Par 3-4 Page 20 of any available deicing system, including pneumatic boots. Proper operation of the boots is necessary to minimize the effect of this ice. The amount of ice increases as airspeed or temperature decreases. At airspeeds typical of small airplanes, it may take many boot cycles to effectively shed layers of ice. It may appear that the boots are not having any effect at all until shedding occurs. In many icing accidents and incidents, loss of airspeed and stall can occur in a span of minutes. Any remaining ice accretion will increase in the stall speed.
(2) A layer of ice that is rough at any thickness on a wing’s leading edge can have a significant effect on aircraft performance, stability, and control. Consequently, some manufacturers now advise that the boots be cycled as soon as icing is encountered, rather than waiting for a prescribed thickness to accrete. The FAA recommends that the deicing system be activated at the first indication of icing as activating the boots early and often never results in having more ice on the wing than waiting for a late activation. It is essential that the pilot consult the AFM or pilot’s operating handbook (POH) (the POH must be consistent with the operating limitations section of the AFM) for guidance on proper use of the system.
(3) At the AOA typical of cruise, this ice should have very little effect on lift.
An increase in stall speed becomes more of a concern at higher AOAs characteristic of approach and landing because the aircraft is operating closer to CLmax. Thus, the pilot should consider continuing activation of the deicing system for a period after exiting the icing conditions so that the wing will be as clean as possible and any effect on stall speed minimized. If the pilot cannot exit the icing conditions until late in the approach or significant icing appears to remain on the wing after activating the system, an increase in the aircraft’s stall speed is a possibility and adjustment of the approach speed may be appropriate. Consult the AFM or POH for guidance.
(4) A traditional concern in the operation of pneumatic boots has been ice bridging.
This is attributed to the formation of a thin layer of ice which forms to the shape of an expanding deicing boot without being fractured or shed during the ensuing tube deflation. As the deformed ice hardens and accretes additional ice, the boot may be ineffective in shedding the bridge of ice.
Studies done in the late 1990s have established that there are few, if any, documented cases of ice bridging on modern boot designs. In addition, several icing tunnel tests sponsored by the FAA since 1999 showed no ice bridging on modern boot designs. Known cases are confined to boots of designs dating back a half century or more.
b. Electroimpact System. The electroimpact system deices a surface using pulses of energy to produce rapid flexing movements of the airplane’s skin surface, which break the bond of accumulated ice and the shattered ice is then carried away by the airflow. This system is the least commonly used.
c. Electrothermal System. The electrothermal system deices a surface by heating the surface to a temperature above freezing to break the bond of accumulated ice. The shattered ice is then carried away by the airflow. The surface is allowed to cool to allow ice to form, and the heat is activated again to shed the ice, thus repeating the cycle. Such systems are common on propellers and helicopter main rotors, and have been recently introduced on wing and tail leading edges.
Par 3-4 Page 21
(1) Propellers are deiced using rubber boots with embedded heater wires to break the adhesion of ice to the propeller blades. Sometimes the blades are heated alternately in sections due to limits of available electrical power. The alternate sections are heated symmetrically to avoid an imbalance of the propeller while sections of ice are being removed and dislodged from the propeller by centrifugal force. Often, on aircraft that have such systems, the skin surrounding the airframe is reinforced with doublers to strengthen the skin where ice is most likely to be flung from the propellers. However, the initial imbalance caused by ice accumulation and the loud noise created by ice shedding and hitting the airframe can be unsettling to passengers and distracting to flightcrews.
(2) Intercycle and residual ice can accrete on airplanes with electrothermal deicing systems. It is typical for these systems also to produce runback ice behind a protected area.
Because other parts of the aircraft, including part of the span of the wing, are not protected from ice, a drag increase from those areas will still be present. This is accounted for in the icing certification process, and the pilot can fly the aircraft safely by following the operating procedures in the AFM or POH. Residual ice and the ice that accumulates between deicing cycles can be expected to have some effect on CLmax, but note that this effect is significant only at higher AOA.
3-5. ANTI-ICING SYSTEMS. An anti-icing system is designed to keep a surface entirely free of ice throughout an icing encounter. Anti-icing protection for wings is normally provided by ducting hot bleed air from the engines into the inner surface of the wing’s leading edge or through an evaporative or running wet system. Anti-icing systems are designed to be turned on prior to encountering icing conditions. Operating them as deicing equipment may result in a system failure or damage.
a. Bleed Air Systems. Bleed air systems are used for larger areas of the aircraft, such as engine nacelles and wing leading edges. Bleed air from turbine engines is the most common type of anti-icing protection for engine nacelles and wings of transport and business turbojets. Hot air is distributed to piccolo tubes, which consist of a perforated pipe installed directly behind the airplane’s skin. Such hot air systems are quite effective in preventing the formation of ice.
NOTE: One drawback of a hot air system is that tapping air from the engine to anti-ice large surfaces affects engine temperature limits that require reduced power settings, which reduce the performance of the engine(s). This may have a significant effect on climb performance (especially one engine inoperative climb performance in multiengine turboprops, turbojets, and turbofans). This performance loss is the reason this system is not common on smaller turbine powered airplanes. Pilots should keep in mind that while cruising or descending with anti-ice systems on, higher-than-normal power settings may be required to ensure sufficient bleed air is being supplied to the anti-ice system, and to prevent engine surges/stalls (see the particular AFM for the appropriate settings.) In addition, in some situations on some airplanes, the hot air system may not fully evaporate all impinging water drops, resulting in runback ice. This may occur with an inoperative engine, and may be the reason your AFM requires a minimum engine power setting on descent.
Par 3-5 Page 22
b. Evaporative/Running Wet Systems. Evaporative or running wet systems are newer designs used on smaller jet aircraft, turbopropeller, and piston aircraft. These chemical systems apply a chemical agent that lowers the freezing point of water found on aircraft surfaces and decreases the friction coefficient of those surfaces to prevent ice from adhering to the surfaces.
Examples of such chemical agents are isopropyl alcohol and ethylene glycol. This wing anti-ice system may be running wet by design, forming runback ice accretions. The effects of these accretions are evaluated during certification, but only in 14 CFR part 25 appendix C icing conditions.
NOTE: While an aircraft’s AFM or POH is the ultimate authority on the operation of anti-icing systems, a good rule of thumb is to activate anti-icing systems at the first signs of visible moisture encountered during conditions conducive to icing. This will prevent the buildup of any appreciable amounts of ice.
3-6. EFFECTS OF ICING ON ROLL CONTROL.
a. Ailerons. This paragraph is in effect a continuation of the previous one, since ice on the wings forward of the ailerons can affect roll control. The ailerons are generally close to the tip of the wing, and generally a stall starts near the root of the wing and progresses outward. In this way, the onset of stall does not interfere with roll control of the ailerons. However, the tips are usually thinner than the rest of the wing, and so they most efficiently collect ice. This can lead to a partial stall of the wings at the tips, which can affect the ailerons and thus roll control.
b. Airflow. If ice accumulates in a ridge aft of the boots, but forward of the ailerons, possibly due to flight in SLD conditions, this can affect the airflow and interfere with the proper functioning of the ailerons, even without a partial wing stall at the tip.
(1) This is the phenomenon that the NTSB found to be responsible for the accident of an ATR-72 turbopropeller aircraft in Roselawn, Indiana in October, 1994. Flight test investigations following the accident suggested two ways in which the ailerons might be affected by ice in front of them.
(2) One has been termed “aileron snatch,” in which an imbalance of forces at the aileron is felt by the pilot of an aircraft without powered controls as a sudden change in the aileron control force. Provided the pilot is able to adjust for the unusual forces, the ailerons may still be substantially effective when they are deflected. The other is that ailerons may be affected in a substantial degradation in control effectiveness, although without the need for excessive control forces.
3-7. TAILPLANE ICING.
a. Downward Lift. Most aircraft have a nose-down pitching moment from the wings because the center of gravity (CG) is ahead of the center of lift. It is the role of the tailplane to counteract this moment by providing downward lift (see Figure 3-8, Tail Down Moment). The result of this configuration is that actions that move the wing away from stall, such as deployment of flaps or increasing speed, may increase the negative AOA of the tail. With ice on the tailplane, it may stall after deployment of flaps (see Figure 3-9, Pitchover Due to Tail Stall).
Par 3-7 Page 23 FIGURE 3-8. TAIL DOWN MOMENT
b. Tailplane Stall. Since the tailplane is ordinarily thinner than the wing, it is a more efficient collector of ice. On most aircraft, the tailplane is not visible to the pilot, who therefore cannot observe how well it has been cleared of ice by any deicing system. Thus, it is important that the pilot be alert to the possibility of tailplane stall, particularly after full flap deflection, on airplanes not evaluated for susceptibility. A no-flap landing should be considered to avoid a tailplane stall, consistent with AFM procedures. Tailplane stall is discussed in detail in Chapter 5, paragraph 5-12.
FIGURE 3-9. PITCHOVER DUE TO TAIL STALL
c. Testing and Analysis. On many transport turbojets, the tailplane has no ice protection.
However, the tailplanes on these aircraft are usually quite thick and therefore are a less efficient collector of ice. Furthermore, these aircraft are subjected to extensive certification testing and analysis to ensure that the tailplane will not be placed at such an extreme angle in actual operations to experience a stall, even with a large ice accretion.
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
Par 3-8 Page 24 (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
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
Par 3-11 Page 25 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.
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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
Par 3-12 Page 27 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.
Par 3-12 Page 28 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
Par 3-13 Page 29 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.
Par 3-15 Page 30
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,
Par 3-16 Page 31 (and 32) 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.