Wing Anti Ice - Dry Air and Natural Icing

The purpose of this post is to provide guidance and advice for engineers involved in planning and conducting Dry Air and Natural Icing (NI) Flight Test of the Wing Anti-Ice System of a Part 25 category aircraft.

Introduction

If certification for flight in icing conditions is desired, the airplane must be able to safely operate throughout the icing envelope defined by the Appendix C of Title 14 CFR Part 25, as indicated in section 25.1419. Additionally, for other parts of the airplane (i.e., engine, engine inlet, propeller) there are more specific icing requirements; however their associated guidance will not be addressed in this flight test guide. It must be noted that some of the icing-related regulations must be complied with, even if the airplane is not certificated for flight in icing (e.g., §§ 25.629(d)(3), 25.903, 25.975, 25.1093, 25.1323(e), and 25.1325(b)).

We are going to focus on Wing Anti-Ice System certification. There are different ice protection technologies available, all of them can be basically categorized in four groups.

●      Thermal (hot air and electrothermal), which can be evaporative or “running wet”

●      Mechanical (pneumatic boots, electro-impulsive)

●      Fluid (weeping systems)

●      Icephobic materials

Selecting a particular kind of ice protection technology will have an influence in the subsequent tests necessary to reach certification. Figure 1 highlights the necessity of completing both dry air and natural icing tests to achieve certification.

Figure 1 – The role of dry air and natural icing tests within the Certification Processes for Wing Anti-Ice Systems

2.2       Purpose of Dry Air Flight Test

●               Purpose of Dry Air testing is primarily to verify functionality (temperature/pressure regulation, CAS indications, instrumentation verification) of the Wing Anti-Ice System in a safe (non-icing) environment. Additionally, temperature data from a dry condition provides additional data for validation of the thermal model. More information can be found in Section 8.2.2. of AC 20-73A.

2.3       Purpose of Natural Icing Flight Test

●               The purpose of NI testing is to verify aircraft functionality in atmospheric icing conditions as specified in 14 CFR § 25.1419. Performance is measured based on continuous maximum (CM) and intermittent maximum (IM) icing conditions as described by Appendix C. NI is a test that impacts most engineering disciplines and requires significant coordination among groups to be performed effectively.

Certification Requirements

a)     14 CFR 25.1419

If the applicant seeks certification for flight in icing conditions, the airplane must be able to safely operate in the continuous maximum and intermittent maximum icing conditions of Appendix C. To establish this -

(a) An analysis must be performed to establish that the ice protection for the various components of the airplane is adequate, taking into account the various airplane operational configurations; and

(b) To verify the ice protection analysis, to check for icing anomalies, and to demonstrate that the ice protection system (IPS) and its components are effective, the airplane or its components must be flight tested in the various operational configurations, in measured natural atmospheric icing conditions and, as found necessary, by one or more of the following means:

(1) Laboratory dry air or simulated icing tests, or a combination of both, of the components or models of the components.

(2) Flight dry air tests of the ice protection system as a whole, or of its individual components.

(3) Flight tests of the airplane or its components in measured simulated icing conditions.

(c) Caution information, such as an amber caution light or equivalent, must be provided to alert the flight crew when the anti-ice or de-ice system is not functioning normally.

(d) For turbine engine powered airplanes, the ice protection provisions of this section are considered to be applicable primarily to the airframe. For the powerplant installation, certain additional provisions of subpart E of this part may be found applicable.

(e) One of the following methods of icing detection and activation of the airframe ice protection system must be provided:

(1) A primary ice detection system that automatically activates or alerts the flightcrew to activate the airframe ice protection system;

(2) A definition of visual cues for recognition of the first sign of ice accretion on a specified surface combined with an advisory ice detection system that alerts the flight crew to activate the airframe ice protection system; or

(3) Identification of conditions conducive to airframe icing as defined by an appropriate static or total air temperature and visible moisture for use by the flight crew to activate the airframe ice protection system.

(f) Unless the applicant shows that the airframe ice protection system need not be operated during specific phases of flight, the requirements of paragraph (e) of this section are applicable to all phases of flight.

(g) After the initial activation of the airframe ice protection system -

(1) The ice protection system must be designed to operate continuously;

(2) The airplane must be equipped with a system that automatically cycles the ice protection system; or

(3) An ice detection system must be provided to alert the flight crew each time the ice protection system must be cycled.

(h) Procedures for operation of the ice protection system, including activation and deactivation, must be established and documented in the Airplane Flight Manual.

b)     14 CFR § 91.527 - Operating in icing conditions

(a) No pilot may take off an airplane that has frost, ice, or snow adhering to any propeller, windshield, stabilizing or control surface; to a powerplant installation; or to an airspeed, altimeter, rate of climb, or flight attitude instrument system or wing, except that takeoffs may be made with frost under the wing in the area of the fuel tanks if authorized by the FAA.

(b) No pilot may fly under IFR into known or forecast light or moderate icing conditions, or under VFR into known light or moderate icing conditions, unless -

(1) The aircraft has functioning deicing or anti-icing equipment protecting each rotor blade, propeller, windshield, wing, stabilizing or control surface, and each airspeed, altimeter, rate of climb, or flight attitude instrument system;

(2) The airplane has ice protection provisions that meet section 34 of Special Federal Aviation Regulation No. 23; or

(3) The airplane meets transport category airplane type certification provisions, including the requirements for certification for flight in icing conditions.

(c) Except for an airplane that has ice protection provisions that meet the requirements in section 34 of Special Federal Aviation Regulation No. 23, or those for transport category airplane type certification, no pilot may fly an airplane into known or forecast severe icing conditions.

(d) If current weather reports and briefing information relied upon by the pilot in command indicate that the forecast icing conditions that would otherwise prohibit the flight will not be encountered during the flight because of changed weather conditions since the forecast, the restrictions in paragraphs (b) and (c) of this section based on forecast conditions do not apply.

 

3.2       Advisory Circular(s)

a)     AC 20-73A

                        i)         This advisory circular (AC) describes an acceptable means, but not the only means of showing compliance with the ice protection requirements of Title 14 of the Code of Federal Regulations (14 CFR) parts 23, 25, 27, 29, 33, and 35. It also offers applicants guidance on how to maintain the aircraft’s airworthiness when operating in an icing environment. This AC is long but it is recommended as the first document to review when beginning to prepare for Wing Anti-Ice Testing.

b)     AC 25-28

                        i)         This advisory circular (AC) describes an acceptable means for showing compliance with the supercooled large drop condition requirements of § 25.773, Pilot compartment view,§ 25.1323, Airspeed indicating system, § 25.1324, Angle of attack system, and § 25.1325, Static pressure systems, as well as the requirements of§ 25.1419, Ice protection, and§ 25.1420, Supercooled large drop icing conditions, of Title 14, Code of Federal Regulations (14 CFR) part 25. Part 25 contains the certification requirements for transport category airplanes. The compliance means described in this document are intended as guidance. They are meant to supplement the engineering judgment that must form the basis of any compliance findings for the supercooled large drop icing conditions in §§ 25.773, 25.1323, 25.1324, and 25.1325, as well as the compliance findings for §§ 25.1419 and 25.1420.

 

c)     AC 25.1419-1A

                        i)         This advisory circular (AC) provides guidance for certification of airframe ice protection systems on transport category airplanes. While this is the primary focus of this AC, the guidance also supplements similar guidance provided in other AC’s concerning icing requirements for other parts of the airplane.

3.3       14 CFR Part 25 Appendix C

a)     Continuous maximum icing (CM icing) is the maximum continuous intensity of atmospheric icing conditions. CM icing is defined by 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 A.1 of the appendix. The limits of the icing envelope in terms of altitude and temperature are given in Figure A.2 of the appendix. The inter-relationship of cloud liquid water content with drop diameter and altitude is determined from Figures A.1 and A.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 A.1, multiplied by the appropriate factor from Figure A.3 of this appendix.

b)     Intermittent maximum icing (IM icing) is the intermittent maximum intensity of atmospheric icing conditions. IM icing is defined by 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 A.4 of the appendix. The limits of the icing envelope in terms of altitude and temperature are given in Figure A.5 of the appendix. The inter-relationship of cloud liquid water content with drop diameter and altitude is determined from Figures A.4 and A.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 A.4 multiplied by the appropriate factor in Figure A.6 of this appendix.


For more info on the overview of the processes that lead up to testing, test requirements as well as applicable regulations together with lessons learned from previous test experience, please reach out to us!

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