VTOL aircraft present a new and emerging technology that differ from conventional rotorcraft and fixed-wing aircraft. The EASA VTOL Special Condition [1] refers to VTOL aircraft as heavier-than-air, person-carrying aircraft in the small-category with lift/thrust units. The distinction from conventional aeroplanes is based upon the VTOL capability of the aircraft, while the distinction from conventional rotorcraft is based upon the use of distributed propulsion, specifically when more than two lift/thrust units are used to provide lift during a vertical take-off or landing.

Certification for flight in icing conditions is complex. The scope of this document is therefore limited to a means of compliance for icing-related regulations applicable to VTOL aircraft that are not certified for flight in icing conditions but may inadvertently encounter icing conditions, and must have provisions to safely operate while exiting the icing conditions. This document also covers snow conditions that are applicable to VTOL aircraft during an inadvertent encounter or for continuous operation.

For aircraft that are not certified to fly in icing conditions, the priority is to avoid operations in conditions where icing is likely to occur. VTOL.2165(b) [1] requires an applicant to provide a means to detect any icing conditions for which the aircraft is not certified to operate in and demonstrate the aircraft´s ability to avoid1 or exit those conditions.

Ground-based weather detecting and forecasting equipment are in development but the accuracy at present is limited and therefore cannot be used as the sole means of compliance for the avoidance of flight into icing conditions. Examples such as current icing potential (CIP) and forecast icing potential (FIP) [2] in the United States, infer icing from various sources such as atmospheric soundings, satellite, radar, surface observations and pilot reports (PIREPS). As a result, the probability of detection of icing conditions is in the order of 90% which is not sufficient for conducting operations over congested areas. Automated surface observations also do not report freezing drizzle or any freezing precipitation. Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are drizzle, rain, snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets [1][23]. When the notice of proposed rulemaking (NPRM) [3] was written, systems being developed such as Terminal Area Icing Weather Information for NextGen (TAIWIN) were envisioned as potentially acceptable weather forecasting products [4]. Airborne-mounted “look ahead” optical based systems are also being developed but the “look-ahead” distance and accuracy remains limited. Currently, there are no readily-identifiable solutions that could be implemented to provide a sole means of compliance to avoid icing conditions with sufficient confidence that an aircraft does not need to demonstrate safe operation during an inadvertent icing encounter. This document, however, provides a possible approach for avoiding icing conditions to ensure that the entire aircraft remains ice-free during operations using an ambient temperature operational limit (section 8).

For the purposes of providing background to the reader, the regulations applicable to rotorcraft (CS-27 and 29) and fixed-wing aircraft (CS-23) that are not certified for icing are presented in APPENDIX A.

This standard addresses icing considerations for the airframe, engines (including electric engines), and propulsors (rotors, fans, or propellers).

¹ This option to ´avoid´ icing conditions was originally added to the performance-based rules of FAA regulations (14 CFR 23) at amendment 23-64 and EASA certification specifications CS-23 Amendment 5.