467 

Federal Aviation Administration, DOT 

Pt. 25, App. O 

PART II—AIRFRAME ICE ACCRETIONS 

FOR SHOWING COMPLIANCE WITH SUB-
PART B OF THIS PART 

(a) 

General. 

The most critical ice accretion 

in terms of airplane performance and han-
dling qualities for each flight phase must be 
used to show compliance with the applicable 
airplane performance and handling qualities 
requirements for icing conditions contained 
in subpart B of this part. Applicants must 
demonstrate that the full range of atmos-
pheric icing conditions specified in part I of 
this Appendix have been considered, includ-
ing drop diameter distributions, liquid water 
content, and temperature appropriate to the 
flight conditions (for example, configuration, 
speed, angle of attack, and altitude). 

(1) For an airplane certified in accordance 

with § 25.1420(a)(1), the ice accretions for each 
flight phase are defined in part II, paragraph 
(b) of this Appendix. 

(2) For an airplane certified in accordance 

with § 25.1420(a)(2), the most critical ice ac-
cretion for each flight phase defined in part 
II, paragraphs (b) and (c) of this Appendix, 
must be used. For the ice accretions defined 

in part II, paragraph (c) of this Appendix, 
only the portion of part I of this Appendix in 
which the airplane is capable of operating 
safely must be considered. 

(3) For an airplane certified in accordance 

with § 25.1420(a)(3), the ice accretions for each 
flight phase are defined in part II, paragraph 
(c) of this Appendix. 

(b) Ice accretions for airplanes certified in 

accordance with § 25.1420(a)(1) or (2). 

(1) 

En route ice 

is the en route ice as de-

fined by part II, paragraph (c)(3), of this Ap-
pendix, for an airplane certified in accord-
ance with § 25.1420(a)(2), or defined by part II, 
paragraph (a)(3), of Appendix C of this part, 
for an airplane certified in accordance with 
§ 25.1420(a)(1), plus: 

(i) Pre-detection ice as defined by part II, 

paragraph (b)(5), of this Appendix; and 

(ii) The ice accumulated during the transit 

of one cloud with a horizontal extent of 17.4 
nautical miles in the most critical of the 
icing conditions defined in part I of this Ap-
pendix and one cloud with a horizontal ex-
tent of 17.4 nautical miles in the continuous 

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