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Icing Certification
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| Article Information | ||
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| Category: | Certification | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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Contents |
Icing Certification
Certification for flight in icing covers three principal aspects:
- Airframe and systems ice protection,
- Aircraft handling and performance,
- Powerplant ice protection,
Certification for flight in icing intended does not necessarily imply fitness for or approval of continuous operations in icing conditions. In many cases, especially for smaller general aviation aircraft, it may be intended to allow for just a temporary period of operation in icing conditions during which their horizontal or vertical extent is vacated.
Airframe Icing Certification
It is important to note that there is no direct correlation between the presence of ice protection equipment and icing certification. Ice protection equipment has existed for considerably longer than the standards for icing certification, and has historically been certificated as a part of the type design. Many aircraft have thus been designed and produced with ice protection equipment, or had ice protection equipment added after the issue of a Supplementary Type Certificate prior to the introduction of an icing certification standard. Although some manufacturers have subsequently opted to obtain icing certification for older designs of general aviation aircraft, others have not.
The idea of certificating the ice protection system as a part of the type design while not certificating that type for flight into known icing is still considered in the US to be a valid design strategy. This is currently seen in the design of the US-built Cirrus SR-22, a single engine aircraft equipped with an ice protection system but not certificated for flight into known icing.
Prior to the existence of a US engineering standard for icing certification, an ice protection system was certificated in that country as a part of the type design by ensuring that it posed no hazard to safe operation. In particular, US Civil Aeronautics Regulations, Part 3 required that a means exist to ensure that the pneumatic de-ice boot would deflate following usage. It was left to the equipment requirements contained within the operating rules to specify what equipage was required for flight into known icing. Vestiges of these equipment requirements are still contained within the operating rules today, and the complicated interface between them and the certification requirements has led to considerable confusion.
In the US, airframe and system ice protection is certificated under FAR/CS 25.1419 for large (transport) aircraft, FAR/CS 23.1419 for small aircraft, FAR/CS 27.1419 for small or normal category rotorcraft, and finally FAR/CS 29.1419 for transport category rotorcraft. In their current version, these rules require that the aircraft be able to operate safely within the specified engineering standard for atmospheric icing. The rules go on to specify methods for determining whether the ice protection is adequate. However, there is no requirement to quantify aircraft handling and performance degradations. The meaning of the term “operate safely” has been extensively debated in the US.
Handling and Performance Evaluation
In the US at present, FAR 23.1419 and FAR/CS 25.21(g) are the only requirements that specifiy a quantitative definition of the term “operate safely”. These rules require that an aircraft should be able to comply with certain requirements of Subpart B of either Part 23 or Part 25, respectively, while operating within the engineering standard for atmospheric icing. However, Parts 27 and 29 have no such definition associated with the rule.
US FAR 25.21(g) was added to the regulations in 2007. The Subpart B requirements to FAR 23.1419 were added in 1993, although it wasn’t until 2000 that this new regulation was incorporated into the certification basis of a new aircraft design. It is therefore important to consider that, of the existing designs that have been certificated for flight in icing, most have been accepted using only a qualitative evaluation of the aircraft handling characteristics and performance degradation in icing. Quantitative thresholds in both performance and handling degradations, beyond which the design would not be accepted for icing certification, have only recently begun to be specified.
The Engineering Standard for Icing Certification
The engineering standard for atmospheric icing is contained in Appendix C of FAR/CS Part 25. This set of criteria was developed following extensive data survey flights made in the United States during the late 1940’s. It consists of two envelopes, the continuous maximum and the intermittent maximum. These envelopes are defined by liquid water content, droplet size and temperature, and specify a horizontal extent for each condition. Between the two, 99.9% of the atmospheric icing environment is characterized.
The engineering standard, comprised of the continuous maximum and intermittent maximum conditions, was introduced into Civil Aeronautics Regulation 4b.640 in August of 1955 along with the original set of requirements for certification in icing. CAR 4b.640 was migrated into Federal Aviation Regulation Part 25 in February of 1965 as FAR 25.1419. The same standard was never a part of Civil Aeronautics Regulation Part 3, which addressed small aircraft. It was introduced into CAR 3’s successor, FAR Part 23, in 1973. Therefore, nearly all transport category aircraft certificated in the United States from the mid fifties forward have been evaluated against Appendix C conditions. On the other hand, a substantial number of smaller aircraft, even though equipped with ice protection systems, were not tested or analyzed to any standard for flight in icing certification prior to 1973.
Neither of these envelopes considers conditions commonly known as freezing rain or freezing drizzle. These conditions, defined as those in which the median volumetric diameter is greater than 50 microns, are collectively referred to as SLD, or supercooled large droplets. Because Appendix C was considered to be an engineering standard, rather than a certification envelope, it is not technically correct to state that operations in SLD lie outside of the bounds of certification. However, SLD conditions do exceed the criteria used for the design and evaluation of the ice protection systems. This is particularly true with regard to the chordwise extent of the airfoil protected surface, which is predicated on Appendix C conditions. Since the accident at Roselawn, Indiana in 1994, it has become increasingly apparent that operations in SLD are extremely hazardous, difficult to differentiate from Appendix C conditions and more frequent that was originally believed. Since 1978, SLD has been involved with 33% of all aerodynamic icing accidents and reported incidents in the United States. To counter this threat, extensive guidance has been developed to identify conditions that exceed the Appendix C criteria and thus may exceed the capabilities on conventional ice protection systems. The FAA issued airworthiness directives in the mid 1990’s to add SLD detection cues and exit procedures to the flight manuals of airplanes approved for icing. Also, considerable research has been conducted over the last ten years aimed at defining an additional certification standard that will quantify SLD conditions. This new standard will ultimately be incorporated into future icing certifications.
Powerplant Certification
Certification for flight in icing for the airframe is optional; however, all turbine engines must be certificated for operation in icing conditions. This is based on the idea that inadvertent icing encounters are always likely even for aircraft not certificated for flight in icing. The criticality of turbine engine induction icing dictates that protection and certification must be present to counter the effects of an inadvertent icing encounter. This is addressed in CS-E 780 of the EASA requirements and in FAR 33.68 of the US FARs. Turbine engine induction icing certification references Appendix C conditions as specified in FAR Part 25 or in CS Definitions.
Interface between Certification and Operating Rules
In some cases, operating rules are ambiguous regarding the equipment and certification requirements for flight in icing. The EU-OPS use the term “certificated and equipped” to avoid any ambiguity regarding those airplanes that, while equipped, have never been certificated. In the Canadian operating rules, only the requirements under which the type certificate was issued are pertinent. In the US operating rules, most of the focus is on icing intensity and equipment requirements. Only in the Part 135 requirements for helicopters is a specific requirement stated for icing certification.
Prior to the introduction of an icing certification standard into FAR Part 23, the FAA issued Bureau of Flight Standards Release No. 434, “Flight Control Hazards and Protection from Icing”, on November 2, 1959. This letter specifically allowed flight into known light icing provided that certain ice protection equipment requirements were met. However, once the FAR Part 23 had incorporated the Appendix C criteria, the FAA stipulated that any subsequently manufactured aircraft that were not certificated under FAR 23.1419 must be placarded to indicate that flight into known icing was prohibited. This placard did not apply to aircraft, perhaps even of the same type, manufactured prior to 1973.
An excellent reference for this discussion in US operations is the paper entitled “A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions” by Dr. Richard Jeck.