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 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 the horizontal or vertical extent of the icing is vacated. The way in which 'icing conditions' are defined for large (transport) aircraft has in recent years come under close scrutiny on both sides of the Atlantic because it has become apparent that icing can occur at static air temperatures much colder than those adopted in the longstanding current definitions.
Airframe Icing Type Certification
It is important to note that there is no direct correlation between the presence of ice protection equipment and certification for flight in icing conditions. Ice protection equipment has existed for considerably longer than standards for icing certification and any such equipment has historically been included in the overall certification process. Many smaller aircraft still in service have thus been designed and manufactured with ice protection equipment installed, or had it added in accordance with a Supplementary Type Certificate (STC) 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 by the FAA to be a valid design strategy for small general aviation aircraft. An example of this approach is the US-built Cirrus SR-22.
Prior to the existence of certification standards for icing certification, an ice protection system was approved as part of the type design process by ensuring that it did not prejudice safe operation. For example, Part 3 of the former US Civil Aeronautics Regulations required that a means existed to ensure that pneumatic de-ice boots 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. Some vestiges of these equipment requirements for small aircraft are still included in operating rules today which has been known to lead to considerable confusion.
Type Certification of large (transport) fixed wing aircraft is nowadays accomplished under 14 CFR 25.1419 by the FAA or under CS 25.1419 by EASA. In their current version, these require that an aircraft should be able to "operate safely" when the stated definition of icing conditions exist. Although there are methods for determining whether the ice protection provided is adequate, there is currently no requirement to quantify aircraft handling and performance degradations. The meaning of the expression “operate safely” has been the subject of much debate, particular in the USA.
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 specified in Appendix C of CS 25 / 14 CFR Part Part 25 has been broadly in its present form since it was first developed in the United States and introduced there in 1955 under the former Civil Aeronautics Board before being transferred into FAR Part 25.1419 in 1965. It consists of two envelopes, the continuous maximum and the intermittent maximum. These envelopes are defined by liquid water content, droplet size and air temperature, and specify a horizontal extent for each condition. Between the two, 99.9% of the atmospheric icing environment is characterized. Smaller aircraft first became subject to a comparable standard only with the advent of FAR Part 23 in 1973.
Appendix C did not address the presence of supercooled large droplets (SLD) - water droplets which persist in subfreezing temperatures and have a median diameter usually defined as greater than 40 microns. Their occurrence is characterised as freezing drizzle where droplet diameters are between 40 and 200 µm and freezing rain where droplet diameters exceed 200 µm. However, because Appendix C was conceived as an engineering standard rather than a certification specification, it is not technically correct to state that operations in SLD lie outside of the bounds of certification even though they are not considered by the criteria used for the design and evaluation of ice protection systems. This is particularly true for the chord-wise extent of the protected surface on a wing or tailplane, which is predicated on Appendix C conditions.
Following the fatal loss of control accident to a large twin turboprop at Roselawn, Indiana in 1994, there was a recognition that SLD could be extremely hazardous and a concerted international effort to improve understanding of their effects and develop corresponding responses occurred. It was claimed during this work that, since 1978, SLD had been involved in around a third of all aerodynamic icing accidents to aircraft of all sizes in the United States. There is now comprehensive guidance on the identification of these conditions in the AFMs of all aircraft engaged in public transport and certificated for operation in icing conditions and work in both Europe and the USA to define an additional certification standard to cover the SLD case is nearing completion.
Although airframe certification for flight in icing for the airframe is optional, all turbine engines must be certificated for operation in icing conditions on the basis that inadvertent icing encounters are always possible, even for aircraft not certificated for flight in such conditions. Turbine engine certification has historically been focussed on inlet ice protection which is addressed in CS E-780 in CFR14- Part 33.68 with reference currently made, as in the airframe case, to Appendix C conditions.
Just as the 'discovery' of the SLD hazard for airframes led to a recognition of the limitations of the definition of icing conditions in Appendix C, a similar 'discovery' of the hazardous effects of Ice Crystal Icing on turbine engines has led to the investigation of this phenomenon in order to inform an effective extension of current certification requirements. Fortunately, the trigger here has been serious incidents rather than fatal accidents.
Interface between Certification and Operating Rules
There has, in the past, been a degree of ambiguity in the relationship between operating rules and type certification requirements for flight in icing conditions. EASA uses the term “certificated and equipped” to preclude such confusion in the case small aircraft that, whilst equipped with some ice protection equipment, have never been certificated for flight in icing conditions. Under US operating rules, most of the focus is on icing intensity and equipment requirements and only in the Part 135 requirements for helicopters is there a specific requirement for icing certification.D
Prior to the introduction of an icing certification standard into CFR 14 Part 23, the FAA standard applicable to small aircraft, a Flight Standards document entitled “Flight Control Hazards and Protection from Icing” specifically permitted such aircraft to enter known light icing provided that certain ice protection equipment requirements were met. However, this 'dispensation' disappeared once Part 23 had incorporated the Appendix C criteria defining icing conditions and the FAA stipulated that any subsequently manufactured aircraft that not certificated under Part 23.1419 must be placarded to indicate that flight into known icing was prohibited. However, this placarding requirement has never been retrospectively applied to any Part 23 aircraft manufactured prior to 1973, even if a type remained in production after this.
A review of the historical background to the FAA approach to aircraft type certification and concomitant operating requirements can be found in a 2001 Paper “A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions”.
- A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions, Richard K. Jeck, FAA, November 2001
- Appendix C 'Icing Conditions' to CFR 14 Part 25, FAA, 2014
- Getting to grips with Cold Weather Operations, Airbus, 2000
- Compliance of Transport Category Aircraft with Certification Requirements for Flight in Icing Conditions, FAA, Advisory Circular (AC) 25-28, 27 Oct. 2014.