Appendix C to Part 25
14:1.0.1.3.12.11.121.1.9 : Appendix C
Appendix C to Part 25 Part I - Atmospheric Icing Conditions
(a) Continuous maximum icing. The maximum continuous
intensity of atmospheric icing conditions (continuous maximum
icing) is defined by the variables of the cloud liquid water
content, the mean effective diameter of the cloud droplets, the
ambient air temperature, and the interrelationship of these three
variables as shown in figure 1 of this appendix. The limiting icing
envelope in terms of altitude and temperature is given in figure 2
of this appendix. The inter-relationship of cloud liquid water
content with drop diameter and altitude is determined from figures
1 and 2. The cloud liquid water content for continuous maximum
icing conditions of a horizontal extent, other than 17.4 nautical
miles, is determined by the value of liquid water content of figure
1, multiplied by the appropriate factor from figure 3 of this
appendix.
(b) Intermittent maximum icing. The intermittent maximum
intensity of atmospheric icing conditions (intermittent maximum
icing) is defined by the variables of the cloud liquid water
content, the mean effective diameter of the cloud droplets, the
ambient air temperature, and the interrelationship of these three
variables as shown in figure 4 of this appendix. The limiting icing
envelope in terms of altitude and temperature is given in figure 5
of this appendix. The inter-relationship of cloud liquid water
content with drop diameter and altitude is determined from figures
4 and 5. The cloud liquid water content for intermittent maximum
icing conditions of a horizontal extent, other than 2.6 nautical
miles, is determined by the value of cloud liquid water content of
figure 4 multiplied by the appropriate factor in figure 6 of this
appendix.
(c) Takeoff maximum icing. The maximum intensity of
atmospheric icing conditions for takeoff (takeoff maximum icing) is
defined by the cloud liquid water content of 0.35 g/m3, the mean
effective diameter of the cloud droplets of 20 microns, and the
ambient air temperature at ground level of minus 9 degrees Celsius
(−9 °C). The takeoff maximum icing conditions extend from ground
level to a height of 1,500 feet above the level of the takeoff
surface.
Part II - Airframe Ice Accretions for Showing Compliance With
Subpart B.
(a) Ice accretions - General. The most critical ice
accretion in terms of airplane performance and handling qualities
for each flight phase must be used to show compliance with the
applicable airplane performance and handling requirements in icing
conditions of subpart B of this part. Applicants must demonstrate
that the full range of atmospheric icing conditions specified in
part I of this appendix have been considered, including the mean
effective drop diameter, liquid water content, and temperature
appropriate to the flight conditions (for example, configuration,
speed, angle-of-attack, and altitude). The ice accretions for each
flight phase are defined as follows:
(1) Takeoff ice is the most critical ice accretion on
unprotected surfaces and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation,
occurring between the end of the takeoff distance and 400 feet
above the takeoff surface, assuming accretion starts at the end of
the takeoff distance in the takeoff maximum icing conditions
defined in part I of this Appendix.
(2) Final takeoff ice is the most critical ice accretion
on unprotected surfaces, and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation,
between 400 feet and either 1,500 feet above the takeoff surface,
or the height at which the transition from the takeoff to the en
route configuration is completed and VFTO is reached, whichever is
higher. Ice accretion is assumed to start at the end of the takeoff
distance in the takeoff maximum icing conditions of part I,
paragraph (c) of this Appendix.
(3) En route ice is the critical ice accretion on the
unprotected surfaces, and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation,
during the en route phase.
(4) Holding ice is the critical ice accretion on the
unprotected surfaces, and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation,
during the holding flight phase.
(5) Approach ice is the critical ice accretion on the
unprotected surfaces, and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation
following exit from the holding flight phase and transition to the
most critical approach configuration.
(6) Landing ice is the critical ice accretion on the
unprotected surfaces, and any ice accretion on the protected
surfaces appropriate to normal ice protection system operation
following exit from the approach flight phase and transition to the
final landing configuration.
(b) In order to reduce the number of ice accretions to be
considered when demonstrating compliance with the requirements of §
25.21(g), any of the ice accretions defined in paragraph (a) of
this section may be used for any other flight phase if it is shown
to be more critical than the specific ice accretion defined for
that flight phase. Configuration differences and their effects on
ice accretions must be taken into account.
(c) The ice accretion that has the most adverse effect on
handling qualities may be used for airplane performance tests
provided any difference in performance is conservatively taken into
account.
(d) For both unprotected and protected parts, the ice accretion
for the takeoff phase may be determined by calculation, assuming
the takeoff maximum icing conditions defined in appendix C, and
assuming that:
(1) Airfoils, control surfaces and, if applicable, propellers
are free from frost, snow, or ice at the start of the takeoff;
(2) The ice accretion starts at the end of the takeoff
distance.
(3) The critical ratio of thrust/power-to-weight;
(4) Failure of the critical engine occurs at VEF; and
(5) Crew activation of the ice protection system is in
accordance with a normal operating procedure provided in the
Airplane Flight Manual, except that after beginning the takeoff
roll, it must be assumed that the crew takes no action to activate
the ice protection system until the airplane is at least 400 feet
above the takeoff surface.
(e) The ice accretion before the ice protection system has been
activated and is performing its intended function is the critical
ice accretion formed on the unprotected and normally protected
surfaces before activation and effective operation of the ice
protection system in continuous maximum atmospheric icing
conditions. This ice accretion only applies in showing compliance
to §§ 25.143(j) and 25.207(h), and 25.207(i).
[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt.
25-121, 72 FR 44669, Aug. 8, 2007; 72 FR 50467, Aug. 31, 2007;
Amdt. 25-129, 74 FR 38340, Aug. 3, 2009; Amdt. 25-140, 79 FR 65528,
Nov. 4, 2014]