Icing Effects, Protections, and Detection (Part Two)

3-8. PROPELLER ICING. Ice buildup on operating propeller blades reduces thrust for the same aerodynamic reasons that wings tend to lose lift and increase drag when ice accumulates on them. The greatest quantity of ice normally collects on the spinner and inner radius of the propeller. However, in one case of suspected large drop icing during a flight test (see Figure 3-10, Propeller Ice Accretion During an SLD Encounter), ice was experienced along the entire span of the propeller blades. This resulted in a 50 knot loss of airspeed in 1 minute, 25 seconds. There was little airframe ice and no indication of propeller icing. As ice accretes on the propeller blades increasing blade drag, the propeller governor of the constant speed propeller flattens the blade pitch to maintain revolutions per minute (RPM). In the cockpit, the pilot sees no change in RPM or torque.

FIGURE 3-10. PROPELLER ICE ACCRETION DURING AN SLD ENCOUNTER

FIGURE 3-10. PROPELLER ICE ACCRETION DURING AN SLD ENCOUNTER

3-9. ANTENNA ICING. Because of their small size and shape, antennas that do not lay flush with the aircraft’s skin tend to accumulate ice rapidly. Furthermore, they often are devoid of an internal anti-icing or deicing capability for protection. During flight in icing conditions, ice accumulations on an antenna may cause it to begin to vibrate or cause radio signals to become distorted. Besides the distraction caused by vibration (pilots who have experienced the vibration describe it as a “howl”), it may cause damage to the antenna. If a frozen antenna breaks off, it can damage other areas of the aircraft in addition to causing a communication or navigation system failure.

 

3-10. COOLING INLET ICING. Some types of electronic equipment generate significant amounts of heat and require independent sources of cooling, which often use external air scoops. These cooling inlets are susceptible to icing and may or may not be heated as part of the icing protection system on older airplanes. Pilots should check their airplane’s AFM to determine if the cooling inlets are protected from ice.

3-11. EFFECTS OF ICING ON CRITICAL SYSTEMS.

a. Pitot Tube. The pitot tube is particularly vulnerable to icing because even light icing can block the entry hole of the pitot tube where ram air enters the system. This will affect the airspeed indicator and is the reason most airplanes are equipped with a pitot heating system. The pitot heater usually consists of coiled wire heating elements wrapped around the air entry tube. If the pitot tube becomes blocked, and its associated drain hole remains clear, ram air no longer is able to enter the pitot system. Air already in the system will vent through the drain hole, and the remaining will drop to ambient (i.e., outside) pressure. Under these circumstances, the airspeed indicator reading decreases to zero because the airspeed indicator senses no difference between ram and static air pressure. If the pitot tube, drain hole, and static system all become blocked in flight changes in airspeed will not be indicated, due to the trapped pressures. However, if the static system remains clear, the airspeed indicator would display a higherthan-actual airspeed as the altitude increased. As altitude is decreased, the airspeed indicator would display a lower-than-actual airspeed.

b. Static Port. Many aircraft also have a heating system to protect the static ports to ensure the entire pitot-static system is clear of ice. If the static port becomes blocked, the airspeed indicator would still function; however, it would be inaccurate. At altitudes above where the static port became blocked, the airspeed indicator would indicate a lower-than-actual airspeed. At lower altitudes, the airspeed indicator would display a higher-than-actual airspeed. The trapped air in the static system would cause the altimeter to remain at the altitude where the blockage occurred. The vertical speed indicator would remain at zero. On some aircraft, an alternate static air source valve is used for emergencies. If the alternate source is vented inside the airplane, where static pressure is usually lower than outside static pressure, selection of the alternate source may result in the following erroneous instrument indications:

(1) The altimeter reads higher than normal.

(2) The indicated airspeed reads greater than normal.

(3) The vertical speed indicator momentarily shows a climb.

 

c. Stall Warning Systems.

(1) Stall warning systems provide essential information to pilots. A loss of these systems can exacerbate an already hazardous situation. These systems range from a sophisticated stall warning vane to a simple airflow-activated stall warning switch. The stall warning vane (also called an “AOA sensor” since it is a part of the stall warning system) has a wedge-like shape, has freedom to rotate about a horizontal axis, and is connected to a transducer that converts the vane’s movements into electrical signals transmitted to the airplane’s flight data computer. Normally, the vane is heated electrically to prevent ice formation. The transducer is also heated to prevent moisture from condensing on it when the vane heater is operating. If the vane collects ice, it may send erroneous signals to such equipment as stick shakers or stall warning devices. Aircraft that use a stall horn connected to the stall warning switch may not give any indication of stall if the stall indicator opening or switch becomes frozen.

(2) Because contamination of the wing reduces lift, even an operational, ice-free stall warning system may be ineffective because the wing will stall at a lower AOA due to ice on the airfoil. Heated or unheated, if the wing is contaminated in any way, an AOA will become unreliable. The stall onset would occur prior to activation of stall warning devices leading to a potential pitch or roll upset. It is imperative that pilots maintain airspeed and monitor AOA closely when in icing conditions.

d. Windshields.

(1) On high-performance aircraft that require complex windshields to protect against bird strikes and withstand pressurization loads, the heating element often is a layer of conductive film or thin wire strands through which electric current is run to heat the windshield and prevent ice from forming.

(2) Aircraft that operate at lower altitudes and lower speeds generally have other systems of window anti-icing/deicing. One system consists of an electrically heated plate installed onto the airplane’s windshield to give the pilot a narrow band of clear visibility. Another system uses a bar at the lower end of the windshield to spray deicing fluid onto it and prevent ice from forming.

e. Engine Pressure Ratio (EPR) Probe (Turbine Engines).

(1) Ice crystals can clog and freeze over turbine EPR probes as well, resulting in unreliable and misleading power indications. These indications may lead a pilot to believe that an engine is producing more or less power than it actually is, and may result in improper throttle adjustments.

(2) There have been several instances where EPR probes became clogged with ice crystals during climb or cruise (e.g., the Air Florida B-737 accident, NTSB accident report NTSB/AAR-82-08). Pilots of turbojet aircraft should calculate a backup N1 setting for takeoff/go-around in icing conditions as a crosscheck for EPR. The activation of engine nacelle anti-ice when flying in heavy clouds usually prevents ice blockage.

 

f. Outside Air Temperature (OAT)/True Air Temperature (TAT) Probe.

(1) Ice crystals can clog and freeze over the heated temperature probe on some aircraft. This tendency to freeze over appears to be sensitive to the location of the probe on the airframe. If the OAT/TAT probe freezes over, the indicated temperature will erroneously rise to 0 ºC and hold. In this situation, some aircraft systems will alert the flightcrew that there is a disagreement between various ambient temperature sensors, thus indicating the presence of ice crystals.

(2) Freezing of the TAT probe has been a precursor in many of the turbine engine power loss events occurring in the area of convective weather systems.

3-12. CERTIFICATION FOR FLIGHT IN ICING CONDITIONS.

a. Current Icing Certification. Aircraft which are “certificated for flight in icing conditions” by Amendment 25-121 or higher go through an extensive procedure intended to ensure that they can safely operate throughout those icing conditions encompassed by the icing envelopes specified by the FAA. The current icing certification process includes extensive analysis (done today with sophisticated computer modeling), tunnel testing, dry-air testing, testing behind an icing tanker, and flight in natural icing conditions. The objective is to verify that the aircraft has functioning ice protection and to ensure that the aircraft will have acceptable performance and handling qualities in all the environmental conditions covered by the icing envelopes for which the aircraft has been tested. For example, certification includes testing and analysis to show that an aircraft can hold in significant icing conditions for up to 45 minutes. Nonetheless, pilots of certificated aircraft should not be casual about operations in icing conditions, particularly extended operations. It is always possible to encounter an unusual condition for which the aircraft has not been certificated, such as Liquid Water Content (LWC) outside the envelopes, which may be indicated by a very rapid rate of accumulation. This can result in runback and ice accumulation aft of protected surfaces.

b. Limits of Icing Certification. SLD may result in drops impinging aft of protected surfaces and causing ice accumulation behind the protected area of leading edges. These surfaces may be very effective ice collectors, and ice accumulations may persist as long as the aircraft remains in icing conditions. Note also that icing conditions can develop very quickly and may not be immediately recognized. The effect on stall speed increase and drag may be large. This can be very hazardous, particularly on approach and landing. On November 4, 2014, part 25, § 25.1420 and part 25 appendix O became effective in order to address SLD certification for new, transport category airplanes, but SLD has not yet been incorporated into the certification of other aircraft types.

NOTE: If an aircraft has certain Supplemental Type Certificate (STC) items installed, these may affect the icing certification of the aircraft as defined by the manufacturer, some of which might not operate correctly when exposed to icing or be certified for icing conditions.

 

c. How to Know Whether a Small Airplane is Certificated for Icing.

(1) The airplane was certificated to 14 CFR part 23, § 23.1419 at Amendment 23-14 or later if your AFM or POH references “part 25 appendix C” icing conditions, or “14 CFR § 23.1419” at Amendment 23-14 or later.

(2) The “Certification Basis” section of your airplane’s Type Certification Data Sheet (TCDS) may reference “14 CFR § 23.1419” at Amendment 23-14 or higher, or “SFAR 23.” The TCDS can be found in the FAA’s online Regulatory and Guidance Library (RGL) at http://rgl.faa.gov.

(3) If there is only a minimum equipment list (MEL) for icing conditions in the AFM or POH, the certification basis of your airplane is prior to Amendment 23-14 (1973).

d. Icing Certification Has Changed Over the Years.

(1) Current part 23 icing regulations have only been applied to new airplane designs certificated since 2000. In these new designs, the stall warning system on an icing-certified airplane is designed and tested with critical ice accretions along the entire span of the wing. In many new designs this results in the stall warning speed biased higher in icing conditions.

(2) Prior to 2000, a clear and unambiguous buffet was accepted for stall warning in icing conditions, even if the airplane was equipped with a stall warning system and a heated stall warning sensor.

NOTE: If the certification basis is Amendment 23-43 or higher, you can be sure the stall warning systems functions in the icing conditions for which the airplane is certified. If it is lower than Amendment 23-43, do not rely on your stall warning system in icing.

(3) Airplanes certified for flight in icing after 1994 have been tested for susceptibility to Ice-Contaminated Tailplane Stall (ICTS). ICTS cannot occur if AFM limitations and procedures are followed.

(4) Prior to 1973, there were no requirements to test part 23 airplanes in icing conditions. Part 23 airplanes were approved for flight in light icing conditions, and moderate icing for limited time, if they were properly equipped. Many of these airplanes remain in the fleet today. The ice protection systems on these airplanes should be considered a means to help exit icing conditions.

e. How Certification Relates to Operating Rules. Operation of an aircraft in known icing is based upon when an aircraft was built and how that aircraft was certified during manufacture. Manufacturers specify how the installed equipment in that aircraft is to be operated in the POH and AFM within certain conditions of limitation.

 

3-13. AIRPLANES NOT CERTIFICATED FOR ICING.

a. Ice Protection. All aircraft are required to have ice protection for their propulsion systems in case of an inadvertent icing encounter, and nearly all aircraft have pitot heat and an alternate source of static air.

b. Avoidance. Airplanes not certified for icing are not tested for inadvertent icing encounters. Pilots of these airplanes must avoid icing conditions and immediately exit icing if inadvertently encountered.

(1) In recent years these airplanes have been involved in more icing accidents than icing-certified airplanes.

(2) Do not believe the myth that thicker General Aviation (GA) airfoils are tolerant of ice accretion. Research has shown that even for small amounts of ice accretion, effects not apparent while operating in the middle of the flight envelope may be noticeable when operating at the edge of the flight envelope. The most common are an increase in stall speed (with a late or no warning) or the inability to climb at altitude.

c. Inadvertent Encounter. Some GA aircraft not certificated for flight in icing conditions have ice protection systems on their wings and tailplane, providing an additional safety margin, should an inadvertent encounter with icing occur. These are intended for emergency use only.

(1) The FAA recommends that aircraft not certificated for flight in icing conditions exit icing conditions as expeditiously as possible.

(2) The differences between these systems and fully certified systems are significant. Airplane performance is unknown, stall warning in icing conditions most likely will not activate prior to stall, controls may jam due to ice accretion, and system features required for known icing may not be present in these “non-hazard” systems.

3-14. MAINTENANCE CONSIDERATIONS. Some anti-icing and deicing systems are known to be very reliable, while others may require a lot of maintenance to remain effective. Pneumatic boots, for example, are known for their susceptibility to damage from many sources and should be inspected carefully. The rubber used for the boots is subject to degradation from atmospheric pollution, which results in the rubber cracking and losing some of its elastic properties. An ice adhesion inhibitor should be applied to pneumatic deicing boots in accordance with the maintenance manual and is highly recommended. Testing in 2005 showed that the proper application of ice adhesion inhibitors improved ice shedding at colder temperatures and a reduced amount of residual and intercycle ice. Any product that is not recommended by the airplane or boot manufacturer should be approved by the FAA. Other problems are defects, delaminations, or tears in the rubber caused by the impact of objects, such as foreign matter found on airport ramps. Pinholes or tears in pneumatic deicing boots will draw in moisture when system vacuum is supplied, and subsequent freezing of this moisture can render the system ineffective. Maintenance personnel should evaluate defects in the boots when they are found.

a. Preflight Inspection. Flightcrews should always examine their airplane’s anti-icing and deicing equipment as part of the normal preflight inspection. A full check of anti-icing and deicing equipment should be performed especially when flight into known or forecast icing is expected as identified in the AFM procedures.

b. Equipment Deficiencies. Flightcrews should consult the airplane’s MEL for details on what is permitted to be inoperable and what equipment deficiencies constitute no-go items.

 

3-15. ICE DETECTION.

a. Electronic.

(1) Many modern aircraft come equipped with electronic ice detectors. A common in-flight ice detector consists of a probe that vibrates at a specific frequency. When ice begins to form on the probe, the frequency of the probe’s vibration will change because of the increased mass of ice on the probe, and an indicator will light in the cockpit. These detectors are activated for a short time period, generally one minute, after which the probe is heated electrically to melt the accreted ice. The process is then repeated. If the aircraft is flying in continued icing conditions, ice will continue to form on the probe, and, for some aircraft, the light in the cockpit will remain on.

(2) Pilots should consult their AFM or POH to determine if their ice detection system is an advisory or primary system. The difference between the systems is the redundancy of the system and testing required for certification. The majority of airplanes have an advisory system, which means the pilot is responsible for detecting ice and ensuring ice protection systems are activated. This is true even for ice protection systems that are automatically activated when the ice detection system detects ice.

(3) Currently, there are no electronic detection systems that can reliably detect ice crystals, although new systems are under development.

b. Visual. Strategically located or unprotected protuberances visible to the crew may also serve as ice indicators. For example, windshield wipers, pod pylons, or landing lights can serve as icing references because they tend to build up ice first, or manufacturers may provide one for this purpose. These ice detectors, referred to as “ice evidence probes,” are typically in plain view of the cockpit. If ice begins to accumulate on such an ice detector, the flightcrew should assume the rest of the aircraft also is accumulating ice and take appropriate action. These detectors only serve their purpose if pilots include them in their scan during flight in potential icing conditions. If possible, pilots should monitor critical surfaces at temperatures near freezing, since ice may form on critical surfaces prior to forming on visual ice indicators.

3-16. VISUAL CUES OF SLD CONDITIONS. If SLD is known to be present, most aircraft with unpowered controls and airframe deicing systems should request a route or altitude change to exit the conditions. This action may be prudent for other aircraft as well. The cues listed below were developed for aircraft with unpowered controls and pneumatic deicing boots as indicative of SLD conditions. Of most concern is the accretion of ice in areas aft of where it would usually be found. Such aft accretions could sometimes be the result of runback due to high liquid water content rather than SLD. Excessive runback icing, however, may have effects similar to SLD, so similar pilot action may be appropriate. The cues are:

a. Wing. Ice may become visible on the upper or lower surface of the wing, aft of the active part of the deicing/anti-icing system. If the wings are visible, pilots should monitor for ice accretion aft of the protected area. Pilots should also look for irregular or jagged lines of ice or for pieces of ice shedding off the airplane. During night operations, pilots should use adequate illumination to observe all areas.

b. Propeller. The aft limit of ice accumulation on a propeller spinner that is not heated will reveal ice extending beyond normal limits, typically back to the blades.

c. Windows. Unheated portions of side windows may begin to accumulate granular dispersed ice crystals or a translucent or opaque coating over the entire window. This icing may be accompanied by other ice patterns on the windows, such as ridges. These patterns may occur from within a few seconds to half a minute after exposure to SLD conditions.

 

d. Engine Nacelles. Ice may form on engine nacelles behind the inlet lip.

(1) Airframe. Ice coverage may become unusually extensive, with visible ice fingers or feathers on parts of the airframe that normally would not be covered by ice. The aircraft’s performance may degrade. Pilots should remain vigilant when icing conditions are present, and any alteration of the aircraft’s performance should be monitored closely as a sign of icing on the airplane.

CAUTION: Pilots should be vigilant for the ice accretions listed above when the following are observed: (1) visible rain or drizzle at temperatures below +5 °C OAT, and/or (2) drops that splash or splatter on impact at temperatures below +5 °C OAT.

CAUTION: Vigilance for SLD ice accretions should also be exercised when flying into or over areas reporting precipitation at the surface, such as rain, freezing rain, sleet, ice pellets, drizzle, freezing drizzle, or snow, where temperatures are near freezing. However, pilots should be aware that SLD could occur aloft without any SLD precipitation on the surface. Current weather information can miss SLD, so it is important to know and watch for cues on the airplane.

(2) While the pilot should be aware of these general cues, there may be specific cues that are characteristic of SLD icing on particular aircraft types. The pilot should consult the aircraft AFM or POH for descriptions of any such cues.