25 (E) Maintain engine speeds of r.p.m. higher to ensure proper operation of the ice protection system.
(ii) Commuter Category Airplanes. Data should be provided so that the balked landing climb limited weight and approach climb limited landing weight can be determined. These data should include the effect of drag due to residual ice on protected and unprotected surfaces, power extraction associated with ice protection system operation, and any changes in operating speeds due to icing. Also, the effect on landing distance due to revised approach speeds, and/or landing configurations, should be shown.
c. Prior to AFM Requirement. If the airplane was certified prior to the effective date of the requirement for an AFM, then the combination of manuals, markings, and placards should adequately address the placard and AFM subjects previously discussed in this AC.
James E. Jackson Acting Manager, Small Airplane Directorate Aircraft Certification Service
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Appendix 1 1 APPENDIX 1. CHECKLIST The left column of this Appendix provides a simplified checklist of the various influence items that could affect the safety of small airplanes while operating in icing conditions. In the right column are suggested considerations for resolving the concerns of each of these influence items. Certain considerations may not be applicable depending on the certification basis of the airplane.
Influence Consideration
1. Crew Visibility a. Conduct evaluations to verify adequate day and night visibility through the protected windshield or the protected windshield segment under dry air and icing conditions.
b. Evaluate the cabin defogging system's capability to clear side windows for observation of boot ice protection system operation and ice accumulation. If a defogging system is not provided, the windows should be easily cleared by the pilot without adversely increasing pilot workload.
c. Minimum light transmittance through the protected windshield or protected windshield segment and effected side windows should consider the requirements in § 23.775(e).
d. Determine that the temperature gradient produced on heated windshields does not adversely affect pilot vision or windshield structural integrity.
2. Engine Installation a. Conduct flight tests, analyses, or refer to substantiation and Cooling data to determine that complete engine installation, including propellers, functions without appreciable loss of power. Verify that engine oil and component cooling is adequate at critical design points throughout the operational and icing envelope. If ice is expected to accumulate at the generator during icing encounters, then cooling air inlet generator cooling tests should be performed with the maximum icing load on the electrical system and critical ice shapes installed on the engine and generator cooling air intake.
Appendix 1 2 Influence Consideration
3. Propeller a. Provide analyses to establish chordwise and spanwise protection required. Aerodynamic heating due to blade rotation, latent heat of fusion, and centrifugal force are important in determining areas requiring protection. Droplet size is the critical parameter for determining chordwise extent of areas requiring ice protection.
b. Where the propeller ice protection system consumes power from the electrical system, pneumatic system, or bleed air system, a load analysis should be provided showing that the power source capacity is adequate to provide ice protection in addition to all other essential loads.
c. Where fluid is required for ice protection, a limitation should be placed in the AFM on flight in icing conditions to prevent exhausting the fluid prior to exiting the icing condition. Sufficient margin in fluid capacity should be maintained to allow for alternate airport landing in accordance with operational requirements.
d. Other specific areas of concern include:
(1) The effect of deicer boot installation upon propeller blade and cuff, and hub structural integrity.
(2) Surface temperature.
(3) Timer or other control system reliability.
(4) Spinner ice accumulation.
e. Perform tests to verify that ice sheds from the blades and to demonstrate compliance. During testing, verify that adequate ice protection is provided, propeller performance degradations are not excessive, vibration characteristics are satisfactory and ice being shed is small enough to avoid detrimental damage to other aircraft components. Tests should include examination of the structural integrity of the propeller assembly and associated equipment with ice protection (heater blankets, slip rings, wiring, etc.) installed.
Appendix 1 3 Influence Consideration
4. Equipment, a. Conduct a study as discussed in Item 9e (failure analysis) Systems, Function, of this AC to ensure that no probable failure or malfunction of and Installation of any power source (electrical, fluid, bleed air, pneumatic, etc.)
will impair the ability of the remaining source(s) to supply adequate power to systems essential to safe operation during icing flight.
b. Conduct a power source load analysis to verify proper power requirements are provided.
c. Verify that power source failure warning is provided to the crew.
d. Demonstrate that the alternator or generator is protected from detrimental ice accumulation.
e. Determine if load shedding can be accomplished after a partial failure condition. If applicable, a load shedding sequence should be provided so the pilot may ensure that adequate power is available to the ice protection equipment and other necessary equipment for flight in icing conditions.
5. Circuit and a. Determine that the design incorporates electrical overload Protective Devices protection that opens regardless of operating control position.
b. Verify that the design is such that no protective device is protecting more that one circuit essential to continued safe flight
(e.g., pitot heat and stall warning transducer heat are considered separate essential circuits and should be provided separate protection). Ice protection monitor and warning circuits should be considered separate from control circuits and each should provide individual circuit protection. On airplanes equipped with dual power sources, a power distribution system having a single bus and a single circuit breaker protecting the ice protection system is not acceptable.
6. Airfoil Leading a. Provide a means to indicate to the crew that the ice protection Edge Protection system is receiving adequate electrical power, bleed air pressure, System vacuum, or fluid, etc., as appropriate, and it is functioning normally.
Appendix 1 4 Influence Consideration
6. Airfoil Leading b. Conduct droplet trajectory and impingement analysis of wing, Edge Protection and horizontal and vertical stabilizers to establish aft limits for ice System (Continued) formation. Areas of concern include adequacy of upper and lower limits of wing and stabilizer protection to allow safe flight in icing conditions.
7. Static Pressure a. Each static port design or location should be such that System correlation between air pressure in the static system and true ambient pressure is not altered when flying in icing conditions. Means of showing compliance include the following:
anti-icing devices, alternate source for static pressure, or demonstration by test that port icing does not occur under any condition.
b. Where the port is thermally protected, a thermal evaluation should be conducted to demonstrate that the protection is adequate.
8. Pitot, Static, a. Provide analysis (thermal analysis in the case of heated pitot Angle-of-Attack, tube and static ports) to establish anti-icing/deicing requirements.
and Stall Warning Sensors b. Perform tests to verify analyses and demonstrate compliance. Use these verified analyses to extrapolate to the critical conditions of Part 25, Appendix C. Several combinations of parameters may be critical test points.
For unprotected components, testing may be conducted to demonstrate that airspeed, altitude, and other indications remain within acceptable tolerances under the critical conditions. In some cases, adequate bench and flight testing may already have been accomplished on other airplanes to establish an approval basis by similarity on a specific airplane.
9. Magnetic Direction a. Designs should minimize magnetic direction indicator (MDI) deviations; however, if MDI deviations greater than 10o exist when operating electrical ice protection equipment, provide placarding.
NOTE: If the ice protection system causes greater than a l0o deviation, then § 23.1327 (Amendment 23-20) should be applied in lieu of previous requirements.
Appendix 1 5 Influence Consideration
10. Ice Detection a. Night flight or dark hangar evaluation of light coverage and glare Light(s) produced by the wing ice detection light(s) should be evaluated.
b. A hand-held flashlight is not acceptable as an ice detection light.
c. The ice detection light(s) should be evaluated in icing conditions to verify that sufficient illumination is provided for the pilot to detect ice accumulation.
11. Antennas and a. Conduct structural analysis to establish that critical ice Other Components build-ups on antennas, masts, and other components attached externally to the airplane do not result in hazards.
b. Tests in natural icing or with simulated ice shapes may be used to substantiate the structural analysis.
c. Ice shedding from these components should be evaluated to verify that size and trajectory do not damage other parts of the airplane.
12. Fluid Systems a. Certain fluids used in ice protection systems are flammable.
Components of these systems must meet the flammable fluid protection requirements of § 23.863. No components of these systems may be installed in passenger or crew compartments without the protection required by § 23.853(d) (prior to Amendment 23-34) or § 23.853(e) (after Amendment 23-34).
b. Fluid capacity should be established based on the operational capability of the airplane and on the ability to fly to an alternate airport and safely land. Means should be provided to monitor fluid capacity and flow rates as they relate to flight. The method for determining ice protection availability should be provided in the operating procedures of the AFM.
c. The Maintenance Manual should list approved fluids and, if pilot and crewmembers are required to replace fluids, these approved fluids should be listed in the AFM. The fluid filler inlet should bear a placard stating that only approved fluids be used. Approved fluids may be listed on this placard or in the AFM.
Appendix 1 6 Influence Consideration
12. Fluid Systems d. The compatibility of the fluid with airframe and engine components (Continued) should be examined to verify that adverse reactions such as corrosion or contamination do not occur, or are prevented through inspection or other measures (e.g., if ethylene glycol is a component fluid, then silver and silver-plated electrical switch contacts and terminals should be protected from contamination by the ethylene glycol in order to avoid a fire hazard).
13. Flight Tests a. The certification rules require analyses and tests to demonstrate that the airplane can safely operate in the icing envelope of Part 25, Appendix C. Compliance can be determined by similarities to previously approved configurations. If it should be necessary to conduct dry air tests with ice shapes, natural icing tests, or simulated icing tests, the goals and results should be in accordance with the guidance provided in Item 11.
14. Flight Manual a. The AFM and appropriate placards in the airplane should be designed and Placards to provide the pilot with sufficient information to safely operate the airplane in an icing environment.
Appendix 2 1 APPENDIX 2. AFM LIMITATIONS AND NORMAL PROCEDURES SECTIONS
1. LIMITATIONS SECTION. In the case of severe icing, the following text and warning information should be used as in the Limitations Section of the AFM:
a. Flight in meteorological conditions described as freezing rain or freezing drizzle, as determined by the following visual cues, is prohibited:
(1) Unusually extensive ice accreted on the airframe in areas not normally observed to collect ice.
(2) Accumulation of ice on the upper surface (for low-wing airplanes) or lower surface (for high-wing airplanes) of the wing aft of the protected area.
(3) Accumulation of ice on the propeller spinner farther back than normally observed.
If the airplane encounters conditions that are determined to contain freezing rain or freezing drizzle, the pilot must immediately exit the freezing rain or freezing drizzle conditions by changing altitude or course.
NOTE: The prohibition on flight in freezing rain or freezing drizzle is not intended to prohibit purely inadvertent encounters with the specified meteorological conditions; however, pilots should make all reasonable efforts to avoid such encounters and must immediately exit the conditions if they are encountered.
b. Use of the autopilot is prohibited when any ice is observed forming aft of the protected surfaces of the wing, or when unusual lateral trim requirements or autopilot trim warnings are encountered.
NOTE: The autopilot may mask tactile cues that indicate adverse changes in handling characteristics; therefore, the pilot should consider not using the autopilot when any ice is visible on the airplane.
2. NORMAL PROCEDURES SECTION. In the case of severe icing, the following text and warning information should be used as in the Normal Procedures Section of the AFM:
a. WARNING. If ice is observed forming aft of the protected surfaces of the wing or if unusual lateral trim requirements or autopilot trim warnings are encountered, accomplish the following:
(1) If the flaps are extended, do not retract them until the airframe is clear of ice;
(2) The flight crew should reduce the angle-of-attack by increasing speed as much as the airplane configuration and weather allow, without exceeding design maneuvering speed;
Appendix 2 2
(3) If the autopilot is engaged, hold the control wheel firmly and disengage the autopilot. Do not re-engage the autopilot until the airframe is clear of ice;
(4) Exit the icing area immediately by changing altitude or course; and
(5) Report these weather conditions to Air Traffic Control.
b. CAUTION. Severe icing comprises environmental conditions outside of those for which the airplane is certificated. Flight in freezing rain, freezing drizzle, or mixed icing conditions (supercooled liquid water and ice crystals) may result in hazardous ice build-up on protected surfaces exceeding the capability of the ice protection system, or may result in ice forming aft of the protected surfaces. This ice may not be shed using the ice protection systems, and it may seriously degrade the performance and controllability of the airplane.
c. THE FOLLOWING SHALL BE USED TO IDENTIFY FREEZING RAIN/FREEZING DRIZZLE ICING CONDITIONS:
(1) Unusually extensive ice accreted on the airframe in areas not normally observed to collect ice.
(2) Accumulation of ice on the upper surface (for low-wing airplanes) or lower surface (for high-wing airplanes) of the wing aft of the protected area.
(3) Accumulation of ice on the propeller spinner farther back than normally observed.
d. THE FOLLOWING MAY BE USED TO IDENTIFY POSSIBLE FREEZING RAIN/FREEZING DRIZZLE CONDITIONS:
(1) Visible rain at temperatures below +5 °Celsius [outside air temperature (OAT)].
(2) Droplets that splash or splatter on impact at temperatures below +5 °Celsius OAT.
e. PROCEDURES FOR EXITING THE FREEZING RAIN/FREEZING DRIZZLE ENVIRONMENT. These procedures are applicable to all flight phases from takeoff to landing.
Monitor the outside air temperature. While severe icing may form at temperatures as cold as −18 °Celsius, increased vigilance is warranted at temperatures around freezing with visible moisture present. If the visual cues specified in the AFM for identifying possible freezing rain or freezing drizzle conditions are observed, accomplish the following:
(1) Exit the freezing rain or freezing drizzle severe icing conditions immediately to avoid extended exposure to flight conditions outside of those for which the airplane has been certificated for operation. Asking for priority to leave the area is fully justified under these conditions;
Appendix 2 3
(2) Avoid abrupt and excessive maneuvering that may exacerbate control difficulties;
(3) Do not engage the autopilot. The autopilot may mask unusual control system forces;
(4) If the autopilot is engaged, hold the control wheel firmly and disengage the autopilot;
(5) If an unusual roll response or uncommanded control movement is observed, reduce the angle-of-attack by increasing airspeed or rolling wings level (if in a turn), and apply additional power, if needed;
(6) Avoid extending flaps during extended operation in icing conditions. Operation with flaps extended can result in a reduced wing angle-of-attack, with ice forming on the upper surface further aft on the wing than normal, possibly aft of the protected area; and
(7) Report these weather conditions to Air Traffic Control.
NOTE: Alternate means of providing this information in the AFM may be approved by the certifying agency.