Supplemental Type Certificate for Beechcraft 76 Duchess

Fluid Ice Protection Systems have been used with varying degrees of success on such diverse aircraft as Russian helicopters, British business jets, and United States (U.S.) general aviation aircraft. The majority of these fluid systems have been employed in propeller ice protection systems. However, the successful use of fluid ice protection systems on the wing and tail surfaces of several aircraft in Great Britain has sparked the interest of several American aircraft manufacturers. The major portion of this Technical Note will concern itself with airframe fluid ice protection systems. The British Civil Aviation Authority (CAA) has certified several aircraft for flight into known icing conditions in which a fluid system under the trade name of TKS was used. These aircraft range from the British Aerospace HS-125 to the Beech Duchess (BE-76). Also, the Federal Aviation Administration (FAA) has certified the Cessna Model S550 with a TKS System. Its certification was received in late 1984. The Beech Starship has applied to obtain FAA certification for flight into known icing conditions and plans to employ a fluid ice protection system. Also, an application for a Supplemental Type Certificate (STC) has been received for approval for flight into known icing conditions utilizing the TKS system for two single-engine aircraft. These aircraft, the Beech Model 33 and the Cessna Model 206, are unique to the icing certification arena in that both are equipped with normally aspirated reciprocating single engines. Bleed-air systems, popular on turbine powered aircraft through the mid 1980's, are no longer the automatic choice. The fuel consumption and power penalties associ- ated with using bleed-air cause higher specific fuel consumption than desired for new generation aircraft. The pneumatic boot systems that have been used on reciprocating and turbopropeller aircraft have exhibited a tendency to seriously deteriorate the aerodynamic performance of the new generation natural laminar flow (NLF) airfoils by creating a discontinuity in the surface at the aft edge of the boot, which can trip the boundary layer, causing it to become turbulent. Under these conditions, the laminar flow exists over only a small percentage of the chord of the wing, i.e., less than 10 percent mean aerodynamic chord (MAC), when possibly 65 percent could be achieved with a smooth surface. with this low percentage of laminar flow is not readily accepted. The drag penalties associated NLF airfoils have been designed to be more Some advanced tolerant of surface steps, insure that the maximum lift coefficient will not decrease significantly with and to transition to turbulent flow near the leading edge. These airfoils, however, still exhibit a large drag increase when the extent of laminar flow is significantly decreased. FUTURE. Recent advances in aircraft systems, combined with changes in the world political climate have created some trends within the aviation industry that the FAA must respond to. Some of these trends that will affect the certification of aircraft for flight into known icing conditions are: 1. Improved and additional systems are being added to single engine aircraft. Turbocharged piston engines combined with pressurized cabins have increased the potential of the single engine aircraft to the point where operators routinely 1 expect to operate in most flyable conditions including known icing conditions. The manufacturers also realize that the operators of single engine aircraft are possi- bly more cost conscious than operators of multi-engine aircraft, so ice protection systems which offer cost advantages over other competing systems are of particular interest. 2. The increased cost of petroleum products in the mid and late 70's has heightened the concern of aircraft fuel efficiency. The aviation industry has responded with new designs to increase fuel efficiency. The fluid ice protection system provides a solution for two problems associated with the above trends; i.e., the fluid system is, in some cases, less expensive than the conventional wing ice protection system and it also provides a very clean wing aerodynamically, which in turn may contribute to increased fuel efficiency. However, the system presents its own new considerations. Natural Laminar Flow (NLF) wings are being developed for some next generation aircraft because they exhibit decreased drag characteristics. However, these wings are more sensitive to airflow disruptions than the conventional turbulent flow wings. These disturbances can be produced by small protrusions such as insect accumulation on the leading edges or the small discontinuities caused by paint edges or minor inservice damage. Recent studies also suggest that heavy rain, and in some cases, condensation, may cause the same type of disturbances. These disturbances can cause significant increases in stall speed, significant reductions in the coefficient of lift, control problems at low airspeed, or flight loads in excess of those expected with laminar airflow. There is no reason to believe that the fluid used in fluid ice protection systems would cause any problem, if it remains a liquid. The possible consequences of these changes in the aerodynamic characteristics has caused the National Aeronautics and Space Administration (NASA) to conduct research flights to better understand this phenomenon. The possibility exists that fluid ice protection systems could be used to keep the wings aero- dynamically clean in flight and reduce the problems associated with the contamina- tion of NLF wings. Also, it now appears that the fluid ice protection system may be used to reduce the possible problems associated with heavy rain by employing a fluid, other than Freezing Point Depressant Fluid (FPD), that decreases the surface tension of water. Other mentions of possible uses of this type of system include a combination of boundary layer control over the entire wing. One concern has been the possibility of dirt of other contaminates blocking the holes perforated in a wing or other airfoils for boundary layer control purposes. A fluid type system could be used to keep these holes clean. If that were the case, then the fluid system could possibly be employed for ground anti-ice operation, too. The schedule requirements of most modern aircraft require that it be capable of flying in almost any weather. This requirement is most obvious for airline opera- tions, because nearly all transport category aircraft are capable of flight in supercooled cloud icing conditions. Most smaller aircraft being developed today are designed with the same type of requirement in mind. Therefore, the ACO's will be seeing new and different types of aircraft ice protection systems on the air- craft that manufacturers present for icing certification. The wing fluid ice protection system is a different type of ice protection system than those certified by the FAA in the past. Therefore, this technical note addresses the wing fluid ice protection system and the problems it brings to personnel associated with the aircraft icing certification process. 2 THEORY There are several chemicals that, when mixed with water, lower the waters' freezing point. Glycol, alcohol, calcium chloride, nitric acid, sodium, sodium chloride, among others (reference 1), exhibit this characteristic. The most common chemicals used in the aviation industry for in-flight and ground deicing or anti-icing are glycol and alcohol. Glycol's freezing point in pure form is approximately 10° Fahrenheit (F) depending upon the type of glycol. When pure glycol is mixed with water, the freezing point lowers to approximately-10° to -40° F for a 50/50 mixture depending upon the type of glycol. A fluid ice protection capability for an airfoil can be achieved by the appropriate mixing of glycol based fluids and cloud supercooled water droplets on the leading edge of an aircrafts' airfoils. Fluid ice protection systems designed for leading edges use porous panels that exude the ice protection fluid undiluted and as supplied by the manufacturer onto the surface where the fluid mixes with impinging supercooled water droplets. This results in a mixture that will contain FPD fluid and water, and possibly ice particles that are swept aft and off the airfoil surface by aerodynamic forces. This ice protection capability involves several variables that determines the type of ice protection. The wariables include the mount of supercooled water droplet striking the wing, the mount of ice protection fluid exuding on the wing surfaces, and the temperatures. A typical arrangement is shown in figure 1. FLOW REQUIRED. There are three different ice protection modes and two thresholds associated with fluid systems (figure 2). They are: 1. Natural Deice Threshold 2. Natural Deice Mode 3. Anti-ice Threshold 4. Anti-ice Mode 5. Deice Mode The above three modes of the airframe ice protection fluid system are in order of increasing fluid flow (though not necessarily total fluid usage). The ice protec- tion fluid flow rate and the amount of water impingement determines the ice protec- tion mode of the fluid system. At the present time, there are basically two methods used to predict the required flow rates. One method is described in ADS-4 and a second method is described in an American Institute of Aeronautics and Astronautics (AIAA) publication (reference 2). The latter has been used in at least one Certification Program (reference 3). These methods are described later in this technical note. NATURAL DEICE THRESHOLD. The natural deice threshold occurs during a gradual transition between no protection and the natural deice mode where there is a period of ice building and ice shedding. If the time between shedding is long enough, the ice protection system can be said to be not working or not providing ice protection. Using the work from reference 4, this time between shedding has been shown to vary from 2 to 7 minutes. The practical considerations for this time will have to be addressed during the certification process to ensure that function occurs. no hazard or loss of intended 3 ANNUNCIATOR PANEL & FLUID QUANTITY GAUGES CHECK VALVE PUSSP SLINGER RING WINDSCREEN SPRAYER PORNO PANELS ICE SETECTOR PORUS PANEL FIGURE 1. TYPICAL ARRANGEMENT OF FLUID ICE PROTECTION SYSTEMS COMPONENTS FOR A LIGHT SINGLE-ENGINE AIRCRAFT NATURAL DEICE THRESHOLD ANTI-ICE THRESHOLD NO PROTECTION NATURAL DEICE ANTI-ICE DEICE CONTINOUS FLOW FLUID FLOW RATE FIGURE 2. FPD FLUID FLOW RANGES INTERMITTENT FLOW TN-86/11-1 NATURAL DEICE MODE. The The natural deice mode is a condition that requires the wing fluid ice protection system to be operating before the aircraft has appreciable ice accumulations. fluid flow rate is lower than required for anti-icing. This lower flow rate allows the freezing point depressant fluid and water mixture at the leading edge to form small spanwise strips of ice along the leading edge stagnation point. The ice will be swept away by aerodynamic forces periodically in the natural deice mode. ANTI-ICE THRESHOLD. - There is a gradual transition between the anti-ice mode and the natural deice mode. The anti-ice mode has an FPD fluid flow rate that is sufficient to prevent any ice from forming on the leading edge. If the fluid flow rate is decreased, there is a point where small accumulations of ice begin to form, this is the anti-ice thresh- old. As the flow rate is decreased (or the amount of water impinging on the airfoil increases), the amount of ice forming will increase. The ice will at first be swept away very rapidly and be imperceptible to the eye. As this transition continues, the ice will begin to form along the leading edge in spanwise bars and be swept away by the airflow every few minutes. At this point, the transition has been completed from the anti-ice mode to the natural deice mode. ANTI-ICE MODE. The anti-ice mode is a condition such that the FPD fluid and the supercooled cloud droplets water mixture on the leading edge has a freezing point low enough to prevent any freezing on the leading edge or any other surface of that airfoil. is never allowed to adhere to the surface being protected. DE-ICE MODE. Ice The deice mode is a condition where ice is allowed to build before the fluid ice protection system is turned on. This allows ice to accumulate which has a bond to the wing surface. When the fluid ice protection system is turned on, a flow is introduced between the ice and the surface to weaken the bond to the extent that ice shedding occurs. There is evidence to suggest that it may not be possible to deice a surface in this manner under some conditions. The testing of the fluid ice protection system should confirm that it operates satisfactorily in the deice mode over the full range of operating conditions that the aircraft is expected to encounter, if certification for deice mode operations is desired. The flow rate for the deice mode is the highest, but that does not mean it requires the most fluid for an icing encounter. Total fluid requirements for the deice mode may be less than the anti-ice mode if the time between deicing cycles is long enough. ICE PROTECTION AREAS The fluid systems used for in-flight anti-icing or deicing are usually only designed for protection of the leading edge of the airfoil. Normally FPD fluid is exuded from a distribution panel that covers the leading edge of the wing. How- ever, in some instances, the wing serves as the distribution panel and is made of a suitable material in which many holes are drilled. The holes may be formed by 5 different methods but one of the methods used at the present time involves using lasers to drill holes nominally 0.0025 inch in diameter, with 0.035 inch between centers. This arrangement yields about 800 holes per square inch. AIRFOILS (WINGS AND EMPENNAGE). The airfoil section usually considered for ice protection systems includes wings, horizontal and vertical stabilizers, canards, winglets, etc. The wing, horizontal and vertical stabilizers have usually been considered one of the primary surfaces to deice or anti-ice. When wing fluid ice protection systems are used they are normally used on the leading edges, but not always. A NASA Lockheed C-140 Jetstar (reference 5) has a spray nozzle mounted inside the Krueger leading edge flap. This nozzle sprays a mixture of water and propylene glycol methyl ether (PGME). Its purpose is to keep the laminar flow section clean of foreign matter and ice. ANGLE-OF-ATTACK LIMITS. The design and installation of a fluid wing ice protection system must take into account the expected angles-of-attack. This is necessary so that the impact areas of the water droplets can be protected (figure 3). If water droplets impinge on unprotected areas and ice accretes, severe problems may develop. An example of a fatal accident attributed to this phenomenon involving a Beech King Air during climb-out is cited in reference 5. Although some pilots are taught that during climb, the best course of action when encountering icing condi- tions is to climb as rapidly as possible, it is not stressed that the entire under-wing area may become exposed to ice accretion which the pilot may not be able to see or be aware of. UPPER IMPINGEMENT LIMIT +S RELATIVE WIND ANGLE OF ATTACK द -S CORD LINE LOWER IMPINGEMENT LIMIT FIGURE 3. WATER DROPLETS IMPINGEMENT LIMITS 6