

(2) For a flared bottom, the pressure 

at the beginning of the flare is the 
same as that for an unflared bottom, 
and the pressure between the chine and 
the beginning of the flare varies lin-
early, in accordance with figure 3 of ap-
pendix B. The pressure distribution is 
the same as that prescribed in para-
graph (b)(1) of this section for an 
unflared bottom except that the pres-
sure at the chine is computed as fol-
lows: 

P

C

K V

ch

S

=

×

3

2

1

2

tan

β

where— 

P

ch

=pressure (p.s.i.) at the chine; 

C

3

=0.0016; 

K

2

=hull station weighing factor, in accord-

ance with figure 2 of appendix B; 

V

S1

=seaplane stalling speed at the design 

water takeoff weight with flaps extended 
in the appropriate takeoff position; and 

b=angle of dead rise at appropriate station. 

The area over which these pressures 
are applied must simulate pressures oc-
curring during high localized impacts 
on the hull or float, but need not ex-
tend over an area that would induce 
critical stresses in the frames or in the 
overall structure. 

(c) 

Distributed pressures. For the de-

sign of the frames, keel, and chine 
structure, the following pressure dis-
tributions apply: 

(1) Symmetrical pressures are com-

puted as follows: 

P

C

K V

S

=

×

4

2

0

2

tan

β

where— 
P=pressure (p.s.i.); 
C

4

=0.078 

C

1

(with 

C

1

computed under § 25.527); 

K

2

=hull station weighing factor, determined 

in accordance with figure 2 of appendix 
B; 

V

S0

=seaplane stalling speed (Knots) with 

landing flaps extended in the appropriate 
position and with no slipstream effect; 
and 

V

S0

=seaplane stalling speed with landing 

flaps extended in the appropriate posi-
tion and with no slipstream effect; and 
b=angle of dead rise at appropriate sta-
tion. 

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pmangrum on DSK3VPTVN1PROD with CFR