Mixed Phase Icing Simulation and Testing at the Cox Icing Wind Tunnel

A new capability was developed for indoor simulation of snow and mixed-phase icing conditions. This capability is useful for year-round testing in the Cox closed-loop Icing Wind Tunnel. Certification of aircraft for flight into these types of icing conditions is only required by the JAA in Europe. In an effort to harmonize certification requirements, the FAA in the US sponsored a preliminary program to study the effects of mixed-phase and fully glaciated icing conditions on the performance requirements of thermal ice protection systems. This paper describes the test program and the associated results. 1.0 Since the ATRINTRODUCTION There are requirements and advisory materials under Joint Airworthiness Requirements (JAR) in Europe that must be satisfied prior to flight into mixed icing and snow conditions [1]. To achieve certification for flight into these conditions, aircraft must demonstrate compliance with those requirements. In the USA, the Federal Aviation Administration (FAA) has issued no such requirements to date other than those restricted to falling and blowing snow with no mention of airspeed [2]. In recent years, several flight programs were conducted in order to re-characterize the atmosphere at different locations around the globe. Most of these programs were geared towards Supercooled Large Droplets (SLD). Among these programs are the NASA Glenn Research Center icing research flights throughout the Great Lakes region, the Canadian Freezing Drizzle Experiment (CFDE) and the Alliance Icing Research Study (AIRS). References [3] through [10] include discussions on those programs. SLD icing conditions are believed to have been a factor in the 1994 fatal crash of an ATR-72 commuter aircraft. There is evidence from those flight research programs that mixed phase icing conditions can be encountered relatively frequently. In mixed icing conditions, ice crystals and supercooled water droplets coexists. -72 accident, the FAA and the JAAhave been working on joint programs in order harmonize the certification requirements. The FAA issued a document detailing its in-flight aircraft icing plan, the purpose of which is to increase flight safety [11]. Among the list of tasks is Task 13. Here, the objective is to characterize SLD aloft and assess mixed phase conditions in the atmospheric flight environment. Specifically, Task 13C states that “The FAA will conduct a study to determine the magnitude of the safety threat that is posed by mixed phase conditions.” Consequently, the FAA sponsored a specialists' workshop on mixed-phase and glaciated icing conditions in Dec 1998. Several presentations were given on the topic as indicated by References [12] and [13]. The experience of authors and experts in the field is that both snow and mixed icing conditions could constitute flight hazards beyond those recognized formally. Insidious accumulations of ice and packed snow in engine inlets and at various locations on the airplane can occur. As already mentioned, the JAA recognizes the hazardous nature of mixed phase and snow conditions and requires testing for engines and air data probes and instrumentation. The effect of these conditions on heated or unheated lifting surfaces has not been systematically evaluated and documented. In order to address this issue, the FAA sponsored a program to investigate the impact of mixed phase and snow conditions on thermal ice protection systems. An exploratory test was conducted in the Cox & Company Icing Wind Tunnel (IWT) in July 2002. This was a collaborative effort between the FAA, Wichita State University, Cox & Company, and NASA Glenn Research Center.