257 

Federal Aviation Administration, DOT 

§ 25.533 

(b) 

Unsymmetrical landing for hull and 

single float seaplanes. 

Unsymmetrical 

step, bow, and stern landing conditions 
must be investigated. In addition— 

(1) The loading for each condition 

consists of an upward component and a 
side component equal, respectively, to 
0.75 and 0.25 tan 

b 

times the resultant 

load in the corresponding symmetrical 
landing condition; and 

(2) The point of application and di-

rection of the upward component of the 
load is the same as that in the sym-
metrical condition, and the point of ap-
plication of the side component is at 
the same longitudinal station as the 
upward component but is directed in-
ward perpendicularly to the plane of 
symmetry at a point midway between 
the keel and chine lines. 

(c) 

Unsymmetrical landing; twin float 

seaplanes. 

The unsymmetrical loading 

consists of an upward load at the step 
of each float of 0.75 and a side load of 
0.25 tan 

b 

at one float times the step 

landing load reached under § 25.527. The 
side load is directed inboard, per-
pendicularly to the plane of symmetry 
midway between the keel and chine 
lines of the float, at the same longitu-
dinal station as the upward load. 

§ 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

S

1

= seaplane stalling speed (knots) at the 

design 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

S

1

= seaplane stalling speed (Knots) at the 

design water takeoff weight with flaps 
extended 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; 

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